Disease diagnosis and treatment benefit greatly from multi-modal approaches. In addition to a range of other benefits, nanoparticles of various compositions can provide these multimodal approaches. For example, nanoparticles can provide treatment through drug delivery along with hyperthermia or photodynamic therapy. Iron oxide nanoparticles (IONPs), which can provide magnetic resonance image contrast and therapeutic heat, are emerging as a key player in this field. Combining these properties with the controlled release of drug further enhances the therapeutic efficacy of the IONP system. However, IONPs on their own are subject to disadvantages such as aggregation and dissolution in biological environments. Additionally, controlled release of drug from IONPs has been limited by low drug loading. In this work, we employ a mesoporous silica coating to confer IONPs with colloidal stability, protection against oxidation and dissolution, and a large pore volume for drug loading, making this platform a valuable tool for multi-modal therapy. In this portion of the study, this platform is used as a controlled-release system whereby IONP-generated heat acts as the controlled release mechanism.
Previous work has demonstrated that surface modification with a short polyethylene glycol chain and a hydrophobic chloro-trimethylsilane group allows high loading of a cancer therapeutic, Doxorubicin. Further, the hydrophobic form of this drug can be retained within the particle for over 24 hours without the use of any additional capping agent simply due to hydrophobic-hydrophobic interactions. Herein, we investigate system-wide temperature increases (via IONP-heating or a water bath) as a driver for increased molecular diffusion resulting in drug delivery. The effects of various drug cargoes, pore diameters, heating profiles, and release media will be presented.

Magnetic nanoparticles, (MNP), are the building-blocks for developing innovative nanodevices with multi-fold therapeutic and diagnostic activities, including magnetic fluid hyperthermia, (MFH), contrast agents for Magnetic Resonance Imaging, (CA-MRI), and targeting of tumor cells. Among the different functionalization strategies developed so far, the approaches using protein-cage structures, like those of the ferritin (Ft) family, are particularly promising, since Fts present a number of favorable properties: they have high solubility and stability in water, blood and buffers, low toxicity, and they can be easily functionalized through genetic engineering and/or chemical reactions involving one of the many chemical groups exposed to the exterior (primary amines, carboxylates, thiols).
In this contribution we present some examples of the application of a human H chain ferritin, HFt, for the realization of magnetic-based theranostic agents for the treatment of melanoma. We will show how, through the fine tuning of the composition of the ferrite nucleus grown within the proteic shell of HFt, this system can become an efficient heat mediator in the tumor treatment via MFH. Indeed, the main constraint of HFt-based MNPs is that their size cannot exceed the protein shell inner diameter (ca. 8 nm). As far as iron oxide is concerned, this size is large enough for MRI application, but it is too small for MFH, as theoretical and experimental studies demonstrated that the maximum MFH efficiency is reached for magnetite MNP of d=16-18 nm, while very poor effects are expected for d<10 nm.[1]
In particular, we will focus on highly monodisperse doped iron oxides NPs mineralized inside a genetically modified variant of HFt, carrying several copies of alfa-melanocyte-stimulating hormone peptide, which has excellent targeting properties towards melanoma cells with high selectivity,[2] and conjugated to polyethylene glycol molecules to increase their in vivo stability.
[1] L. Lartigue et al. J. Am. Chem. Soc. 133, 10459 (2011)
[2] L. Vannucci et al., Int. J. of Nanomed. 7 1489 (2012)

Radiotherapy bears the risk of long-term adverse effects as being severe cardiac complications. Strategies to improve radiotherapy therefore aim to increase the impact on the tumor or to decrease the radiation dosage in order to prevent the healthy tissue from damage. We could show that superparamagnetic iron oxide nanoparticles (SPION) may increase the therapeutic efficiency of radiotherapy by catalyzing and, thereupon, enhancing the X-ray induced production of reactive oxygen species (ROS). Two different synthesis routes were followed to prepare citrate-coated SPION that have maghemite and magnetite structures and sizes between 3 and 20 nm. Human breast cancer (MCF-7), mouse human colon cancer cells (Caco-2) and mouse fibroblasts (3T3) cells were incubated with uncoated or citrate-coated SPION and exposed to X-rays at a single dose of 3 Gy or left non-irradiated. The ablation effect of X-ray irradiation for SPION surface structures was verified using XANES spectroscopy. The ROS concentration was measured via the fluorescence intensity of the 2&’,7&’-dichlorofluorescein dye. Obviously, all internalized SPION species resulted into an enhancement of ROS formation and an additional enhancing effect due to X-ray treatment was as well manifest. Citrate-coated SPIONs in X-ray treated cells were observed to provide an enhancement in ROS formation for 240 % when compared with X-ray treated cells without internalized SPION. On the other hand, SPION internalized in non-irradiated cells caused an increase of ROS concentration up to 77 %. This implies that SPIONs do not only enhance the efficiency of X-rays on ROS formation. They also may act via their surfaces as catalyst for the Haber-Weiss cycle and moreover, contribute via releasing iron ions to the generation of ROS through the Haber-Weiss and Fenton reaction. The substantial enhancing effect due to the high-energy X-ray treatment is explained with efficient ablation of the SPION surface chemistry. The freshly formed surfaces consist now of easier accessible iron ions and thus may more effectively catalyze the ROS production than completely coated surface. The catalytic function of SPION surfaces has a promising potential for radiotherapy.

Different techniques can be used to image the biodistribution of a molecular marker in vivo, each with its own advantages and limitations. The combination of complementary modalities has attracted much attention in the past few years. For example, MRI allows in-depth whole body imaging albeit with a low sensitivity. Near infrared fluorescence imaging is more sensitive and provides resolution down to sub-cellular levels but is limited in depth due to the diffusion of light by tissues. The design of multimodal magnetic and NIR-emitting nanoparticles offers the opportunity to combine advantages from both imaging modalities.
We present here the solvothermal synthesis of multimodal probes based on Zn-Cu-In-(S,Se)/ZnS quantum dots (QDs). Their emission is tunable over the whole “optical therapeutic window” (700-900nm). The growth of manganese containing materials (Mn(x)Zn(1-x)S or MnOx) on these QDs confers them a (super)paramagnetic character. The nanoparticles are characterized by transmission electron microscopy, X-ray diffraction, electron paramagnetic resonance, inductively coupled plasma atomic emission spectroscopy and SQUID. Surface modification with polyethylene glycol allows the transfer of our nanoparticles in water. They exhibit longitudinal relaxivities on the order of 1000mM-1(QD).s-1. We finally show that these nanoparticles can be used in vivo to image lymph nodes by both MRI and NIR fluorescence imaging.

Cardiovascular disease remains the leading cause of death worldwide. As such, clinicians increasingly use magnetic resonance imaging (MRI) as a diagnostic tool to visualize the vasculature. To enhance the diagnostic capability of the imaging modality, it would be of great benefit to introduce a contrast agent that would allow for early and local diagnosis of diseased blood vessels within a patient. MRI, however, has low sensitivity to its imaging probes, thus requiring high accumulation of contrast agent in target tissue. This challenge has prompted extensive efforts to improve the contrast efficiency, or relaxivity, of superparamagnetic iron oxide nanoparticles (SPIONs), often used as negative contrast agents in MRI. A particularly successful strategy is the controlled clustering of SPIONs, which allows the overall diameter of the clustered material to be tuned within an optimal range for size-dependent relaxivity enhancement. Most clustered SPION formulations, however, still achieve a relaxivity of only 70% of their theoretical maximum. We hypothesize that a major reason for the limited enhancement is due to the properties of the cluster-inducing molecular layer. Here, we present a unique polymer that can tailor the SPION cluster to a desired size and impart a surface hydrophilicity to enhance interaction with surrounding water and bring the relaxivity within 3% of the theoretical maximum for the magnetic material. The resulting SPION cluster represented a six-fold improvement compared to commercial, unclustered SPIONs and was able to locally highlight a site of ischemia within a mouse hindlimb. We believe that the hydrophilic SPION cluster will be broadly useful in improving diagnostic quality of various acute, chronic, and malignant diseases.

Each year, considerable number of people are diagnosed with cancer and the risk is predicted to be increased in developing countries in the coming decade. Having a share of 13%, cancer is still the number one reason of death in the world. Anti - Cancer drugs that are used for treatment may have toxic effects which can cause severe complications in cancer patients. As a consequence it is necessary to design a drug delivery system to reduce the side effects and the amount of drug administered in the body.
Our work focuses on carrying and releasing the anti - cancer drug temozolomide which is widely used against a brain tumor, 4th degree astrositoma at the tumor site. Due to the drugs high hydrophobicity, the bioavailability is increased by the use of cyclodextrins, a type of oligosaccarides composed of glucose units. Cyclodextrin - Temozolomide complex is then carried via magnetic nanoparticles that are targeted to the tumor via external magnetic field.

Synthesis of magnetic iron oxide nanoparticles is performed by aqueous co-precipitation method in the presence of cyclodextrins. High pressure liquid chromotography (HPLC) and UV spectroscopy show effective capture of temozolomide in the hydrophobic core of cyclodextrins and its corresponding release. Results further show that it is possible to target the complex that is flowing inside a channel at a specified location by applying an external magnetic field in a reversible manner.
This work shows that cyclodextrin-magnetite complexes are potential candidates for the targeted delivery of toxic hydrophobic drugs using an external magnetic field.

Magnetic Particle Imaging (MPI) is a novel non-invasive biomedical imaging modality that uses safe magnetite nanoparticle contrast agents as tracers. MPI generates positive contrast images directly originated from the tracers and its predicted spatial resolution (up to sub-milimeters) and tracer mass sensitivity (~ nanograms of the tracers) make it a potential technique for cancer imaging and stem cells tracking. Whilst controlled synthesis of iron oxide nanoparticles (NPs) tracers with tuned size-dependent magnetic relaxation properties has contributed significantly to the development of MPI, additional functionalization of the same tracers for other imaging modalities such as MRI and fluorescent imaging would accelerate screening of the synthesized tracers based on their in vitro and in vivo performance in animal experiments. Here, we synthesized highly monodispersed NPs, used ligand exchange to introduce carboxyl groups on their surface and then conjugated three different types of PEG (NH2-PEG-NH2, NH2-PEG-SH and NH2-PEG-FMOC) to these NPs by amide bonding. Further, we labeled these NPs by cy5.5 near infra-red (NIRF) molecules through a click chemistry reaction. Bi-functional PEG (NH2-PEG-NH2) resulted in a larger hydrodynamic size (~100nm) of the tracers, due to linkage between some particles by amide bonding at both ends of the PEG molecules. We found that formation of these aggregates changes the MPI performance and pharmacokinetics of these multimodal contrast agents in mice animal models. Conventional T2 MRI protocols helped to quantify the biodistribution of these MPI tracers, while NIRF scanning of the excised tissues and organs enabled anatomical mapping of the tracers biodistribution and clearance pathway in reticuloendothelial organs. We believe that combination of the unique capabilities of each of these imaging modalities with the synthesis-dependent MPI performance of these NPs expedites the MPI progress toward future clinical applications.

Recently our group developed several kinds of MRI contrast agents using uniform-sized iron oxide nanoparticles. For example, using 3 nm-sized iron oxide nanoparticles, new non-toxic MRI contrast agent was realized for high resolution MRI of blood vessels down to 0.2 mm, which can be potentially applied to early diagnosis of cancers, stroke, and cardiovascular diseases.
We fabricated several kinds of magnetite nanoparticle-based nanostructured materials for magnetically separable and recyclable catalysts. A simple synthesis of noble metal (< 6 nm-sized Pd or Rh)- 20-30 nm-sized Fe3O4 heterodimer nanocrystals was achieved by controlled one-pot thermolysis of a mixture solution composed of Fe(acac)3 and Pd(acac)2 (or Rh(acac)3) in oleylamine and oleic acid. The nanocrystals catalysts exhibited good activities for various organic coupling reactions such as Suzuki coupling reaction and reduction of nitroarenes and alkenes. We reported one-pot synthesis of magnetically recyclable mesoporous silica catalyst for tandem acid-base reactions. Highly active magnetically recyclable hollow nanocomposite catalysts with a permeable carbon surface have been prepared for selective reduction of nitroarenes and Suzuki cross-coupling reactions. These catalysts could be easily separated by a magnet, and recycled consecutively.

Superparamagnetic magnetite (Fe3O4) nanoparticles have been extensively studied for hyperthermia therapy and as MRI contrast agents. By the introduction of porosity in Fe3O4 we can produce a multifunctional biomedical agent capable of hyperthermia treatment, controlled drug release triggered by magnetic heating, as well as imaging contrast enhancement. There are various synthetic methods for Fe3O4 nanoparticles, but only limited reports on the introduction and control of porosity in Fe3O4 without using sacrificial templates. Herein we used an iron complex with a mildly explosive organic ligand as the precursor and successfully obtained hollow and porous microspheres composed of Fe3O4 nanocrystals (~26 nm) using a one-step, template-free ultrasonic spray pyrolysis synthesis. The specific surface area of hollow Fe3O4 microspheres was measured to be as high as ~200 m2/g, which is comparable with the highest value reported to date. The porous structure is mostly due to the gas products produced during the decomposition of the energetic iron complex, which was also studied using TGA/DSC. Radiofrequency magnetic heating tests on the porous Fe3O4 were performed in a homemade solenoid with alternating current magnetic field (62 KHz). A heating rate of more than 1°C per minute was achieved for the aqueous suspension of porous Fe3O4, which is practical for hyperthermia therapy. Drug release and further structural/functionality optimization studies are underway to achieve slow initial drug release, stimulated drug release with radiofrequency magnetic heating, as well as higher magnetic heating rate.

Owing to their stability, non-toxicity, and high saturation magnetization, superparamagnetic iron oxide nanoparticles (SPIONs) have been an increasingly attractive contrast agent in magnetic resonance imaging (MRI) in various biomedical applications, in which SPIONs&’ surface is required to be hydrophilic and biocompatible. We report here the design and synthesis of a small and water-soluble zwitterionic dopamine sulfonate (ZDS) ligand. Following ligand exchange, the ZDS coated SPIONs (ZDS-NPs) showed small hydrodynamic diameters (~10 nm), high saturation magnetization (74 emu/g [Fe]), and stability with respect to time, pH, and salinity. ZDS-NPs also exhibited only small non-specific uptake into HeLa cancerous cells in vitro and demonstrated low non-specific binding to serum proteins in vivo in mice. Furthermore, following conjugation with streptavidin and dye the resulting ZDS-NPs nano-complexes were able to specifically target biotin receptors.
References:
1. Wei H, Insin N, Lee J, Han HS, Cordero JM, Liu W, Bawendi MG. “Compact Zwitterion-Coated Iron Oxide Nanoparticles for Biological Applications.” Nano Letters 2012, 12: 22-25.
2. Wei H, Bruns OT, Chen O, Bawendi MG. “Compact zwitterion-coated iron oxide nanoparticles for in vitro and in vivo imaging.” Integrative Biology 2013, 5: 108-114.
3. Chen O, Wei H, Maurice A, Bawendi MG, Reiss P. "Pure colors from core-shell quantum dots." MRS Bulletin 2013, 38: 696-702.

Geometric confinement is paramount to understand the magnetization processes observed in nanoscale ferromagnets, as the typical size of objects approaches characteristic magnetic length scales such as the exchange length or the domain wall width. Among the most interesting geometries for its potential application in ultrahigh density storage are continuous magnetic thin films with periodic arrays of holes, the antidots structures [1,2]. Antidot arrays allow modifying the magnetic properties of ferromagnetic films such as anisotropy, coercivity, remanence and magnetoresistance, by tuning the geometry of the arrays, in particular their size, shape and periodicity [3].
Macroscopic characterization provides indirect information on the magnetization states of antidots arrays. On the other hand, most microscopy imaging techniques provide partial information or lack the spatial resolution for a detailed characterization of closely spaced antidots arrays [4]. Particularly in Lorentz microscopy (LM), the superimposition of pure geometrical contrast limits the information that can be retrieved from the magnetic contrast. In this work, we have implemented a new approach to extract magnetic information by filtering the contrast produced by the periodic structure of the holes in LM images.
Cobalt antidots have been fabricated by focused ion beam etching of arrays of holes in 10-nm-thick cobalt films grown by sputtering on a 50-nm-thick Si3N4 membrane. Rectangular lattices of circular holes with 40 nm of diameter were produced where the distance between holes was varied from 560 to 120 nm. In arrays with a hole separation below 160 nm, Fresnel fringes mask the domain wall structures inside the arrays. Applying Fourier filtering of the diffraction contribution due to the rectangular lattices, we improve the visualization of the magnetic contrast and observe the presence of magnetic superdomains separated by horizontal and vertical stripes of magnetic contrast (superdomain walls). We have applied the Transport-of-Intensity Equation procedure to reconstruct the in-plane magnetization of the antidots arrays in Fourier-filtered LM focal series and obtain magnetization maps revealing the true orientation of magnetization inside the superdomains. In situ LM upon magnetic field has also been carried out to analyze the magnetization reversal processes of the antidot array. The magnetization switching along the easy axis occurs in two stages: nucleation and propagation of superdomains walls along the direction of the magnetic field, therefore along the perpendicular axis. These results correlate with the predictions of micromagnetic simulations.
[1] Z. L. Xiao, et. al. Nanotech, 3. 357 (2003)
[2] L. Torres, et. al. Appl. Phys. Lett., 73, 3766 (1998)
[3] C. C. Wang, et. al. Nanotechnology, 17 1629 (2006)
[4] Magnetic Microscopy of Nanostructures, ed. by H. Hopster and H. P. Oepen (Springer-Verlag Berlin Heidelberg, 2004).

Magnetic nanoparticles and nanowires present a large variety of magnetic properties. For instance, magnetic domain walls (DW) in nano-cylinders are model systems to go beyond the classical Walker limit. Fe nanocubes are very interesting for their hyperthermia applications. We developed a method combining magnetic electron holography with micromagnetism to obtain unprecedented resolution of the 3D structure at remanence of the internal DW structure in 55 nm and 85 nm diameters Ni nano-cylinders, and magnetic mapping inside of Fe nanocubes.
Ni nanowires grown by electro deposition have been recovered after dissolution of the membranes on a carbon foil for structural and magnetic TEM imaging. They present a polycrystalline structure with randomly oriented grains, leading to a random distribution of crystalline anisotropy and grain sizes roughly equal to the wire diameter. Off-axis electron holography experiments have been performed on single nano-cylinders. Before imaging, a 2 Tesla magnetic field was applied perpendicular to the nanowire axis (same direction than the electron path) using the TEM objective lens to favour the nucleation of transverse walls. Micromagnetic 3D calculations have then been computed using the OOMMF code to simulate the DW configuration for the two wire diameters. To fit the electron holography experiments, the electron beam phase shift images induced by the resulting magnetic state in the whole (X, Y, Z) space has been calculated from the micromagnetic simulations.
This method was applied to demonstrate the occurrence of a magnetic transition at remanence from a transverse wall (55 nm) to a hybrid magnetic state (85 nm) depending on the nano-cylinder diameter. Our electron holography experiments evidence the appearance of transverse walls in Ni nanocylinders which can be easily nucleated by saturating the sample perpendicular to the wire axis. As these are perfect objects to test the massless DW concept we believe they can be used to develop future spintronics devices.
Single crystal Fe nanocubes elaborated by organo-metallic chemistry have been studied by the same process using the new Hitachi TEM dedicated to Lorentz microscopy and electron holography with an unprecedent spatial resolution of 0.5 nm. Fe particles present sizes in the 15-30nm range. If monodomain states (flower state) have been observed, we will show unexpected vortex states as a function of the cube size and the applied magnetic field. The micromagnetic simulation of these magnetic states indicates changes in Fe properties compared to the bulk ones.
N. Bizière, C. Gatel, R. Lassalle-Balier, M.-C. Clochard, J.-E. Wegrowe and E. Snoeck, Nano Lett. 13, 2053-2057 (2013)
L.-M. Lacroix, S. Lachaize, F. Hue, C. Gatel, T. Blon, R. P. Tan, J. Carrey, B. Warot-Fonrose, and B. Chaudret, Nano Lett. 12, 3245-3250 (2012)
E. Snoeck, C. Gatel, L.-M. Lacroix, T. Blon, S. Lachaize, J. Carrey, M. Respaud and B. Chaudret, Nanoletters 8, 4293-4298 (2008)

A magnetic skyrmion is a nanometer-scale vortex-like spin structure with topological stability. Since discovery of skyrmions in a B20-type compound of MnSi[1], various properties have been revealed: Lorentz transmission-electron-microscopy (TEM) studies provided firm evidences of skyrmion formation and also found that thin films of B20-type compounds ubiquitously host stable skyrmions over a wide temperature - magnetic field region[2]. Near room-temperature formation of skyrmion crystal was observed in B20-type FeGe[3]. Moreover, motions of skyrmions at low current density (10^5 - 10^6 A/m^2) indicate possible electrical manipulation[4]. Such electrical controllability as well as their stable particle nature highlights potential applications for novel spintronic devices. In this talk, we will present bulk magnetic properties of B20-type germanium compounds, including Lorentz TEM observation of skyrmions[5]. Emergence of 3-dimensional skyrmion crystal state is also discussed in terms of topological Hall effects induced by Berry-phase mechanism[6]. We will further show robust formations of skyrmions in epitaxial thin films of B20-type compounds, which also produce the topological transport phenomena[7]. This work was done in collaboration with a FIRST program “Quantum Science on Strong Correlation” led by Yoshinori Tokura.
[1] S. Mühlbauer et al., Science 323, 915 (2009).
[2] X. Z. Yu et al., Nautre 465, 901 (2010).
[3] X. Z. Yu et al., Nautre Materials 10, 106 (2011).
[4] F. Joneitz et al., Science 330, 1648 (2010).
[5] K. Shibata et al., Nature Nanotech. 8, 723 (2013).
[6] N. Kanazawa et al., Phys. Rev. Lett. 106, 156603 (2011); N. Kanazawa et al., Phys. Rev. B 86, 134425 (2012).
[7] Y. Li et al, Phys. Rev. Lett. 110, 117202 (2013).

Magnetic oxide nanoparticles and hetero-systems of reduced dimensionality have attracted a lot of attention both from the stand point of basic research and for their foreseen applications. The exchange coupling in bi-magnetic core/shell nanoparticles has attracted considerable attention since the reduced dimensionality environment along with the presence of the interface may result in novel behaviors not present in bulk. In this work we analyze bi-magnetic core/shell nanoparticles based on nominal Fe3O4 and Mn3O4 oxides, by means of aberration-corrected scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) and theoretical calculations. As we will show, atomic scale studies are challenging because this system is very sensitive to the electron beam, and low voltages are required. Simultaneous studies of the structure, the chemistry and also the electronic properties show that the samples are very high quality, with coherent interfaces between core and shell, and that the composition is indeed Fe3O4 / Mn3O4. The Mn-oxide shell is not continuous, and is easily reduced to MnO at 200 kV, we will discuss this phase transformation and its effects on the system structure and chemistry.
Research supported by the European Research Council Starting Investigator Award STEMOX # 239739 (M.R. and J. S.),by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SJP, MV) and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Although lattice defects are known to severely degrade device properties, they may be utilized to locally apply functionalities in bulk materials. As one of the important lattice defects, dislocations -one-dimensional lattice defects with locally distinct atomic-scale structures - sometimes exhibit unique functional properties. For example, conductive atomic-scale wires are formed in insulating materials by segregating dopant atoms into dislocations [1]. In addition, we have succeeded in applying magnetic property to non-magnetic material dislocations [2]. In our previous study, high-density of dislocations were introduced into antiferromagnetic NiO single crystalline thin film. The dislocations show local ferromagnetic property without any dopant. The ferromagnetic dislocations show extremely large magnetic coercive force over 4 T and high Curie temperature around 520 K. By combining atomic-scale local electron energy loss spectroscopy (EELS) analysis and first principle calculations, we have revealed that the ferromagnetic property at the dislocations is according to Ni vacancies introduced along dislocation cores. For the effective usage of dislocations as functional structural elements in crystalline materials, it is desired to have a guideline to control their densities and arrays inside materials. However, there is no such guideline available at hand.
In the present study, we show the method to control the density and array of magnetic dislocations in NiO thin films by changing several factors such as lattice mismatch between single crystalline NiO thin film and substrates, post annealing temperatures and annealing atmospheres. NiO single crystalline thin film was deposited by PLD on SrTiO3 and KTaO3 single crystalline substrates. The lattice mismatch between NiO and substrates (SrTiO3 and KTaO3) is 7% and 5%, respectively. Crystallinity was improved by post annealing in air after the film deposition. The density of dislocations are successfully controlled by changing substrate. The density is 5 times higher in the film deposited on SrTiO3 substrate compared to the film on KTaO3 substrate. The relative position of dislocations are controlled by changing post annealing temperature. The dislocations are randomly distributed in the sample annealed at 1373K and they lined up along sub-grain boundary in the film annealed at 1423K. The controlled dislocations also show ferromagnetic property. Coercive force and Curie temperature of the dislocations are found to be the same with the previous results and EELS profile also show the same tendency with the previous one, so the distance or density of the dislocations is not affecting the magnetic property.
[1] A. Nakamura, et al., Nature Mater. 3 (2003) 453.
[2] I. Sugiyama, et al., Nature Nanotechnol. 8 (2013) 266.

Magnetic domain structures of the sintered Nd-Fe-B permanent magnets have been investigated in detail using Kerr microscopy, whose magnetic field has reached 2T, which is the highest magnetic field produced by the same measuring technology. The high-coercivity magnet has reached the saturated state so as to observe the magnetic domain structures easily. The reversal of domain structure in magnetization and demagnetization process of Nd-Fe-B magnets was researched. The change in microstructure caused by the distribution of Nd-rich leads to a decoupled magnetization reversal. The following phenomena was clearly observed that the magnetization reversal appears independently in each grain in a high-coercivity magnet which contains adequate Nd-rich phases along the grain boundaries, whereas the magnetization reversal appears simultaneously in a few grains in NdFeB magnet in which the Nd-rich phase along the grain boundaries is absent. It concludes that the domain passing through grain boundaries was primarily reason for making coercivity of NdFeB magnets decrease.

Magnetic properties of perovskites and related oxides are strongly affected by strain, chemical composition, and octahedral tilts. In thin films and heterostructures, distortions and local composition can vary on the scales down to atomic, making atomic-scale characterization necessary for understanding magnetic behavior. Quantitative aberration-corrected scanning transmission electron microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) can provide direct structural and chemical information at the unit cell level. First principles calculations can further be used to uncover the underlying mechanisms and develop strategies for engineering the desired behaviors.
Using this approach, we were able to investigate magnetic dead layer phenomenon in a series of LaCoO₃/SrTiO₃ thin films and heterostructures, connecting it directly to octahedral tilt behavior. We have also demonstrated that differences in magnetic behavior between La_0.5Sr_0.5CoO₃ films grown on two different substrates can be traced back to differences in the overall oxygen stoichiometry.[1] Finally, we have discovered a patchwork of different magnetic states at the interface of brownmillerite La_0.5Sr_0.5O_2.5 with NdGaO₃, while the interface with LSAT was found to be magnetically homogeneous. Prospects of atomic scale determination of magnetic order will also be discussed.
Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE.
References:
[1] Y.M. Kim et al., Nature Mater., 11, 888 (2012).

Among the ferrites, the cobalt ferrite CoFe2O4 has attracted much attention due to its unique and interesting structural, magnetic and optical properties which have great potential in many applications such as magnetic data storage, sensors, catalysts and drug delivery. In the present work, nanocrystalline CoFe2O4 samples have been synthesized with the help of co-precipitation method by taking pH = 7, 10 and 13 to study the effect of pH variation on structural characteristics, optical and magnetic properties of the samples. X-ray diffraction (XRD) results indicate the formation of nanocrystalline single phase mixed spinel structure with space group Fd-3m in the grown sample. Rietveld refinements of the XRD data reveal structural distortion at octahedral site due to the migration of Co2+ ions from tetrahedral to octahedral site and Fe3+ ions from octahedral to tetrahedral sites. The transmission electron microscope (TEM) and field emission scanning electron microscope (FESEM) results show that particle size decreases as value of pH increases. The UV-Vis spectra show two band gaps in all samples and the observed values of band gaps are ~ 1.43, 0.6 eV for pH=7; 1.48, 0.62 eV for pH=10 and 1.64, 0.95 eV for pH=13. The field dependence of magnetization (M-H) curves, measured at room temperature, show the ferromagnetic behavior with saturation magnetization ~ 82.36, 81.54 and 76.36 emu/gm for pH=7, 10 and 13, respectively. Thus, the value of saturation magnetization decreases with decreasing particle size. The temperature dependent magnetization (M-T) of CoFe2O4 (pH=7, 10, 13) samples measured at the applied magnetic field ~ 100 Oe show bifurcation in field cooled and zero field cooled M-T plots at TB. The value of TB decreases as the pH of samples increases, i.e as particle size decreases. The M-H plots measured at 5 K show an interesting metamagnetic transition in all samples. The correlation of the observed optical and magnetic properties with the structural characteristics of the samples will be described and discussed in this paper.

CoFe₂O₄ possesses attractive properties owing to its high coercivity, magnetocrystalline anisotropy and chemical stability[1]. When nanometer sized particles of this material are used, additionally superparamagnetic properties can be coupled to its applications. Thin films made from this material offer applications in high density storage areas as well as magnetoelectric composite manufacturing[1]. For fabrication of films, particle based solution deposition is advantageous since it provides control over the film thickness from tenth to hundreds of nanometers at shorter times with a high control over crystallinity, defined stoichiometry. In addition, possibilities for large scale up and low costs are attractive towards other vacuum-based techniques[2]. In this study, we report the structural, electrical, optical and magnetic properties of thin films manufactured via spin coating of nanoparticle dispersions of CoFe₂O₄. The nanoparticles are produced through an efficient and fast microwave assisted non-aqueous sol-gel route, yielding cobalt iron oxide particles of approx. 5 nm range[2]. For structural and morphological characterization, high resolution scanning electron microscopy, atomic force microscopy, confocal Raman microscopy and X-ray reflectometry are applied. The obtained results indicate that several hundred nanometer thick, uniform and crack free films of roughness below 1 nm are produced. Conductivity of the films is studied by making use of 4-probe electrical conductivity and discussion of determined activation energy relative to film processing. Electronic band gap of the films is determined through UV-vis measurements. Through the combination of electronic band gap probing and determination of overall electrical conduction, incl. electronic and ionic carriers, role of carriers types is discussed. Magnetic characteristics are examined via SQUID vibrating sample magnetometer to carry out room temperature B-H measurements, field cooling and zero-field cooling magnetic measurements. Conclusively, all characterization results showed that with this method CoFe₂O₄ films are manufactured with high quality and magnetic properties, which are possible to incorporate in several composite geometries with BaTiO₃ for multiferroic applications.
References
[1] Sun J, Wang Z, Wang Y, Li F. Structure and magnetic properties of CoFe2O4 nanocrystal thin films prepared by sol-gel method. 2011. p. 756-8.
[2] Kubli M, Luo L, Bilecka I, Niederberger M. Microwave-Assisted Nonaqueous SolGel Deposition of Different Spinel Ferrites and Barium Titanate Perovskite Thin Films. CHIMIA International Journal for Chemistry. 2010;64:170-2.

This study is mainly discussed the magnetic properties of nano-magnetite with morphologies of nano-particles, nano-rods, and nano-tubes. Three crystal morphologies of magnetite are prepared by using nano-hematite via carbon reduction method. The nano-particle of magnetite has a granular shape with the crystal size about 70 nm. The particle size of nano-rod is 75 nm width and 300 nm length, while nano-tube has a inner-diameter of 65 nm width and 250 nm length. It shows that all of magnetites have the ferro-magnetism by the superconducting quantum interference device magnetometer (SQUID) measurement. The coercivity of nano-particles, nano-rods, and nano-tubes is 78, 235, and 192 Oe, respectively, which has a remanence ratio (Mr/Ms) of 11/110, 17/77 and 17/81. The spatial distribution of magnetism are characterized by magnetic force microscopy (MFM). The MFM phase images exhibit bright and dark areas, implying ferro-magnetic domains in our samples. Furthermore, nano-particles, nano-rod and nano-tubes of magnetites show a complicated magnetic domain arrangement.

Inorganic nanoparticle/organic hybrid materials attract increasing attentions because of their beneficial properties of each phase. The authors reported the syntheses of ferrite nanoparticle (NP)/organic hybrids from metal-organics [1,2]. Manganese zinc ferrite is one of the soft ferrites and characterized by its high initial permeability, high resistivity, and low power loss. Manganese zinc ferrite has high frequency applications, such as magnetic recording materials, multi-layer chip inductor, and electromagnetic interference shielding. This paper describes the in situ synthesis of manganese zinc ferrite NP/cellulose hybrid nanocomposite from metal acetylacetonates below 100 centigrade. A mixture of manganese (II) acetylacetonate (MA), zinc acetylacetonate (ZA) and iron(III) 3-allylacetylacetonate (IAA) was hydrolyzed and polymerized yielding spinel oxide NP/organic hybrid. The hybrid was analyzed by FT-IR, UV-visible spectroscopy, DTA-TG, powder XRD, VSM, and SQUID. The formation of (Mn,Zn)Fe2O4 NPs in an organic matrix was confirmed by XRD analysis. The crystallite size of spinel particles was dependent upon the hydrolysis conditions of MA-ZA-IAA. Crystalline manganese zinc ferrite NPs below 5 nm were uniformly dispersed in the organic matrix. The magnetization of hybrid increased with increasing water amount for hydrolysis. The magnetization versus field curve for the manganese zinc ferrite NP/organic hybrid showed neither remanence nor coercivity above 11K. The magnetization versus H/T curves from 40 to 100K were superimposed on the same curve, and dictated by the Langevin equation. The remanent magnetization and coercive field of the hybrid were 2.1 emu/g and 20 Oe, respectively, at 4.2K. A manganese zinc ferrite NP/cellulose hybrid was free-standing, and transparent. The absorption edge of the hybrid was blue-shifted compared to that of bulk ferrite.
1. K. Hayashi, K. Maeda, M. Moriya, W. Sakamoto, and T. Yogo, J .Mag. Mag. Mater., 324, 3158 (2012).
2. K. Hayashi, R. Fujikawa, W. Sakamoto, M. Inoue and T. Yogo, J.Phys. Chem.C, 112, 14255 (2008).

Iron oxide nanoparticles (NPs) have attracted a lot of interest due to their many potential applications in areas including optoelectronics, magneto-optics, high density data storage, etc. In particular, iron oxides (Fe₃O₄ and Fe₂O₃) are also well suited for biomedical applications. We have investigated Faraday Rotation (FR) response for two types of Fe₂O₃) NPs (in aqueous suspension) that are of the same average diameter (10 nm) but differ in one important respect; one group consists of uncoated particles whereas the other group is functionalized with caffeic acid. This system is being investigated and characterized for use in tumor imaging applications. Faraday rotation (FR) refers to the rotation of the polarization vector of a light beam as it passes through a sample in the presence of a magnetic field. FR can reveal interesting material properties such as saturation magnetization and wavelength dependent Verdet constant of the material under investigation. The latter is a measure of the magnetically induced birefringence of the material. Typically FR setups rely on AC or DC magnetic fields. While these are valuable techniques with their own advantages, this work focuses on a pulsed field setup that can reveal dynamic information about the resulting magnetization, as the magnetic response of the sample is measured in the presence of short intense fields on the order of 0.6 Tesla and lasting approximately 100 milliseconds. All experiments are carried out at excitation wavelength of 633 nm (He-Ne wavelength).
The two NP samples show very different response to the field pulses. The NP systems investigated in this work show very unique short term and long term behavior revealing various time scales of interest. These unique characteristic times for the functionalized vs. uncoated particles provide valuable clues about the magnetization response of the NP and its relationship to the detailed structure of the NPs (core vs. shell). Magnetic response from these systems persists long after the magnetic field pulse has subsided. This can be related to the relaxation modes (Neel vs. Brownian) and as possible evidence of NP size dispersion. Additionally, the possibility of agglomeration is also discussed. We show how this analysis can be used to investigate the details of saturation magnetization. We will also present results from the more conventional techniques like TEM and magnetometry based magnetization measurements, and will thereby compare the results obtained from FR to those obtained from techniques that are more widely used for NP characterization. We hope to show that FR can be a reliable and complimentary technique that can be used to develop a detailed picture of the magnetic response of these NP systems.

Magnetic nanoparticles (MNP) currently appears as a subject of great interest in different scientific areas since size-dependent quantum effects, high surface area to volume ratio and single domains regimen usually result in unique magnetic properties. Among such areas, biomedicine seems to be the most promising because MNP can be applied simultaneously in disease diagnostics and therapies. This multifunctionality is usually achieved by performing different coatings on the MNP surface with functional compounds, such as inorganic shells, biocompatible molecules, fluorescent dyes, antibodies, genetic material, drugs, and others. However, such compounds present diamagnetic behavior, which reduces the magnetic response of the functionalized particle. An alternative is to use MNP with high saturation magnetization (M_S), such as metallic iron and cobalt. But an inconvenient of using such materials is their high reactivity in oxidizing environments and toxicity compared with magnetic oxides. To overcome this inconvenient, we have synthesized silica-coated iron MNP in a core-shell structure by using a three step experimental route. The route is composed of magnetite MNP synthesis, silica coating and thermal reduction under H₂ atmosphere steps. Spherical magnetite MNP with 15.2 nm were obtained by thermal decomposition of iron(III) oleate previously synthesized and purified in 1-octadecene solvent. Silica coating was performed by alkali-catalyzed hydrolysis of tetraethylorthosilicate (TEOS) in a reverse microemulsion system. By varying reaction time, TEOS concentration and magnetite core amount, silica shell thickness was tuned between 5 and 14 nm. In the reduction step, time, temperature and H₂ gas flow were varied in order to find the optimized condition that provides a completely reduced iron core. Reduced samples were passivated with ethanol vapor. Temperatures up to 600 °C and H₂ flow of 90 L/h are required for a completely reduce magnetite to metallic α-Fe. Passivation process renders a wüstite layer onto the particles surface. silica-coated iron MNP presented a M_S increasing of 21.4 % compared to the non-reduced sample and coercivity of 43 Oe, which means a behavior close to the superparamagnetism. The same reduction process performed for silica-coated magnetite MNP with a 31 nm magnetic core resulted in a M_S increasing of 93.4%. The higher α-Fe/wüstite ratio and minimized spin canting effect explains the higher M_S increasing for larger MNP. The core-shell structure remains unchanged after thermal treatments, which states the ability of silica shell in the prevention of sintering process. In addition, the silica shell onto the particle surface confers the MNP colloidal stability at pH values higher than 2.2. By combining the improved magnetic properties of the α-Fe core with the hydrophilicity, biocompatibility and colloidal stability of the silica shell, we believe to be found a promising material for in vitro and in vivo biomedical applications.

The current demand of technological devices requires the continuous stabilization of new compounds at the nanometer scale, their chemical and physical properties allowing the development of novel applications. Mn perovskite related mixed nanooxides constitute a promising system in the field of nanoscience and nanotechnology as a result of the structural variety and outstanding properties found in their bulk counterparts [1]. We report for the first time the stabilization and structural and magnetic characterization of single crystalline 4H-SrMnO3-δ nanoparticles. Stoichiometric 4H-SrMnO3.0 nanoparticles have been successfully synthesized from thermal decomposition, at 850 C under oxygen gas flowing atmosphere, of a new heterometallic precursor [SrMn(edta)(H2O)5]3/2H2O which crystal structure has been resolved by single crystal X-ray diffraction. From this precursor, highly homogeneous 4H-SrMnO3.0 nanoparticles with average particle size 70 nm are obtained. Local structural information, provided by atomically-resolved microscopy techniques, shows that 4H-SrMnO3.0 nanoparticles exhibit the same general structural features than the bulk material, although structural disorder, due to edge-dislocations, is observed. The nanosize of particles enables a topotactic reduction process at 220 C stabilizing a metastable 4H-SrMnO2.82 phase while a cubic related phase is always stabilized for anionic compositions below SrMnO2.98 in bulk material [2]. Although the insertion of cubic layers in bulk hexagonal perovskites has been previously established as a mechanism of anion vacancies accommodation, the use of local probe techniques allows observing the pathway of such process in the corresponding nanoparticles. SrMnO2.82 shows a very complex microstructure with a high concentration of extra cubic layers breaking the (hchc)-4H sequence and leading to a local re-arrangement of the cubic and hexagonal layers in the defect area. This complex process involving both anionic and cationic sublattices is here seen for the first time. Magnetic characterization of nano-SrMnO3.0 shows significant variations with respect to the bulk material. Besides the dominant AFM interactions, a weak FM contribution as well as exchange bias and a glassy-like component are present. After the reduction process, the stabilization of Mn3+ in the 4H-structure gives rise to magnetic anomalies in the 40-60 K temperature range.

Recently, magnetic nanoparticles (NP) have been studied as a consequence of the various applications that can take due to their different physical and chemical properties. Some of the possible applications are catalytic processes, ultra high density magnetic recording, and diagnosis and therapy in biomedicine. Ferrites are important nanoscale materials that can have their properties easily manipulated, therefore studies based on synthesis of ferrite NP and their properties is fundamental. In this work, the syntheses of MFe₂O₄ (M = Mn, Co or Co_(1-x)Mn_x) NP were studied and the control over chemical composition, morphology and magnetic properties was evaluated. The NP were synthesized by adaptation of the modified polyol process, which is based on the thermal decomposition of a metal precursor in high boiling point solvents, in the presence of surfactants. The reaction parameters, such as temperature and the reactants order of addition, were evaluated in order to obtain NP with the desired properties. The synthesized manganese, cobalt and mixed cobalt and manganese ferrites had the expected crystalline structure and, in general, also presented control over shape, size and size distribution according to the experimental conditions. CoFe₂O₄ NP showed octahedral shape and size about 40 nm, while MnFe₂O₄ and Co_(1-x)Mn_xFe₂O₄ NP were spherical with average size around 5 nm. Hot-injection synthesis must be used in order to obtain MnFe₂O₄ with narrow size distribution. However, the heating up process showed as good results as hot-injection for the synthesis of Co_(1-x)Mn_xFe₂O₄ and CoFe₂O₄, and all systems had good control over chemical composition. The samples of MnFe₂O₄ showed saturation magnetization (M_s) above 40 emu.g^-1, the cobalt ferrite exhibited value of M_s ~76 emu.g^-1 and the Co_(1-x)Mn_xFe₂O₄ NP showed intermediated M_s, around 50 emu.g^-1. Despite having the highest value of M_s, CoFe₂O₄ NP presented high value of coercive field, near 1 kOe, which is assigned to NP shape and the high value of CoFe₂O₄ anisotropy constant. Co_(1-x)Mn_xFe₂O₄ and MnFe₂O₄ NP showed magnetic hysteresis curve close to superparamagnetic behavior. The addition of Mn in the CoFe₂O₄ decreased M_s and changed the morphology of CoFe₂O₄ from octahedral to spherical. Even having almost the same size, M_s of Co_(1-x)Mn_xFe₂O₄ was higher than observed for MnFe₂O₄ and can be easily manipulated changing the proportion between Co and Mn in the ferrite composition. In general, the modifications employed in polyol process are efficient to obtain NP with control over morphology and chemical composition, and enable to manipulate NP magnetic behavior by varying reaction conditions, as well as to achieve narrow size distribution in all synthesized systems.

In this work, magnetite thin films were prepared by sol-gel method on glass substrates followed by calcinations at 300 °C for an hour. The effect of glucose on the optical properties of the films was studied. The optical properties were studied by a UV-visible spectrophotometer. The results show that some of the prepared magnetite thin films have new optical properties due to a proper amount of glucose introducing. After introducing the glucose additive in Fe3O4 colloids, the intensity of the transmittance and the optical band gap of the Fe3O4 thin films increases because of the enhanced Fe3O4 crystallization. On the contrary, the absorbance, the film thickness, and the surface root-mean-square (RMS) roughness of the Fe3O4 thin films decreases. The glucose additive could not only improve the surface RMS roughness and microstructure of Fe3O4 thin films, but also enhance the transmittance and the energy band gap more easily.

The field of magnetic nanomaterials has garnered increased attention because of the promise to tailor properties based on the engineered size and shape of the constituent components. Nanostructured magnetic materials (including both discrete nanoparticles and nanocomposite compactions) offer the chance for new ranges of material performance. Additionally, nanomaterial research is beginning to show the chance for substitutions towards less-expensive and environmentally sustainable alternative elements for common consumer applications. To capitalize on such opportunities of new materials, we report here the formation and material properties of complex alloyed nanoparticles. A new wet-chemistry synthesis is reported for the important ternary-alloy FeCoV nanoparticle, with the in situ formation of encapsulating silica shells. Simple metal nanoparticles and/or metal-oxide nanoparticles do not exhibit the necessary convergence of material properties for practical nanomaterial application. The advantages of the nanoparticles presented here includes: facile synthesis of large quantities of these complex nanomaterials, expressed superparamagnetism at room temperature, high magnetic saturation values, and relative stability of such nano-featured materials during thermal annealing. Detailed analysis of directly synthesized FeCoV alloyed nanoparticles coated in nanometers-thick silica shells revealed the following interesting qualities. By TEM and XRD analysis showed that the FeCoV nanoparticle cores were consistently less than 10nm in diameter with an average of approximately 5nm diameter. BET analysis of these silica-coated binary-alloy nanoparticles (using both coordinating and non-coordinating gases) confirmed that >99% of the metal surfaces are covered with silica. The ternary-alloyed nanoparticles were thermally annealed under a variety of conditions. Trends in resulting nanomaterials became evident through TEM and M(H) curve data collected for the species. Minimal nanoparticle growth, with remarkable gains in magnetic saturation of over 125% (exceeding 100 emu/g) was determined empirically and is reported.

Rapid isochronal annealing of FeSiNbBCu amorphous alloys at heating rates of the order of 100 K/s results in massive heterogeneous nucleation of Fe3Si nanocrystals with an estimated 60-100% increase in their number densities than their conventionally annealed counterparts. Amorphous precursor ribbons when subjected to such rapid annealing under an applied uni-axial tensile stress along the long axis of the ribbon with values ranging from 50 - 600 MPa exhibit strong creep induced anisotropy. Such induced anisotropy has its origin from the associated elongation of Fe3Si nanocrystals induced during stress annealing. The microstructural investigations at various length scales aimed at understanding the effects of rapid stress annealing on the soft magnetic properties and the morphology of emerging soft magnetic nanocrystals using transmission electron microscopy and atom probe tomography will be presented.

Research in nanoparticles (NPs) has become a hot topic in the last decades. In the case of magnetic nanoparticles, they can be used in a wide range of potential applications in medicine or high-density data storage among others (1,2). To design magnetic nanoparticles for a specific application is required to control size, composition and distribution and to be able to modify their magnetic properties. The Fe-B system is especially attractive due to its ability to tailor its magnetic behavior with continuous composition variations (3,4). In fact, magnetization measurements in Fe1minus;x Bx glasses indicate that the saturation magnetic moment depends on their composition (5).
The present work analyzes Fe-B nanoparticles (NPs) of different sizes formed in a single process by gas aggregation from Fe80B20 targets. Crystal structure studies were carried out by electron diffraction pattern simulation, using Fast Fourier Transform (FFT) of the high resolution transmission electron microscopy (HTEM) images. Compositional information was achieved by means of Z- contrast scanning transmission electron microscopy (STEM), Energy Dispersive X-Ray (EDX) and energy-filtered TEM (EFTEM).
The experimental data reveal that all NPs, regardless their size, are covered by a less than 3 nm thick amorphous Fe-B alloy. Conversely, the crystal structure and chemical composition of the NPs core depend on their size. More concretely, NPs with a diameter above 30 nm are monocrystalline and are identified as tetragonal Fe3B (I 4), NPs with a diameter ranging from 20 to 30 nm are not monocrystalline showing amorphous areas together with different phases -orthorhombic FeB (pnma) and tetragonal Fe3B (I 4)-, and finally, NPs with a diameter below 20 nm are fully amorphous.
The variety of crystal structures measured in Fe-B nanoparticles can be explained as different stages of amorphous Fe80B20 crystallization being an indicative of the important role of thermal diffusion in the cooling process. It is expected that the external amorphous layer gets cold easily, as well as the core of small nanoparticles not permitting the full rearrangement of the atoms, whereas the core of large NPs is cooled down more slowly favouring its crystallization.
Thanks to the observed size dependence of the structure and composition of the Fe-B nanoparticles it can be inferred that their magnetic properties could be mass selected, opening new possibilities in the development of Fe-B applications.
1 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 (2000).
2 R. Bennewitz, J. N. Crain, A. Kirakosian, J. L. Lin, J. L. McChesney, D. Y. Petrovykh, F. J. Himpsel. Nanotechnology 13, 499 (2002).
3 C.L. Chien, D. Musser, E.M. Gyorgy, R.C. Sherwood, H.S. Chen, F.E. Luborsky, J.L.Walter, Phys. Rev. B, 1979, 20, 283
4 N.A. Blum, K. Moorjani. T.O Poehler, F.G. Satkiewicz, J. Appl. Phys. 1982, 53, 2074
5 R. Hasegawa, R.Ray, J. Appl. Phys. 1978, 49,4174

In the recent decades, wide gap oxides diluted magnetic semiconductors (ODMS) have attracted much attention due to their importance to the basic magnetism and the great potential applications in spintronic and optoelectronic devices [1]. In particular, the ferromagnetism (FM) has also been found in some oxides semiconductors even without intentional doping of magnetic transition metal impurities [2-4]. However, the origin of room-temperature FM in ODMS still remains controversial. It is proposed that the defects should be the potential sources to generate the FM. Up to now, there is no solid evidence identifying which one essentially associates with the FM mechanism especially in p-type Cu2O system.
In order to further provide evidence and look into the physical mechanism for the FM in undoped oxides semiconductors, we have fabricated the Cu/Cu2O granular films and core-shell nanoparticles by magnetron sputtering. Room-temperature FM is found in all the samples. The saturation magnetization (MS) in films and nanoparticles is much larger than that of the similar reported systems. The photoluminescence spectra illustrate that the FM originates from Cu vacancies in the Cu/Cu2O composite structures. Thus, the FM can be modulated by the amount of Cu vacancies through the Cu/Cu2O interface engineering. Furthermore, according to the contact of metal Cu with semiconductor Cu2O and the band alignment, the FM can be understood fundamentally by the charge-transfer theory based on Stoner model [5].
This work is supported by NSFC of China (Nos. 11104148, 5171082, 51101088 and 11304161), Tianjin Key Technology R&D Program (No. 11ZCKFGX0130), and the Fundamental Research Funds for the Central Universities.
References
[1] T. Dietl, Nature Mater. 9, 965 (2010).
[2] M. Venkatesan, et al., Nature (London) 430, 630 (2004).
[3] L. Y. Li, et al., Nanotechnology, 21, 145705 (2010).
[4] X. Zhang, et al., Phys. Rev. B 80, 174427 (2009).
[5] J. M. D. Coey, et al., J. Phys. D 41, 134012 (2008); J. M. D. Coey, et al., MRS Bull. 33, 1053 (2008).

In the present days, ferrofluid are used as magnetic contrast agent in MRI and in magnetic fluid hyperthermia treatment for killing tumor cells. In ferrofluid each MNP is coated with the surfactant or long chain polymers to inhibit clumping. Particles in ferrofluid are suspended by relative Brownian motion and will not settle down under normal conditions, however to increase the stability of the ferrofluid some common surfactants such as Oleic acid, Citric acid, poly acrylic acid are used. Since every surfactant possesses head and tail, one of them sticks out to the surface of the MNPs and other one adsorbs into the carrier liquid forming a regular micelle around the particles, then steric repulsion prevents these MNPs from getting agglomerate [1]. The polar nature of the nanoparticles decide the solubility in the carrier liquid; ionic polymer like poly(acrylic acid) attached to the MNP makes the neutral nanoparticle polar due to which the MNPs soluble in water to form a stable ferrofluid. Polyacrylic acid (PAA) coated cobaltferrite nanoparticles were synthesized using co-precipitation method and surfactant thickness was varied by varying the precursor to surfactant concentration [2]. Nanoparticles with size ~ 17 nm were synthesized with different surfactant thickness. XRD and SEM analysis carried out confirm the phase purity and grain size. PAA surfactant bonding and surfactant thickness were studied by FTIR and thermogravimetric analysis. At room temperature the nanoparticles show superparamagnetism and saturation magnetization varies from 32 emu/g to 40 emu/g with increase in the polymer thickness from 0.6 nm to 1.8 nm. Increase in the magnetization upon bonding of polar polyacrylic acid with increase in the polymer thickness could be due to the stoichiometric oxide surface in the magnetic inert layer and decrease in the polymer content was systematically seen through FTIR spectra. Ferrofluids were prepared by dispersing the MNPs in deionized water at concentrations viz, 10 g/L, 5 g/L, 2.5 g/L and 1.25 g/L. Magnetic hyperthermia studies on these ferrofluids were performed by varying alternating magnetic field (AMF) strength at 275 kHz frequency. Ferrofluids at a concentration of 1.25 g/L, with 10 min of AMF exposure of strength ~15.7 kA/m show stable temperatures ~ 48, 58 and 68 ^oC with increase in the surfactant thickness. A maximum specific absorption rate of 132 W/g for ferrofluid with a surfactant thickness of 0.6 nm and at 1.25 g/L concentration was observed. MTT assay cytotoxicity studies on subconfluent monolayer of L-929 cells were performed with the poly acrylic acid coated cobaltferrite nanoparticles show viability upto 10 mu;g/mu;L and is above the NSAI standards for invivo studies [3].
[1] de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189-209.
[2] Krishna Surendra, M.; Kannan, D.; Ramachandra Rao, M. S. MRS Proceedings 2011, 1368, mrss11-1368-ww11-40-45.
[3] ISO 20776-1:2006

In the recent time spinel ferrite magnetic nanoparticles have been largely studied owing to various applications of these materials in the information storage, ferro-fluid technology, magnetocaloric effect, refrigeration and medical diagnostics. In this category cobalt ferrite (CoFe2O4) nano particles specifically gained huge research attention and prepared by various chemical methods. However, further investigations are still needed on the substituted CoFe2O4 (CFO) nano particles to explore their various characteristics. In this paper we present our results on Mn and Zn substituted cobalt ferrite (Co0.6Zn0.4Mn0.3Fe1.7O4 ) nano particles prepared by chemical co precipitation method. The x ray diffraction patterns and transmission electron micrographs of as prepared Co0.6Zn0.4Mn0.3Fe1.7O4 (CZFMO) nano particles indicated their particle size in 20-25 nm range. The properties of these nano particles before and after thermal annealing have been compared. Magnetization (M) vs. field (H) loop measurements on as prepared and thermally annealed CZFMO nano powders revealed some unusual features contrary to CFO nano particles prepared under same conditions. The saturation magnetization (Ms) decreases whereas the remnant magnetization (MR) and coercivity (Hc) values increase after thermal annealing process. These features might be related to the spin canting effect at B site induced by diamagnetic Zn+2 ions at A site. These nano sized powder samples are further characterized by low temperature magnetic measurements, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA) and Brunauer Emmett Teller (BET) analysis.

Graphene oxide-Fe3O4 nanoparticle composite has recently become a hot topic in various fields such as MRI, drug delivery, battery, and removal of heavy metal from waste water. For secure attachment on graphene, Fe3O4 nanoparticles are modified by aromatic compounds to get phi-phi interactions on graphene or synthesized on graphene oxide by a chemical precipitation method. In this study, we investigate a simple and cost-effective method for the composite fabrication through non-specific interactions, e.g., hydrophobic interaction assisted by ultrasonication. Stability of the composite in an aqueous solution was confirmed by multiple precipitation-dissolution cycles and repetitive heavy ion (chromium and lead ions) removal tests from waste water using voltammetry where the ion-adsorbed composite was easily removed by a magnet. And we found that the efficiency of the test was comparable to previous reports. The chemical structure was measured by UV-vis, FTIR, and Raman scattering, and its morphology was monitored by TEM, SEM, and XRD.

Magnetic metal/metal oxide nanocomposites have received increased attention because of their unique electrical, mechanical and magnetic properties. In particular, magnetic nanocomposite powders with fine microstructure by mechanical alloying (MA) are of interest because of possible applications as permanent magnets and soft magnetic material for high frequency.
In the present work, solid state reduction of Fe₂O₃ by Zn element during ball milling has been investigated at room temperature. It is found that nanocomposite powders in which ZnO is dispersed in α-Fe matrix with nano-sized grains are obtained by ball milling. Also, we report the results of a study of the structure and magnetic properties of nanocomposite compacts prepared by spark plasma sintering (SPS). Magnetic measurement supports the characterization of the solid-state reduction between metal and metal oxide due to MA process: it can be used to obtain the information on magnetic phase composition and microstructure of nanocomposite powders. Acknowledgements This work is financially supported by the Ministry of Knowledge Economy (MKE) of Korea.

Due to their chemical stability, their high specific surface area and their reducing potential, Fe3O4 nanoparticles appear to be a promising alternative in the effort to mitigate drinking water contamination by heavy metals and, especially, species in high oxidation states, like hexavalent chromium. However, high removal efficiency is not the only prerequisite for the successful, competitive and economical viable transfer of these nanomaterials in large-scale conditions of application. The main drawback for their use lies on their small size and the inherent difficulty for complete recovery after treatment. In this point, the spontaneous magnetic properties of Fe3O4 nanoparticles introduce an extra possibility for separation by an external magnetic field. This study tries to provide a methodology for the production of Fe3O4 nanoparticles in large quantities needed for water treatment applications and to develop an integrated system for the purification of water from Cr(VI) and the magnetically induced recovery of used nanoparticles. For this reason, the co-precipitation of Fe2+ and Fe3+ salts at alkaline conditions was optimized in a continuous flow reactor which supplies 30-40 nm Fe3O4 nanoparticles at a kilogram-scale under strictly steady conditions. A water treatment facility is proposed to test the ability of the obtained nanoparticles for an efficient Cr(VI) reduction consisting of a contact reactor tank outflowing into a horizontal tube placed on the center of a magnetic field generated by parallel oriented permanent magnets.
The research project is implemented within the framework of the Action “Supporting Postdoctoral Researchers” of the Operational Program "Education and Lifelong Learning" (Action&’s Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund and the Greek State.

La(FexSi1-x)13 alloys are concerned due to its magnetocaloric effect (MCE) and the low price of raw materials, which make it a key materials for promising room temperature magnetic refrigerant [1]. The NaZn13-type phase (hereinafter 1:13 phase) paly a key important role to the MCE of La(FexSi1-x)13 alloys [2-3]. Many reports found that the 1:13 phase was not formed directly from the melt upon cooling but via a peritectic reaction between the pro-peritectic γ-Fe and the La-rich phase [4]. In this paper, the 1:13 phase is found to form directly during solidification process. Three kinds of solidification microstructure were formed because a competitive nucleation occur between the 1:13 and a-Fe phase during solidification process of LaFe11.5Si1.5 alloy: (1) In a high cooling speed, a large amount of NaZn13-type phase with equiaxed grains and a small amount of α-Fe phase are formed because of an sufficient nucleation rate of 1:13 phase (see Fig.1.b). (2) when cooling rate decreasing, a large number of α-Fe phase with dendrites are formed because nucleation rate of α-Fe phase is large that of 1:13 phase (see Fig.1.c). (3) When nucleation rate of 1:13 phase is equal to that of a-Fe, The transition region are formed between the 1:13 and a-Fe phase(see Fig.1.d). So nucleation rate is a key factor for the phase composition and microstructure of LaFe11.5Si1.5 alloy.

A non-monotonic size change of monodisperse Fe3O4 nanoparticles (NPs) with the diameter of 3-20 nm is
observed in the scale-up synthesis. It is interestingly observed that the particle size does not decrease monotonously with either the increase of precursor concentration, or the molar ratio of surfactant to iron precursor. The time-dependent Fourier transform infrared spectrum (FTIR) and Transmission electron microscopy (TEM) images reveal that the non-monotonic size change results from the different influence of surfactant on the three reaction stages including monomer formation, nucleation, and growth with increasing the amount of precursor. Furthermore, we have demonstrated that the high frequency properties of superparamgnetic Fe3O4 NPs can be tuned via dipolar interaction. The cut-off frequency can exceed the natural resonance frequency by superparamagnetic/ferromagnetism transition. In addition, the magnetic loss of microwave is significantly reduced via SiO2 coating. The finding of non-resonant mechanism in superparagnetic NPs provides us a new approach to tune high frequency beyond the Snoek limit.

Magnetic iron oxide nanoparticles (MIONPs) have been widely used in numerous applications in the biomedical field ranging from contrast agents in magnetic resonance imaging (MRI) to heating agents in cancer hyperthermia. In these applications, magnetic nanoparticles are coated with surfactants and polymers to enhance biocompatibility, prevent agglomeration, and enhance functionality. However, the coating interacts with the surface atoms of the magnetic core leading to the formation of a magnetically disordered layer. This effect reduces the total amount of the magnetic phase and could have profound implications on the final application. The magnetic phase reduction can also be attributed to the interaction between the coating and the solvent that is used to suspend the magnetic nanoparticles. For example, we have previously demonstrated that the thickness of the nonmagnetic phase depends strongly on the type of the organic coating as well as suspension media.
In the current study, we have focused on the interaction between the surfactant and the suspension media. Iron oxide nanoparticles were confirmed to be magnetite via Raman spectroscopy and X-ray diffraction experiments. The size of the oleic acid coated iron oxide nanoparticles was found to be 8 nm from TEM experiments. Thermogravimetric analysis (TGA) experiments revealed that the iron oxide nanoparticles were fully covered with oleic acid. To understand the effect of surface coating thickness on the saturation magnetization, the quality of the solvent was varied by changing the concentration of poor solvent in a good+poor solvent mixture. As the solvent quality changed from good to poor, the thickness of the oleic acid coating decreased. In addition, the hydrodynamic radii of the nanoparticles in various poor and good solvents were measured by dynamic light scattering. The saturation magnetization, which was obtained from vibrating sample magnetometry (VSM) measurements, was used to determine the effective concentration of magnetic phase as a function of poor solvent concentration. The actual iron oxide concentration was determined by Tiron chelation test. The difference between the VSM and Tiron test concentrations indicates the reduction of magnetic phase of the magnetic core in the presence of different suspension media. The behavior of the oleic acid under different solvent qualities was also investigated by atomistic simulations.

Biocompatibility, low toxicity, and magnetic properties have made iron oxide nanoparticles (NPs) a widely explored field. Potential applications include targeted drug delivery, localized therapy, and as magnetic resonance imaging (MRI) contrast agents. For each of these applications, NPs need to be injected intravenously, and directly interact with the human body&’s immune system. Protein absorption and non-specific binding to the NP surfaces, in vivo, may trigger an undesirable immune response. Systematically modifying the NP surfaces may potentially reduce the immune response and, in conjunction, increase blood circulation time and targeting efficiency. Several small molecules including cysteine, lysine, glutathione (GSH), and tris(hydroxymethyl)- aminomethane (Tris), along with polyethylene glycol (PEG) were conjugated to the surface of dopamine coated Fe2O3 NPs. Each of these molecules either have zwitterionic or hydrophilic properties thought to reduce protein absorption. Ligands, covalently bonded to quinone dopamine, were desired, in order to maintain functionality, in vivo.
The surface chemistry was measured using Fourier transform infrared spectroscopy (FTIR). FTIR data confirmed conjugation of the ligands to the NP surfaces. Dynamic light scattering (DLS) will be used to measure size distribution and zeta potential of the conjugated NPs. The pH of the NP solution will be altered to determine the pH effects on particle charge and stability. The results will be compared to known properties of the conjugated molecules to determine effectiveness of the covalent conjugation of ligands onto the NP surfaces.
This work is in part supported by NSF DMR-0907204 and CAREER: DMR 1149931

Among all the emerging research wings focusing on Ferroelectric Liquid Crystals (FLCs), one of the latest emerging research areas being the study of composites of FLCs dispersed with magnetic nanoparticles with an idea of developing multifunctional optical devices. The dielectric and diamagnetic properties of FLCs enable their optical properties to be controlled by applying electric or magnetic fields [1-3]. However, in practice most liquid crystal based devices so far have been driven by electric fields. Although some early studies suggest the possibility of controlling LC&’s optical properties by external magnetic field, but generally it can not be employed due to their feeble sensitivity to magnetic field and very low diamagnetic permeability anisotropy. Therefore, to enhance the functionality of LC based devices, steps are being taken towards developing new magneto-electric or multi-ferroic soft materials which can be easily tuned by various external stimuli [4-6]. In current study, we have dispersed different concentrations of iron oxide (Fe2O3) nanomagnetic particles into FLCs and characterized their optical texture, electro-optic and dielectric behavior with respect to changes in temperature and electrical bias conditions. Temperature dependent dielectric permittivity has shown a moderate increase in the dielectric constant with magnetic nanoparticle addition. Rotational viscosity, which is one of the vital parameters for display application, continuously decreases with increase in electrical bias in iron oxide NPs dispersed FLCs which indicates an indirect coupling between the magnetic dipole moment and liquid crystal director field. As a result, response time of composite material also decreases with increasing electrical bias above saturation bias voltage. Phenomenon of dielectric and static memory effect in these composite is also observed which yet again indicates a coupling of magnetic NP&’s field with FLC&’s director orientation. Detailed analysis of above results will be discussed at the time of presentation.
References:
[1] G. W. Taylor, Ferroelectric Liquid Crystals-Principles, Preparations and Applications (Gordon &Breach, New York, 1991).
[2] P. Kopcansky et al., Czechoslovak Journal of Physics 51, 59 (2001).
[3] Y. D. Gu et al., Phys. Rev. Lett. 85, 4719 (2000).
[4] P .Goel, et.al.,Liquid Crys. 39, 1(2012)
[5] Rozca, B., Jagodicb, M., Gyergyeka, S., Drofenikb, M., Kraljc, S., Lahajnara, G., Jaglicicbb, Z., Kutnjak, Z. Ferroelectrics, 410:37-41, (2011).
[6] Goel P; Biradar A.M. Appl.Phys.Lett.101, 074109-4, (2012).

Anisotropic magnetic nanostructures reorient themselves parallel to the external magnetic fields to minimize their magnetic potential energy and dipole-dipole interactions, and are considered as promising building blocks for self-assembly towards various applications. Based on the self-assembly of these anisotropic magnetic nanostructures, we demonstrated the instant and reversible control of the intensity and polarity of light by external magnetic fields herein.

Magnetically recoverable nanoparticles (NPs) represent an easy and environmentally benign means for catalyst recovery, providing catalytic properties intermediate between homogeneous and bulk heterogeneous materials. This presentation will discuss the development of a novel bimetallic Cu-Fe nanoparticle synthesis which provides a recoverable heterogeneous catalyst for the Cu(I)-catalyzed alkyne azide cycloaddition, known as “click” reaction (CuAAC). In this system, the Fe(0) core is believed to play three distinct roles: i) to provide a means for magnetic recoverability, ii) to serve as a source of electrons to reduce Cu(II) into the catalytically active species, and iii) to act as a support to prevent the liberation of soluble ions, enabling a heterogeneous mechanism. The synthesis of the catalyst proceeds without the use of reducer, or ligand, making this reaction very atom economical. The scope of the use of this new Cu-Fe catalyst was assessed by promoting the reaction between small alkynes and azides, but also between larger molecules, as for example the preparation of a rhodamine-labeled hormone, which could eventually be used the development of novel aromatase enzyme assays. This type of fluorescence-based assays could lead to new straightforward ways of identifying endocrine disrupters from our environment.

In this work, an old-fashioned plastic additive - polypropylene grafted with maleic anhydride (PP-g-MA), has been utilized as polymeric surfactant and stabilizer for synthesizing magnetic nanoparticles including iron oxide and cobalt nanoparticles through a facile one-pot bottom up method. The novel function of this long known PP-g-MA has been successfully demonstrated, i.e., for simultaneously controlling the size, shape, crystalline phase, and self assembly patterns of these aforementioned magnetic nanostructures. By simply varying the ratio of PP-g-MA to organometallic precursor, such as Fe(CO)5; or or changing the back bone chain length of PP-g-MA, such as numerical molecular weight of 800, 2500, and 8000 g/mole, the as prepared iron oxide or cobalt nanoparticles with tunable morphologies (hollow vs. chain-like, and different building blocks for chain-like structures) and crystalline phases (α- vs. γ- phase for Fe2O3, and fcc- vs. ε- phase for cobalt nanoparticles) can be achieved; which has rarely been reported before through the bottom up wet chemistry method. In addition, the different crystalline phases and self assembly patterns can in turn lead to significant differences in the magnetic properties (including saturation magnetization and coercivity) of these magnetic nanostructures. The formation mechanisms were proposed and discussed in detail.

Rare-earth intermetallics such as R2Fe17 (Rare Earth), R2Fe14B and RCo5 find their applications as permanent magnets. The magnetic properties these intermetallics can be improved by the incorporation of interstitial and substitution atoms for Fe or the combination of rare earth substitution and Fe substitution. We choose Gd2Fe17 due to the coexistence of rhombohedral and hexagonal structures and comparatively higher magnetic anisotropy. Proper selection of the doped atom and their content in Gd2Fe17 may further enhance its magnetic properties. In the present study, Gd2Fe17 compound is co-doped with non-magnetic Al and Nb atoms for Fe. Al substation in R2Fe17 have has shown to increase the Tc but reduce magnetization while Nb substitution helps in removing free iron which often reduces the energy product of the magnet.
In this study, the structural and magnetic properties of Gd2Fe17-x-yNbxAly compounds were investigated by x-ray diffraction, room temperature magnetometry and Mossbauer spectroscopy. A series of Gd2Fe17-x-yNbxAly (x=0, 0.25, 0.5 and y=0, 1, 2, 3) compounds were prepared by arc melting techniques in high pure Ar atmosphere. X-ray diffraction show formation of pure Th2Zn17 phase. Rietveld analysis show that that Al prefers 18h site while Nb prefers 6c site. Substitution of Al for Fe in Gd2Fe17 brings in lattice contraction due to smaller Al radii compared to Fe, while further partial replacement of Al with Nb increases the lattice volume. Substitution of Al and Nb enhanced the magnetic properties of Gd2Fe17 due to the change of anisotropy in the compound. The observed RT saturation magnetization for Gd2Fe17 is Ms~67 emu/gm and decreases with increasing Al content. On the other hand, the Ms increases with increasing Nb content in Gd2Fe17-x-yNbxAly for a give Al content.. In Gd2Fe17-x-1NbxAl Ms is increased from 58.97 emu/gm to 62.96 emu/gm for increasing the Nb content from x=0 to 0.5. The increase in Ms with Nb content could be associated with the magnetovolume effect affecting the density of Fe (3d) band. Therefore, Nb and Al have been observed to have reverse influence on the magnetization. Room temperature Mossbauer analysis show a decrease in hyperfine field with Al content. The isomer shift of Nb doped Gd2Fe17-x-yNbxAly show a linear increase with Nb substitution. The increase in isomer shift is associated to the increase in the unit cell volume with Nb substitution which decreases the s electron density at the Fe nucleus. Further temperature dependent Mossbauer analysis and Curie temperature of these compound is in progress.

The work reports the formation and characterization of Ni nanostructures embedded in Al2O3 (sapphire) matrix. Single crystals of sapphire were implanted with 80 keV negative Ni ions at a fluence of 1 × 1016, 5 × 1016, 7 × 1016 ions cmminus;2 with current density of 15 µA/cm2 and post annealed in air at 600 °C for 4 hrs. X-ray diffraction, AFM/MFM and UV-Visible absorption, investigations confirmed the formation of embedded nickel nanostructures. XRD spectrum for Ni-implanted Al2O3 sample clearly indicates the presence of Ni(111) metallic phase along with the NiOx phase at fluence of 7 x 106 ions/cm2. The size of the nanostructures calculated using Scherrer&’s relation has been estimated to be ~ 8.2 nm. AFM image indicates the rms surface roughness around 12.5 nm in the Ni implanted sample and ~1.7 nm for smooth surface of Al2O3. The magnetic phase contrast observed in MFM measurements reveals the formation of magnetic domain due to Ni nanostructure formation. Three absorption bands at 3.69, 4.38 and 5.15 eV have been observed with the UV-VIS studies of Ni-Al2O3 composite matrix. These absorption bands can be designated as broad band of Ni nanoparticles, NiOx absorption edge and L2 - L1 interband transition of Ni, indicating the formation of Ni-NiOx core-shell morphology.
Keywords : Implantation, Al2O3, XRD, AFM, MFM and Optical Absorption

Thanks to their remarkable magnetic properties [1] the use of Cobalt-Nickel nanowires is proposed in different field of application such as the manufacture of permanent magnets [2] . In order to exploit the potential of these nanowires, it is mandatory to establish a new process which can be able to assemble these nanowires and emphasize their magnetic properties. This work focuses on the development and study of nanostructured magnetic materials (based on nanowires assembly) by a "bottom-up" strategy allowing the modulation of their magnetic behavior (soft - to - hard). The originality of the project is to combine a soft chemistry process for nanowires elaboration with unconventional compaction technique (Spark Plasma Sintering) for their sintering. The first goal of this work was to control the morphology, size and magnetic anisotropy of 3d alloys nanowires (based on Co & Ni) elaborated by polyol process synthesis. The second goal was to induce organization of nanowires inside of nanostructured bulk material thanks to magnetic field assisted SPS sintering. This has been achieved under the combined effects of pressure and magnetic field. The use of a low sintering temperature preserves the nanoscale character of the nanowires and thus the confinement of their peculiar magnetic properties. This confers to the as-obtained material its hard magnetic behavior and high coercitivity. Conversely, an increase in the sintering temperature has shown microstructural upheavals and led the decrease in the value of the coercive field of the elaborated bulk materials.
This new sintering process should favor a high saturation magnetization to the nanostructured bulk system and more of all it should be at the origin of controlled anisotropy and coercitivity. The choice of temperature, pressure and external applied field parameters during the sintering process allow to tune the magnetic material. Indeed, the most significant result is that this work open the way for new nanostructured magnets with higher performances and stabilities between 300K and 400K then the NdFeB or SmCo magnet commonly used [3].
[1] D. Ung et al., Adv. Mater., 17, 338 (2005).
[2] T. Maurer et al., PRB 80, 064427 (2009)
[3] T. Maurer et al., Appl. Phys. Lett. 91, 172501 (2007)

For the clean energy industry, permanent magnets are essential. They are used in devices like electric vehicles, wind turbines and advanced batteries, among many other green high-tech applications. Currently, the best high-performance permanent magnetic materials are based on neodymium, an expensive and increasingly unavailable rare-earth element. As an alternative, samarium-cobalt (SmCo) hard magnets have regained importance; however, their modest saturation magnetization represents a problem that needs to be overcome before industrial use. Magnetic nanocomposites, which combine exchange-coupled magnetically hard phases -such as SmCo- and soft phases, could offer all the required properties for industrial applications: large coercivity, high Curie temperatures, and high magnetic moment provided by the soft phase. Here, we report our recent progress on the synthesis of high-performance samarium-based magnetic nanocomposites by chemical methods. In this approach, SmCo oxide nanoparticles (as the hard phase precursor) and soft magnetic nanoparticles (3d transition metals) are chemically synthesized and protected before reductive annealing at high temperature. To fabricate the composite, different techniques such as self-assembly and electrophoretic deposition will be evaluated as potential avenues to optimize the exchange coupling of the materials. Enhancement of the magnetic properties and the stability against environmental conditions is expected and will be analyzed and compared to previous reports in the area.

Hybrid materials, as iron oxide/polymer, provide the opportunity to perform materials with predefined properties. This work presents two multifunctional magnetic hybrid materials with potential applications as micro-actuators. The first one consists of iron oxide (Fe2O3) nanoparticles embedded in polyvinyl butyral (PVB). For the second one, Fe2O3 nanoparticles, coated with carboxymethyl cellulose (CMC), were embedded in PVB. The synthesis of both Fe2O3/PVB and (Fe2O3-CMC)/PVB hybrid material films are first described. Then, we show their structural, morphological and magnetic characterization. Finally, we evaluate the deformation of the hybrid materials against a variable magnetic field. The maximal deformation is obtained by the (Fe2O3-CMC)/PVB beam-shaped structure (28.37 mm x 2.6 mm x 0.183 mm) with 843 mu;m. The maximal electric power applied to the system has been 1.14 W. The levels of displacement induced in both hybrid materials as a response of the external magnetic field, besides the low electric power required, let us conclude that the magnetic hybrid materials presented here could be considered as good candidates to contribute organic/inorganic multifunctionalities to micro-actuators and microsystems applications.

Metal α-Fe nanoparticles have long been of scientific and technological interest because of their large saturation magnetization (218 emu/g) and the controllability of magnetic characteristic by particle size or shape. However, it is difficult to keep the metallic state of α-Fe nanoparticles because α-Fe nanoparticles are easily oxidized at the moment of exposure to the atmosphere, and form various iron oxides. In order to make highly stable metallic α-Fe nanoparticles for various applications such as biomedical imaging, sensing or soft phase of the nanocomposite magnet with high-density magnetic energy, the prevention of oxidation is indispensable. In this study, Fe₃O₄ coated α-Fe nanoparticles were prepared by reduction of Fe₃O₄ nanoparticles and successive oxidation process of their surface.
Fe₃O₄ nanoparticles were synthesized based on our previous report [1]. The obtained Fe₃O₄ nanoparticles (average diameter : 8.2 ± 0.6 nm) were coated with uniform SiO₂ shell through the formation of water-in-cyclohexane reverse microemulsion and hydrolysis of tetraethyl orthosilicate, in order to prevent aggregation of Fe₃O₄ nanoparticles by heat treatment. The resulting Fe₃O₄@SiO₂ nanoparticles were treated to α-Fe@SiO₂ nanoparticles under 100%-H₂ atmosphere, followed by the partial oxidation to form the oxide layer under 2%-O₂/N₂ atmosphere at 300-400 °C.
The TEM image of partially oxidized nanoparticles, bright surface layer on dark core in the particle was observed. The XRD pattern after reduction resulted in only the observation of α-Fe peaks. On the other hand, the patterns of not only α-Fe but also Fe₃O₄ phase were observed partial oxidation treatment at 400 °C. From these results, α-Fe/Fe₃O₄ core-shell structure might be formed by partial oxidation treatment. The oxidation resistance of the resulting nanoparticles was investigated by measuring the change of saturation magnetization value (M_s) with time. In the case of the α-Fe nanoparticles without surface oxidization, M_s was reduced by 40% after 1 day from the treatment. By contrast, for the partially oxidized α-Fe nanoparticles, no significant change in Ms was observed even after 80 days. The XRD measurements resulted in the observation of α-Fe phase even after 80 days. These results show the surface oxidation treatment must be effective to protect α-Fe phase from oxidization under atmosphere for long time.
[1] M. Nakaya, M Kanehara and T. Teranishi, Langmuir, 22, 3485 (2006)

Diluted magnetic semiconductors (DMSs), which can use both the charge degree and spin degree of carriers have attracted much attention of the researchers due to their potential application in spintronics [1]. Recently, rear-earth (RE) ions doped DMSs have also evoked a great deal of interest because the colossal magnetic moment of Gd in was observed. Like GaN, ZnO based DMSs, which were theoretically suggested as a wide band gap semiconductor with room temperature ferromagnetism (RTFM), were also a focus in the field of spintronics [2]. However, there is still no room temperature ferromagnetism found in Tb-doped ZnO DMS. Moreover, rare earth doped nanocrystals exhibit specific properties and ZnO is regarded as an excellent host material for the doping of the rare earth because their 4f intra-shell transitions give a sharp and intense emission which makes them better luminescent material. The luminescence of Tb3+ is particularly interesting among all the rare-earth (RE) elements, because here the major emission band is near 544 nm [3], showing a green emission which is one of the three primary colors. In this work, the Tb-doped ZnO nanorods (i.e. Zn1-xTbxO, x= 0.025, 0.05, 0.075, 0.100) were successfully synthesized by direct thermal decomposition route of Zinc and Terbium acetated in air at 300 C for 6 h. Structural, magnetic and optical properties were studied. The prepared products were characterized by XRD and TEM. The XRD result indicates that the patterns of Tb-doped ZnO with different doping concentrations are pure phase with the wurtzite structure ZnO. No any impurities were detected. TEM images reveal that nanoparticle and nanorods are several hundreds of nanometer in length. High-resolution TEM micrographs show the interplanar distance of fringes are ~ 0.26 nm which is corresponding to the (002) plane of wurtzite ZnO. The SAED show spotty ring patterns, revealing their highly crystalline ZnO wurtzite structure (JCPDS, 36-1451). Which is consistent with those of (100), (101), (102) and (110) planes of the XRD results. The optical properties of the samples were investigated by PL.The results indicate that the emission spectra of the peaks at 490 nm and 544 nm respectively are of Tb3+ions. The O1s XPS spectrum is presents in a sharp peak centering around 530 eV and 531.5 eV ascribed to the coordination of oxygen in Tb-O-Tb and Tb-O-Zn. High-resolution spectrum of Tb 4d peak evidently shows the peak located at ~ 148.5 eV revealing the Tb-related phase of Tb2O3. Hence the valence state of Tb in the samples is mainly Tb3+. The magnetic properties of the samples were investigated by VSM. Magnetic measurements indicate that all of the Tb-doped ZnO samples exhibit room temperature paramagnetic behavior. We suggest that it is possibly due to the substitution of Tb3+ ion into the Zn site. These phenomena may play a major role in the room temperature magnetism of the Tb-doped ZnO samples.

Magnetite nanoparticles (MNPs) are suitable materials for hyperthermia induced by radiofrequency magnetic fields, provided their size is carefully tailored. MNPs of 4 and 8 nm are obtained by thermal decomposition of Fe(acac)3 in 1,2-hexadecandiol using oleic acid and oleylamine as stabilizers at different temperature. In particular, 8 nm particles are synthesized by heating at 200 °C for 2 h and 300 °C for further 1 h [1]. The black-brown mixture obtained was precipitated with EtOH and dispersed in hexane, giving a colloidal solution. Then, 10-undecynoic acid was added to introduce the alkyne groups. TEM studies confirm the crystal structure of the MNPs and show that they have nearly spherical shape with homogeneous size distributions.
The RT best superparamagnetic properties are found for 8 nm MNPs which present a saturation value of the magnetic moment per unit volume of 60 emu/g [2].
The MNPs hypertermic properties are measured by applying a radiofrequency magnetic field (f=250KHz H=0.016 T) and then by measuring the specimen temperature variation as a function of time. The biocompatibility is tested on lung adenocarcinoma A549 cells internalized at intracytoplasmatic level (endocytosis process). No inhibited proliferation and cell death is found after 72 hs for a concentration of 50 mg/ml.
To improve the hyperthermic effect of the Fe3O4 MNPs, they are then anchored via “click- chemistry” reaction to functionalized semiconducting core-shell cubic SiC/SiO2 biocompatible [3] nanowires (NWs). (100) SiC NWs are also interesting for their high thermal conductivity. This last property can in particular improve and optimize the heat distribution generated by MNPs along the NWs and in turn inside the cancer cell.
Both MNPs and NWs are properly functionalized to allow their mutual link by chemical bonds, exploiting the Huisgen 1,3-dipolar cycloaddition reaction (click-chemistry reaction). To this purpose azide groups are introduced on the surface of the silica shell of the NWs and C-C triple bonds are introduced in the stabilizing agents of the MNPs.
The NWs are functionalized by reacting the free hydroxyl groups of the silica shell with 3-azidopropyltrimethoxysilane and are then decorated with MNPs via click-reaction. The Cu(I)-catalyzed cycloaddition leads to the easy stereoselective formation of 1,4-substituted triazole. The new nanosystem, made by NWs covered with MNPs is then obtained through the reaction at RT of the alkyne group on the MNPs and the azide on the NWs. TEM studies demonstrate the successful functionalization.
Finally the comparison between the hyperthermic properties of MNP and MNP/SiC/SiO2 system is presented.
1 S. Sun et al J. Am. Chem. Soc. 126, 273 (2004)
2 V. Chiesi et al, JEMS 2012, Sept 2012, Parma , Italy
3. S. E. Saddow Editor Silicon Carbide Biotechnology: A Biocompatible Semiconductor for Advanced Biomedical Devices and Applications, copy; 2011 Elsevier LTD, UK ISBN 0-12385-906-9.

The design and fabrication of sophisticated bifunctional luminescent magnetic nanomaterials containing Fe3O4 functionalized with rare earth ions inclusion complexes of calixarene and b-diketonate ligands have been reported. Their preparation is accessible thought facile one-pot methodology. These novel Fe3O4@CC-Eu(TTA) and Fe3O4@CC-Tb(ACAC) luminescent magnetic nanomaterials present very interesting superparamagnetic and photonic properties. The magnetic properties were explored at temperatures of 5 and 300 K and come to know that the extent of coating and crystallinity affect the saturation magnetization values of nanoparticles. Although magnetite is a strong luminescence quencher, the coating of the Fe3O4 nanoparticles with synthetically modified calixarene macrocycle has overcome this difficulty. The intramolecular energy transfer in the nanomaterials from the T1 excited triplet states of TTA and ACAC ligands to the 5D0(Eu3+) and 5D4(Tb3+) emitting levels have been determined. The experimental intensity parameters Omega;lambda; (lambda; = 2 and 4) were determined for the Fe3O4@CC-Eu(TTA) system from the emission spectral data. The Omega;lambda;, also known as Judd-Ofelt parameters, are determined by the intensities of the transitions 5D0→7FJ (J = 2 and 4) of Eu3+ ion, where the mechanisms of forced electric dipole and dynamic coupling are considered simultaneously. The obtained intensity parameters values were compared to the reported Eu(TTA)3.2H2O complex and were found completely different. Similarly the mixed complex of calixarene macrocycle with TTA ligand is in process of preparation. Later on we will calculate the experimental intensity parameters and quantum efficiency for the complex and will compare with Fe3O4@CC-Eu(TTA) system in order to guess the quenching phenomena operated by Fe3O4, H2O and may be hydrophobic cavity of calixarene molecule. In addition to the supraparamagnetic behavior, these nanophosphors may be act as emitting layer of red and green light for converting molecular devices (LCMDs).

Iron oxide nanoparticles, mostly magnetite and maghemite, are probably the best-studied magnetic nanomaterials due to their biocompatibility and nontoxicity properties which make them highly attractive for diverse applications in medical, biological or even catalysis and ecological fields. Nevertheless, the formation process of these magnetic nanoparticles is still not clear for many reaction systems. This research presents the development and optimization of the synthesis of magnetic nanoparticles of iron oxides by the solvothermal method with benzyl alcohol using iron(III) chloride hexahydrate as metallic precursor in substitution of iron(III) acetylacetonate.
The synthesis parameters were studied and varied in order to obtain a high yield of magnetite. Metallic iron was evaluated as reducing agent and iron source, providing the charge balance of the magnetite&’s structure. Furthermore, the use of urea as a precipitation agent was crucial to the increase in yield of magnetite once it decreased the acidity of the medium and induced the precipitation of the Fe²⁺ ions. The time of solvothermal treatment and the effect of magnetic stirring were also evaluated. It could be noticed that longer times of treatment and no stirring led to an increase in the yield of magnetite, whose formation process followed the Ostwald step rule, considering the phases&’ stability in the set of conditions applied. The particles were characterized by X-ray diffraction, Mössbauer spectroscopy, thermogravimetric analysis, infrared spectroscopy (FTIR) and transmission electronic microscopy. The phases were quantified by Rietveld method and Mössbauer spectroscopy, and a 95% maximum yield of magnetite in the phase mixture was achieved.
Benzyl alcohol polymerized during the solvothermal treatment leading to the formation of a dark and sticky resin with water release. The polymerization took place by Friedel-Crafts alkylation reaction, since FeCl₃.6H₂O is known as a catalyst for this reaction. The resin was characterized by infrared spectroscopy and nuclear magnetic resonance (NMR-¹H). A mechanism for the polymerization reaction was purposed in order to elucidate the synthetic route and its relation with the particles&’ formation. The FTIR spectra showed that after the washing steps the resin remained adhered at the particle&’s surface stabilizing them against oxidation.

Carbon/Cobalt ferrite (C/CoFe2O4) composite nanofibers have been successfully fabricated to evaluate their magnetic and electrochemical behavior. The CoFe2O4 nanoparticle embedded carbon nanofibers without synthesis of CoFe2O4 nanoparticle first were as-sisted by using 0, 20, 40 and 60 wt.% Cobalt and Iron nitrate (1:2 mol) dispersed into carbon-based solution (PAN) followed by electrospining and heat treatment, respectively. The final products are designated as C/CoFe2O4_0 (CNF), C/CoFe2O4_20, C/ CoFe2O4_40, and C/CoFe2O4_60. All of samples were characterized by means of XRD, SEM, TEM, Raman spectroscopy, XAS, BET, and VSM. Moreover, the electrochemical properties were determined using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode configuration. The key process to generate mixed phase of carbon and CoFe2O4 from the polymer and metal precursor solutions is the flow rate ratio of air and argon atmosphere under the heat treatment process. We have found that at the rate ratio of 1:10, the composite phase was formed depending on heat treatment tem-perature but not on the metal content composition. For the other samples, it has been found that mixed phases of carbon and CoFe2O4 were formed at 500 oC. The content of CoFe2O4 nanoparticles strongly affected to the morphology, structure, magnetization and specific capacitance of CNFs. The morphology of fiber appeared straight having porous and uniform in cross section and there were ~20-40% shrinkage after carbonization due to the difference combustion of organic PAN matrix. The crystallite size is not linear depen-dent with the increasing of CoFe2O4 content. XANE spectra at the Fe (7112 eV) and Co (7709 eV) K-edge were used to confirm the Fe3+ and Co2+ oxidation states of CoFe2O4 nanoparticle. Pure CNF exhibited paramagnetic, while composite nanofibers exhibited ferrimagnetic with the saturated magnetization values of 1.04, 7.5, and 43.4 emu/g for C/CoFe2O4_20, 40 and 60, respectively. The coercivity value was also increased with increasing of CoFe2O4 content. The Hc values of 590 Oe, 1051 Oe, and 1384 Oe were obtained for the samples of C/CoFe2O4_20, C/CoFe2O4_40, and C/CoFe2O4_60, respec-tively. The CV curve of the composite sample showed the redox peak related to the re-duction/oxidation transition of CoFe2O4 nanoparticles, while nearly rectangular shape was observed in pure CNF. Due to the reinforcing effects coming from CoFe2O4 nanoparticle, the BET specific surface area, the current density and the capacitance performance of the composite nanofibers are significantly improved after adding the CoFe2O4 nanoparticles.

In this paper, the conductive polypyrrole (PPy) polymer nanocomposites (PNCs) reinforced with different magnetite (Fe3O4) nanoparticle loadings have been synthesized using a facile surface initiated polymerization (SIP) method. The morphology of the fabricated PNCs was conducted by scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM). The chemical structure of the PNCs was studied by Fourier transform infrared (FT-IR) spectroscopy, the thermal stability was performed by thermogravimetric analysis (TGA), and the crystallization structure of the PNCs was studied by X-ray diffraction (XRD). The optical properties, magnetic properties, temperature dependent resistivity, and magnetic properties of the PNCs were systematically studied. The electrical conduction mechanism was explored by the introduced Mott variable range hopping mechanism. The permittivity and magnetoresistance behaviors are investigated as well. The negative permittivity was observed at certain frequency for all the synthesized nanocomposites. The positive magnetoresistance at room temperature in the pure PPy was analyzed by the wave-function shrinkage model. The negative magnetoresistance at room temperature in the nanocomposites was interpreted by the forward interference model.

Core/shell nanostructured materials with controlled size and shape have been acknowledged for their unique physical and chemical properties, which can be used for technological and biomedical applications as well in fundamental research. Although a wide variety of nanoparticles were prepared and tested for diagnosis and treatment of diseases, lots of studies still must to be performed to reach these goals. Currently, superparamagnetic iron oxide nanoparticles (SPION) are the most studied magnetic nanoparticles aiming biomedical applications, but SPION have low saturation magnetization which prevent some applications in vivo. Then, there is an actual demand to develop new nanostructures with high magnetization, low toxicity and easy surface functionalization with biocompatible molecules. Core/shell nanostructures based in a magnetic core presenting high magnetic emanation coated with a thin layer of a biocompatible material can also be used in both therapy and diagnosis of cancer, also known as theranostic nanoparticles. In this work, superparamagnetic metallic cobalt nanoparticles (CoNP) were synthesized and a gold coating was performed to protect the magnetic core against oxidation leading to multifunctional nanoparticles with both magnetic and surface plasmon resonance (SPR) properties suitable for future biomedical applications. The CoNP was synthesized using cobalt(II) acetylacetonate as precursor and sodium borohydride as reducing agent via hot-injection method at 493 K in dibenzylether in the presence of oleic acid and oleylamine as surfactants. Gold nanoshell was performed using tetrachloroauric(III) acid and oleylamine as surfactant and reducing agent at 353 K. CoNP showed a mixed crystalline structure of hexagonal compact (hcp) and face-centered cubic (fcc) phases. No ε-Co phase was observed as generally reported when conventional thermal decomposition method is used. HRTEM analysis showed that the as-synthesized CoNP core had spherical shape with 5.3 nm of diameter, narrow size distribution and low structure defects, in a typically nanocrystal structure. Magnetic characterization indicated high saturation magnetization (M_s) without any correction due the surfactant adsorbed, 115.4 emu.g^-1, and the superparamagnetic behavior at 300 K was investigated by fitting the Langevin equation resulting in a magnetic diameter of 4.7 nm. The M_s of Co/Au nanostructures (71.6 emu.g^-1) was significantly increased compared with the naked SPION in the same range of size. These nanostructures also presented SPR properties consistent with gold nanoshell coated a spherical dielectric core leading to red-shift in the plasmon band, which can be understood by the plasmon hybridization method. This behavior agrees with the reported core/shell dielectric interactions. In conclusion, the Co/Au nanostructure presented here have enhanced magnetic and SPR properties showing good potential to be used in biomedical application as bifunctional optical-magnetic sensor.

The ability to control the shape of metal nanocrystals is very important to application such as catalysis, magnetism and plasmonics. Here we report on the synthesis of monodisperse and shape control Ni (i) and NiPt nanocrystals and electroless deposition of Pd, Ag, Pt and Au on selective areas of the nanocrystals. Metallic core are synthesized from metal-organic precursors while galvanic displacement and/or chemical reduction allows for the second metal decoration.
Depending on the phase diagram and diffusion processes, the particles rearrange at low temperature (below 150°C) by alloying or demixing processes. This soft chemical route yields core/shell, dendrites or hollow morphologies.
Each bimetallic system shows very different growth pathways which we interpret as three different growth mechanisms: galvanic displacement at low temperature, metal assisted growth and overgrowth at high temperature. Combined high resolution transmission electron microscopy (HRTEM), XRD and magnetic measurements demonstrate the diffusion processes taking place in the nanocrystal. This result is a nice example of chemical rearrangement which we apply to hydrogen oxidation electrocatalysis.
(i) Nickel nanocrystals: cubes, pyramids and tetrapods from fast reaction M. Shviro and D. Zitoun RSC Adv., 2013, 3 (5), 1380 - 1387

Synthesis and Characterization of Shape-controlled Iron Oxide Nanoparticles
Yuping Bao, Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, Al 35487.
Shape control is one of the most exciting aspects of the nanoparticle synthesis, which allows for manipulating the chemical and physical properties of nanomaterials. The control of the nanoparticle physical properties also leads to new types of nanoparticle applications. In this talk, we will discuss the synthesis of various shaped iron oxide nanoparticles via a modified “heat-up” method. These shapes include ultrathin nanowires, nanoflowers, nanoplates, and nanoworms. In particular, we will discuss the importance of the reaction temperature and ligands to the shape control of iron oxide nanoparticles.
This work is in part supported by NSF DMR-0907204 and CAREER: DMR 1149931

We report the controlled synthesis of exchange-coupled fct-FePd/α-Fe nanocomposite magnets with various Fe con-centration converted from Pd/Fe3O4 core/shell nanoparticles via high-temperature annealing process. The shell thick-ness of Pd/Fe3O4 core/shell nanoparticles can be readily tuned for the preparation of different nanocomposite magnets. Under annealing conditions, the magnetically hard fct-FePd phases formed by the interdiffusion between α-Fe and fcc-Pd phases, while the excessive α-Fe phases exist around the fct-FePd phases, realizing the exchange coupling between the soft and hard magnetic phases. Magnetic measurements show the variation of the magnetic properties of different nanocomposite magnets, indicating the distinct exchange coupling at the interfaces. The coercivity of the exchange-coupled nanocomposites is tunable from 0.75 to 2.8 kOe and the saturation magnetization can be controlled from 93 to 180 emu g-1. This work provides an example to synthesize exchange-coupled nanocomposites via the bottom-up ap-proach with controllable magnetic properties for advanced magnets.

Despite their potentially large utility in applications such as theranostics and batteries, shape control in hollow iron oxide nanostructures has been challenging. In this work, we describe the formation of silica-encapsulated, hollow Fe3O4 tetrapods via the Kirkendall effect at a relatively low temperature of 120oC. This was achieved by first selectively removing CdS from silica-encapsulated core/ultrathin-shell CdS/PtS tetrapods via acid treatment, followed by exposure to iron precursors. Surprisingly, the heterogeneous nucleation and growth of iron took place exclusively within the PtS interior while oxidation of the resulting iron tetrapod under mild conditions produced a hollow iron oxide shell which preserved the shape of the original tetrapod with remarkably high precision. The silica-encapsulated hollow Fe3O4 tetrapods produced were found to exhibit exceptional colloidal stability and relatively good magnetic properties, with saturation magnetization at ~29 emu/g and a blocking temperature of ~144K. This work demonstrates the shape-preserving properties of the nanoscale Kirkendall effect, which can be exploited to produce hollow iron oxide nanostructures of unprecedented geometry.

Magnetic single domain nanoparticles can be seen as the “nano” analogue of permanent magnets and are found in a wide variety of applications ranging from engineering (i.e. smart nanocomposite materials that respond to an applied magnetic field) to nanomedicine (i.e. MRI, magnetic hyperthermia in cancer therapy or magnetically assisted drug delivery). An important property of these particles, that proved to be crucial for an increasing number applications, is their magnetic anisotropy energy, that is the strength of the energy barrier that opposes the fluctuations of the orientation of the particle&’s magnetic moment. While weak anisotropy energies yield room temperature superparamagnetism with a reversible magnetization process and no remanent magnetization nor coercivity, large anisotropy energies give the possibility to trap a remanent magnetization with significant coercivity and thus create permanent nanocomposite magnets.
In this presentation, we will show how it is possible to tune the magnetic anisotropy energy of magnetic nanoparticles or materials made with magnetic nanoparticles. We will describe a first approach that consists in synthesizing and dispersing bi-magnetic core-shell nanoparticles that include a core made with with a weak anisotropy energy material (soft ferrites such as magnetite or manganese ferrite for instance) and a shell made with a large anisotropy energy material (hard ferrites such as cobalt ferrite). This strategy allows us to tune the magnetic anisotropy energy of a particle without altering its magnetization and with a good control of its size.
Another route for the control of the magnetic anisotropy that we will also present is the simple mixture of soft and hard magnetic nanoparticles to create binary mixtures in a particular matrix that can be liquid or solid. In this case, it is the mixing ratio that allows one to adjust the properties of the final material.
Finally we will discuss how these particles can be useful for the improvement of existing applications and for the development of new ones.

The development of power efficient electronic components could be dramatically improved by the availability of nanomagnetic composites that can be solution-deposited as back-fills or integrated as conformal layers into a variety of devices. Composites of superparamagnetic particles with high magnetic permeability, and extremely high resistance to suppress and eddy current would be ideal targets. In this presentation we share our progress in investigating single components and multi-component nanoscale assemblies of superparamagentic nanoparticles for high frequency applications. We will present preliminary results on composites comprised of iron oxides or related ferrite nanoparticles in which our current best practices in synthesis, structural characterization, and functional DC and AC magnetic testing are reported.

Soft magnetic nanofilms with large permeability and resonant frequency are potentially applied to various GHz frequency applications. Electrodeposition can prepare these films fast and cost-effectively. As plating parameters affect microwave properties largely, understanding relationship between each parameter and microwave properties will enhance the film development. Herein through series of experimental and theoretical analyses, some detailed parameter effects on microwave properties of CoFe nanofilms were revealed. For example, plating solution composition and temperature could vary permeability(mu;) and resonant frequency(f_r). Previous work merely attributed this to the change of magnetic moment(M_s). However, the root-cause was attributed to which the two parameters determined over-potentials of E_Fe²⁺/_Fe and E_Co²⁺/_Co according to Nernst Equation during the ion reductions on the cathode, thereby leading to different deposition rate of Fe or Co and Fe:Co ratio in the as-prepared film. With Fe content of 30-60%, the CoFe alloy possessed high M_s of up to 2.45T. As thin films from electrodeposition, especially under in-plane magnetic field induction, exhibited in-plane anisotropy(H_k), according to Acher&’s law, this film can result in large mu; and f_r. Suitable organic additives could also significantly improve microwave properties, due to reduced oxygen content in the film via dispelling the O₂ adsorbed on cathode (e.g., for S-organic additive) or reducing Fe³⁺ to Fe²⁺ in the solution (for dimethylamine borane). Hence the additive reduced crystal defects and improved particle uniformity by decreasing O-doping, which in turn decreased the film coercivity(H_c) and damping parameter(α). For CoFe alloys, as H_k << M_s, the characteristic relaxation frequency could be as omega;_α = 2πγ&’H_k/α. Hence lower α led to larger omega;_α. According to Standard Lorentzian Eq. µ = 1 + (µ_s - 1)/[1 + iomega;/omega;_α - (omega;/omega;_r)²], larger omega;_α results in higher µ. Plating current density(I_d) also largely influenced µ and f_r. This was due to I_d also greatly varying the over-potentials. It affected not only the Fe:Co ratio and O-content, but also particle size, nanomorphology of the film. Lower I_d resulted in lower Fe- but higher O-content, thus the deposited film had lower M_s and larger H_c. On the other hand, although higher I_d led to lower O-content, the resulted higher crystal defects and larger particle size caused poorer crystallinity. As the crystallite size (calculated from XRD peaks using Scherrer Equation) did not vary significantly, more amorphous CoFe components formed with the I_d increase, which led to lower crystalline H_k. According to f_r = omega;_r/(2π) = [γ&’/(2π)²] ×(4πM₀H_k)^1/2 and above equations, lower H_k resulted in lower f_r and µ. Thus only optimized middle-range I_d could produce desirable microwave properties. Based on the analyses, the effects of film thickness and annealing were also revealed. CoFe nanofilms with quite large µ and f_r of up to 1200 and 5 GHz have been prepared as well.

The propulsion in micro/nanoscale has attracted considerable attention owing to its great promise for a wide range of future technological applications. Magnetically-controlled motion, inspired by the motility of natural microorganisms, represents an attractive route for addressing the challenge of efficient nanoscale locomotion. Two different kinds of magnetic nanopropellers are demonstrated here: firstly, magnetically-powered flexible nanowire propeller which exploits the flexibility of nanowire artificial flagella for propulsion can be mass-produced using a simple template electrodeposition protocol. Under an alternating magnetic field, they can display a high propulsion velocity in real biological environment which is comparable to natural microorganisms. Secondly, plant-based bio-inspired magnetic helical nanopropellers can be fabricated using an extremely simple, cost-effective fabrication route. The helical microstructures used here are derived from spiral water-conducting vessels of different plants, harnessing the intrinsic biological structures of Nature. Geometric variables of the spiral vessels, such as the helix diameter and pitch, can be controlled by mechanical stretching for the precise fabrication and consistent performance of helical nanopropellers. Mechanical properties of the xylem vessels of a wide variety of different plants were evaluated for the consistency and reproducibility of their helical parameters. Sequential deposition of thin layers of Ti and Ni on the spiral vessels followed by dicing leads to an extremely simple and cost-efficient mass-production of functional helical nanopropellers. The resulting plant-based magnetic nanopropellers display efficient propulsion, with a speed of over 250 µm/s. The magnetically driven nanopropellers provide an attractive approach for the targeted drug delivery: they can transport the PLGA drug carriers through a microchannel to the HeLa cancer cells in biological media for cancer therapy. Another important potential application of these magnetic nanopropellers is motion based nanofabrication. An attractive biocatalytic surface patterning route approach for generating three-dimensional Au microstructures with the high topological complexity (e.g., helical shape) using enzyme (e.g. glucose oxidase) modified magnetic nanopropellers is presented. A judicious control of the magnetic actuation allows the fine tuning and tailoring of the resulting microstructures. From these research advances, we expect that such magnetic propellers can play an important role in diverse future applications ranging from drug delivery to nanoscale fabrication.

Magnetic nanoparticles display interesting features which can find applications ranging from biomedicine over magnetic sensors to storage devices. Among them pure metallic Fe nanoparticles are of special interest due to their high magnetization, close to the bulk value (2.2 mu;B per Fe atom) [1, 2] or even higher for small-sized nanoparticles (2.6 mu;B per Fe atom) [3]. In this work, we focus on the influence of the chemical environment on the growth mechanism of Fe(0) nanoparticles and their corresponding morphologies as well as on their magnetic properties. In parallel with experiments, we have performed series of density functional theory calculations to investigate the thermodynamics properties of selected ligands, present in the reaction medium in mild conditions. An important conclusion is that the cubic shape of the Fe nanoparticles can not be solely governed by thermodynamics, since in our calculations the adsorbed species, despite efficient co-adsorption process, can not promote (100) facets instead of the most stable (110) oriented surfaces. Contrarily, we show that the iron precursor dissociation on the presented facets, with a strong preference for the (100) surface in terms of activation energy barrier and adsorption energy, is the key process that governs the morphology. This is especially true during the healing mechanism of nanocubes [4], or for the reformation of cubes from stars that can be obtained in some specific conditions. [5]
Besides, we also report on the theoretical magnetic properties of small iron nanoparticles and larger nanocubes, as synthesized in our laboratory [4]. We confirm that large (> 7nm) cubic Fe nanoparticle should present a magnetization close to the bulk value. Indeed (100) surfaces present the largest magnetic moment per surface atom due to their coordination number below to the and more importantly since ligands yielded from the dissociation of hydrochloric acid, palmitic acid and hexadecylamine on the surface have only weak influence on the surface magnetic moments. For small laquo;nakedraquo; nanoparticles or dispersed in polymer, the effect of surface hydrides is also investigated, and proved to also contribute with an extra spin part to the large magnetization.
[1] Science 303 (2004) 821.
[2] J. Mater. Chem. 21 (2011) 13464.
[3] Beilstein J. Nanotechnol. 1 (2010) 108.
[4] J. Am. Chem. Soc. 131 (2009) 549.
[5] Manuscript in preparation.
[5] Phys. Rev. B submitted.

Spin-based transport in semiconductor systems has been proposed as the foundation of a new class of spintronic devices. For the practical realization of such devices, it is important to identify new magnetic systems operating at room temperature that can be readily integrated with standard semiconductors. A promising class of materials for this purpose is the magnetic chromium-based chalcogenide spinels with high Curie temperature. Our recent band structure calculations indicate the exciting possibility of inducing half-metallicity in some of the mixed chalcospinels - ((Cu(Cd)Cr2S(Se)4-x, CuCr2S(Se)4-xEx (E=F, Cl, Br), and CdCr2S(Se)4-xDx (D=N, P, As) - and a possible spin symmetry filter effect in CdCr2Te4. These predictions motivate us to experimentally demonstrate and study the half-metallicity and spin filter effect in these compounds and utilize them for spin-based devices. We have developed solution-based synthesis methods for nanocrystals of CuCr2X4 (X = S, Se, Te) and the mixed systems CoxCu1minus;xCr2S4 and CuCr2SexS4-x. In the bulk, the CuCr2X4 (X = S, Se, Te) compounds are ferromagnetic metals with TC's of 377, 430 and 360 K, respectively, while CoCr2S4 is known to be a ferrimagnetic semiconductor at low temperatures (TC ~ 220 K). The syntheses of the nanocrystals involve hot injection of an excess of chalcogenide containing solutions into a boiling coordinating solvent containing CuCl2/CoCl2 and CrCl3.6H2O. Systematic changes in the lattice parameter, size, and magnetic properties of the nanocrystals are observed with composition. The nanocrystals are magnetic over the entire range, with the transition temperature dependent on the size and composition. Band structure calculations have been carried out to determine the electronic and magnetic structure as a function of composition. The results suggest that ferrimagnetic alignment of Co and Cr moments result in a decrease in magnetization with increasing Co concentration. The details of the syntheses methods, structural and magnetic characterizations and band structure calculation will be presented.

Synthesis and Structural Characterization of Ferromagnetic Au/Co Nanoparticles
Nabraj Bhattarai, Subarna Khanal, Daniel Bahena, Arturo Ponce, Robert L. Whetten, and Miguel Jose-Yacaman
Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA
The fabrication of bimetallic magnetic nanoparticles (NP) smaller than the size of single magnetic domain is very challenging because of the agglomeration, non-uniform size and possible complex chemistry at nanoscale. In this paper, we present an alloyed ferromagnetic 4±1 nm thiolated Au/Co magnetic NPs with decahedral and icosahedral shape.1 The NPs were characterized by Cs-corrected scanning transmission electron microscopy (STEM) and theoretically studied by Grand Canonical Monte Carlo (GCMC) simulations. Comparison of Z-contrast imaging and energy dispersive X-ray spectroscopy (EDS) used jointly with STEM simulated images from theoretical models uniquely showed an inhomogeneous alloying with minor segregation. The magnetic measurements obtained from superconducting quantum interference device (SQUID) magnetometer exhibited the ferromagnetic behavior. This magnetic nanoalloy in the range of single domain is fully magnetized and carries significance as promising candidate for magnetic data recording, permanent magnetization and biomedical applications.
Reference:
1. N. Bhattarai, G. Casillas, S. Khanal, D. Bahena, J.J. Velazquez-Salazar, S. Mejia, A. Ponce, V.P. Dravid, R.L. Whetten and M.M. Mariscal: Structure and composition of Au/Co magneto-plasmonic nanoparticles. MRS Communications 3, 177 (2013).
2. The author would like to acknowledge the valuable interaction from Prof. MM Mariscal, Dr. G. Ajithkumar, Prof. S. Mejia, and Prof. V. Dravid.

The coupling between different magnetic layers in magnetic thin film bi-layers and multilayer systems is usually ferromagnetic (FM) (i.e., the layers are aligned parallel to each other). However, other types of couplings such as antiferromagnetic (AFM) (i.e., layers antiparallel to each other) have also been reported. Interestingly, the diverse couplings, in particular the AFM one, cause numerous significant effects, which have given rise to important magnetic devices. In contrast, the magnetic properties of bi-magnetic core/shell nanoparticles remain relatively unexplored. While Monte Carlo simulations have probed the effects of different types interface couplings from the theoretical point of view (e.g., FM vs. AFM coupling), experimental work so far has only reported ferromagnetic coupling between the counterparts. Here we present the existence of an interfacial AFM coupling in ferrimagnetic (FiM) soft/hard and hard/soft core/shell nanoparticles based on iron and manganese oxides. Narrow size distributed Fe₃O₄/Mn₃O₄ and Mn₃O₄/Fe₃O₄ core/shell, soft/hard and hard/soft, were synthesised by seeded growth.[1] In contrast to conventional systems, the temperature dependence of the magnetization, M, and the ferromagnetic resonance field, H_R, and amplitude, A_FMR, show a downturn at the magnetic ordering temperature of the hard Mn₃O₄ phase (T_C(Mn₃O₄) ~ 40 K). This decrease in M, H_R and A_FMR can be attributed to an antiferromagnetic coupling between both phases. Moreover, X-ray magnetic circular dichroism (XMCD) spectra and element-selective hysteresis loops confirm that the magnetization of the Mn-containing phase lies opposite to the Fe-containing phase. Magnetometry hysteresis loops show that for small cooling fields the loop shifts towards negative fields similar to exchange bias in conventional FM/AFM systems.[2] However, for large cooling fields the loops shift to the opposite direction, i.e., positive exchange bias, as expected from an AFM interface coupling.[2,3] Finally, Monte Carlo simulations clearly confirm that an AFM interface coupling leads to a magnetization decrease at low temperatures and a positive exchange bias for large cooling fields.
[1] G. Salazar-Alvarez et al. J. Am. Chem. Soc. 133, 16738 (2011).
[2] J. Nogués et al. Phys. Rep. 422, 65 (2005).
[3] J. Nogués et al. Phys. Rev. Lett. 76, 4624 (1996).

Synthesis and self-assembly of ferromagnetic/ferrimagnetic nanoparticles are important for magnetic data storage applications in hard disk drive and magnetic tape. In order to get high density data storage, uniform size and shape of the nano-building blocks are needed. As an important ferrimagnetic material, cobalt ferrite (CoFe2O4) shows very high magnetic anisotropy and coercivity, which is a desirable system for these applications. Here I will discuss the solution-based synthesis of uniform cobalt ferrite nanocubes. The magnetic properties can be easily tuned by precisely controlling the size of the cube and composition of Co in the ferrite. Using the self-assembly process at the water-air interface, large area monolayer assemblies can be fabricated and those monolayer assemblies are used as magnetic recording medium for the data storage demonstration.

Among a variety of candidates for rare-earth-free permanent magnets, MnBi compound has a potential to be applied in practical applications due to its positive temperature coefficient of magnetocrystalline anisotropic field. However, fabrication of bulk MnBi magnets is very challenging because the MnBi low temperature phase (LTP) is metastable and easy to decompose. In this work, the preparation and magnetic properties of MnBi bulk magnets have been studied systematically. To facilitate the formation of the low temperature phase in MnBi alloys, the annealed arc-melted MnBi alloys are treated with a profile heat treatment (PHT) process, in which a temperature gradient and heating steps are established. The MnBi alloys after the PHT are crushed into powder with a size of 3-5 micrometer via a low temperature (liquid nitrogen) ball milling (LTBM) technique, which substantially reduces the decomposition of LTP occurred during room-temperature ball milling process. The as-prepared MnBi powder via PHT and LTBM is aligned with a magnetic field and pressed into a green bulk sample before it is undertaken a warm compaction process. The obtained anisotropic bulk magnet of MnBi shows a magnetic properties of Br=5.9 kGs, Hci=5.5 kOe and (BH)m=7.8 MGOe at 300K and a magnetic properties of Br=4.5 kGs, Hci=23.0 kOe and (BH)m=5.0 MGOe at 473K.

We have created single domain nanocrystalline single metal (e.g. Fe and Ni) and bimetals (e.g. Fe-Co, FePt) nanoparticles in 5-10 nm size range in amorphous as well as crystalline thin film matrices using a pulsed laser deposition method (PLD). By restricting the nanoclusters/nanoparticles size in the ensembles to a single domain region and by carrying out various magnetic measurements, a better understanding of magnetization configuration in a magnetic system has been developed. For example, the peaks in the coercivity isotherms delineate a critical grain size dc which identifies the crossover from single domain to multi domain behavior. The presence of dipolar interactions and their diminishing influence with increasing temperature is believed to be responsible for the observed dependence of dc on temperature and is in good qualitative agreement with our modification of existent theory of interacting particles. Results obtained from scanning transmission electron microscopy with atomic number contrast (STEM-Z) and energy loss spectroscopy (EELS) were also used to understand the atomic structure of these magnetic nanoparticles and interface between the nanoparticles and the surrounding matrices. Since Z-contrast imaging and EELS could be performed simultaneously, we were able to make direct correlations between structure and chemistry of the magnetic nanoparticles. The other part of this talk will focus on the fabrication and characterization of TiN nanowires using the same (PLD ) method where Ti-N based gaseous reactants in the laser plume supersaturate the catalytic gold (Au) liquid dots located on the substrate surfaces. Growth of TiN continues as long as the dissolution rate of material into the catalyst matches the extrusion of solid material at the liquid/solid interface. This bottom-up approach gives rise to a one-dimensional TiN nanowire structure (length: 200-300 nm and diameter: 20-30 nm) capped with a catalytic Au seed. The ascent of Au nanodots to the top of TiN nanowires can be explained based on breaking of weaker bonds and formation of stronger bonds. These TiN wires have shown promising biological properties in terms of cell adhesion and cell viability. The most potential clinical impact of this work is expected to be the in the development of wear and corrosion free (or with harmless level of corrosion) orthopaedic implants which have improved osseointegration properties.

Many efforts have been done in the synthesis of magnetic nanowires in past decades due to their potential application in the field of permanent magnets [1], sensors [2], high-density magnetic recording [3] and spintronic devices like magnetic memory [4] or microwave circuits [5]. Some of those applications are the natural exploitation of intrinsic properties of the anisotropic nano-objects like their large shape magnetic anisotropy which gives rise to an intrinsic large coercive field. Two main effects [6] have been demonstrated (mainly by micromagnetic simulations) to change drastically this intrinsic property: exchange bias effect rising from air-contact surface oxidation of the nanowires and inhomogeneity of demagnetizing field at the nanowires edges coming from different morphologies. Here we present a detailed study of the morphology effect of Co(80)Ni(20) nanowires on the magnetic coercivity properties. Changing synthesis conditions, we succeeded in having a good control of the nanowires edge morphology. Over a large statistics, we found the so elaborated nanowires to have an average length inside the range L= (100-200) nm and an average diameter (in the middle of the nanowires) of D=(7-8) nm. A conic head diameter (named T) was compared to the diameter D of the nanowires. We were able to vary the D/T parameter from a value of 0.36 (so-called diabolo-like nanowires) to 0.8 (quite cylinder-like nanowires). Magnetic measurements have shown few interesting features: (i) an exchange bias effect occurred at 110K likely related to the transition of CoO nanoshell from superparamagnetic to antiferromagnetic behaviour, (ii) this transition is strongly dependent from the thickness of the CoO shell (i.e. oxidation) and its nano-grains fluctuations/viscosity (iii) an increasing of coercivity with D/T ratio is observed (limit value being 7000 Oe to take into account for applications) and gave insights on the magnetization reversal mechanisms.
[1] T. Maurer et al., PRB 80, 064427 (2009), N. Ouar et al., accepted for publication in JAP, DOI: 10.1063/1.4827199 Date: 23-October-13
[2] P. D. McGary et al. J. Appl. Phys. 99, 08B310 (2006)
[3] F. Dumestre et al. Angew. Chem Int. Ed. 42, 5213-5216 (2003), A.I. Gapin et al. J. Appl. Phys. 99, 08G902 (2006)
[4] S. S. P. Parkin et al. Science 320, 190 (2008)
[5] B. Ye et al. J. Magn. Magn. Mater. 316, E56 (2007)
[6] F. Ott et al., J. of App. Phys. 105, 013915 (2009), T. Maurer et al. Phys. Rev. B 80, 064427 (2009)

Colloidal heterostructured nanocrystals (HNCs) represent last-generation breeds of wet-chemically synthesized inorganic particles, in which distinct material sections are interconnected via direct bonding (heteroepitaxial) interfaces in elaborate onion-like or oligomer-type topologies [1-3]. The development of HNCs embodies a generic approach to multi-component nanoscale entities, whereby an increasingly higher level of structural-architectural sophistication opens access to enhanced and/or diversified capabilities by combining control over the geometry and composition of the constituent domains with the engineering of their relative spatial arrangement. In this talk I will present recent progress made by our research group in the fabrication and characterization of various prototypes of elaborate HNCs that entail magnetic materials combined with semiconductosr and/or plasmonic metals sharing tailoired heterojunctions [4-13]. The distinctive chemical-physical properties as well as the technological potential offered by such multifunctional HNCs will be highlighted.
References
[1] P. D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 2006, 35, 1195
[2] M. Casavola, R. Buonsanti, G. Caputo, P.D. Cozzoli. Eur. J. Inorg. Chem. 2008, (6), 837
[3] L. Carbone, P.D. Cozzoli Nano Today 2010, 5, 449
[4] L. Carbone, P.D. Cozzoli, O. C. Kappe, Angew. Chem. Int. Ed. 2011, 50, 11312-11359
[5] R. Buonsanti et al. J. Am. Chem. Soc. 2006, 128, 16953-16970
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The synthesis of nanoscale structures capable of moving in liquids represents a major nanotechnological challenge. Significant progress has been made recently towards the fabrication of micro/nanomotors that rely on local chemical fuel or on external stimuli. Among the different types of micro/nanomotors, magnetically actuated ones are extremely promising for diverse in vivo biomedical applications owing to their attractive swimming performance. In particular, helical magnetic micro/nanoswimmers - inspired by bacterial flagellum propulsion - transform a rotation around their helical axis into a translation along the helical axis to offer an efficient locomotion behavior. However, till recently the large-scale preparation of helical micro/nanostructures has been challenging, since traditional microfabrication techniques - based on the deposition or removal of thin layers of material - have not been compatible with the preparation of complex three-dimensional (3D) helical micro/nanostructures. Taking advantage of recent advances in the electrosynthesis of nanosprings, we demonstrated a templating route can lead to the large-scale low-cost preparation of remarkably small magnetically driven helical nanoswimmers (down to 100 nm in diameter and 600 nm in length) that display efficient propulsion behavior. Our study demonstrates that such template synthesis provides convenient control the dimensions, geometry and composition of the helical swimmers, as desired for optimizing the swimming performance. Geometrically tunable helical nanostructures - with varied diameter, length and spiral pitch - can be readily fabricated via judicious selection of the membrane template pore size, composition of the plating solution and electrodeposition parameters. Thousands of helical nanoswimmers can thus be prepared within few hours. The nanoscale dimensions and efficient propulsion behavior of these template-prepared helical nanoswimmers make them ideal candidates for future miniature devices in the human body.

Complex oxides have served as promising candidates for discovery of novel functional materials. One of the most successful examples is the mixed-valence lanthanum manganite in which very large magnetoresistance (MR) effects are observed. Among them, La1-xSrxMnO3 (LSMO) is one most-studied compound of the mixed-valence manganite family for its robust and large MR effect above room temperature. However, the tradeoff between the high-required magnetic fields and significant MR values has hindered the potential applications.
In this work, a general approach to derive large MR characteristics of colossal magnetoresistance (CMR) materials in both low-field and high-field regimes through highly correlated nanostructures is successfully demonstrated. Such goals are achieved by 1) magnetically- and structurally- coupled ferromagnetic/CMR nanostructures (here, CoFe2O4/LSMO), where the polarized spins can transfer smoothly through the interfacial boundaries and epitaxial structures, enabling us to modulate the exchange couplings among the CMR material; 2) the tunable spacing and size of the CoFe2O4 (CFO) nano-pillars, which dominate the spin-polarized transportation and the magnetic interactions that lead to the enhanced MR effects in the system.
Firstly, self-assembled La1-xSrxMnO3- CoFe2O4 (LSMO-CFO) nanostructures are deposited by using pulsed laser deposition. The spinel-structured ferromagnetic CFO nano-pillars are uniformly and epitaxially embedded in the perovskite-structured LSMO matrix. X-ray magnetic circular dichroism as functions of magnetic field and temperature has further revealed the similar hysteresis-loops with respect to specific elements (Co, Mn and Fe) in the nanostructure, implying a highly coupled magnetic nature. Such an elegant system enables us to gain advanced control of the CMR materials since the magnetic couplings and 3-D epitaxial strain have served as the bridges across the borders of two functional materials. Magnetotransport as functions of temperatures and magnetic fields have been executed. Our results have revealed that the magnetic coupling in high-CFO-density nanostructures results in the extremely high colossal magnetoresistance (up to 400% CMR change) in high magnetic field regime. On the contrary, the decoupling feature and enhanced spin-polarized tunneling process, which give rise to the significantly increased low-filed MR (up to 35%), has been discovered when it comes to the nanostructure with loose CFO density. In this study, an elegant approach has been demonstrated to engineer both high- and low- field MR phenomena in a single material by such magnetically and structurally coupled ferromagnetic/CMR architecture. Our results have further shed the light on controlling the intriguing physical properties through the correlations between functional materials, which lead to new generation multifunctional applications and devices.

The manipulation of the magnetic properties of nanoscale materials with external electric fields is of considerable interest both as fundamental research subject and in view of developing nanoscale magnetoelectronic devices. Two-dimensional monolayer graphene (MLG) with its unique electronic properties is one of the most promising materials for future applications. Therefore, using MLG as a substrate for magnetic 3d transition metal (TM) nanoclusters should offer new opportunities in spintronics. This contribution presents the results of ab-initio density-functional theory of the magnetic properties of 3d TM adatoms and dimers on MLG as a function of applied external electric field E. Special attention is paid to field-induced changes of the magnetic order, the magnetic anisotropy energy and of the orientation of magnetization. For instance, in the case of Co dimers on MLG the electric field induces a transition from FM to AFM ordering as well as a change of the direction of the easy axis. Trends for other TMs are also discussed.

The coupling between ferroelectric and ferromagnetic order parameters in multiferroic thin films is a fascinating prospect for fundamental scientific research and potential multifunctional device applications. Among many multiferroic materials, BiFeO3 has potential for the development of novel spintronic devices as its unrivaled room-temperature multiferroic properties can be exploited in exchange-coupled magnetic tunnel junctions (MTJs). Until now, the fundamental problem in implementing these devices has been that exchange interactions between BiFeO3 and a ferromagnetic overlayer have been observed only in the presence of domain walls. We now have new evidence of an intrinsic exchange interaction between monodomain BiFeO3 [1] and a cobalt ferromagnetic overlayer that is not mediated by domain walls. We have developed a quantitative understanding of the magnetoelectric properties of monodomain BiFeO3 films using advanced synchrotron techniques (XMCD and XMLD). Such an approach would provide the basis for a deterministic and scalable device design that has been difficult in domain-wall-based approaches due to the inherent difficulty in precisely controlling domain wall formation and movement.
This work has been done in collaboration with W. Saenrang, S. Ryu, S.H. Baek, J.W. Freeland, B.A. Davidson and M.S. Rzchowski. This work is supported by the Army Research Office under Grant No. W911NF-10-1-0362.
[1]. “Ferroelastic Switching for Nanoscale Nonvolatile Magnetoelectric Devices” S. H. Baek, H. W. Jang, C. M. Folkman, Y. L. Li, B. Winchester, J. X. Zhang, Q. He, Y. H. Chu, C. T. Nelson, M. S. Rzchowski, X. Q. Pan, R. Ramesh, L. Q. Chen and C. B. Eom Nature Materials, 9, 309 (2010)

Magnetized nano-pillars are fabricated by conformally coating multi-walled carbon nanotubes (CNTs) with ferromagnetic metals, in order to deliver magnetostrictive structures with high strains, and also to achieve controlled morphology of nano-pillars within composites. Currently, conventional and giant magnetostrictive alloys are limited with their saturation strain. Micro-structures consisting of the magnetic CNT nano-pillars with high aspect ratio (up to 1000) can potentially improve the actuated strain range. Separately, nano-composites, consisting of nano-particles and matrices, have been investigated as multi-functional materials, but their property scaling has not been optimal due to poor control of nano-particles&’ micro-structure within matrices. With the magnetized CNT nano-pillars, organized morphology can be achieved by applying magnetic fields during composite fabrication. Multi-walled CNTs are a convenient structural component, for their unique, tailorable dimensions, and for multi-functional properties such as high strength and thermal/electrical transport properties. The metal-coated CNTs are to be sintered to form nano-composites; there composites&’ applications include highly conductive and thermally stable electrodes, thermal interface materials, and electrocatalysts.
The authors plan to experimentally realize these novel magnetized nano-pillars. Three types of ferromagnetic metals (iron, cobalt, and nickel) are conformally deposited on vertically and horizontally aligned MWCNTs using either evaporation beam deposition or electroless plating. Metal-deposited CNTs will be evaluated using electron spectroscopy and atomic force microscopy for metals&’ morphologies and geometries. The quality of the deposited metals (stoichiometric ratio, oxidization degree, CNT-metal surface bonding, etc.) will be inspected using X-ray photoelectron spectroscopy (XPS). These nano-pillars will then be magnetized and characterized using vibrating sample magnetometer. Composites consisting of aligned CNTs and metal matrices will be fabricated by sintering, and their anisotropic electrical and thermal conductivities will be characterized. By comparing these material and property data, the choice of metal kind and deposition method for the above applications will be evaluated and improved.

A dynamic and reversible control of magnetic properties via applied electrostatic field (surface charge) is relevant to application areas concerned with the manipulation, storage, and transfer of information by means of electron spins. For various nanostructures and many different ferro- and ferrimagnetic materials it has been reported that an applied surface charge can evidently affect magnetization or magnetic anisotropy.
In this work we present the results of studies on the magnetoelectric coupling in the artificial multiferroic La1minus;xSrxMnO3/Pb(Zr,Ti)O3 (LSMO/PZT) heterostructures. This multiferroic was - for the first time - in-situ investigated in a superconductive quantum interference device (SQUID). To get sample size suitable for the in-situ magnetometry, specially developed, large-distance magnetron sputtering was employed to deposit epitaxial thin films with remarkably high lateral uniformity, yielding ferroelectric device areas of several square millimeters.
The response of the magnetization to the ferroelectric switching of the PZT film evidences a purely electrostatic coupling mechanism with negligible piezoelectric influence. Temperature dependence of the magnetic modulation upon the ferroelectric stimulation indicates a field-effect dominated magnetoelectric coupling mechanism, confirming the concept of electrostatic hole doping of LSMO. For a small surface charge concentration at low temperature, a remarkably large tuning coefficient of about 4 mu;B/hole was determined suggesting the inducement of a ferromagnetic to antiferromagnetic phase transition in LSMO. Simultaneously, a shift in the magnetic transition temperature at higher surface charge concentration indicates coexistence of the ferromagnetic and antiferromagnetic phases at the LSMO/PZT interface. The dynamic magnetization modulation, resulting upon charging, is interpreted and discussed in the context of the existing phase diagrams.

Almost all matter expands with heating, as a result of the asymmetric nature of interatomic potential. The first exception to this general rule was discovered in a face-centred-cubic (FCC) Fe65Ni35 alloy, which exhibits a nearly zero thermal expansion over a wide temperature range and thus named “Invar” (denoting Invariance). Despite the fact that the discovery of Invar effect was crowned by a Nobel Prize in 1920, its origin remains one of the most challenging puzzles in physics. Fe100-xNix alloys undergo a displacive martensitic transition from FCC austenite/parent phase to BCC (body-centred-cubic) martensite at low Ni side of x<33at.%, and Invar effect occurs in the adjacent “non-martensitic” compositions of x>33at.%, where the FCC phase does not transform into martensite even down to 0K, but there exists a sluggish magnetic ordering at high temperature, from which Invar effect starts to appear. It is generally agreed that there exists a volume-compensation mechanism that offsets the otherwise normal thermal expansion, but its nature is still unclear. In this study we show that the “FCC phase” of Fe-Ni Invar alloys actually undergoes a new kind of magneto-structural transition - a specific strain glass transition. Phase field simulations for such a strain glass transition reproduces well the Invar effect and are consistent with the experimental observations. This transition provides a straightforward explanation for the Invar effect and other related Invar anomalies. Our work not only explains the long-standing Invar puzzle, it may also open a new vista to design novel multi-functional materials for advanced applications.

Codeposition of immiscible oxides on a single crystal substrate can produce two-phase nanocomposites in which each phase grows epitaxially on the substrate, forming columnar structures with well-defined vertical interfaces. The most widely studied system consists of spinel pillars grown within a perovskite matrix on a (100) perovskite substrate. Combinations of a ferroelectric perovskite such as BiFeO3 and a ferrimagnetic spinel such as CoFe2O4 yield multiferroic nanocomposites.
Vertical oxide heterostructures grow spontaneously, and the random positions of the crystals of the individual phases limit the utility of the nanocomposite. However, substrate patterning can direct the nucleation to form well ordered structures. We describe here templated self-assembly of spinel/perovskite (Co or Mg)Fe2O4/BiFeO3 nanocomposites to form both periodic and aperiodic arrangements with period of ~40 nm and above. The flexibility of the method in obtaining nanocomposites with a range of geometries and compositions, using templates formed by top-down or self-assembly methods, is illustrated, and the magnetic and structural properties of ordered nanocomposites are compared with their untemplated counterparts. Composition control in the vertical direction is also demonstrated by forming composites with a layer of CoFe2O4 pillars under a layer of MgFe2O4 pillars. The switching behavior of the magnetic pillars is explained in terms of a position-dependent magnetoelastic anisotropy resulting from epitaxy with the BiFeO3. The effects of magnetic interactions between pillars are analysed, and possible device applications are discussed.

The advanced properties of bulk multiferroic materials arise from the magneto-electric (ME) coupling which occurs between the ordered ferroic phases inside the multiferroic materials. Multiferroics suitability for a wide range of applications has been already demonstrated. The next development stage in the physics of solid-state multiferroics is their transition from bulk to thin film structures. This will ensure their commercialization and integration to the existing micro/nano silicon and data storage technologies. The following questions arise: Is the fundamental ME coupling retained at micro/nano scale and how multiferroics behave at such small dimensions? Some of these questions have already been addressed in a number of recent studies, reporting the electric field-induced magnetic domain formation observed via Kerr microscopy [1] and local ME coupling measurements via scanning probe microscopy of continuous surfaces [2,3].
In this work we report the study of a thin-film multiferroic composite examined via magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE) and interferometry measurements. The multiferroic composite studied consists of Terfenol-D / PNZT (200 nm) / Pt (150 nm) / TiO2 (40 nm) / SiO2 / Si, with Terfenol-D thickness ranging from 50 nm to 200 nm with controlled growth temperature ranging from 350 K to 600 K. Using simultaneous MOKE and interferometry measurements, changes in magnetic anisotropy of the ferromagnetic Terfenol-D layer are shown to arise from the strain-induced coupling of the ferroelectric PNZT layer. The changes in magnetic properties of the Terfenol-D layer are further correlated to changes in the nanoscale domain structure using MFM measurements and micromagnetic simulations. The magnetic anisotropy is found to be highly dependent on the growth temperature and using MFM imaging we also show the magnetic domain dependence on deposition temperature.
[1] T.H.E. Lahtinen, J.O. Tuomi, S.van Dijken, Adv. Mater. 23, 3187 - 3191 (2011)
[2] T.K. Chung, G.P Carman, K.P. Mohanchandra, Appl. Phys. Letters 92, 112509 (2008)
[3] D.V. Karpinsky, R.C. Pullar, Y.K. Fetisov, K.E. Kamentsev, A.L. Kholkin, J. Appl. Phys. 108, 042012 (2010)

Instantaneous and reversible tuning of the plasmonic property of metal nanostructures remains a challenge, which however holds great promise for developing novel optoelectronic devices and more effective chemical and biomedical sensors by allowing instant selective excitation or quenching of specific plasmon modes. For the first time, we report here the dynamic and reversible tuning of the plasmonic property of colloidal anisotropic metal nanostructures by controlling their orientation using external magnetic fields. By using gold nanorods as an example, we demonstrate that magnetic orientational control of the nanostructures can be achieved by binding them to superparamagnetic iron oxide nanorods in a parallel manner so that the resulting hybrid nanostructures tend to align along the external magnetic field in order to minimize the magnetic potential energy, thus enabling selective excitation of the plasmon modes of AuNRs through the manipulation of the field direction relative to the directions of incidence and polarization of light.

Presented is the first demonstration of devices utilizing spin wave interference to construct transmission holograms. Our group presents the experimental results of two spin wave device structures, both a two-terminal single-cross structure and a four terminal double-cross structure. Wave-guides are used to excite spin waves in the magnetic structure, allowing interference to take place in one or both of the junctions. Transmission holograms can be constructed from the phase data provided by the device. By utilizing special ferromagnetic elements on top of the structure it is possible to influence the interference of the spin waves and gain a memory effect from the device. Magnonic holographic devices are of great potential to complement the conventional general-type processors in special task data processing and may provide a new direction for functional throughput enhancement. Also discussed is the potential for scalability and integration into current CMOS technologies with the goal to reach densities as high as 1TB/cm2 as well as performing some special data processing tasks using these elements.

It was recently shown that the partial oxidation of a permalloy thin film in lateral spin valve devices can drastically increase the performance of such devices [1]. In order to apply Permalloy oxide (PyO) in magnetic sensor devices, the electric and optical properties must be known. The optical properties would allow the oxide thickness and quality to be monitored in production lines while the electric properties would allow optimization of devices.
Films were prepared by reactive dual ion beam sputtering on various substrates including glass microscope slides, silicon epilayers, thermally grown SiO2, and silicon nitride layers. The substrates were mounted on a water cooled substrate holder that was rotated during deposition at a rate of 50 rpm. 80 Watts of rf power was used for the argon gun at a flow rate of 15 sccm. The oxygen flow rate was varied during deposition and films were made with either molecular or atomic oxygen. The atomic oxygen was created with an ion-assist gun that was pointed directly at the substrate. The deposition rate was approximately 1 A/sec. The surface of the films showed a smooth feature-less surface by electron microscopy.
The magnetic properties of the films were determined by a Quantum design PPMS system from 15-300 K. The diamagnetic and paramagnetic contributions of the substrate were subtracted in order to isolate the magnetic properties of PyO. Films sputtered at an oxygen flow rate of 7.5 sccm showed a small ferromagnetic moment (Ms=48 kA/m) which suggests that the oxidation is not complete at those oxygen flow rates. For samples sputtered at larger oxygen flow rates (10 sccm), no ferromagnetic properties were found.
The optical properties of the samples were determined from 200-1000 nm by spectroscopic ellipsometry using a Woollam M2000 spectroscopic ellipsometer. Ellipsometric measurements of substrates and samples were done at three different angles (60, 65, and 70 degrees) in addition to optical transmission measurements at perpendicular incidence. PyO appears to be strongly absorbing above 4 eV while the absorption is negligible under 2 eV. The film thickness was determined from Δ and Psi; spectra below 2 eV using a Cauchy model.
The room temperature electrical properties of the samples were determined by a Jandel linear four point probe using tungsten carbide electrodes. A large contact resistance was measured indicating a non ohmic contact. The current was sourced with a Keithley 6221 current source and the 100 TOhm buffer amplifiers of a Keithley 7065 Hall card between the sample and the voltmeter. The thickness obtained from ellipsometry measurements was used to calculate the resistivity (4E3 Ohm cm). The resistivity was measured as a function of temperature by van der Pauw measurements. It was found that the resistivity decreased as a function of temperature, which is characteristic of semiconductors.
[1] Goran Mihajlovic, et al., Applied Physics Letters, 97, 112502, 2010

CoFeB/MgO/CoFeB is, as of today, the most thoroughly studied system for spintronics applications, due to its expected performance for perpendicularly magnetized spin-transfer torque MRAM (STT-MRAM). Its strength lies in its capability to follow a solid-state epitaxy process using industrially-compatible sputtering equipment, and relies on a subtle balance between competing growth and diffusion processes. Layers adjacent to CoFeB are of particular importance not only because they can enhance or inhibit the crystallization of CoFeB in the desired body-centered cubic (001) orientation required to obtain high tunnelling magnetoresistance, but also because they can lead to a perpendicular magnetic anisotropy (PMA) originating from the interface between CoFeB and MgO. While the exact mechanism is still unclear, Ta is generally required to observe an optimum value of the PMA[1]. However, Ta diffusion through CoFeB in Ta/CoFeB/MgO has been shown to be detrimental to the interfacial anisotropy during post-growth annealing processes[2]. In this paper, we have studied the influence of the interdiffusion (intermixing) processes which arise during the sputtering of a typical perpendicular magnetic tunnel junction (MTJ).
Ta 5/TaN 20/Ta 5/CoFeB 1.1/MgO 1/CoFeB 0.5/Ta tTa/overlayer tOL samples (all thicknesses in nm) were sputtered on SiO2 substrates and post-growth annealed at 240°C. CoFeB layers were grown from a Co40Fe40B20 target. Magnetization curves were obtained with an alternating gradient force magnetometer (AGFM). Our results reveal two key points: (1) the effective perpendicular uniaxial anisotropy energy density Keff decreases with increasing Ta thickness, indicating that the capping layers above MgO can strongly affect PMA, and (2) the overlayer material above Ta does not affect the Keff trend, clarifying that it is the interdiffusion of Ta and MgO playing the central role.
Furthermore, there is also an asymmetry in the degree of PMA degradation when Ta is sputtered above or below MgO, as evidenced by AGFM measurements of Ta 5/MgO tMgO/CoFeB 1.2/Ta 0.3/CFB 1/MgO tMgO/Ta 15 samples annealed at 240°C. We find that Keff is hardly affected by the thickness of the bottom MgO layer but increases significantly with the thickness of the top MgO layer. These results highlight the greater impact of deposition kinetics vis-a-vis thermal annealing in the thickness range of interest for perpendicular STT-MRAM, particularly in systems utilizing Ta.
1. D. C. Worledge, et al., Appl. Phys. Lett. 98, 022501 (2011).
2. N. Miyakawa, D. C. Worledge, and K. Kita, IEEE Mag. Lett., 4, 1000104 (2013).

This paper addresses introduction and precise control of electrical and magnetic properties of oxides via pulsed laser annealing, vaccum annealing and high-energy ion implantation. These oxide thin film heterostructures involving ZnO, NiO, MgO,YSZ (yttria stabilized zirconia) and VO2 are integrated epitaxially with silicon and sapphire substrates using domain matching epitaxy paradigm. The domain matching epitaxy paradigm involves matching of integral multiples of lattice planes across the film-substrate interface, where misfit across the scale is accommodated by principle of domain variation. By controlled introduction of intrinsic defects, it is possible to tune room-temperature ferromagnetism and electrical transport properties in these oxide materials, where the thickness of modified regions can be varied by laser parameters. High-power pulsed laser annealing will be used to introduce defects in the top few layers of ZnO and NiO and modify the electrical, optical and magnetic properties in a controlled way. In ZnO, ferromagnetism can be introduced into the surface layers; In NiO, n-type layers can be created in the near-surface regions of p-type films, thus leading to oxide p-n junctions in the same material; In YSZ/Si(100) thin films, a conductivity increase over several orders of magnitude is achieved by pulsed laser irradiation. These two-dimensional metamaterials are expected to exhibit novel properties with an exciting potential for next-generation solid-state devices.

In this work we investigate the effects of a discrete thickness variation in a permalloy film on spin waves by means of time-resolved Scanning Kerr Microscopy [1].
A straight step edge is preparated by partially covering a permalloy film by a second permalloy film. Spin waves are excited on both sides of the step edge by the signal line of a coplanar waveguide. A transition between refraction of the spin wave from the thinner region and reflection of the spin wave from the thicker region at the step edge is observed at a critical angle of the orientation of the external magnetic field.
Furthermore, if the external field is applied perpendicular to the step edge, the spin wave is trapped along the step edge by the inhomogeneous stray field of the step edge.
We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the Graduiertenkolleg 1286.
[1] M. R. Freeman and J.F. Smyth, J. Appl. Phys. 79, 5898 (1996)

For some time there has been interest in exploiting the magnetism of the rare earth elements. Most of the elements only show significant magnetic moment at cryogenic temperatures that then limits applicability.
To overcome this attempts have been made to couple the rare earth 4f electrons to conventional ferromagnets in various multilayer schemes to maximize magnetisation and Curie temperature. One of the most interesting and promising routes is to exploit ferromagnetic coupling between a conventional ferromagnet and the rare earth via an ultra thin antiferromagnetic metal such as Cr.
Building on promising suggestions from atomistic simulation and XCDM measurements of increased Curie temperature in a Gd/Cr/Fe system [1], we recently demonstrated that in Gd/Cr/Fe70Co30 the magnetisation, M(T), was impaired by the crystalline quality of the Gd which exhibited initial fcc nucleation before reverting to the desired hcp [2].
In this paper we report on our recent advance to use a Y seed layer to promote enhanced magnetisation and quality in the Gd layer. In combination with elevated deposition temperature we find a 10 nm Y seed layer yields optimal M(T) behaviour with a magnetisation of 2.62 T in Gd films at 4K exceeding that previously reported for high quality Gd films [3]. We then present our recent results on temperature dependence of magnetisation in tailored stacks of Y/Gd/Cr/Fe70Co30 that establish if there is enhanced magnetization in the system.
[1] B. Sanyal et al, Phys. Rev. Lett. 104, 156402 (2010)
[2] C. Ward et al, Appl. Phys. Lett. 102, 092403 (2013)
[3] G. Scheunert et al, Appl. Phys. Lett. 101, 142407 (2012)

There are new device opportunities at the interface of complex oxide heterostructures due to the interplay of charge, orbital, and spin degrees of freedom. A model system is the low dimensional conducting layer generated at the interface of LaAlO₃ and SrTiO₃, which has demonstrated high mobilities and tunable carrier densities. For spintronics applications, there has been a significant push in understanding all oxide based heterostructures for potential TMR applications in FM/I/FM type structures. However, little has been explored towards employing these high mobility interfaces as spin transport channels. Such conducting interfaces could be practical routes for realizing efficient spin transistors in which spin manipulation functionality could be epitaxially incorporated. First, spin injection, a key requirement of the spin transistor, must be explored. Here, we report our investigations of spin injection from ferromagnetic materials into the LaAlO₃/SrTiO₃ interface in a three-terminal geometry. Complex oxide films are grown by pulsed laser deposition and patterned into devices through lithography and hard-mask techniques. Using Hanle spin precession, we have observed spin lifetimes in the range of 80 - 100 ps. Notably, the devices exhibit unusual bias dependence. For negative applied biases (electron injection), no spin signal is observed. However, for positive biases (electron extraction) the spin signal increases with increasing bias. These results provide a building block in the field of oxide-based spintronics.

Magnetic sensors have wide applications in many fields with different conditions, and it is necessary to know the temperature dependence of magnetic sensors especially magnetic tunnel junction (MTJ) sensors. Here we report our work on MTJ sensors with temperature, and hope more understanding can be obtained about the performance of the MTJ sensor in a wide temperature range.
Here MgO-barrier MTJ sensors with both CoFeB layers pinned by IrMn have been fabricated and their temperature dependence of field sensitivity and low frequency noise has been investigated. At room temperature, the field sensitivity of 2.6%/Oe and the noise detectivity about 90 nT/ Hz0.5 have been observed in such MTJ sensors.1 With the decrease of temperature, field sensitivity increases while the low frequency noise level decreases gradually. High field sensitivity and low noise level are appreciated for their applications, which suggest MTJ sensors have a good performance at low temperatures.
Reference
1J. Y. Chen, J. F. Feng, and J. M. D. Coey Appl. Phys. Lett. 100, 142407 (2012).
This work is supported by the joint projects between China and Russia, China and Ireland.

Rare-earth-based alloys have been the standard for magnetostrictive materials since the development of TbFe (Terfenol) in the 1970&’s. For certain applications, however, these alloys have limitations such as brittleness, use of rare earth elements, and they are only available as bulk materials or vacuum-deposited thin films. Here, a relatively new magnetostrictive alloy, GaFe (Galfenol), has been electrochemically deposited as a wide variety of structures, such as nanowires and thick films on non-planar substrates wires. In addition, Galfenol overcomes many of the limitations of standard magnetostrictive alloys in that it is rare-earth-free and ductile. Our group has developed the chemistry to deposit GaFe in high-aspect ratio, nanoporous alumina templates for nanowires and on planar substrates of metals, Si, GaAs, and glass for sensors. Recently, a unique process has been developed to electrodeposit GaFe on cylindrical shafts using a rotating cylinder electrode (RCE) and hydrodynamic modeling. Specifically, rotating electrodes result in a uniform linear diffusion boundary layers over the samples, but cylindrical conditions vary greatly from planar rotating disk electrodes (RDE). Similar boundary layers could be obtained by increasing the rotation speeds for RCE vs RDE. EDS results of as-deposited nanowires and films show that the composition can be controlled for the entire range of Galfenol alloys (10%-40% at.% Ga) by manipulating deposition potential from 1.4 to 1.8V using rotation rates from 100 to 1000rpm. X-ray diffraction (XRD) analysis showed that the nanowires and films are polycrystalline which agrees well with capacitance bridge measurements which determined magnetostriction values of 50 to 120ppm for the films. Previous nanowire measurements by atomic force microscopy yielded about 40ppm magnetostriction for GaFe nanowires. Additionally, data from vibrating sample magnetometry (VSM) first order reversal curves (FORC) were used to measure the distribution of coercitivies in thin film grains and nanowire arrays. This unique in situ deposition capability enables magnetostrictive films to be used in new areas such as contactless torque sensors, implantable devices for remote sensing and energy harvesting devices.

Intimate contact between a ferromagnet (F) and an antiferromagnet (AF) can result in exchange bias, a phenomenon used in magnetic sensors and data storage devices where the strong magnetic anisotropy of the AF causes a shift (HE) in the magnetic behavior of the softer, ferromagnetic material. These systems are characterized by pinning of magnetic spins at the F/AF interface, requiring large fields to reverse the moment of the ferromagnet. In addition, the bias field is directly proportional to the number of spins at the interface; as such, controlling the microstructure of biphasic F/AF materials can be used to tailor magnetic response (e.g., switching field, remanence, and coercivity).
In this work, we use a novel microplasma-based spray deposition technique [1-5] to realize nanostructured NiFe2O4/NiO films for exchange bias applications. Organometallic precursors are broken down in a flow-stabilized hollow cathode jet to form a directed flux of active growth species (atoms, ions, clusters, etc.), which can be spray deposited on any surface. The unique operating conditions of the microplasma, namely high Telectron and low Tion, high pressure (10 to 1000 Torr), and short space time (~mu;s), provide an ideal environment for cluster nucleation and growth of NiFe2O4/NiO nanogranular films.
This talk will highlight microplasma deposition of NiFe2O4/NiO nanogranular systems with emphasis on how film microstructure, grain size, relative phase fraction, and interfacial structure affect exchange bias phenomena. Film morphology (SEM and TEM), crystal structure (XRD), and magnetic properties (SQUID) as a function of microplasma operating conditions and post-growth annealing will be discussed. Large coercivity enhancements of NiFe2O4 films were realized by introduction of the NiO phase (~500 Oe to ~1050 Oe), and exchange fields (HE) up to 80 Oe were observed in biphasic NiFe2O4/NiO films, indicating the presence of exchange coupling.
[1] T. Koh and M.J Gordon, J. Phys. D: App. Phys., accepted (2013).
[2] T. Koh, I. Chiles, and M.J Gordon, Appl. Phys. Lett. 103, 163115 (2013).
[3] T. Koh and M.J Gordon, JVST A 31, 061312 (2013).
[4] T. Koh and M.J. Gordon, J. Crystal Growth 363, 69 (2012).
[5] T. Koh, E. O'hara, and M.J. Gordon, Nanotechnology 23, 425603 (2012).

Nanoscale devices are typically fabricated from thin film multilayers patterned with photolithography and ion milling. Applications such as magnetic recording field sensors and MRAM commonly use exchange biased thin films to optimize device performance. During ion milling, device films are exposed to energetic ions which can modify their magnetic and exchange bias properties. Clearly device edges are most susceptible to damage during this process, but distinguishing the changes in magnetic film properties from random variations in device performance is not trivial. To simplify this problem and quantify the effect of milling damage on magnetic materials, we investigate the properties of exchange biased planar structures exposed to ions from above, through a non-magnetic cap layer. Exchange biased structures provide a high resolution means to study damage given their sensitive nature. The conclusions drawn from this work provide insight into the effects of ion milling in related but more complex geometries, such as non-vertical sidewalls and varying ion beam angles.
We used DC magnetron sputtering to make thin film samples which were measured with vibrating sample magnetometry (VSM) before and after milling. To set a baseline for the thickness dependence of magnetic properties without milling damage, we made control samples: underlayer(UL)/55Å IrMn/t CoFe/40Å Ag/50Å Ru using CoFe thicknesses ranging from 4 to 30Å. Test films to study the impact of ion milling are similar: UL/IrMn/t CoFe/40Å Ag, with three starting (unmilled) thicknesses of CoFe: 15, 22, and 30Å. All samples were annealed at 280oC to set the exchange bias. We milled the test films using low power Ar+ ions at a 10o angle from the surface normal, stopping at various depths either in the Ag cap or in the CoFe below. After a sample is ion milled, it is covered with a 50Å Ta cap deposited in-situ to prevent exposure of the sample surface to atmosphere. We used X-ray Reflectivity (XRR) to determine the ion milling depth.
We observed significant sample modification after Ar ion milling. The modifications of sample magnetization and pinning strength depend upon the initial CoFe thickness and the distance from the milling front to the IrMn/CoFe interface. In comparison with our data, simulations yield a reasonable value for the milling depth where the onset of damage occurs. Pinning can decrease by as much as 40% before the entire Ag cap is removed. Structures that include an additional CoFe free layer and Ru capping layer show much greater loss of pinning as milling progresses. Regardless of the thickness of the CoFe reference layer, pinning decreases by nearly 50% before the CoFe reference layer has even been milled. We suggest that damage similar to what we observe occurs laterally at device edges formed by typical photolithography and ion milling processes, which could play a significant role in modifying film properties and enlarging the spread of device performance variation.

Fe16N2 is a mystery but a unique magnetic material that possesses both giant saturation magnetization and large magnetocrystalline anisotropy, which make it a promising candidate for next generation rare-earth-free magnet. In this talk, I will first review the history and analyze the previous inconsistencies and obstacles of the Fe16N2 topic in the past 40 years. Then I will present our effort to address those inconsistencies during past ten years. From X-ray magnetic circular Dichorism (XMCD) experiment, polarization-dependent x-ray absorption near edge spectroscopy (EXANE), polarized neutron reflectivity (PNR) and first-principle calculation, it has been both experimentally and theoretically justified that the origin of giant saturation magnetization and the large magnetocrystalline anisotropy is correlated with the formation of highly localized 3d electron states in this Fe-N system. The synthesis of Fe16N2 thin films, powders and bulks will be presented in details. Most importantly, we demonstrated that its thermal stability could be up to 250 oC. Then, specifically, x-ray photoemission spectroscopy, high-resolution imaging, electron diffraction and polarized neutron reflectivity and diffraction methods have been used to characterize the presence and structure of the Fe16N2 ordered iron nitride phase in both FeN thin-film samples produced by the facing-target deposition method, bulk powder aggregate samples resulting from a nitridation reaction and bulk FeN magnet from a special alloying process. Coupled with reporting a relatively large magnetic energy product up 10 MGOe at room temperature, the observation and confirmation of significant Fe16N2 ordered phase in different FeN samples, by magnetometry, direct imaging and electron microdiffraction and polarization neutron methods will be discussed in details.

Lanthanide ions are well-known to exhibit large magnetic moments and strong magnetic anisotropy and therefore they are considered as good candidates for the elaboration of Single Molecule Magnets (SMMs). Complexation of these particular metal ions by redox active ligands derived from TTFs led to electroactive SMM with antennae effect of the ligands as well as very good correlation between magnetism and luminescence at the molecular scale. We report in this lecture several compounds exemplifying these features [1,3].
Among them, dinuclear complexes of lanthanides associating both 4,5-Bis(thiomethyl) -4&’-carboxylictetrathiafulvalene and 4,5-Bis(thiomethyl) -4&’-ortho-pyridyl -N-oxide -carbamoytetrathiafulvalene ligands have been elaborated. Dc magnetic susceptibility measurements highlight ferromagnetic interactions between the metallic centres. The two Dy(III) and Yb(III)-based analogues display SMM behaviour. Experimental and theoretical magnetic and photo-physical investigations have confirmed that a multi-electroactive luminescent SMM is obtained in the case of the Yb(III) analogue [3].

References
1. F. Pointillart, et al., Chem. Eur, 2011, 17, 10397
2. F. Pointillart, et al., JCS Chem Commun., 2012, 48, 714
3. F. Pointillart, et al., JCS Chem Commun., 2013, 49, 615

Ultra soft magnetic materials like Fe(Co)-Si(Zr)-B-Cu based alloys with a nanocrystalline phase embedded within an amorphous matrix have been developed. In contrast, reports on Ni based permalloys, consisting of nanocrystalline fcc solid solution embedded in amorphous matrix are very limited. The present paper deals with the structural evolution of melt spun Ni(Fe)-Zr-B-Cu based alloys during rapid solidification using melt spinning technique and correlation with their soft magnetic properties. Alloys with nominal compositions (Ni1-xFex)88Zr7B4Cu1, where x = 0.2, 0.3, 0.4 and 0.5 were prepared using a vacuum arc meting furnace. Rapidly solidified ribbons were produced using a vacuum melt spinner to obtain amorphous phase followed by controlled crystallization for obtaining nanocrystalline phase in amorphous matrix. The as-spun and annealed samples were then characterized using X-Ray diffraction (XRD), differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM) and Coercimeter.
The XRD results revealed that the as spun alloy ribbons with x= 0.2, 0.3 and 0.4 exhibit the presence of both amorphous and fcc solid solution phases, whereas the ribbon with x=0.5 is completely amorphous. Detailed thermal analyses revealed that ribbons with x = 0.2 to 0.4 undergo two stage phase transformations; one at 419-425 and other at 567-587oC. The first stage corresponds to the transformation of metastable fcc solid solution into ordered Ni3Fe phase and the second stage corresponds to the transformation of amorphous to eutectic Ni3Fe and Ni3Zr phases. In case of the ribbon with x = 0.5, the amorphous phase is transformed into eutectic Ni3Fe and Ni3Zr phases upon crystallization at 579oC, which is similar to the transformation of other alloys. On annealing at 620oC, it has been found that the grain size of the nanocrystalline fcc Ni(Fe) phase, calculated from Scherrer&’s formula decreases with Fe content. The saturation magnetization values of all 620oC annealed ribbons decrease as compared to the as-spun condition due to the precipitation of non-magnetic Ni3Zr phase. The coercivity values initially decrease to 0.2 - 0.3 Oe on annealing at 450oC and then further increase to high values on annealing at 620oC.

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Nanocrystalline alloy of Fe(73.5)Si(13.5)B(9)Nb(3)Cu(1), known as FINEMET, is a very attractive soft magnetic material. It exhibits excellent permeability, high saturation magnetization, low coercivity, and high electrical resistance. This material is prepared by annealing a melt-spun amorphous Fe(73.5)Si(13.5)B(9)Nb(3)Cu(1) alloy at temperatures in the range of 520-580°C. It is believed that the presence of Cu and Nb dramatically affects the growth kinetics of the nanocrystalline, hence the microstructure and material performance. In this work, we present a mesoscale phase field model to investigate the effect of Cu precipitation on the nucleation and growth kinetics of bcc FeSi nanograins during annealing process. During the past two decades, phase field method has matured as a method of choice to simulate microstructural evolution under different material processes. This approach has the well-known advantage that it avoids explicitly tracking a sharp boundary by smearing the interface region over limited thickness. In addition, phase field method is formulated to address non-equilibrium states. The developed model considers three phase equilibrium: Cu-rich phase, bcc FeSi phase and amorphous matrix phase. The driving forces of the nucleation and growth of the bcc crystal phase are described by the local Cu concentration. The impact of the nucleation and growth kinetics of the Cu rich phase on the bcc FeSi phase morphology is systematically examined.

Integrating magnetic nanocrystals (NCs) with discrete energy levels into magnetic tunnel structures is of significant interest due to expectation of novel properties from their spin selective transport and single electron features. When size and shape dispersed NCs are arranged on suitable templates they can have strong particle interactions and show behaviors corresponding to their ensemble. Ordered superstructures by colloidal magnetic NCs with translational and orientational order and interesting collective properties can be prepared by solution casting through sensitive interparticle and particle-substrate interactions. In this talk, I discuss the study on magnetic field induced assembly of mono-dispersed iron oxide NCs sandwiched between gold electrodes to obtain spin selective transport across the superlattice array. The superlattice formation by mixed phase Fe3O4@γ-Fe2O3 NCs on SiO2/Au surface proceeds through slow solvent evaporation and is studied for controlled interparticle spacing. For specific NC solution concentration, the ordering is also dependent on the substrate chemistry and the ligands passivating the NC surface, that affects the concentration of cluster nuclei formed. The tunnel junctions exhibit enhanced positive tunneling magnetoresistance (TMR) below NC blocking temperature and a switching from negative to positive TMR across the Verwey transition. The electron transport across the array show interesting features of conduction through parallel and discrete conducting channels even at room temperature. A good correlation is identified between the magnetoresistance, the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance.
References
1. I. C. Lekshmi, C. Nobile, R. Rinaldi, P. D. Cozzoli and G. Maruccio, Sci. Adv. Mater. 2013, 5, 1.
2. I. C. Lekshmi, R. Buonsanti, C. Nobile, R. Rinaldi, P. D. Cozzoli and G. Maruccio, ACS Nano 2011, 5, 1731.
3. Z. Nie1, A. Petukhova, and E. Kumacheva, Nature Nanotech. 2010, 5, 15.
4. C. T. Black, C. B. Murray, R. L. Sandstrom, and S. Sun, Science 2000, 290, 1131.

When a material having interfaces of ferromagnetic - antiferromagnetic , ferromagnetic - ferrimagnetic or antiferromagnetic - ferrimagnetic is cooled through the Neel temperature (TN) of the antiferromagnetic phase (TN < TC) in presence of magnetic field, an unidirectional anisotropy is induced in the material which is called Exchange bias (EB). The EB phenomenon is usually observed as a shift of magnetic hysteresis loop or asymmetric hysteresis loop. EB was first observed by Mieklejohn and Bean in Co/CoO particle system. In more than five decades of its discovery, exchange bias effect has been studied in a variety of magnetic systems and the phenomenon is useful for applications like recording media, sensors, read heads etc. The maximum EB field of ~10 kOe has been reported at 5 K in Ni/NiO core-shell systems at 5 K core-shell systems for cooling field 20 kOe. EB behavior in Heusler alloys have also been studied. In Heusler alloys EB is attributed to coexistence of ferromagnetism and antiferromagnetism in the martensite phase. The highest value of exchange bias field (HEB) of ~480 Oe in Heusler alloys have been reported for the Ni45Co5Mn38Sb12 alloys by Nayak et al. at 3 K for cooling field of 50 kOe.
In this work we would present the EB behavior of Ni56Mn21Al22Si1 (NMAS) ribbons. The room temperature structure of NMAS evaluated was noted as orthorhombic 14 M with a=4.14 Å, b=29.84 Å, c=5.72 Å (martensite phase). We report the exchange bias of 1.8 kOe in NiMAS polycrystalline ribbons at 5 K in cooling field of 20 kOe, which is the highest exchange bias field reported so far in any Heusler alloy.

We report the observation of magnetic and resistive aging decays in a self assembled nanoparticle system produced in a multilayer Co/Sb sandwich. Aging is characterized by an initial slow decay followed by a more rapid decay in both the magnetization and resistance. The decays are large accounting for almost 70-80% of the magnetization and between 20-50% change in the sample resistance. During the more rapid part of the decay, the derivative of the slope of the decay changes sign and this inflection point can be used to provide a characteristic time. The characteristic time is strongly and systematically temperature dependent. The relationship between the characteristic time and aging temperature fits an Arrhenius law indicating thermally activated dynamics. Both the temperature scale and time scales are in potentially useful regimes. Measurements have been made on samples produced with varying deposition temperatures, Co thicknesses and Sb thicknesses. Magnetic, electronic, AFM, MFM and STM measurements will be reported.

Controlling the orientation and stability of the magnetization direction in transition-metal nanostructures is crucial for the development of new magnetic materials to be applied, for example, in high-density recording, spintronic devices and medical treatments. Understanding the magnetization dynamics and thermodynamics in the presence of spin-orbit interactions and electronic correlations poses also a most interesting challenge to fundamental condensed-matter physics. In this contribution we first determine the magnetic anisotropy energy (MAE) of small monodomain Fe, Co and Ni clusters by performing full-vectorial calculations in the framework of a self-consistent tight-binding theory. The minimum energy paths (MEPs) corresponding to the relevant magnetization reversal processes are derived by means of a nudged elastic-band method. Complex multiaxial MAE surfaces are discussed, which can be correlated to bond-length relaxations and reduced cluster symmetry. In the second part the theory is extended to finite temperature equilibrium states by using a functional integral formalism. The application of the theory to Fe ((001) monolayers reveals, in agreement with experiment, a remarkable temperature induced reorientation, from out-plane to in-plane magnetization, at relatively low temperatures (T/Tc = 0.2). The microscopic origin of the change in magnetization direction is analyzed from a local perspective by comparing the temperature dependence of the anisotropy of the electronic energy and entropy.

Magnetic nanostructures have attracted interest due to the enhancement in magnetic properties and their potential application in data storage, sensing, and beyond CMOS devices. We herein propose spin quantum cross (SQC) devices, in which organic molecules or inorganic quantum dots are sandwiched between two edges of magnetic thin films whose edges are crossed. In SQC devices, the junction area is determined by the thickness of the magnetic thin films, in other words, 1-20 nm thick films could produce 1x1-20x20 nm² nanoscale junctions. This method offers a way to overcome the feature size limit of conventional lithography. Moreover, a high magnetic field can be locally generated in the organic molecules or inorganic quantum dots sandwiched between the two edges of the magnetic thin films due to the contribution of the stray field from their edges. Since the high magnetic field produces a large Zeeman effect, the energy splitting of the organic molecules or inorganic quantum dots can be enhanced. Therefore, large spin filtering effects can be expected. As the first attempt towards the creation of such novel magnetic nanostructural devices, we investigate magnetic properties of SQC devices experimentally and theoretically.
Co thin films were deposited on borate glass substrates by electron beam evaporation. A borate glass was pressed onto the deposited Co/glass at a deformation temperature using a glass mold pressing technique. The surface of glass/Co/glass was polished by chemical mechanical polishing (CMP) methods. The microstructures of samples were evaluated using scanning electron microscopy (SEM). The surface morphology and the roughness of the polished glass/Co/glass were analyzed by atomic force microscopy (AFM). The stray fields contributed from the edges of the Co thin films were observed by magnetic force microscopy (MFM). The local magnetic fields generated between the two edges of the Co thin films in SQC devices were calculated by micromagnetic simulation.
The ultrasmooth surface with a roughness of 0.29 nm can be realized in the polished glass/Co/glass using the CMP technique. Under this situation, the position of the Co edges has been identified from the SEM observation. Also, the stray magnetic fields contributed from the Co edges have been successfully observed by MFM. The stray fields are uniformly distributed, indicating that the single domain structures can be formed. According to the theoretical calculation, the local magnetic fields generated between the two Co edges in SQC devices increase with decreasing the distance between the two Co edges and increasing the Co thickness. The local magnetic field exhibits as high as 7000 Oe under the condition that the distance between the two Co edges is 5 nm and the Co thickness is 19 nm. These results indicate that SQC devices utilizing stray magnetic fields can be expected as novel magnetic nanostructural devices, leading to spin filtering devices and beyond CMOS devices.

Template-assisted electrodeposition has been used to fabricate arrays of polyethylene glycol polymer (PEG) coated ferromagnetic nickel nanowires. During the process, metal and polymer were co-deposited into the pores of an anodic alumina membrane (AAO). The formation of the PEG (outer layer) with the Ni (core) occurs when PEG adsorbs on the pore walls followed by deposition of nickel metal along the pore channel. Unlike conventional nickel nanowires, PEG-coated Ni (PEG-Ni) nanowires have a tendency to show greater resistance to breakage and less of a tendency to agglomerate after dissolution of the template. After AAO removal, these polymers coated nanowires are embedded in polyethylene glycol di-acrylate (PEGDA) where polymerization is carried out under UV light in a magnetic field. The solvent active polymer PEGDA can be expanded and contracted, varying the interactions of adjacent magnetic nanowires and hence the magnetic hysteretic properties as a function of wire arrays orientation. Details on the systematic studies of these materials will be presented in terms fabrication and magnetic characterization and the origins of these behaviors will be discussed.

Energy dispersive X-ray-spectroscopy (EDS) using scanning transmission electron microscopes (STEM) and scanning electron microscopes (SEM) was performed to demonstrate the capabilities and suitability of the available instrumentation for composition and structure analysis to understand the properties of magnetic nanomaterials. Various samples were studied, including single nanoparticles, complex magnetic nanostructures for data storage arrays and meteorite material.
Silicon drift detectors (SDD) for EDS are a widely established technology now, also for analyzing light elements. SDD-EDS offers quantification accuracy on the level of few atomic percent or better. For example, distinguishing between magnetite and hematite by EDS is routinely possible. High collection and high take-off-angle EDS in combination with conventional and aberration-corrected STEM allows nm- and atomic scale element analysis [1-3]. In case of atomic columns containing different element species the acquired spectroscopic data need to be combined with theoretical simulations of radiation effects to quantify the composition correctly [4].
In a simple experiment we show that nm-sized Ni- and Co-catalyst nanoparticles in carbon nanotubes [5] can be easily identified by EDS using a TEM-sample in SEM. Another example is the investigation of magnetic hedgehog-like nanostructures designed for high density magnetic data storage. To produce these nanostructures granular CoCrPt-SiO2- films were deposited onto spherical SiO2 nanoparticles using texture enhancing Ta/Ru seed layers [5]. Here a combination of EDS analysis in conventional STEM and electron energy loss spectroscopy (EELS) in aberration corrected STEM helped to understand the formed peculiar film morphology and the thus beneficially modified magnetic reversal behavior. All elements of interest used for the design of these hedgehog-like nanostructures are predestined for EDS-analysis, whereas only few of them are well suited for EELS. Furthermore, we demonstrate versatile EDS analysis tools for fast data acquisition, identification and quantification of magnetic material compositions and structures in 2D and 3D.
[1] S. von Harrach et al., Microsc. Microanal. 15 (Suppl. 2), 208 (2009).
[2] P. Schlossmacher et al., Microscopy Today 18(4), 14-20 (2010).
[3] T.C. Lovejoy et al., Appl. Phys. Lett. 100, 154101 (2012).
[4] B. D. Forbes et al., Phys. Rev. B 86, 024108 (2013).
[5] S. Hermann et al., Microelectronic Engineering Vol. 87 (3), 438-442 (2010).
[6] C. Brombacher et al., Appl. Phys. Lett. 97, 102508 (2010).

Magnetic granular films have been attracted attention as one of promising candidate for magnetic memory and sensor elements because of their technical advantages. In this study, we report the electric and magnetic effects of inserting the Fe nanoparticle into the MgO layer of CoFeB/MgO/Fe/MgO/CoFeB structures. The sample&’s structure is Ta(25)/Co₂₀Fe₆₀B₂₀(1.3)/MgO(1)/Fe(x)/MgO(1)/Co₄₀Fe₄₀B₂₀(2.2)/Ta(5), and were deposited on SiO₂ substrates at room temperature with various Fe thickness. The unit is in nanometer. The structures were then annealed at temperature 300°C up to two hours. The electric and magnetic properties were investigated with various temperature from 50 k to 300 k. Varieties of magnetic and transport properties were found with various Fe thickness and annealed temperature. We found that the low deposition rate and thinner iron layer thickness are improved pMTJ structure magnetic properties. Smaller size and few numbers iron particles were improve magnetic properties at low temperature. The coercivities and shift fields for with Fe particle structures were larger than without Fe particle structures. Perpendicularly magnetic anisotropy of full structure was increased after annealing without applied field. In our case, the Fe 0.1 nm has best electric and magnetic properties. Furthermore, we found that the MR ratio increases dramatically over 111% with annealed at 300°C and embedded 0.1 nm Fe particle. In addition, the effect of varieties of Fe particles and annealing temperature on perpendicular anisotropy energy, MR ratio, and TEM microstructure properties will be presented and meticulously discussed.
This work was supported by the Department of Industrial Technology, Ministry of Economic Affairs, Republic of China, under Contract No. 98-EC-17-A-01-S1-026, and by the National Science Council of Taiwan, Republic of China, under Contract No. NSC 98-2112-M-224-002-MY3.

Ferromagnetic/ferroelectric multiferroic thin film heterostructures offer important opportunities for use in spintronic devices as electrically controllable ferromagnetic elements[1]. In this work, we demonstrate clear electric field switching effect of the magnetic anisotropy of Ni thin layer/ poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer film heterostructures, enabling us to develop a promising energy-saving way to manipulate the magnetization orientation of a multiferroic heterostructure by electric field.
Samples used consist of a ferroelectric P(VDF-TrFE) thin film deposited on an Au/Ti/SiO₂-coated Si substrate by spin coating and a ferromagnetic 10-nm-thick Ni thin layer grown by molecular beam epitaxy under a base pressure of 10^-9 Torr. Magnetization curves were collected using a magneto-optical Kerr effect measurement setup as a function of electric field applied normal to the film plane using the bottom Au electrode. While all the magnetization curves show square shaped ferromagnetic behavior, the magnetic coercivity of the Ni layer exhibits a clear electric field dependence; the magnetic coercivity rapidly changes at around the negative polarization switching field (~ -500 kV/cm) of the P(VDF-TrFE) by sweeping electric field down from a positive field sufficient to saturate the polarization of P(VDF-TrFE). A similar tendency is also seen at the positive switching field when increasing electric field from the negative to positive. Such hysteretic electric field dependence unambiguously indicates that a change in the magnetic anisotropy of the Ni layer occurs, accompanied by the switching of the ferroelectric polarization of P(VDF-TrFE), presumably due to possible interface strain arising from the inverse piezoelectric effect of P(VDF-TrFE). While the interface strain is believed to be the predominant cause for the modulation of the magnetic anisotropy, electric field induced charge accumulation or chemical bonding, which should vary the spin dependent electronic structure of the ferromagnetic Ni layer in the very vicinity of the interface, could also be alternative origin of the magnetic modulation. A slight difference between the magnetic coercivities for positive and negative polarization orientations that we observed is likely associated with such electronic effects although the effects are not significant for a Ni layer thickness exceeding the screening length (~ 1 nm ) of the polarization charges of ferroelectrics by conduction electrons in the Ni layer.
This work was supported in part by the Advanced Materials Development and Integration of Novel Structured Metallic and Inorganic Materials Project of MEXT, Industrial Technology Research Grant Program in 2009 from NEDO of Japan, and JSPS KAKENHI (Grant No. 25^.03065).
Reference
[1] G. Venkataiah, Y. Shirahata, M. Itoh, and T. Taniyama, Appl. Phys. Lett. 99, 102506 (2011).

Among R2Fe17-xMx (M=Al, Si, Ga) compounds, the Ga substituted R2Fe17compounds show high Curie temperature (Tc) values. Furthermore, enhancement in Tc for Ga substituted R2Fe17 compounds is obtained by introducing carbon or nitrogen such as R2Fe17GaCx. However, the improvement in Tc is overshadowed by a concomitant deterioration in saturation magnetization. This is because the improvement in Tc was achieved at high value of doping in R2Fe17-xMx (M = Al, Ga, Si) compounds. The high level of doping for Fe brings in magnetic dilution effect. The metalloid doped compounds such as R2Fe17Nx or R2Fe17Cx have high Tc but are difficult to process at high temperature due to dissociation of compounds. In view of this the present study investigates effect of doping R2Fe16Ga0.5RM0.5 with refractory metals (RM) Ti, V, Zr, Nb, Mo, and W.
A series of refractory metal doped Gd2Fe16Ga0.5RM0.5 were prepared via arc-melting to understand the role of refractory element doping on the phase formation and its effect on magnetic properties. Phase analysis of these intermetallic compounds shows that these compounds have hexagonal Th2Ni17 structure (space group, P63/mmc). Rietveld analysis reveals c-axis expansion with very little effect on a-axis as a function of atomic weight of dopants. A complex interplay of coordination and electronegativiety between Ga, Fe and refractory elements dictates the lattice. The room temperature saturation magnetization (Ms) were observed to decrease with atomic weight of dopant refractory element (from 66.49 emu/g for Gd2Fe16Ga1 to 56 emu/g for Gd2Fe16Ga0.5Mo0.5 to 56 emu/g). The change in Fe-Fe exchange interaction resulting from lattice changes along with alteration to (3d, 4s) - 4p hybridization between Fe and Ga atoms upon dopant elements could bring in reduction in Ms. Further evidence of magnetovolume effect comes from the observed reduction in Tc (from 560 K for Gd2Fe16Ga to 540 K for Gd2Fe16Ga0.5Mo0.5) with dopant elements. The magnetovolume effect is also echoed in hyperfine parameters as well were a linear increase in isomer shift (IS) and decrease in hyperfine field (HF) is observed. Furthermore increased number of d- electrons increases the screening of s- electrons at Fe nucleus due to hybridization between Fe (3d) - RM (3d) bands. This hyberdization also leads to band broadening resulting in lower hyperfine field and Ms values.

The substitution effect of Al, Ga and Si on magnetic properties of R2Fe17-xMx (M=Al, Ga, Si) is well documented in the literature. The substitution of Al and Ga for Fe brings lattice expansion while substitution of Si for Fe brings lattice contraction. In spite of the observed variation on unit cell volume, the enhancement in the Tc was observed and it was found that the observed Tc is independent to the substituted elements. Among Al, Si and Ga, Ga substituted compounds show high Tc (e.g Sm2Fe16Ga ~485 K, Dy2Fe16Ga 462 K). This improvement in Tc is achieved at high value of doping exceeding x greater than 5 in R2Fe17-xMx (M = Al, Ga, Si). The high level of doping for Fe brings in magnetic dilution effect. Improvement in Tc is observed in metalloid substituted R2Fe16GaCx compounds. However, these compounds are unstable at high processing temperature. In view of this the present study investigates the effect of doping R2Fe16Ga0.5TM0.5 with transition metals (TM), Cr, Mn, Co, Ni, Cu, and Zn.
A series of TM doped Gd2Fe16Ga0.5TM0.5 compounds were prepared via arc-melting. The XRD phase analysis of these intermetallic compounds shows that these compounds have hexagonal Th2Ni17 structure (space group, P63/mmc). Rietveld analysis reveals volume expansion of the unit cell as a function of atomic weight of dopants. However, the observed variations in lattice expansion was not linear, for example, the maximum volume was observed for Gd2Fe16Ga0.5Cu0.5 (524.289 Å3) and minimum volume was observed for Gd2Fe16Ga0.5Ni0.5 (521.897 Å3). A complex interplay of coordination and electronegativiety between Ga, Fe and TM elements dictates the lattice. In comparison of Gd2Fe16Ga (Ms~67 emu/g), highest Ms was observed for Cu substitution (Ms ~77 emu/g) and lowest for Mn (Ms~56 emu/g). The change in Fe-Fe exchange interaction resulting from lattice changes along with alteration to (3d, 4s) - 4p hybridization between Fe and Ga and 3d (Fe)-(3d)(TM) atoms could bring in observed changes in Ms. In comparison of Gd2Fe16Ga (Tc~ 559 K), highest Tc was observed with Co substitution (Tc~587 K) and lowest Tc was observed for Mn substitution (Tc~526 K). The reduction in Tc for Mn substituted sample could be attributed to the antiferromagnetic exchange coupling between Fe and Mn.
The Mossbauer spectral analysis show that the weighted hyperfine field decreases for TM = Cr and Mn doping but increases for TM = Co, and Ni doping due to higher magnetic moment of these dopant elements. The variation in isomer shift with TM doping was observed which may arise from the combined effect of (i) volume change (ii) change in net electron density with TM doping and (iii) screening of s-electron with ‘d&’ electrons of TM. The 3d (Fe)-(Co, Ni, Cu) hybridization with Fe (3d) increases the screening of ‘s&’ electrons at Fe nucleus and hence shows an increase in the isomer shift. The study clearly demonstrate benefits of doping TM in R2Fe16Ga0.5RM0.5 in terms of Ms and Tc.

Binary rare-earth (R)-iron inter-metallic compounds with Th2Ni17 and Th2Zn17 structure have been extensively studied for potential application of these materials as permanent magnets. The main drawbacks of these materials are low Curie temperature (Tc) and magnetic anisotropies. In the recent years, many efforts have been made to solve these problems substituting smaller or bigger magnetic or non-magnetic elements for iron atoms in R2Fe17 or introducing interstitial atoms into it which in turn brings in change in Tc and magnetocrystalline anisotropy. In the present work the effect of substitution of non-magnetic atom Zr on Dy2Fe17-xZrx (0 le; x le; 1 with 0.25 increment) and effect of substitution of Zr in place of Ga in Dy2Fe16Ga1-xZrx (0 le; x le; 1 with 0.25 increment) is studied. The samples Dy2Fe17-xZrx and Dy2Fe16Ga1-xZrx are prepared by arc melting in an argon atmosphere of high purity. The structural and mangetic properties of these compounds have been studied by x-ray diffraction, vibrating sample magnetometer (VSM) and 57Fe Mossbauer Spectroscopy. Rietveld analysis of XRD patterns has shown that these compounds form the hexagonal Th2Ni17 structure. Partial substation of Zr for Fe in Dy2Fe17-xZrx compounds shows an increase in unit cell volume at the rate of ~0.42 per Zr atom. The Curie temperature (Tc) of Dy2Fe17-xZrx reaches a maximum value of 505K at x = 0.5 Zr substitution. The magnetization value of Dy2Fe17-xZrx decreases at the rate of 1.8emu/g per Zr atom. Furthermore the unit cell volume expansion was observed in Dy2Fe16Ga1-xZrx with Zr substitution. The Tc of Dy2Fe16Ga1-xZrx first increased to a maximum value of 505.1K for x=0.5 and then decrease with further increase in Zr concentration. The observed Tc in Dy2Fe16Ga0.5Zr0.5 is 40K higher than that of Dy2Fe16Ga1. This improvement in Tc is due to improved Fe-Fe exchange interaction upon unit cell volume expansion. However, lower magnetization values were obtained for Dy2Fe16Zr1 (52.02 emu/g) as compared to Dy2Fe16Ga1 (61.50 emu/g). The difference in magnetization values between Zr and Ga doped samples may be attributed to the d electron density. Zr with higher atomic number contributes more electrons to the d band thus lowering the net magnetic moment of iron atoms. The rapid decrease in isomer shift, especially for a 4f site, with the substitution of Zr further corroborates the increase in electron density at iron nucleus. In terms of Tc our results are better than previously published results Ga and Mn doped Dy2Fe17.

Binary rare-earth (R) - iron intermetallic compounds with Th2Ni17 structure have been extensively studied for their potential application as permanent magnets. The main drawbacks of these materials are low Curie temperature (Tc) and magnetic anisotropies. In recent years, many efforts have been made to solve these problems including substituting smaller or bigger magnetic or non-magnetic elements for iron atoms in R2Fe17 and introducing interstitial atoms into it which in turn brings in changes in magnetocrystalline anisotropy. Among various non-magnetic substitution such as Al, Ga, and Si, the highest Tc enhancement has been reported for Ga substituted intermetallics. For example, Tc value of Dy2Fe16Ga1 is ~466 which is 25.94% higher than the parent Dy2Fe17 compound. It was also reported by Fidler et. al [1] that substitution of refractory elements such as ( Ti, V, Mo, Nb) can bring about grain refinement and hence further enhance the magnetic properties of R2Fe17 intermetallics. Additionally the refractory element doping prevents the formation of alpha-Fe, a soft phase which often lowers the coercivity of the permanent magnet. In view of the above, this work presents the effects of substituting Nb atoms for Ga in R2Fe16Ga1-xNbx with non-magnetic refractory Nb atom and reporting the structure and magnetic properties and Mössbauer spectroscopy (MS) studies of the Dy2Fe16Ga1-xNbx (0le;xle;1.00) compounds.
The samples Dy2Fe16Ga1-xNbx (x=0.0 to 1.00) were prepared by the process of arc melting in high-purity argon atmosphere. The phase and structure of samples were determined using XRD with Cu Kα radiation. The magnetic properties like Ms and Tc of the samples were investigated using Vibrating Sample Magnetometer (VSM).
The X-ray powder diffraction (XRD) results show that the samples have Th2Ni17-type structure (space group, P63/mmc). XRD patterns show that samples are single phase up to x = 0.6. The XRD patterns also show impurities phases DyFe3 and NbFe2 at higher Nb content (x>0.6) which were also detected in the MS. Rietveld analysis show a linear unit-cell volume expansion with increasing of Nb content. The reduction in saturation magnetization (Ms) of Dy2Fe16Ga1-xNbx at 300 K with increasing Nb content is attributed the magnetovolume effect. For Dy2Fe16Ga0.4Nb0.6 the maximum in Tc was observed around 524 K which is 58 K higher than the Dy2Fe16Ga compound and 154K higher than its parent compound Dy2Fe17. In MS studies, the hyperfine fields (HF) of Dy2Fe16Ga1-xNbx were decreased upon Nb substitution. The observed increase in isomer shift with Nb substitution for Ga may have resulted from the decrease in s electron density due to the unit cell volume expansion.
In conclusion, of Dy2Fe16Ga1-xNbx compounds were observed to have pure phase up to x = 0.6 with absence of any alpha-Fe phase and have 12% higher Tc value then the Dy2Fe16Ga1 compound.

Ferromagnetic nanotubes represent intriguing nanomagnets as they show magnetization reversal via vortex wall formation while avoiding the Bloch point structure relevant for magnetic nanowires. Tubes support stray-field-free vortex states that might be useful for 3D magnetic memory architectures. To understand the intriguing properties in detail experiments on individual nanotubes need to be performed. We have prepared nanotubes by both atomic layer deposition and magnetron sputtering of a few 10 nm thick Ni and CoFeB, respectively. The tubes contained a core of a 10 to 20 µm long GaAs nanowire. Transferred to a separate substrate we attached Au leads and integrated coplanar wave guides to study the magnetic states via the anisotropic magnetoresistance effect and the GHz response. Using electrically-detected ferromagnetic resonance we find a characteristic series of eigenexcitations depending on the applied magnetic field. The eigenfrequencies observed for CoFeB are typically larger compared to Ni and above 10 GHz attributed to the large saturation magnetization of about 1.5 T. The eigenfrequencies might reflect standing spin waves of different order in different segments of the nanotubes. Focused-laser heating was used to explore thermoelectric properties. Seebeck voltages of a few µV were observed. The signal strength depended characteristically on the magnetic state suggesting a magneto-Seebeck effect observed on an individual CoFeB nanotube for the first time. The results are important to evaluate the potential of tubular nanomagnets for applications in magnonics and sensing. The work has been funded by the DFG via GR1640/5-1 in SPP 1538.

In order to investigate the magnetoelectic coupling between ferroelectric and magnetic materials, the heterostructured Fe₃O₄/BaTiO₃ films were grown on SrTiO₃(001) single crystal substrate by laser molecular beam epitaxy, in which, top Fe₃O₄ layer thickness is 80nm and bottom BaTiO₃ layer thickness is 45nm. X-ray diffraction patterns indicate that both Fe₃O₄ and BaTiO₃ layers are epitaxially grown and behave good textures, and the peaks at 2theta; of 42.9° and 45.3° are attributed to Fe₃O₄(004) and BaTiO₃(002), respectively. Temperature dependent magnetic measurements ( M-T) show that there are three jumps in M-T curve with the temperatures of 120K,620K and 815K. Here, the temperatures of 120 K and 815 K are corresponding to the Verwey temperature and Curie temperature of Fe₃O₄. However, it is found that there is a sharp jump at the 620K, which is different from any characteristic temperatures of Fe₃O₄ and BaTiO₃. In our case, the strain should exist due to the lattice mismatch between SrTiO₃(001) substrate and BaTiO₃ (002) layer, which may be the main reason for inducing the increase of the transition temperature from ferroelectric to paraelectric phase, this result is agreement with the published literatures. From our results, the ferroelectric transition temperature has shifted from 393 K (bulk BaTiO₃ single crystal) to 620 K, and enhanced about 230 K . In the curve of M-T, this jump around 620 K is possibly beacuse the presence of strong magnetoelectric coupling between ferromagnetic Fe₃O₄ and ferroelectric BaTiO₃ layers, at which the lattice parameters of BaTiO₃ can be enlarged due to phase transition, producing the strong interface strain and further inducing the change of magnetic domain structures of Fe₃O₄ layer. The remarkable increase of ferroelectric transition temperature of BaTiO₃ would provide a broad range of temperatures for the magnetoelectric coupling films in practical application of spintronics devices.

The importance of quasiperiodic structures can be estimated by the large bibliography published about this subject in the past decades [1]. A quite interesting characteristic of these quasiperiodic crystals is the fact that they display collective properties, due to the presence of long-range correlations that are not shared by their constituents. On the one hand, from a theoretical perspective, quasiperiodic systems present a variety of interesting features such as, fractal spectra, scaling laws, localization of states, oscillatory specific heat [2], etc.
From a experimental point of view, the advances in experimental growth techniques made possible to synthesize multilayers and superlattices of impressive quality, by means of sputtering and molecular beam epitaxy. In particular, layered magnetic systems have been attractive objects of research in the past years because of the discovery of physical properties such as the giant magnetoresistance [3], large out-of-plane magnetic anisotropy [4], etc. On the other hand, materials with simultaneously negative permittivity and negative permeability, yielding a negative refractive index, have recently been extensively studied in several distinct artificial physical settings, inspired by Veselago&’s work [5] many years ago. Veselago coined these peculiar materials as left-handed materials (LHMs) because they support backward waves for which the electric field E, the magnetic field H, and the Poynting vector S form a left-handed triplet. LHMs are of great interest for a variety of potential applications [6].
It is the aim of this work to investigate the behavior of magnetic-polaritons on a one dimensional nanolayers composed of ferromagnetic material and LHM layers arranged in a quasiperiodical fashion, which follows the Fibonacci, Thue-Morse, and double-period sequences. The transfer-matrix is used to calculate the dispersion relation for bulk and surface modes. Numerical examples are presented, whose main features are discussed, including the appearing of a negative group velocity of the bulk and surface&’s modes. We discuss also the appearance of gap-less region at the low wave vector, near the plasmon polariton resonance.
Acknowledgments: This work was partially financed by the Brazilian Research Agency CAPES and CNPq.
[1] E.L. Albuquerque, M.G. Cottam, Polaritons in Periodic and Quasiperiodic Structures, Elsevier, Amsterdam, 2004.
[2] E.L. Albuquerque, M.G. Cottam, Phys. Rep. 376, 225 (2003).
[3] M.N. Baibich et al. Phys. Rev. Lett. 61, 2472 (1988).
[4] P.F. Carcia, A.D. Meinhaldt, A. Suna, Appl. Phys. Lett. 47, 178 (1985).
[5] V. G. Veselago, Sov. Phys. Usp. 10, 509 (1968).
[6] R. A. Shelby, D. R. Smith, and S. Schultz, Science 292, 77 (2001).

In the past decade, nanomaterials research have drawn much attention for their unique physical and chemical properties in both science and industry. For example, iron oxide magnetic nanoparticles (MNPs) show great potential in applications of information storage, electronic devices, medical diagnostics, and drug delivery. In the application of information storage or electronic devices, the iron oxide nanoparticles are normally dispersed in a polymer thin film and in most cases the aggregation of the nanoparticles may cause a degradation in both physical and chemical properties. In this study, we have developed a pure inorganic SiO2-TiO2 thin film containing MNPs. The MNPs/SiO2-TiO2 thin film is solution-processable and shows high thermal and chemical stabilities. Fe3O4 MNPs nanoparticles with diameters about 5-20 nm were fabricated by a facile wet-chemical method according to XRD and TEM analyses. The prepared MNPs were dispersed into a SiO2-TiO2 gel, which was prepared by the sol-gel technique. The MNPs have a good dispersion and form a continuous phase in SiO2-TiO2 thin film. The eddy current effect and anisotropy properties of the nanocomposite thin film were studied with varying concentrations of MNPs in thin films. The hybrid Fe3O4/SiO2-TiO2 thin film exhibited ferromagnetic properties. This Fe3O4/SiO2-TiO2 composite thin film has shown a potential in some applications such as magnetic devices or EM wave absorbers.

Advanced magnetic materials are required for enhanced performance of power electronics and electric machines and drives. The soft magnetic materials and permanent magnets that are used in these applications benefit from nano-engineered microstructures. For power electronics, the complementary needs of high power density and high system efficiency demand soft magnetic components that exhibit low power loss at high frequencies. This supports the trend moving towards modular power electronic systems with high frequency transformers. New magnetic materials operable from the MW-level-20 kHz range up to the kW-level -MHz range with operating temperatures up to 300 °C will be required. Advanced electric machines and drives, often with permanent magnet architectures, are being developed to operate at continually higher speeds and temperatures. Soft magnetic laminates used in rotating components require high tensile strength and low power loss. The recent volatility of the price of rare earth elements used to manufacture permanent magnets has highlighted the importance of a co-ordinated strategy to mitigate their supply risk. Several parallel technology development paths are being pursued to develop rare earth alternatives in electric machines. Material development efforts include nanocomposite permanent magnets that partly substitute non-critical elements for all rare earths as well the on-going search for new compounds that contain no rare earths. Strategies for improved system performance blend nanoscale structure control, novel device geometries, new alloy and compound development, and improved processing methods to maintain a sustainable value chain.

In the early 1980&’s, global instability in the price and availability of samarium and cobalt kindled research efforts to identify replacements for Sm-Co permanent magnets, culminating with the discovery of Nd2Fe14B. Nd-Fe-B magnets have become the world&’s premier permanent magnets, with ubiquitous applications in high performance devices including smart phone speakers and hard disk drive motors. Today a similar scenario is playing out for Nd-Fe-B: recent growth in demand for large quantities of low cost, high performance magnets for electric and hybrid vehicle traction motors and for wind generators has stimulated a search for alternatives to Nd-Fe-B magnets. At GM R&D, we exploit rapid quenching as a fast and expedient synthesis route for exploring a variety of potential permanent magnet materials. The hard magnetic properties of melt-spun ribbons arise from the unique nanostructure comprised of grains of the primary magnetic phase with dimensions of 100 nm or less, and affords assessing materials properties in a variety of systems. For example, we are exploring magnets based on Ce2Fe14B because Ce is much more abundant than Nd in rare earth ores and is in global oversupply. To date we have achieved Ce-(Fe,Co)-B materials with technical magnetic properties exceeding those of ferrite, even after Co substitution for Fe to increase the Curie temperature Tc. We have also been participating in two projects funded by the ARPA-E Rare Earth Alternatives for Critical Technologies (REACT) program. In a project led by Ames Lab we seek to develop iron-rich Ce-Fe magnets based on compounds having the ThMn12 crystal structure. Such materials have promising intrinsic permanent magnet properties, especially when nitrogen is infused interstitially into the lattice. By substantially expanding the crystal lattice, nitrogen insertion enhances the Fe-Fe exchange and greatly increases Tc. The Tc of CeFe11MoNx, for example, is ~60 K higher than in Nd2Fe14B, implying reduced loss in magnetic properties at elevated temperatures. Co substitution for Fe also increases Tc; replacing one Fe by Co in CeFe11Ti increases Tc by 100 K. Another project led by Northeastern University explored a fascinating FeNi phase having the L1₀ body-centered tetragonal crystal structure (an ordered variant of fcc Fe) found in certain high-Ni meteorites. From magnetic evaluation of a piece extracted from meteorite NWA-6259, we estimate the anisotropy field to be 1.6-2.0 T, with a Tc of at least 800 K. Furthermore, the meteorite sample itself, although far from microstructurally optimized, already has an intrinsic coercivity Hci = 0.1 T. In keeping with the high Tc, the magnetic properties are extremely stable with temperature; Hci decreases by < 1% up to 180 °C, compared with a loss of at least 64% in (Nd,Dy)-(Fe,Co)-B magnets.
This research is funded in part by the Advanced Research Projects Agency - Energy under grant numbers 0472-1526 and 0472-1537.

Permanent magnet alloys containing Al, Ni, Co and Fe (alnico) represent a class of functional nanostructured alloys that arise from spinodal decomposition. We show inconsistencies in the data that suggest that the coercivity mechanism is more complex than previously thought, involving the interplay of grain size and shape, chemistry and possibly stress at the phase interfaces. The resulting phase separation is sensitive to alloy chemistry and the crystallographic orientation of the primary phase to an imposed magnetic field during decomposition. The saturation magnetization can be predicted by the Fe:Co in the magnetic bcc phase, the phase fraction of the bcc phase, and the degree of crystallographic alignment relative to the applied field during annealing. With improved coercivity, theoretical energy density can rival the best commercial Nd-based alloys above 200°C. Refining of the nanostructure to improve the coercivity will rely on understanding the delicate balance between the chemistry and the thermal history of the alloy. Understanding how these various factors come together to control the nanostructure and the theoretical and practical limits for optimal coercivity will be required to develop an alnico alloy capable of replacing rare earth-based permanent magnets at elevated temperatures.

A permanent magnet with large energy product can store high density magnetic energy. Such a magnet is required to have a large coercivity and a high magnetic moment. In this talk, I will summarize our efforts in using solution phase chemistry and self-assembly to fabricate monodisperse magnetic nanoparticles and nanocomposite magnets. I will first highlight the synthesis of FePt, SmCo, FePd and Fe nanoparticles and their assemblies into composite nanostructures. I will then describe how to use controlled thermal annealing to achieve efficient exchange-coupling between magnetically hard and soft phases within the composite structure. Our work demonstrate the great advantage of using chemical synthesis and self-assembly to fabricate exchange-spring nanocomposite magnets with enhanced energy product for high performance permanent magnet applications.

MnBi attracts great attention in recent years for its great potential as permanent magnet materials. It is attractive because its coercivity increases with temperature, reaching 23 kOe at 475 K. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because the melting temperature of Bi is much lower than that of the MnBi phase. In addition, the eutectic reaction L→Bi+MnBi at 535 K limits the temperature at which the bulk magnet is fabricated. Here, we report our effort on developing MnBi single phase bulk permanent magnet. Conventional powder metallurgical approaches were used for MnBi alloy synthesis. MnBi particles with >92% purity can be routine produced. The aligned powder secured with wax exhibited coercivity of 13.6 kOe and energy product of 11.3 MGOe at room temperature. Thermal stability of the obtained MnBi powder with various particle sizes was investigated using TGA, DSC, XRD, and VSM. It is found below 200 C, the powder is stable in air. Bulk magnets, about 5 grams in weight, were fabricated using press and sintering technique. The obtained bulk magnet exhibits energy product of 7.8 MGOe.

Shock-compression is a viable approach for the fabrication of bulk magnets made from soft, hard, and nanocomposite magnetic powders. The rapid and localized deposition of shock energy at inter-particle regions results in inter-particle bonding and densification of powders, with limited thermal excursions thereby retaining the starting material phase and grain structure. Bulk magnets can thus be produced starting with nano-sized single- or multi-component particles or nano-grained polycrystalline as well as composite powders. The shock compression experiments are performed using a three-capsule fixture designed with use on our 80-mm gas gun to produce compacts of approximately 10-12 mm diameter and 2-4 mm thickness. The precursor powders are statically pressed into the sample die, and impacted at conditions necessary to achieve good inter-particle and complete densification. We have studied the shock compaction of various soft and hard magnetic materials, as well as nanocomposite exchange-coupled magnets. However, challenges remain due to possible stress-induced phase transformations at high pressures, strain-induced phase changes from severe plastic deformation, and thermal decomposition caused by bulk temperature increases arising from densification starting with low packing density, i.e., below 50% TMD. Our work on compacts of Fe4N and Fe16N2 soft-phase magnets, show evidence of phase transformation, with the FCC Fe4N partially transforming to HCP Fe3N, consistent with previous reports of the transition occurring at static pressures of ~3 GPa, and Fe16N2 decomposing to α-Fe, Fe4N, and FeN due to the combination of thermal effects associated with dynamic void collapse and plastic deformation. Compacts of hard-phase magnets, Sm2Fe17N3 and Nd2Fe14B/α-Fe, show retention of phase and grain structure in the recovered bulk compacts, and appear to accommodate the plastic deformation primarily by grain size reduction, especially in the case of larger grained materials. In this presentation, the key results in terms of the retained microstructure and measured magnetic properties, as a function of the conditions of compaction of the hard and soft magnets will be presented. The challenges and possibilities of using the shock-compression approach for fabrication of bulk magnets will also be highlighted.

This presentation examines the work done toward the fabrication of a 3-D magnonic crystal, the magnetic analogue to a photonic crystal, using a bottom-up, self-assembly technique. Magnonic crystals have the potential to control the propagation of spin waves on the nanometer scale. Thus, they are recognized as important components in next-generation spintronics and magnetic data storage technologies. Our fabrication approach starts with assembly of microspheres into a colloidal crystal film, which was subsequently used as a template and infiltrated with different magnetic nanoparticles via dip-coating. The composite structure was then heat-treated to fuse the nanoparticles and enforce stability of the superlattice. This process was repeated to maximize the fill fraction of the magnetic components. Controlled heat-treatment under oxidizing and reducing atmospheres can also be used to tune the magnetic properties of superlattice. The samples were characterized by electron microscopy, optical reflectance spectroscopy, XPS, and computational simulations. These intermediate materials provide a sturdy foundation for the fabrication of a 3-D magnonic crystal.

Magneto-optical activity is based on the rotation or ellipticity that a magnetic medium exerts on the polarization of light. Here we show that by combining magnetic materials with metals at the nanoscale, a huge increase of the magneto-optical activity emerges, associated to plasmon resonances. Thus, an outstanding increase of the rotation and ellipticity of the light polarization is observed at the frequencies of the plasmon resonances of magnetic nanoparticles. This phenomenon has a significance that goes beyond its fundamental interest, with obvious relevance for applications in sensing and optical communications [1]. Yet, the lack of a general theoretical framework able to anticipate over the whole visible spectrum and with high accuracy the rotation/ellipticity of light after interacting with a magnetoplasmonic medium is a serious hindrance towards an efficient development of new materials for the abovementioned applications. Here we have addressed this paucity of knowledge by formulating an innovative theoretical approach that endows our model with a powerful predictive character [2]. Based on measurements of the magneto-optic activity in nickel nanoparticle colloids, we have found a striking quantitative agreement between experiment and theory. Even more, large enhancements of the Verdet constant are found at high fields, again in very good accordance to the expected values from our model. We note that the applicability of our model goes well beyond the particular case of colloidal metals, as other systems such as metal inclusions in polymers or glasses containing small magnetic clusters can be as well considered. In addition, the observed large Verdet constant enhancements allow envisioning the exploitation of light polarization, instead as the commonly used reflectance, as a probe for plasmon-sensing devices. Our results provide new routes for plasmon-based biological and chemical detection, as well as new avenues for largely transparent tunable isolators for optical communications.
[1] G. Armelles et al., Adv. Optical Mater. 2013, 1, 10
[2] O. Vlasin et al., submitted.

Magnetic field has been regarded as an effective tool for synthesizing functional nanostructured materials, controlling their properties, and fabricating novel devices. The key is to induce sufficient magnetic interactions among neighboring nanostructures or surroundings. In this presentation, I will use a number of examples recently developed in my group to demonstrate that magnetic field can be utilized to dynamically tune the optical properties such as diffraction and plasmonic properties of nanostructured materials. Such dynamic tuning is realized through effective control over the assembly and disassembly behaviors or the orientation of the nanoscale building blocks using magnetic fields.

The conjugation of magnetic and plasmonic moieties at the nanoscale offers a wide variety of opportunities: magnetic-plasmonic hybrids have been proposed as bi-functional systems for a variety of applications in theranostics, in which the gold shell acts as a protecting agent for the magnetic core, [1] as a highly functonalizable surface, [2] as an optical heater [3] or as an active optical beacon. [4] The most interesting aspect of this type of systems however, is probably that of conjugating the two functions to obtain novel effects: active magneto-plasmonics (the modulation of plasmon resonance with an external magnetic field) has been demonstrated recently, [5] and several reports of plasmon-mediated enhancement of the magneto-optical response of hybrids have been described. [6]
I will show how ordinary gold nanoparticles' optical properties are modified by the magnetic field and describe the origin of such magnetoplasmonic behavior. [7] The modulation of the optical properties of plasmonic systems can improve the detection of small shifts of the plasmon resonance, which is critical in refractometric sensing. Magneto-optical spectroscopies proved themselves invaluable to study this effect, and to give an experimental comparison to theoretical models. However, the effect of the magnetic field on non-magnetic plasmonic materials -such as gold or silver- is rather small: in order to try and boost this effect, we have been working on colloidal hybrid nano-architectures in which a magnetic moiety flanks the plasmonic one. Using a combined approach with the aid of magneto-optics, magnetometry and x-ray spectroscopy we investigated several routes to design magnetic-plasmonic hybrid nanoparticles containing gold and iron oxides with different geometries, exhibiting different levels of conjugation, thus a variable extent of interaction of the magnetic and plasmonic functions, as well as AuFe alloys. We found that the chemical composition of the magnetic part critically affects the synergy of the two moieties. [8]
This work has been financed by the Italian MIUR through FIRB projects “NanoPlasMag” (RBFR10OAI0) and “Rete ItalNanoNet” (RBPR05JH2P), by the EC through FP7-214107-2 NANOMAGMA, FP7-NMP4-LA-2008-213631 NANOTHER and ERC Advanced Grant “MolNanoMas” (267746), and by Fondazione Cariplo through Project No. 2010-0612.
[1] L. Wang et al., J. Mater. Chem., 2005, 15, 1821.
[2] X. Zhao et al., Anal. Chem. 2008, 80, 9091.
[3] L. Wang et al., Angew. Chem. Int. Ed. 2008, 47, 2439.
[4] Q. Wei et al., J. Am. Chem. Soc., 2009, 131, 9728.
[5] V. Temnov et al., Nature Photon., 2010, 4, 107.
[6] P. Jain et al., Nano Lett., 2009, 9, 1644. Y. Li et al., Nano Lett., 2005, 5, 1689. G. Shemer et al., J. Phys. Chem. B, 2002, 106, 9195.
[7] Pineider et al., Nano Lett. 2013, 13, 4785.
[8] Pineider et al., ACS Nano 2013, 7, 857.

Metallic inverse opals have attracted much interest in recent years due to their interesting and unique properties including optical absorption, thermally stimulated emission and plasmonic physics. Potential applications for such materials range from high-efficiency light sources and photovoltaic energy conversion to battery electrodes. Here we will present our approach to optimization of the fabrication to obtain large-area metallic, ferromagnetic inverse opals, with special focus on cobalt structures. They can be used in magneto-photonic devices with the potential to control propagation of light by magnetic field. Moreover, they may present a starting point in inexpensive fabrication of 3-D magnonic crystals. Finally, we will discuss some of the unique properties of ferromagnetic inverse opals, obtained by using a wide range of microscopy and magneto-optic spectroscopic techniques.

We have developed a method to create magnetically actuated colloidal micromirrors, the orientation of which can be readily controlled by an applied magnetic field. The mirrors are comprised of gold microplates, which are very reflective and have a high aspect ratio (up to ~150), which are conjugated to amine functionalized paramagnetic Fe2O3 nanoparticles. Due to the combination of the properties of the Fe2O3 nanoparticles and the gold microplates, the resulting material is both highly reflective and exhibits a strong, anisotropic response to an applied magnetic field. Due to the high aspect ratio of the micromirrors, the extinction and reflectance of the bulk colloidal solution is strongly dependent on the mirror orientation. Thus, by varying the relative orientation of an external magnetic field, the optical properties of the mirror solution can be dynamically tuned as the particles align with the magnetic field lines. In addition to observing the movement and on-off contrast of the micromirrors, we also characterize the optical response of the material to a rapidly rotating magnetic field to demonstrate its rapid on-off switching. Unlike conventional responsive micromirrors, this material does not require sophisticated microfabrication techniques to produce, can rotate freely, and since the orientation is controlled via magnetic field, contactless orientational control is possible. We believe this material has the potential to be useful for a number of applications, notably in new types of optical switches and smart window technologies.

Magnetic anisotropy plays a key role in detrermining the long range magnetic order of low dimensinal magnetic system and overcoming the superparamagnetic limit of magnetic recording media. In this talk, we adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.
The current trend in the spintronics devices with faster response demands the study of the magnetization dynamics of magnetic nanostructures on very small time scales. The laser-induced ultrafast demagnetization of CoFeB/MgO/CoFeB magnetic tunneling junction is exploited by time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electrons tunneling current. It opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions.

In recent years, the development and promise of spintronics has served as the driving force for numerous research endeavors. In particular, spin-based effects (namely the inverse spin Hall effect, ISHE) have been examined in most common semiconductors, such as silicon. However, low spin-orbit coupling severely limits the potential of silicon, thereby making it necessary to investigate other materials. One such material is ZnO, a widely studied and well understood transparent semiconductor with the added benefit of having a large exciton binding energy for room temperature lasing and optoelectronics applications. Herewith we are reporting the observation of the ISHE in ZnO thin films. Thin films of ZnO were deposited on sapphire (c-plane) using a pulsed laser deposition (PLD) technique. An ultrathin MgO tunnel barrier for spin injection was fabricated via subsequent deposition. The ZnO/MgO thin films were then patterned and etched. Afterward, a NiFe permalloy film, patterned using photolithography, was deposited via e-beam deposition. After magnetizing the NiFe electrode, spins were injected from the NiFe into the ZnO by applying current along the length of the channel. Voltage was measured in a secondary channel, perpendicular to both the spin injection as well as the charge current direction. By utilizing this setup, in an ideal situation, voltage contributions from sources other than ISHE can be eliminated. Testing was performed by measuring the transverse channel voltage as a function of applied magnetic field. Analysis of the transverse voltage data indicated the presence of the ISHE in the ZnO film. These results have also allowed us to determine the degree of spin-orbit coupling in ZnO and compare it to other common semiconductors. The observation of the ISHE in ZnO further opens a path towards the realization of transparent devices capable of coupling spintronic and optoelectronic functionalities into a single device.

As a fundamental effect of the magnetic solids, anomalous Hall effect (AHE) has gained much interest [1]. So far, no material can satisfy large anomalous Hall resistivity (ρAH), large longitudinal resistivity (ρxx), and low switching field all at the same time [2]. In this work, we present a systematical study on the AHE in a perpendicularly CoFeB sandwiched by MgO and Ta layers, which recently has been used to fabricate a perpendicular magnetic tunnel junction [3]. Following our previous result on the origin of perpendicular magnetic anisotropy in MgO/CoFeB/Ta thin films [4], we report that there is no strong relationship between PMA and the thickness of the tantalum or MgO cap layer. We further demonstrate the large AHE in the perpendicular CoFeB thin film. Compared to low ρxx (about several tens mu;Omega;cm) for the traditional Pt based AHE materials, ρxx for all the reported Ta based AHE materials is larger than 200 mu;Omega;cm. The optimized ρAH and ρxx have been obtained as high as 4.7 and 270 mu;Omega;cm, respectively, providing a new pathway for device applications.
[1] A. Gerber, J. Magn. Magn. Mater. 2007, 310, 2749.
[2] J. Moritz, B Rodmacq, S. Auffret, B. Dieny, J. Phys. D: Appl. Phys. 2008, 41 135001.
[3] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, Nat. Mater. 2010, 9, 721.
[4] T. Zhu, Y. Yang, R. C. Yu, H. Ambaye, V. Lauter, and J. Q. Xiao, Appl. Phys. Lett. 2012, 100, 202406.

Spin-electronics and -caloritronics are rapidly developing research fields and many new phenomena such as the Spin-Seebeck-Effect (SSE, [1, 2]) and the Tunneling-Magneto-Seebeck-Effect (TMS, [3, 4]) have been observed.
Spin-Seebeck effects such as the Longitudinal Spin-Seebeck effect (LSSE) are often detected indirectly via the Inverse Spin Hall Effect in, e.g., Pt and varying amplitudes of the detected thermopower have been reported. Moreover, spin-dependent thermal transport phenomena can be contaminated by other spin thermoelectric effects such as the Nernst effect (Anomalous and Planar Nernst Effect, ANE and PNE). We first present results on the LSSE on semi-conductive nickel ferrites [5], where the ANE can be strongly influenced by changing the conductivity of the ferromagnet and thus a direct comparison of LSSE and ANE becomes possible. We additionally varied the direction of the temperature gradients in a well-defined way to elucidate the influence of the PNE on the results of LSSE measurements.
The second part will discuss the tunneling magnetoresistance (TMR) and the tunneling magneto-Seebeck effect (TMS) detected on magnetic tunnel junctions. We present results on the TMS obtained by two heating methods, discuss time-resolved experiments [6] and the influence of an uncertainty of input parameters such as temperature profiles on the effect. In addition, we demonstrate that an applied bias voltage strongly changes the observed TMS and discuss the results on the base of the electrode&’s band-structures. For TMR devices with ultrathin MgO barrier, we demonstrate a critical switching current for classical Spin Transfer Torque Switching down to around 0.2 MA/cm² . In relation with the thermally driven spin currents, the possibility to achieve sizeable thermally driven spin transfer torque will be evaluated [7].
[1] K. Uchida, et.al., Nat. Mat. 9, (2010) 894
[2] K. Uchida, et.al., Appl. Phys. Lett. 97 (2010) 172505
[3] N. Liebing, et.al., Phys. Rev. Lett. 107 (2011) 177201
[4] M. Walter, et.al., Nat. Mater. 10 (2011) 742
[5] D. Meier, et.al., Phys. Rev. B 87 (2013) 054421.
[6] A. Boehnke, et.al., Rev. Sci. Instrum. 84 (2013) 063905
[7] J. C. Leutenantsmeyer, et.al., SPIN 03 (2013) 1350002.

La1-xSrxMnO3 (x=0.3) is a well-known material that is often used in spintronics research as an electrode for spin-injection. In general for spintronic devices, spin-injection is achieved either by a spin-polarized electrical current passing from a magnetic material into a non-magnetic one, or by spin pumping via microwave resonance in a magnet. In this paper, we show the possibility of injecting pure spin currents into semiconductor channels by utilizing the phenomenon of spin Seebeck effect (SSE). The SSE separates spins in a magnetic material along the direction of an applied thermal gradient. A pure spin current can be drawn from the magnetic material by attaching a non-magnetic material perpendicular to the gradient in the magnetic material. In our study, the LSMO used in the SSE testing was deposited by a pulsed laser deposition (PLD) technique onto SrTiO3 (100) substrates. Platinum wires were employed as the top contact and were deposited using e-beam evaporation with a shadow mask. The thermal gradient was applied across the length of the LSMO by heating one end of the sample with a micro-heater. The SSE voltage generated within the platinum (by means of the inverse spin Hall effect) is measured using a nanovoltmeter. Analysis of SSE data clearly depicted the half-metallic nature of LSMO at low temperatures. Specifically, in a normal ferromagnet, the sign of the SSE voltage is expected to be opposite on the hot and cold ends of the material due to the separation of the spins in opposite directions. In contrast to the above, we found that the sign of SSE voltage in our test device did not change between the hot and cold sides; just a reduction in the magnitude of SSE voltage was observed. This suggests the presence of only one type of spin in the material.

The anisotropic magneto-thermal resistance (AMTR) effect is the thermal analogue of the anisotropic magneto-resistance (AMR) effect observed in ferromagnetic conductors. We have investigated the AMTR effect in Ni nanowires [1]. We present measurements of electrical and thermal transport properties of cylindrical Ni nanowires in the temperature range between 78 K and 380 K. The determined AMTR ratios lie below the AMR ratios, resulting in an anisotropic Lorenz number. To explain this observation, we apply a simple model that considers spin mixing due to electron-magnon scattering. In comparison, we will present measurements of the magneto-thermal resistance (AMTR) on GMR Co/Ni multi-layer multilayers under thermal transport in-plane [2], where a Field-independent Lorenz number over the entire temperature range is observed.
The magneto-thermopower (Seebeck coefficient) is measured and correlated to the anisotropic magneto-resistance of Co-Ni alloyed nanowires [3]. By a micrometer setup, three magneto-thermoelectric quantities are determined along the nanowire in magnetic fields applied perpendicularly to the nanowire axis: temperature difference, thermovoltage and electrical conductivity. The highest absolute and relative variation of the Seebeck coefficient are determined to be 1.5 µV/K at RT for Co_0.24Ni_0.76 nanowires and 10.9 % at 100 K for Ni nanowires. Power factors of 3.7 mW/mK² have been achieved, which is competitive with common thermoelectric materials like TiS₂ and Bi₂Te₃. For Co-Ni nanowires containing up to 40 % Co a linear relationship between the magnetic field dependent change of the Seebeck coefficient and the electrical conductivity is found following the Mott relation.
Acknowledgements:The authors gratefully acknowledge the financial support by the German Science Foundation via the German Priority Program SPP 1538 on Spin Caloritronic Transport - SpinCAT.
References:,
[1] J. Kimling, J. Gooth, K. Nielsch, “Anisotropic Magnetothermal Resistance in Ni Nanowires”, Phys. Rev. B 87, 094409 (2013).
[2] J. Kimling, K. Nielsch, K. Rott, G. Reiss, “Field-dependent thermal conductivity and Lorenz number in Co/Cu multilayers”, Phys. Rev. B87, 134406 (2013).
[3] V. Vega, T. Böhnert, S. Martens, M. Waleczek, J.M. Montero-Moreno, D. Görlitz, V.M. Prida1, K Nielsch, “Tuning the magnetic anisotropy of Co-Ni nanowires: Comparison between Single Nanowires and Nanowire Arrays in Hard-Anodic Aluminum Oxide Membranes”, Nanotechnology23, 465709 (2012).

Spintronics and Spin Caloritronics are the two new paradigms of electronics which utilize both the electron&’s charge as well as its spin degrees of freedom. They have the potential to facilitate a new generation of logic and photonic devices having high-speed, large memory and ultra-low power consumption. The most important and critical step in the functioning of these devices is the injection and detection of spin-polarized carriers at the ferromagnet-semiconductor interface. Despite considerable efforts, efficient injection of spins into nonmagnetic semiconductors still continues to be a major hurdle in this field. All the possible routs of injecting spin in semiconductors rely on oxides. In this talk, I will present some of our very exciting research going on in this field in my group at the University of Utah. Particular focus will be on the observation of room temperature ferromagnetism and dynamic superparamagetism in insulating oxides. Successful injection and detection of spin polarized carriers in semiconductor channels by electrical and thermal methods will be discussed.

Ferrites, double oxides of iron and another metal, represent an important group of ferrimagnets from a technological point of view. With high electrical resistivity and low eddy current losses they are attractive for high frequency applications where power losses must be minimized. Due to its wide band gap, high Curie temperature, relatively high saturation magnetization and moderate magnetostrictive effect, nickel ferrite (NFO) is an interesting material for a number of applications, including sensors, microwave and spin filter devices, memory and magnetoelectric composites.
Textured nickel ferrite films were deposited at room temperature onto c-plane sapphire substrates via sol-gel deposition. A nanoimprint lithography technique using a polydimethylsiloxane (PDMS) stamp was used to transfer a pattern from a master to the thin film which was subsequently annealed to crystallize the NFO. The pattern from the master was transferred completely and with high degree of precision over large areas without affecting the nucleation, grain growth or texturing due to insufficient thermal budget at the PDMS interface. X-ray diffraction revealed the films remained textured single phase inverse spinel NiFe2O4. Thus we have a fast, inexpensive, easy, versatile and scalable method for large scale surface design of sol-gel derived textured thin films.
In addition, magnetic measurements showed, to the authors&’ knowledge, a first ever report on large reduction in coercivity with the surface patterned samples. This means the coercivity of films may be tuned by surface modification at the nanoscale, making them particularly interesting in high frequency applications where further reduction in the power losses can be obtained.

Near room temperature magnetic cooling systems based on the magnetocaloric effect have significant advantages compared to conventional gas-compression cooling systems, e.g., high energy efficiency as well as no greenhouse gases. Gadolinium and other rare earth based alloys have been intensively studied but they are expensive, available in limited quantities and corrode easily. Magnetocaloric nanoparticles (MCNPs) are of considerable interest because nanoparticles can be suspended in a solution, providing unique applications such as self pumping. The present work focuses on chemically synthesized Fe based MCNPs. These nanoparticles are affordable and more environmentally friendly compared to commonly studied magnetocaloric materials such as Gd. X-ray diffraction (XRD), energy dispersive X-ray (EDX), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) techniques were used to determine the magnetocaloric characteristic of these nanoparticles. The effects of particle size and material composition on the Curie temperature and magnetocaloric properties were determined. Fe-Ni alloys reveal high magnetization and soft magnetic properties, the addition of few weight % Mn or Cr results in a desirable reduction in Curie temperature. At Curie temperature (T_C) ~ 370K, the maximum change in magnetic entropy |ΔS_M | reach 1.35 J Kg^-1K^-1 for a field change of 5T. From the area under the curve relative cooling power (RCP) is calculated and it is found to be ~550 J Kg^-1. Moreover, The RCP value is higher than that reported for rare earth based alloys. These MCNPs were suspended in suitable liquids for self pumping and magnetic cooling applications.
We acknowledge support from the National Research Foundation, Singapore through the CREATE program on Nanomaterials for Energy and Water Management.

Giant magnetocaloric materials like GdSiGe compounds are very promising for thermal applications such as magnetic refrigeration [1]. Generally, energy change associated with magnetic phase transitions are known to be small, for example ~10 cal/g for nickel at its Curie temperature of 358°C [2]. However, new magnetocaloric materials create a large change of energy due to the superposition of structural phase transition with ferromagnetic phase transition. In this research, magnetic oxide nanoparticles have been investigated to see whether a large effect can be observed during the phase transition of antiferromagnetic nanoparticles, as well.
Materials chosen for the study are chromium oxide (Cr2O3) and cobalt oxide (CoO) which have Neel temperatures close to the room temperature in the bulk form, 47°C and 18°C, respectively. The research has been carried out with quantum mechanical calculations using the DFT method with B3LYP functional and Pople type basis sets augmented with polarization functions. Atomic models consist of nanoparticles carved from the bulk crystal maintaining symmetry as much as possible.
Optimal geometry of the nanoparticles are calculated in two ways: (a) Non-magnetic geometry where total spin and spin densities of individual atoms are zero. (b) Magnetic geometry where total spin is zero, but spin densities of individual atoms are non-zero. As expected, calculated energy of the magnetic geometry is lower than that of the corresponding non-magnetic geometry. There are measurable differences in bond angles and bond lengths between the two geometries. Spin densities of the atoms of the magnetic nanoparticle exhibit plus and minus values confirming the antiferromagnetic behavior.
A phase transition energy is estimated for each nanoparticle based on the energy difference between the magnetic geometry and the non-magnetic one. For the smallest nanoparticles (CoO)4 and Cr2O3, phase transition energy is 270 cal/g and 360 cal/g , respectively, which is considerable for a magnetic phase transition. Calculated phase transition energy decreases with increasing particle size. For example (CoO)6 and (Cr2O3)2 yield a phase transition energy of only 60 cal/g and 85 cal/g , respectively. Although a large effect is observed, it seems limited to small sizes.
[1] V.K. Pecharsky and K.A. Gschneidner, “Advanced Materials for Magnetic Cooling,” Material Matters, Vol. 2, No. 4, 2007, pp. 4-7
[2] R.M. Bozorth, Ferromagnetism, IEEE Press, Piscataway, NJ, 1993