Symposium MD9 : Magnetic Materials---From Fundamentals to Applications

Giant magnetoresistive (GMR) biosensors consisting of many rectangular stripes are being developed for high sensitivity medical diagnostics of diseases at early stages, but many aspects of the sensing mechanism remain to be clarified. Using e-beam patterned masks on the sensors, we showed that the magnetic nanoparticles with a diameter of 50 nm located between the stripes predominantly determine the sensor signals over those located on the sensor stripes. Based on computational analysis, it was confirmed that the particles in the trench, particularly those near the edges of the stripes, mainly affect the sensor signals due to additional field from the stripe under an applied field. We also demonstrated that the direction of the average magnetic field from the particles that contributes to the signal is indeed the same as that of the applied field, indicating that the particles in the trench are pivotal to produce sensor signal. Importantly, the same detection principle was validated with a duplex protein assay. Also, 8 different types of sensor stripes were fabricated and design parameters were explored. According to the detection principle uncovered, GMR biosensors can be further optimized to improve their sensitivity, which is highly desirable for early diagnosis of diseases.

The GMR biosensors require well-tailored magnetic particles as detection probes, which need to give rise to a large and specific biological signal while showing very low nonspecific binding. This is especially important in wash-free bioassay protocols, which do not require removal of particles before measurement, often a necessity in point of care diagnostics. We show that magnetic interactions between magnetic particles and magnetized sensors dramatically impact particle transport and magnetic adhesion to the sensor surfaces. The dynamics of magnetic particles’ biomolecular binding and magnetic adhesion to the sensor surface is revealed in microfluidic experiments. We elucidate how flow forces can inhibit magnetic adhesion, greatly diminishing or even eliminating nonspecific signals in wash-free magnetic bioassays, and enhancing signal to noise ratios by several orders of magnitude. Our method is useful for selecting and optimizing magnetic particles for a wide range of magnetic sensor platforms.

This work is supported in part by NIH through the Center for Cancer Nanotechnology Excellence (U54CA151459) and the Autoimmunity Center of Excellence (ACE) at Stanford (U19AI110491).

References:
1. Jung-Rok Lee, et al., “Experimental and theoretical investigation of the precise transduction mechanism in giant magnetoresistive biosensors,” Sci. Rep., in press.
2. A.D. Henriksen, et al., “On the importance of sensor height variation for detection of magnetic labels by magnetoresistive sensors,” Sci. Rep., 5, 12282, 2015.
3. D.J.B. Bechstein, et al., “High performance wash-free magnetic bioassays through microfluidically enhanced particle specificity,” Sci. Rep., 5, 11693, 2015.

MRI is a widely used and valuable imaging tool for non-invasive clinical diagnosis and disease monitoring. In particular, in combination with therapeutic agents, MRI can offer in in-vivo tracking of drug delivery and release. To enable in-vivo MRI therapy tracking, efficient contrast agents are critical for resolution and tracking time. In this presentation, the development of effective contrast agent for MRI tracking will be presented, mainly T1 contrast agents based on the size and shape variation. For any biomedical application, the surface properties are key to defining the interfacial interactions. This talk will also address the facile method developed in our lab to attach various molecules onto nanoparticle surfaces for hydrophilicity and desirable functionality. Subsequently, drug encapsulation and release will be discussed.

Magnetic particle imaging (MPI) is a real-time, quantitative and clinically safe imaging technique, potentially applicable for future clinical applications such as cancer imaging, cardiovascular imaging or stem cell labeling and tracking. MPI performance (i.e. spatial resolution and sensitivity) is highly dependent on nanoparticles (NPs) size distribution, magnetization and environment. Therefore, NPs MPI signal is different after distribution into tissues or binding to the cells. Here, using in vitro and in vivo studies, we present the potential capabilities of MPI for tissue targeted imaging applications such as cancer and atherosclerosis imaging. To do this, first we used two preliminary experimental models to predict the performance of the tracers in a tissue equivalent environment and an acidic lysosome-like solution. For efficient targeting and specific binding of the NPs to the cells we need to conjugate additional biomolecules that can be recognized by receptors on the cells membranes. Therefore, as the next step, we introduced functional groups to the surface of the optimized NPs that can be used for bonding of the targeting peptides for imaging of the cancers or heart lesions. We also conjugated near infra-red fluorescent (NIRF) molecules to these functional groups and evaluated NPs performance as multimodal MPI tracers. This additional modality enabled us to study the targeting ability, biodistribution and pharmacokinetics of the NPs in mice more accurately, since NIRF has a high tracer mass sensitivity and reveals more details of microstructural distribution of the NPs in cancer, heart lesions and other organs. In addition, we investigated the pharmacokinetics and biodistribution of these NPs after their intravenous injection and evaluated their MPI performance after their accumulation in reticuloendothelial system (RES) organs (e.g. liver and spleen). We also present preliminary results of our in vivo cancer and atherosclerosis imaging in mice models (i.e. glioma xenografts in nude mice, prostate cancer in trabsgenic mice and heart lesions in C57BL6 LDL receptor mice). These results pave the ways for future applications of MPI for pre-clinical tissue-targeted magnetic imaging and diagnostic applications.

Acknowledgements:
This work was supported by NIH grants 1RO1EB013689-01 (NIBIB), 2R42EB013520-02A1 and 1R41EB013520-01.

Spin-dependent transport is of interest for spintronic applications.[1-3] Novel experimental techniques for the study of spin-dependent transport include near-zero field magnetoresistance (MR)[1-3] and electrically detected magnetic resonance (EDMR).[2,4,5] EDMR has been used to study a variety of defects in organic[1,3] and inorganic[2,4,5] semiconductors and dielectrics. We demonstrate a ubiquitous nature of MR and EDMR phenomena. We use EDMR, which can be used to study defect chemistry and energy levels, to link defects to MR phenomena in a variety of semiconductor and insulator systems. We complement the aforementioned methods with an analysis of EDMR and MR in inorganic materials as a means of understanding the relationship between spin-dependent transport and quantum effects such as electron-nuclear hyperfine interactions and spin-orbit coupling.
Films in our study include a-C:H, diamond-like carbon (DLC), a-B:H, a-SiC:H, and a-Si:H, and range in thickness between 10 and 500 nm. Films are grown on p-Si with Ti gate contacts. In all cases, strong EDMR and MR responses are observed. The EDMR responses, which we later link to MR responses, are believed to be a result of spin-dependent recombination (SDR) and/or spin-dependent trap-assisted tunneling events (SDTAT) through point defects. These transport phenomena have been observed in a variety of organic and inorganic systems.[1.4,5] Responses in this study are almost always broad and featureless, making defect identification via EDMR somewhat difficult. However, we circumvent this problem, to some extent, by making line width comparisons of spectra at high and low EDMR field/frequency combinations. As will be explained, these comparisons allow for an elementary understanding of line width contributions from defect hyperfine interactions and spin-orbit coupling. We also perform EDMR and MR measurements as functions of heterojunction bias, and compare with calculated band diagrams. Variable bias measurements allow for a rough calculation of defect energy levels. We find, via variable bias EDMR and MR comparisons, that defect energy levels are very similar in both cases. This provides a strong circumstantial connection between the nature of EDMR and MR phenomena. Finally, we make EDMR and MR measurements as functions of temperature. We find that the EDMR and MR versus temperature responses to be similar. This provides more evidence linking the underlying EDMR and MR phenomena.
[1] J. M. Lupton and C. Boehme, Nature Materials 7, 598 (2008).
[2] C. J. Cochrane et al. J. Appl. Phys. 112, 123714 (2012).
[3] N. J. Harmon et al., Phys. Rev. B 85, 075204 (2012).
[4] D. Kaplan et. al, J. Phys. Lett. 39(4), L51-L54 (1978).
[5] J. T. Ryan et al., J. Appl. Phys. 108, 064511 (2010).

Strong magnets are required to be installed in high efficiency motors for energy conservation. Nd-Fe-B magnets are known as the strongest permanent magnets and widely used in various kinds of materials, for example hybrid vehicles, wind turbines, hard disk drives, and cell phones. However, thermal stability of these magnets, especially at high temperature, is a known issue that is challenging for material designs. Recent experiments examining the microstructures of Nd-Fe-B magnets show that controlling the structural and magnetic properties of interfaces between subphases and main phases is important to improve the coercivity. As in Ref. [1], Nd-Fe-B magnets can be actually improved by the Nd-Cu grain boundary diffusion process and there is a relationship between the Cu concentration at the grain boundary and the coercivity improvement[2].
To understand this coercivity improvement we simulated Cu-doped Nd₂Fe₁₄B/NdO_x systems, which were discovered around triple junctions in annealed Nd-Fe-B magnets and Cu is thought to be around main-phase (Nd₂Fe₁₄B) grains[1]. We used the computational code OpenMX within density functional theory[3]. We simulated various model structures containing more than 200 atoms each and checked their stability by comparing their total energies after structural optimization. For computational costs we used the open core pseudopotential of Nd atoms in which well-localized 4f electrons are treated as spin-polarized core electrons. From the total energy analysis we found that substituting the Nd site with Cu at the main-phase interface gives a structure that is more stable than the one obtained by substituting at the subphase (NdO_x) interface. We analyzed the anisotropy of Nd atoms by calculating the crystal field parameters (CFP) of Nd atoms from first principles since the coercivity of Nd-Fe-B magnets is basically related to the anisotropy of Nd atoms. In addition, the anisotropy of Fe atoms were calculated in the manner determined by T. Zahra et al.[4]. We found the anisotropy of Nd and Fe atoms at the interface are basically negative as reported in a previous study of the surface case for Nd atoms[5]. Considering these negative anisotropy and the total energy analysis, we conclude that Cu substitution works effectively in terms of improvement of the coercivity.

[1] H. Sepehri-Amin, et al., Acta Mater. 61, 6622 (2013).
[2] A. Yasui, et al., J. Appl. Phys. 117, 17B313 (2015).
[3] http://www.openmx-square.org
[4] T. Zahra, T. Ozaki, S. Tsuneyuki, and Y. Gohda, Appl. Phys. Lett. 104, 242403 (2014).
[5] S. Tanaka, et al., J. Appl. Phys. 109, 07A702 (2011).

Spintronics based on magnetic and non-magnetic elemental metals generated functionalities that are employed in nanoscale devices such as switches, memories, and sensors. Ferrimagnetic electric insulators such as man-made yttrium iron garnets form another class of versatile materials with high magnetic quality. K. Uchida, E. Saitoh c.s., demonstrated that they can be actuated thermally and electrically and thereby integrated into conventional electronics and thermoelectric devices, raising the hope for a new and green spintronics. After an elementary introduction into the basic physical concepts, I will review a number of recent theoretical insights and experimental evidence on magnetic insulators and its bilayers with non-magnetic metals.

Co:ZnO films with Co concentrations of up to 60% of the cationic lattice have been grown by reactive magnetron sputtering. The wurtzite crystal structure was maintained even for these high dopant concentrations beyond the coalescence limit [1]. By measuring the x-ray absorption at the near edge and the linear and circular dichroism of the films at the Zn and Co K-edge it could be shown that Co substitutes predominantly for Zn in the lattice. No hints of metallic Co have been found in the samples. At low Co concentrations the films are paramagnetic, but with increasing Co content the films show antiferromagnetic next neighbor interaction [2] and develop magnetic order with increasing characteristic temperature. Uncompensated spins interact with the antiferromagnetic configurations and lead to a vertical exchange bias like effect. In addition, the single ion anisotropy [3] is lost with increasing Co percentage and the effective magnetic moment is strongly reduced.

[1] B. Henne, V. Ney, K. Ollefs, F. Wilhelm, A. Rogalev, A. Ney, Scintific reports, in revision (2015)
[2] A. Ney, V. Ney, F. Wilhelm, A. Rogalev, K. Usadel; Phys. Rev. B, Volume 85, 245202/1-8 (2012)
[3] A. Ney, T. Kammermeier, K. Ollefs, S. Ye, V. Ney, T. C. Kaspar, S. A. Chambers, F. Wilhelm and A. Rogalev; Phys. Rev. B 81, 054420 (2010

Since its discovery in 1970s, α″-Fe16N2 has drawn the attention of the scientists for its high saturation magnetization and uniaxial magnetocrystalline anisotropy.[1-2] However, due to the discrepancies between the results reported by several groups, its potential application was always debated and scientists couldn’t find a common ground till 2010.[3-6] Nian Ji et. al. proposed a cluster model for the first time to describe the high moment of α″-Fe16N2, which was widely accepted in the scientific world.[7-8] However, the exact value of magnetocrytalline anisotropy of α″-Fe16N2 was also reported differently in different papers.[1,6,9] In our lab, we calculated the magnetocrystalline anisotropy of α″-Fe16N2 by using DFT calculation and validated the result from experimental results. Due to its high magnetocrystalline anisotropy, α″-Fe16N2 has a potential application as a rare-earth-free permanent magnet.[10-11] Thus, we analyzed the potential coercivity mechanism of the bulk α″-Fe16N2 magnet and proposed a model based on random anisotropy model with some extension. We also calculated the effect of grain size, stress annealing and composite phase mixture in the coercivity of α″-Fe16N2 bulk magnet. Finally, we analyzed the effect of nanostructure on the magnetic properties by using analytical equations.

References:
1.Kim, T. K., and Minoru Takahashi, Applied Physics Letters 20.12 (1972): 492-494.
2.Annual Conference on Magnetism and Magnetic Materials, 1996 (Session on Fe₁₆N₂)
3.Sugita, Y., et al. , Journal of applied physics 70.10 (1991): 5977-5982.
4.Coey, J. M. D. , Journal of Applied Physics 76.10 (1994): 6632-6636.
5.Huang, M. Q., et al. , Journal of Applied Physics 75.10 (1994): 6574-6576.
6.Ji, Nian, et al. , Physical Review B 84.24 (2011): 245310.
7.Ji, N., X. Liu, and J. P. Wang. , New Journal of Physics 12.6 (2010): 063032.
8.Wang, Jian-Ping, et al. , Magnetics, IEEE Transactions on 48.5 (2012): 1710-1717.
9.Uchida, S., et al, Journal of Magnetism and Magnetic Materials 310.2 (2007): 1796- 1798.
10.Wang, Jian-Ping, Shihai He, and Yanfeng Jiang. , U.S. Patent Application 14/238,835.
11.Ogawa, Tomoyuki, et al, Applied Physics Express 6.7 (2013): 073007.

Rutile RuO₂ is a metallic 4d transition metal oxide with applications in electrochemistry, catalysis, and as diffusion barriers and strip-line connectors in microelectronics. Its electronic and chemical properties have therefore been studied extensively. However, its magnetic properties have received little attention.
We have theoretically and experimentally investigated the magnetic properties of RuO₂. DFT+U calculations show that for modest local interactions the moments within the Ru₂O₄ unit cell strongly prefer to align antiferromagnetically, while leaving the electronic structure metallic. Magnetic susceptibility measurements show an approximately linear increase over a large temperature range, with a broad maximum near 925 K, suggesting a surprisingly high Neel temperature. This is confirmed with single crystal neutron diffraction experiments revealing structurally forbidden Bragg peaks that are absent in X-ray diffraction data. These magnetic peaks gradually disappear above 925 K, in agreement with the susceptibility data. This establishes RuO₂ as a 4d binary oxide exhibiting itinerant antiferromagnetism at temperatures far surpassing the ordering temperatures of elemental Cr or doped pnictides.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. Tunnel barriers to date have relied upon oxides which often exhibit defects, trap states and interdiffusion which compromise performance and reliability. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We fabricate graphene-based magnetic tunnel junctions, and demonstrate electrical spin injection/detection in silicon and graphene using graphene as a tunnel barrier.

Graphene is grown by chemical vapor deposition and incorporated as the tunnel barrier by physical transfer and standard lithographic processes to form Co / graphene / NiFe magnetic tunnel junctions (MTJs) [1]. Non-linear I-V curves and weak temperature dependence of the zero-bias resistance provide clear evidence for tunneling. The magnetic field dependence exhibits the classic signature of an MTJ, and the structures exhibit tunneling magnetoresistance to 425 K, in good agreement with theory [2].

Graphene successfully circumvents the classic issue of conductivity mismatch between a metal and a semiconductor for electrical spin injection / detection. Hanle spin precession measurements performed on non-local spin valve (NLSV) devices with NiFe / single layer graphene / Si contacts exhibit the classic Lorentzian lineshape due to spin injection and dephasing in Si. The spin lifetimes correlate with Si carrier concentration, and the contact resistance–area products are 2-3 orders of magnitude lower than those achieved with oxide barriers [3]. This reduction of contact resistance enables NLSV spin transport and Hanle measurements of spin lifetimes in Si nanowires [4].

Finally, we demonstrate homoepitaxial tunnel barrier structures in which graphene serves as both the tunnel barrier and the transport channel [5,6]. We fluorinate the top graphene layer to decouple it from the bottom layer, so that it serves as a tunnel barrier for spin injection into the lower graphene channel. We demonstrate lateral transport of spin currents in NLSV structures, high spin injection efficiency with record tunneling spin polarization >45%, and determine spin lifetimes with the Hanle effect. Similar effects are observed with hydrogenated graphene [6].

[1] Cobas, Friedman, van’t Erve, Robinson, and Jonker, Nano Letters 12, 3000 (2012).
[2] Karpan et al, Phys. Rev. Lett. 99, 176602 (2007); Phys. Rev. B 78, 195419 (2008).
[3] van’t Erve, Friedman, Cobas, Li, Robinson and Jonker, Nature Nano. 7, 737 (2012).
[4] van ‘t Erve, Friedman, Li, Robinson, Connell, Lauhon and Jonker, Nature Comm. 6, 7541 (2015).
[5] Friedman, van ‘t Erve, Li, Robinson and Jonker, Nature Comm. 5, 3161 (2014).
[6] Friedman, van ‘t Erve, Robinson, Whitener and Jonker, ACS Nano 9, 6747 (2015).

Most of natural processes occur under out-of-equilibrium conditions. However, the lack of general principles governing non-equilibrium behavior only allows to treat it in a few special cases. We introduce generalized non-Hermitian Hamiltonian approach for description of out-of-equilibrium phase transitions in the exemplary context of dissipative non-equilibrium dynamics of an open quantum spin system. The imaginary part of the proposed Hamiltonian describes effects of damping and the applied Slonczewski spin-transfer torque (STT). In the classical limit, our approach reproduces Landau-Lifshitz-Slonczewski dynamics of a large macrospin. We reveal the STT-driven parity-time (PT) symmetry-breaking transition corresponding to a phase transition from precessional magnetization dynamics to controlled switching. Micromagnetic simulations for nanoscale ferromagnetic disks demonstrate the predicted effect. Our findings break ground for a general quantitative description of out-of-equilibrium phase transitions.

The use of organic semiconductors (OSCs) in spintronics has aroused considerable interests, owing to their much longer spin-relaxation times of OSCs than those of inorganic counterparts. The most studied example is the organic spin valve (OSV), in which magnetoresistance (MR) effect is frequently reported. However, studies on pure spin current injection and transport in OSCs are scarce. Recently, the pioneering work by Watanabe et al. demonstrated that pure spin current can be pumped into and propagates in semiconducting polymers (Nature Phys. 10, 308 (2014)). In the present work we extend the study to small molecule OSCs, and demonstrate that pure spin current can be injected into Alq₃ from the adjacent magnetic insulator Y₃Fe₅O₁₂ (YIG) by spin pumping. The pure spin current is detected by inverse spin Hall effect (ISHE) in Pd after propagation through Alq₃. An unusual angular dependence of the inverse spin Hall effect is found. It, however, only appears when the microwave magnetic field is neither fully perpendicular to nor fully within the sample plane, excluding the presence of the Hanle effect. Together with quantitative temperature-dependent measurements, these results provide compelling evidence that the pure spin current transport in Alq₃ is dominated by the exchange-mediated mechanism.

In this work we report reversible reduction in coercivity of Co/Pd multilayer thin films under high-density direct current biasing. We carried out in-situ focused magneto optic Kerr effect based hysteresis measurement while the specimen was under DC bias. The experiments show a reversible reduction in coercivity during the application of direct current. We propose this reduction occurs due to the spin-orbit torques (Rashba) generated at high current densities. Using an in-situ transmission electron microscope biasing experiment, we also showed the presence of dissymmetric lattice structure of Co/Pd multilayers. Our results suggest that the Rashba torque is the dominant spin-orbit torque since coercivity change is a bulk phenomenon as compared to spin Hall effect.

Irradiation of ferro- and ferri-magnetic metals with high-energy femtosecond laser pulses leads to ultrafast heating induced changes to the magnetization. In ferrimagnetic metals such as GdFeCo or TbFeCo, reversal of the magnetization can occur if heating of the electrons is sufficiently large and fast. Ultrafast magnetic switching via heating of the electrons opens the possibility to generate ultrafast switches for memory and logic. However, a comprehensive and quantitative description of all optical switching (AOS) remains elusive due to the complexity of the problem. Following laser irradiation, energy and angular momentum is rapidly exchanged between numerous subsystems that cannot all be simultaneously probed, e.g. thermal electrons at the Fermi level, 4f spins, majority and minority 3d spins, and phonons. In this work, we perform a combination of single shot and time-resolved magneto optic Kerr effect (TR-MOKE) measurements of ferri-magnetic GdFeCo and TbCo alloys as a function of laser fluence and ambient temperature. At low absorbed laser fluence, e.g. less than 0.4 mJ/cm², the demagnetization rate as a function of ambient temperature of the FeCo sublattice is qualitatively similar to that of a single element ferromagnet such as Ni, Fe, or Co. Additionally, the demagnetization rate of the CoFe sublattice displays a dependence on ambient temperature consistent with the predictions of a simple thermal model. For absorbed fluences greater than ~1 mJ/cm², the magnetization dynamics of the FeCo sublattice show dramatic differences from our low fluence experiments. These differences include magnetization reversal for fluences above a critical value, e.g. ~2 mJ/cm² at room temperature. The critical switching fluence decreases substantially with increasing ambient temperature. The speed of both the initial switching and subsequent magnetization recovery depend strongly on the ambient temperature of the sample. Finally, we experimentally determine the peak electron and lattice temperatures required for AOS to occur.

This work shows how the magnetic properties of intermetallic MnBi nanocomposite are influenced by effect of heat treatment and crystallite size controlled by mechanical milling. Mn_xBi_100-x (x= 45, 50, 55) compounds with 5%, 10% and 15% of Bismuth excess were synthetized by arc melting technique, and annealed under Ar atmosphere at temperature T equal to 573 and 623K for 12 and 10h. Another nanocomposite of Mn_xBi_100-x (X= 45, 50, 55) were synthetized in the same way, grinded during 4 and 8 hours in a high energy mill and annealed at 1073 K for 10 minutes and quenched. X-Ray Diffraction patterns confirmed the presence of MnBi phases. The microstructure were characterized by Scanning Electron Microscopy. Hysteresis loops collected by Vibrant Sample Magnetometer (VSM) showed an increment of coercive field from 5 KOe (freshly melted) to 10 KOe after the annealing for no-grinded samples. Grinded samples showed and increment to 16KOe in coercive field due to the small size of crystallite. So, samples with small average crystallite size reduced my milling have better magnetic properties than samples just synthetized and annealed.

For a permanent magnet, high magnetocrystalline anisotropy, high magnetic moment and coercivity are required. Fe-Co alloy is proposed as an alternative to rare earth permanent magnets, as it is proposed to exhibit both, a high magnetocrystalline anisotropy energy [1] along with a high magnetization.
In this work we present our approach of inducing magnetocrystalline anisotropy and coercivity in Fe-Co/Au-Cu multilayer films, produced by magnetron sputtering. The main idea is the stabilization of metastable tetragonal phases and the induction of anisotropy by straining the FeCo unit cell [1]. We also doped FeCo with Carbon since by this way, we can stabilize strain and increase the magnetocrystalline anisotropy [2, 3]. However, with increasing thickness of the films, a partial strain relaxation is expected [4]. In order to induce strain from both sides in the FeCo layer and thus prevent the strain relaxation, we deposited Fe-Co/Au-Cu multilayers on single crystalline MgO (100) substrate. The thickness of the layers varied between 1-3nm for FeCo and 1-2nm for AuCu.
X-ray,SQUID and FMR measurements were performed. The multilayer films grow epitaxially. A peak shift of 0.5^o was observed in the XRD patterns compared to the theoretically expected value for FeCo-C alloys, which is a strong indication of tetragonal distortion. A high magnetocrystalline anisotropy value of about 0.4 MJ/m³ was measured using FMR for the [Fe₄₅Co₅₅(3 nm)/Au₃₀Cu₇₀ (1 nm)]₁₂ system. Theoretical calculations have also been performed using full potential linearized augmented plane wave method (FLAPW) with generalized gradient approximation (GGA). An anisotropy value of 0.43MJ/m³ was estimated according to the computational results for the AuCu/FeCo/AuCu system.
In our multilayers, the up-sclaing of the thickness, a reduced strain relaxation, along with the induction of coercivity in the order of 900 Oe, is a first promising step towards rare earth free permanent magnets.






Acknowledgements
We acknowledge funding of the EU through FP7-REFREEPERMAG

References
[1] T. Burkert, L. Nordström, O. Eriksson, and O. Heinonen , Physical Review Letters 93, 027203 (2004)
[2] E. K. Delczeg-Czirjak, A. Edstrom, M. Werwinski, J. Rusz, N. V. Skorodumova, L. Vitos, and O. Eriksson, Physical Review B 89, 144403 (2014)
[3] G. Giannopoulos, R. Salikhov, B. Zingsem, A. Markou, I. Panagiotopoulos, V. Psycharis, M. Farle, and D. Niarchos, APL Materials 3,041103 (2015)
[4] L. Reichel, G. Giannopoulos, S. Kauffmann-Weiss, M. Hoffmann, D. Pohl, A.Edstroem, S. Oswald, D. Niarchos, J. Rusz, L. Schultz, and S. Fähler, Journal of Applied Physics 116, 213901 (2014)

Magnetically controlled MEMS switches, the functional analogue of small reed switches and Hall sensors, are highly competitive products. The important target is to increase the switches lifetime and sensitivity on condition of keeping inertial stability.
In present work for the study of magnetic sensitivity in COMSOL program package a number of 3D models of magnetically controlled MEMS switches for 3 design options has been developed: with a fixed beam, with a torsion bar beam, with the magnetic concentrator. The models are based on the equations of magnetic field distribution, action of magnetic field force on ferromagnets, on equations describing mechanical stresses. The properties of electrodeposited permalloy (75% Fe, 25 % Ni) as ferromagnetic material of the magnetic beam, magnetic concentrators and torsion levers have been investigated in the models.
In the course of modeling the following results were received: 1) profiles of mechanical stresses distribution, magnetic field inside MEMS switch design and dynamics of geometry change of moving parts during commutation are defined; 2) dependences of magnetic sensitivity and shock tolerance at various design geometries are received; 3) it is established that the use of magnetic concentrators is an effective method of MEMS switch sensitivity increase. 4) It is established that the use of elbowed torsion bars for fastening is an effective method for magnetic sensitivity increase. 5) It is shown that electrodeposited permalloy consisting of 75% Fe and 25 % Ni is the suitable material for production of basic ferromagnetic materials of MEMS switches.
The results of the completed calculations show that the design including a traveling magnetic ram and fixed magnetic concentrator provides 2 times increase of magnetic sensitivity. The use of magnetic concentrators is the effective method for improvement of operational characteristics of magnetically controlled MEMS switches. It is shown that the use of elbowed torsion bars is the effective way of magnetic sensitivity increase providing the possibility of considerable decrease of mechanical stresses appearing at commutation.
The obtained results are used for the development of new MEMS switches types.

The R₂Fe₁₇ compound is the most iron-rich phase in the binary R-Fe alloy system. The main drawbacks of these materials are low Curie temperature (T_c) and magnetic anisotropies which restrict the possible applications of these materials as high temperature permanent magnets. The substitution of non-magnetic atoms (C, N, H) in Fe sites increases ferromagnetic coupling which in turn increases the Tc and magneto-crystalline anisotropy but the nitrogen and carbon materials are not stable at high temperatures.

Furthermore, Unless removed by a homogenization treatment, the iron reduces the coercivity of the subsequently fabricated magnets.Thus 2:17 alloys are subjected to long isothermal annealing of several days, in order to remove the free iron. This makes the production less cost effective. In an attempt to reduce the manufacturing costs additional elements such as Nb, and Si have been added to the ingot to create an alloy without free iron.These additions lead to the formation of a eutectic mixture consisting of the 2:17 phase, Nb-Si phase and the DyFe₃ paramagnetic Laves phase. Majority of the work on 2:17 pure and substituted intermetallics are limited to single atom substitution but in The doubly substitution improves the Tc at lower level doping without much deterioration in magnetization. Interestingly, Nb and Si doped compounds are light-weighted and corrosion resistant, which are desired in many potential applications of permanent magnet.

In this study the effects of double substitution of nonmagnetic metallic atom Nb and Si for Fe on the structural and magnetic properties of Dy₂Fe_17-(x+y)Nb_xSi_y compounds were investigated. A series of Dy₂Fe_17-(x+y)Nb_x Si_y (x= 0, 0.25,0.5 and y =0, 1, 2, 3) compounds were prepared by arc melting in an argon atmosphere. The structural and magnetic properties of these compounds have been studied via XRD,VSM and ⁵⁷Fe Mössbauer spectroscopy. The substitution of Nb and Si in Dy₂Fe_17-(x+y) compound was found to have an important effect on their structure and magnetic properties. An increase in unit cell volume with the increase in Nb and Si content is observed in Dy₂Fe_17-(x+y)Nb_x Si_y. Rietveld analysis of XRD patterns show that these compounds form the hexagonal Th₂Ni₁₇ structure. The saturation magnetization of Dy₂Fe_17-(x+y)Nb_x Si_y shows an approximately linear increase with the increase of x (for y=0) up to 95.86 emu/g for x=0.25 and y=0. However, lower magnetization values were obtained for Dy₂Fe_15.75Nb_0.25 Si₁ (51.99 emu/g) and Dy₂Fe_14.75Nb_0.25 Si₂ (24.36 emu/g). The room temperature Mössbauer analysis showed that the hyperfine field values decreased with Nb and Si substitution due to magnetic dilution upon substitution.

Thus, the present study clearly demonstrates Dy₂Fe_17-(x+y) Nb_x Si_y compounds achieve high Tc in low level doping of Nb and Si without much reduction in Ms. This magnet may find application in the industry for its high T_c, high energy product,and corrosion resistance properties.

Zr₂Co₁₁-based nanomaterials are promising candidates for the development of rare-earth-free permanent magnets with strong magnetocrystalline anisotropy energy, high Curie-temperature, and hard magnetic performance. Addition of suitable external element Mo, Si, or B into Zr₂Co₁₁-based alloys has shown significant improvement in the magnetic properties, such as a significant increase in coercivity and energy product [1-4]. Although Boron element doping favors the formation of the hard-magnetic phase and increases the coercivity of melt-spun Zr-Co alloys, how B addition affects magnetic microstructure and what is the correlation of magnetic microstructure and magnetism are still unclear. The investigation of the magnetic microstructure and property relationship is important for the understanding of these improvements and further for the development of high-performance magnets. The effect of B addition on magnetic domain structures of nanocrystalline Zr₁₆Co_82.5−xMo₁B_x (x = 0, 1, 2, 3 and 4) melt-spun ribbons was investigated by Magnetic Force Microscopy (MFM). We find that element addition is a feasible way to modify magnetic domain structure and enhance coercivity for Zr₂(Co, Mo, B)₁₁ nanocrystalline alloys. The best magnetic properties were obtained with a significant increase in coercivity Hc and isotropic maximum energy product (BH)_max for the x = 1 sample with Hc = 5.4 kOe and (BH)_max = 4.1 MGOe . A detail MFM image analysis of the B-content dependence of magnetic domain structures in this class of non-rare-earth Zr₁₆Co_82.5−xMo₁B_x (x = 0, 1, 2, 3 and 4) nanocrystalline ribbons is provided to further advance the insight into the underlying microscopic mechanisms. Surface roughness analysis of the melt-spun ribbons was also performed by Atomic Force Microscopy (AFM). This research was supported by DOE/Ames under Grant no. DE-AC02-07CH11358 and SC-15-423. The research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by the National Science Foundation under Award ECCS: 1542182, and the Nebraska Research Initiative.

The manipulation of droplets is of significant interest for a broad range of applications from lab-on-a-chip devices to bioinspired functional surfaces. To manipulate a droplet, micro- or nanopatterned surfaces have been suggested as a passive approach, in which unique structural features or chemical gradients enable liquid wetting and spreading in a specific direction. Although these approaches enable control over liquid wetting and spreading without external forces, they are typically slow and not reversible, presenting inherent limitations. A set of active droplet manipulation techniques have also been proposed, including electrowetting, dielectrophoresis and thermocapillary force. Compared with the passive approaches, these active techniques provide enhanced control over droplet position and motion in a fast and reliable fashion. Nonetheless, the electrowetting platform requires electrodes formed on a substrate, as well as an external power source, for the manipulation of liquid droplets, which limits the scalability and applicability of the technique.

To this end, magnetically actuating surfaces with micro- or nanoscale structures have considerable potential to be used for the active and dynamic manipulation of droplets because of their reversible and instantaneous structural tunability in response to a remote and non-destructive magnetic field. However, previous droplet manipulation techniques based on magnetic force are mostly limited to irreversible transitions of wetting states without the ability to control the position and motion of discrete droplets. Although some studies reported positional control of droplets based on using a magnetic field, they utilized a droplet mixed with magnetic nanoparticles, which significantly limits broad application of the techniques.

Here, we report a novel technique that enables active and dynamic control over the position and motion of a pure discrete droplet by utilizing a magnetically responsive flexible film comprising reversibly actuating hierarchical pillars on the surface. In this approach, a discrete droplet of water can be rapidly manipulated to arbitrary target locations on the flexible film with only the use of a permanent magnet. The flexible film with actuating hierarchical pillars is simply fabricated by a mouldless self-assembly of a solution comprising precured polymers and magnetic particles under a magnetic field. This magnetically responsive surface exhibits reliable actuating capabilities with immediate field responses and maximum tilting angles of ~90°. Furthermore, the magnetic responsive film exhibits superhydrophobicity regardless of the tilting angles of the actuating pillars. Using this novel magnetically responsive film with a superhydrophobic nature, we demonstrate highly precise and dynamic manipulation of a discrete droplet in an active and instant manner using only a permanent magnet.

The anisotropic magnetoresistance (AMR) effect is a phenomenon in which the electrical resistivity depends on the relative angle between the magnetization direction and the electric current direction [1-9]. The AMR ratio, which is the efficiency of the effect, is defined by (ρ_∥-ρ_⊥)/ρ_⊥, where ρ_∥ (ρ_⊥) is a resistivity for the case of the electrical current parallel to the magnetization (a resistivity for the case of the current perpendicular to the magnetization). The AMR ratio has been experimentally measured for various ferromagnets since about 150 years ago. Systematic analyses for their AMR ratios, however, have been scarce so far. In addition, the intuitive explanation about the AMR effects has seldom been reported.
In this study, we first derived a general expression of the AMR ratio [3,4]. We here used the two-current model with all the s-d scattering processes. The d states were obtained by applying the perturbation theory to a Hamiltonian with the exchange splitting and spin-orbit interaction V_SO. Using the expression, we next analyzed the AMR ratios of Fe, Co, Ni, Fe₄N, and half-metallic ferromagnet. We found that Fe, Co, and Ni exhibit the positive AMR ratio, while Fe₄N and half-metallic ferromagnet show the negative AMR ratio. Such a tendency agreed with the experimental results. In addition, their AMR effects could be intuitively explained by using the d orbitals, which were distorted by V_SO. We finally showed that the negative AMR ratio is a necessary condition for half-metallic ferromagnet [3-6]. Details will be reported in our presentation.

[1] Masakiyo Tsunoda et al., Appl. Phys. Express 2, 083001 (2009).
[2] Masakiyo Tsunoda et al., Appl. Phys. Express 3, 113003 (2010).
[3] Satoshi Kokado et al., J. Phys. Soc. Jpn. 81, 024705 (2012).
[4] Satoshi Kokado et al., Adv. Mater. Res. 750-752, 978 (2013).
[5] Fujun Yang et al., Phys. Rev. B 86, 020409 (2012).
[6] Yuya Sakuraba et al., Appl. Phys. Lett. 104, 172407 (2014).
[7] Kazuki Kabara et al., Appl. Phys. Express 7, 063003 (2014).
[8] Satoshi Kokado et al., phys. stat. solidi (c) 11, 1026 (2014).
[9] Satoshi Kokado et al., J. Phys. Soc. Jpn. 84, 094710 (2015).

In this work, we present the experimental results on magnetic behavior and thermal transport behavior of ferromagnetic multilayer thin films. We have developed a unique micro-electro-mechanical systems (MEMS) based experimental setup that has freestanding specimen and allows us to measure electrical and thermal conductivity of thin films. We measured the thermal conductivity using 3ω method. We also investigated the ferromagnetic behavior under high density current using magneto-optic Kerr effect measurement and transmission electron microscope. The techniques presented in this work allows us to have a comprehensive characterization of the multilayer ferromagnetic thin films.

An iterative-seeding based magneto-plasmonic Au@CoFe₂O₄ yolk-shell nanoparticles using ascorbic acid and cobalt ferrite as reducing nanotemplates has been designed. We have proposed a novel aspect of site-specific targeting of Doxorubicin using iterative Au coated CoFe₂O₄ yolk-shell nanoparticles as a nanopayload and folic acid as a targeting agent for cancerous cells. The multiple iterations and different shell size and shapes have been well explained using XRD and HRTEM. One single layer of Au on Cobalt ferrite nanoparticles enhances the capability of binding drugs, but multiple coating further augments the physiological stability and tunes surface plasmon resonance as well as dielectrics for proficient loading of drugs as well as pH-dependent release in specific microenvironment. The functionalization of folic acid and Doxorubicin was confirmed by FTIR and TGA. SQUID explained the efficacy of iterative method by confirming that even after 5 Au iterations, Au@CoFe2O4 was highly superparamagnetic. MRI showed that the yolk-shell possess enhanced T₂ contrast for imaging both normal and cancer cells. Magnetic Hyperthermia studies exhibited an overwhelming augmentation in the temperature rise for yolk-shell nanoparticles. Doxorubicin release kinetics profile has been fit based on Zero-order, First Order, Higuchi and Hixson-Crowell model. This nanocarrier as an efficient MRI contrast agent, Magneto-hyperthermal and drug-delivery nanofleet is a step ahead towards the success of cancer theranostics. This drug delivery system can act as an efficient nanocarrier in the cancer micromilieu for synaphic targeting and assassination of the cancer cells, thus rescuing the life of patients.

Multiferroics are the materials in which two or more than two ferroic orders(ferroelectric, ferromagnetic and ferroelastic) occur in the same phase. Among these compounds, ABO₃ perovskites where A = Bi and B = transition metal of the first row, i.e., Fe, deserve much attention because of their multiferroic possibilities, where spontaneous magnetic order and ferroelectric polarization might be combined in a single phase material. These materials offer wide potential applications in information storage and sensors. It has been already reported that during the synthesis of BiFeO₃, results in the appearance of phases. Many alternative strategies have been (are being) attempted to prepare pure BiFeO₃. In order to obtain a pure phase, a temperature and heat treatment are critical.
Single particle 40 to 60 nm of the multiferroic rhombohedral structure BiFeO₃ were synthesized via co-precipitation method using cetyltrimethyl ammonium bromide(CTAB) as a surfactant, and different heat treated in air (at 550°C, 600⁰C and 650⁰C) for 2 hrs. The effects of CTAB contents (0, 3, 6 and 9 wt.%) on formation, structure, morphology and magnetic property of the BiFeO₃ particles were investigated. Pure phase Multiferroic BiFeO₃ powder were obtained in the existence of CTAB surfactant at 550⁰C, whereas secondary phase Bi₂₅FeO₃₉ was detected for the sample without presence of CTAB and in the samples with CTAB annealed at higher temperature.
Our vibrating sample magnetometer results suggest that a saturation magnetic response in BiFeO₃ can be initiated when the size of the system is less and coerivity increases with increasing of particle size due to the uniaxial crystalline anisotropy.
The Mossbauer study of BiFeO₃ gives quadrupole splitting of sample with increased sample size, which could suggest a change in atomic displacement accompanied by modified polarization properties. At room temperature, a superposition of quadrupolar (doublet) and magnetic (sixplet) absorption spectra is observed. The spectra become progressively more magnetic with increasing annealing temperature. The collapsed quadrupolar spectra are associated with the subset of smallest particles within the size distribution synthesized within a given sample. Least-square fits of our experimental data to theoretical spectra give values for the isomer shift (δ), quadrupole splitting (ΔE_Q), and magnetic hyperfine field (H_hf) consistent with the presence of high-spin ferric ions . BiFeO₃ has been reported to exhibit a single doublet with a quadrupole splitting of about (ΔE_Q) 0.08 mm/s, indicating a slightly distorted octahedral symmetry at the Fe³⁺ site above Neel temperature. For larger particles, two inequivalent magnetic subsites are observed with slightly different values of magnetic hyperfine fields (H_hf1 ) 471.9 kOe and (H_hf2 ) 494.7 kOe for the 50 nm, 600 °C annealed sample. These result implies that CTAB may act as a crystalisation master, controlling the nucleation and growth of the crystal of BiFeO₃.

Since the demonstration of magnetic vortex oscillators [1] driven by a spin-polarized current, there has been an increased interest in the dynamic properties of geometrically confined spin vortices. The frequency of the gyrotropic mode of the vortex core depends on the geometry (circular vs. elliptical dots) and the dot dimensions [2] Apart from this, it has been reported that the intrinsic film defects may affect the dynamics of magnetic vortices [3]. In this work, we demonstrate that a relatively small alteration to the dot topography could also serve as an artificial pinning site for the vortex core, and thus significantly influence the resonant properties. The pinning sites were introduced by inserting a concentric nonmagnetic disk under the magnetic element. The experimental microwave absorption spectra clearly indicate that the behavior of the vortex core is significantly modified due to the presence of the vortex barrier. A stepwise jump upwards in frequency (~0.2 GHz) of the vortex mode is observed in the modified dot structure when the field magnitude is decreased, while the reference dot demonstrates a very weak field independence, as expected. The stepwise jump in the frequency can be attributed to the change in the vortex core position. The micromagnetic simulations are in good agreement with the experiment, suggesting that the frequency difference between the remanent state and the displaced vortex could be further increased with the increase of the aspect ratio of the barrier. The described approach can be used to design spintronic devices with distinct magnetic field dependence of the resonant response.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

[1] V.S. Pribiag, I.N. Krivorotov, G.D. Fuchs, P.M. Braganca, O. Ozatay, J.C. Sankey, D.C. Ralph, and R. a. Buhrman, Nat. Phys. 3, 498 (2007).
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Manipulating the spin state of electrons has driven much interest due to the potential applications of spintronics devices. Diluted magnetic semiconductor is one of the promising candidates to combine both semiconducting and magnetic properties into spintronic devices. Recently, MoS₂ has attracted much attention because of its unique semiconducting properties with an indirect bandgap of 1 eV for the bulk form and a direct bandgap of 1.8 eV for the monolayer. It has been widely explored due to its potential applications in transistors, sensors and energy materials. Theoretical calculations have indicated that MoS₂ are diamagnetic and transition-metal-doping could result in ferromagnetic coupling. However, only a few experimental works have been conducted to study the magnetic properties of transition-metal-doped MoS₂. In this work, Co doping is performed by ion implantation. Co ions with a dose concentration of 1.65×10¹⁶ ions/cm² are implanted into a MoS₂ single crystal and produce a 15nm-thick surface layer doped with 4 at% of Co. The effect of Co doping on the magnetic properties of MoS₂ is investigated in detail. XRD and Raman analysis indicate that the MoS₂ single crystal has a hexagonal structure and no secondary phases are detected in the Co-implanted sample. The temperature dependent M-H loops show that the pure MoS₂ single crystal is diamagnetic. Furthermore, the addition of Co ions in MoS₂ single crystal turns its magnetic behaviour into ferromagnetic at room temperature. ZFC and FC curves also confirm the ferromagnetic ordering beyond room temperature. Our theoretical results based on first principle calculation elucidate that the Co substitution in the crystal lattice is the origin of ferromagnetism.

MoS₂ is one of the 2D materials that have shown great potential for the new generation low-dimensional transistors, photo-emitting devices and spintronics devices due to their unique structural and electronic properties. Monolayer MoS₂ (1H-MoS₂) has drawn much attention due to the unique properties from its bulk counterpart. Previous researches have shown that pristine bulk MoS₂ is nonmagnetic; however recent theoretical and experimental results indicate a likelihood of magnetism in 1H-MoS₂.
In this work, we carefully investigate the structural, electronic and magnetic properties of Co doped monolayer MoS2 by considering a variety of defects including all the possible defect complexes using first-principles calculations. A 4×4 MoS₂ monolayer supercell structure containing 32 S and 16 Mo atoms is adopted in this study. For comparing the size effect of the supercell on the electronic and magnetic properties and simulating different doping levels, 3×3 and 5×5 MoS₂ monolayer supercells with the same lattice constants and calculation details are also employed.
The results indicate that pristine 1H-MoS₂ is nonmagnetic. The materials with the existence of S vacancy or Mo vacancy alone are non-magnetic either. Further calculation demonstrates that Co dopants can induce robust magnetic moment in this system and the magnetization originates from the d orbitals of the Co, NN Mo atoms and the p orbitals of the NN S atoms via p-d hybridization. The magnetic moment is strongly dependent on the doping concentration. 1H-MoS2 with lower doping concentrations (4 at% or 6.25 at%) has a stable magnetic moment of 3 μB, which is higher than that at higher doping level (8 at%, or 11.1 at%, or 12.5 at%). The substitutional Co_Mo defects tend to cluster at higher doping concentration. Subsequently the substitutional CoMo defects will not result in magnetic state. However, if Mo vacancies exist in the system, the dopants tend to separate from each other with a particular distance and the system shows ferromagnetic state. In addition, this tendency is energy favorable.
Based on our calculations, we find that the magnetism in Co-doped 1H-MoS2 system can only be obtained when the dopants are rather dispersed and uniform, suggesting intrinsic ferromagnetism of a new kind of diluted magnetic semiconductor. The work may pave ways for achieving intrinsic diluted magnetic semiconductor based on MoS₂ semiconductors.

Cancer is among the major causes to human deaths. Magnetic hyperthermia therapy is one of the promising technologies to treat cancer. Therefore, the goal of this research is to develop electrospun magnetic iron/polyaniline nanofibers for application in hyperthermia therapy. In this study, we synthesized metallic iron (Fe⁰) nanoparticles at room temperature with assistance of polyvinylpyrrolidone to tailor their particle size. The Fe nanoparticles were subsequently incorporated with conducting polyaniline (PANI) to form Fe⁰ / PANI nanofibers using electrospinning technique. The PANI shells prevent the possibility of Fe⁰ oxidation and further improve the hyperthermia effect, benefiting from the high conductivity of the PANI polymer. The method developed in this study could provide a new method to design magnetic nanocomposites with enhanced hyperthermia effect to treat cancers.

Wireless power transfer(WPT) involves various technologies and one of the crucial elements is the use of magnetic materials. Various magnetic materials can be used for WPT or wireless power charging. Currently Fe based metallic materials are employed in various devices but regardless of the high permeability the use of Fe based metallic materials is limited to a few kHz due to low resistivity and large eddy current losses. Thus in order to provide high permeability with use in higher frequency range such as megahertz(MHz) range the use of ferrites is necessary. In this study Mn-Zn ferrites with high permeability is fabricated with Co as primary additive and their properties investigated in MHz range. Also the formation of thin layers using thermal spray coating process and its properties with regard to MHz range is studied.

A considerable amount of research has been carried out on Ni-Zn ferrites because of their innumerable applications in non-resonant devices due to their high magnetization Ms. Similarly, Al doped SrFe_12-XAl_XO₁₉ has their distinct magnetic properties such as their high T_c, high coercive force, have made them popular for industrial application such as microwave device in recording media.
Energy product as a combination of magnetization and coercivity essentially determines the quality of the permanent magnets. But unfortunately, magnetic materials tend naturally to have one rather both; soft magnetic materials usually have relatively high magnetization, while most of hard magnets have high coercivity, but quite low magnetization. Exchange spring principle can identify a route to provide the possibility for improving remanence and energy product of the permanent magnets by making the composite of suitable material. The present study is to obtain Pure phase exchange-coupled nanocomposites of hard-soft magnetic oxides, (hard) SrFe_12-XAl_xO₁₉ - (soft)Y Ni_0.5Zn_0.5Fe₂O₄ with X=0, 0.5, 1, 1.5 and 2 and Y=wt.10%, wt.20%, wt.30%, wt.40% and wt.50% were prepared via autocombution method. The X-ray powder diffraction (XRD) patterns of as prepared nanocomposite peaks are in good agreement with the hexagonal and cubic phases and their peaks are broadened due to their nanometer sizes. VSM measurement report that a _~19% increase in Ms value for nanocomposite with x=0 (69.5 emu/g) for hard and 50 Wt.% of the soft phase. A linear increase in Mr /Ms with soft-phase content indicates the presence of enhanced exchange-coupling between hard and soft phases of the nanocomposite. The highest Mr /Ms ratio of 0.60 was obtained for nanocomposite containing wt. 10.% of the soft-phase with X=1.5 Al doped sample which is significantly larger than the value of 0.5 predicted for non-interacting permanent magnet. This indicates the existence of intergrain exchange coupling between hard and soft phases. The observed reduction in coercieve field values of the nanocomposite with increase in soft-phase content is explained on the basis of competition between exchange and dipolar interaction between hard-soft and soft–soft phases of the nanocomposite.
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We report a facile organic-phase synthesis of monodisperse barium-doped iron oxide (Ba-Fe-O) nanoparticles (NPs) through thermal decomposition of Ba(stearate)₂ and Fe(acac)₃ in 1-octadecene with oleic acid and oleylamine as surfactants. The Ba/Fe composition (from 0.04 to 0.095) is controlled by the Ba(strearate)₂/Fe(acac)₃ ratio or by the oleylamine and 1-octadecene volume ratio. The as-synthesized Ba-Fe-O NPs, especially the Ba_0.082-Fe-O NPs, can be easily converted into hexagonal barium ferrite (BaFe) by annealing in O₂ atmosphere at 700 °C for 1 h, showing strong ferromagnetic properties with Hc reaching 5260 Oe and Ms of 54 emu/g. More importantly, these monodisperse Ba-Fe-O NPs are well dispersed in hexane and can be easily assembled into densely packed 2D arrays and further converted into oriented BaFe magnets. Our reported synthetic method and self-assembly approach can also be extended to Sr-Fe-O and ferromagnetic SrFe NPs, providing a unique way of fabricating ferromagnetic ferrite arrays that may be important for magnetic energy storage and data storage applications.

The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe3O4) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe3O4, which could be stabilized by strain in films or nanostructures.

[1] K. P. McKenna, F. Hofer, D. Gilks, V. K. Lazarov, C. Chen, Z. Wang and Y. Ikuhara, Nature Communications 5, 5740 (2014)

Heat assisted magnetic recording (HAMR) technology based on L1₀-ordered FePt magnetic media is a promising magnetic recording technology to overcome the areal density limit of current perpendicular magnetic recording technology and thereby, extend the areal density beyond 1.5 Tbit/in². To develop high performance HAMR media, it is essential to have granular structured FePt media layers with good L1₀ chemical ordering and strong (001) texture. In this work, we systematically investigate the influence of the lattice mismatch between FePt and the substrate crystal on the structural and magnetic properties of FePt films. Granular structured, ~10 nm thick, epitaxial FePt films with and without carbon segregant were prepared on different single crystal substrates of (La,Sr)(Al,Ta)O₃ (LSAT), SrTiO₃ (STO), MgAl₂O₄ (MAO), and MgO, causing an in-plane lattice mismatch (LM) with FePt to vary from 0.4% up to 9.6%. The films on STO, MAO, and MgO show strong (001) texture with no misoriented FePt grains while the film on LSAT, the least lattice-mismatched substrate, additionally has FePt grains with (110) orientation. Although all of the films are in-plane tensile-strained, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) confirm that the strain level and the related interface defect density impacting the texture, L1₀ chemical ordering, and magnetic properties are significantly affected by the degree of LM. Detailed quantitative and statistical XRD and HRTEM results on the FePt strain state and interface defect structure and their impact on the ordering and magnetic properties will be presented in this talk.

Eutectoid decomposition of A1 Co-Pt alloys near 60% Pt can result in formation of the unique nanochessboard structure. The nanochessboard represents a self-assembled 2+1D-periodic stacking of magnetically hard L1₀ nanorods embedded in a soft L1₂ matrix, all separated by coherent interfaces. Lateral lengthscales of the chessboard of order 20 nm. As such, the chessboard is a fascinating structure in which to examine exchange coupling, of intermediate complexity between epitaxial bilayers and nanocrystalline aggregates. Two key process parameters that govern chessboard formation are the thermal cooling rate used to pass through the A1—L1₀+L1₂ eutectoid, and the final temperature reached during the ramp, prior to rapid quenching. At slower cooling rates, 40 ^oC/day, we varied the final temperature, and measured both M-H loops and first order reversal curves (FORCs). Formation of the hard L1₀ phase is clearly evident in both the major-loop susceptibilities and in the FORC density plots. The data suggest incomplete exchange coupling between the hard and soft phases, including residual A1, likely due to lengthscales that are slightly too large relative to the hard-phase domain wall thickness. At higher cooling rates, 80 ^oC/day, however, single-phase behavior is obtained, even though x-ray diffraction confirms the presence of both hard and soft phases. Increases in both coercivity and remanence ratio are observed, relative to the slower-cooled specimens. This results from a reduction in chessboard lengthscale by more rapidly cooling through the eutectoid, resulting in rigid exchange coupling. FORC density plots from chessboards can show rich substructure indicating significant local and mean field interactions. Some aspects resemble “paradigmatic” features observed in idealized systems, but with interesting and non-trivial differences arising from the more complex structure of the chessboard. Support of the National Science Foundation through grant DMR-1105336 is gratefully acknowledged.

Advances in nanoscience have allowed us to tune material properties by controlling the nanoscale architecture. Here, we report that by controlling film porosity, annealing temperature and building blocks (i.e. nanocrystals and sol-gel precursors) we are able to tune the static magnetic properties of cobalt ferrite (CoFe₂O₄) thin films over a wide range. Bulk cobalt ferrite is a magnetically hard material with a coercivity around 3600 Oe. However, its coercivity can be lowered by making it nanocrystalline, mesoporous or both. Furthermore, the magnetic properties can be fine-tuned by varying the annealing temperature of the films. By combining these methods, we have shown that thin films of cobalt ferrite can be made with coercivities ranging from 3200 Oe down into the superparamagnetic regime. In addition to controlling the coercivity, the saturation field, and thereby effective permeability, can be controlled by changing size and spacing in nanocrystal based films. Finally, we find that despite the large change in static properties, the dynamic properties, as probed by ferromagnetic resonance (FMR) studies, remain unchanged over the measureable samples. These results suggest that by precisely controlling the nanoscale structure, the magnetic properties of thin films can be easily tuned and tailored toward a variety of device applications. Of particular interest is the possibility of using cobalt ferrite and other hard magnetic materials in areas such as high-frequency applications where their use has traditionally been limited

Molecular spintronics, an emerging research field at the frontier between organic chemistry and spintronics, has opened novel and exciting opportunities in terms of functionalities for spintronics devices. Beyond, plasticity and low cost, carbon based materials were first seen as very promising for spintronics devices due to their expected long spin lifetime and potential for spin transport. It was only very recently that new spintronics tailoring opportunities that could be brought by molecules and molecular engineering were unveiled. Among them, it was shown that spin dependent hybridization at the metal/molecule interface could lead to a radical tailoring of spintronics properties [1,2]. In this direction Self-Assembled Monolayers (SAMs) appear to be a very promising candidate thanks to their impressive molecular scale crafting properties. Despite all the promising possibilities to build molecular spintronic devices, up to now less than a handful of experiments on the use of SAMs as spin-dependent tunnel barriers have been reported [3], still already highlighting promising potential at low temperatures [4]. In an effort to achieve room temperature spin signal, we have developed magnetic tunnel junctions based on alkanethiol (C12 to
C18) and conventional “3d” ferromagnets (such as Co,NiFe…). In this direction, we have developed a novel process to recover the oxidized ferromagnet from oxidation [5].
We will present NiFe/alkanethiol SAMs/Co devices and discuss electerical transport properties.


[1] C. Barraud et al., “Unravelling the role of the interface for spin injection into organic semiconductors,” Nat. Phys., vol. 6, no. 8, pp. 615–620, 2010.
[2] M. Galbiati et al., “Spinterface: Crafting spintronics at the molecular scale,” MRS Bull., vol. 39, pp. 602–607, 2014.
[3] W. Wang et C.A. Richter, “ Spin-polarized inelastic electron tunneling spectroscopy of a molecular magnetic tunnel junction”, Applied Physics Letters, 89, 153105 (2006)
J.R. Petta et al., “Spin-Dependent Transport in Molecular Tunnel Junctions”, Phys. Rev. Lett., 93, 136601
[4] M. Galbiati et al., “Unveiling Self-Assembled Monolayers’ Potential for Molecular Spintronics: Spin Transport at High Voltage,” Adv. Mater., vol. 24, no. 48, pp. 6429–6432, 2012.
[5] Marta Galbiati, Sophie Delprat et al. “Recovering ferromagnetic metal surfaces to fully exploit chemistry in molecular spintronics“, AIP advances 5, 057131 (2015)

Magnetic nanoparticle systems may be considered a new class of magnetic materials, since their properties may be very different from conventional magnetic materials and can be tailored by means that are not easily accessible in other systems. Magnetostatic (dipolar) interactions between nanoparticles may open new ways to design nanocrystalline magnetic materials and devices if the collective magnetic properties can be controlled at the nanoparticle level. Dipolar interactions are sufficiently strong to sustain magnetic order at ambient temperature in assemblies of closely-spaced nanoparticles with magnetic moments of ≥ 100 µ_B.

Through evaporation of dense colloids of ferromagnetic 13 nm ε–Co particles onto carbon substrates, anisotropic magnetic dipolar interactions can support formation of elongated “rope-like” nanoparticle structures with aggregate widths of 100–400 nm and lengths of up to some hundred microns [1]. We have observed that unusual domain structures arise in these systems [1]. The domain structures are deemed “unusual” because, unlike conventional/continuous materials, there is no exchange energy cost proportional to the cross-sectional area of a domain wall, thus walls can be very long in dipolar-coupled magnetic materials. They can also feature a 180-degree magnetization rotation between neighboring particle chains corresponding to zero wall width.

We have verified that longitudinal domain-walls can be stabilized by dipolar inter-particle interactions in elongated nanoparticle assemblies with aspect ratios (length/width) of 2–7, in the absence of inter-particle exchange coupling. The energy of transverse/vortex walls is comparable in continuous film and nanoparticle assemblies while the energy of longitudinal walls is very large (proportional to the wall length) in continuous films and very low in nanoparticle assemblies. We find that the propagation of longitudinal domain walls occurs in the transverse direction with reverse field and that domain structures are stabilized during reversal by imperfections and packing disorder.

The domain behavior has been captured visually by electron microscopy experiments, and the results interpreted on the basis of numerical simulations, providing a coherent and consistent physical picture that highlights how the domain behavior of nanoparticle-materials does not involve transverse (or vortex) domain walls typically observed in thin film strips of Co of comparable dimensions.

[1] Scientific Reports 5, Article number: 14536 (2015)

In recent years, research on interfaces of various oxides has gained considerable interest because of some exciting spin-dependent phenomena¹. Due to very low magnetic damping, ferrimagnetic yittrium iron garnet (YIG) is a promising material for spin injection². The magnetic property of YIG thin film can be tuned by introducing a non-magnetic layer on top of the YIG layer. In this work, we show the structural and magnetic properties of bilayer and multilayers consisting of a ferrimagnet (Bi doped YIG) and a non-magnetic (Ga doped ZnO) layers. 10% bismuth was doped in YIG, in order to lower the sintering temperature of YIG compound to prepare the target for pulsed laser deposition (PLD).
Bi:YIG/Ga:ZnO multilayers have been grown on YAG substrate by PLD. Structural analysis was carried out using high resolution XRD (Rigaku, Smartlab).Thickness and roughness of each layer has been calculated from X-ray reflectivity (XRR) measurement. Magnetic properties were studied using SQUID VSM (Quantm Design). ZFC and FC have been recorded on the samples in the temperature range from 300 K to 20 K by applying a field of 500 Oe. Compared to the only Bi:YIG layer, the multilayers (repetitive YIG/ZnO layers (1 to 5)) show two transitions at ~ 45 K and ~110 K. The separation between ZFC and FC curve increases with the number of interfaces and this may be due to some interesting scattering phenomenon occurring at the interface. Results will be discussed in detail.
References:
Sujit Das et. al., Phys. Rev. B. 91, 134405 (2015).
Matthias Althammer et. al., Phys. Rev. B. 87, 224401 (2013).

As humans in recent years undergo frequent RF exposure from electronic devices, the development of electromagnetic interference (EMI) shielding layer that is flexible, thin, and effective is now in high demand. Conventional metal coatings or polymer/nanoparticle composite layers are not suitable for flexible uses in concerns of delamination and agglomeration. To optimize the EMI shielding effectiveness (SE) and increase flexibility, the magnetic metal nanoparticles dispersed evenly in carbon (C) conducting path is considered to be one of the best forms of EMI shielding material, which we targeted for. We have developed a novel fabrication method, which is simple and cost effective, to control the size and distribution of transition metals inside C fibers through oxygen partial pressure (P_O2) controlled calcination. The resulting SE was higher than the required commercial level of 90%. Amongst common magnetic metals, nickel and cobalt were applied due to their high permeability that enables greater shielding effectiveness. The greatest advantages of our material not only lies on its effectiveness but also on that we minimized coercivity to eliminate local heat produced when EM field is applied and, at the same time, optimized to achieve anisotropic thermal conductivity in plane direction for heat sink.

Ni/Co/C hybrid nanofibers were prepared by electrospinning followed by heat treatment technique under P_O2 ranging from 0 to 1.3 Torr at 700°C. The Ni/Co ratio in weight was kept to be 5:5, and Ni/C and Co/C fibers were also prepared for comparison. The static magnetic properties of nanofibers were determined using vibrating sample magnetometer (VSM). Vector network analyzer (VNA) and 4-point probe technique were used to measure permeability, permittivity, and sheet resistance to analyze the EMI-SE in the EM frequency range of 300 MHz- 8GHz.

FE-SEM and HR-TEM results showed that the average size of metal nanoparticles grows from 5nm to 52nm as P_O2 increases. Applying P_O2 to combust C out to leave pores, but not enough O₂ to oxidize the metal composite, enables the diffusion of metal particles into the C pores through certain mechanisms, which allowed us to control morphology of the nanofibers. XRD and Raman confirmed phases and crystallinity of the fibers, respectively. The permeability, which could be judged by the slope in M-H curve, was optimized at P_O2=0.4 Torr, yet the coercivity was kept at a negligible level with the enlarged metal particles. This result is meaningful in that the increase in coercivity had been unavoidable when the nanoparticle size increases due to the loss of superparamagnetic behavior, however, for this heat treatment technique under the mechanism in which nanoparticles diffuse to agglomerate, the metal particle forms multi magnetic domain that results in a low coercivity. Calculating from the permeability and permittivity over the X-band EM frequency range, we obtained the EMI-SE of at least 93% throughout the whole range.

Nanomagnetism research which aims to understand and control magnetic properties and behavior on the nanoscale through proximity and confinement, is currently shifting its focus to emerging phenomena occurring on mesoscopic scales. New avenues to control magnetic materials open up through enhanced complexity and new functionalities, which can impact the speed, size and energy efficiency of spin driven applications.
Magnetic soft X-ray spectro-microscopies [1] provide unique characterization opportunities to study the statics and dynamics of spin textures in magnetic materials combining X-ray magnetic circular dichroism (X-MCD) as element specific, quantifiable magnetic contrast mechanism with spatial and temporal resolutions down to fundamental magnetic length and time scales.
I will review recent achievements and future opportunities with magnetic x-ray spectro-microscopies. Examples will include the dynamics of magnetic vortex structures [2,3] with potential application to novel magnetic logic elements [4], magnetic spectromicroscopy of domain walls [5], and approaches to image the 3dim magnetic domain structures in rolled-up thin films with x-ray tomography [6].
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy Contract No. DE-AC02-05-CH1123.
[1] P. Fischer, and H. Ohldag, Report on Progress in Physics 78 094501 (2015)
[2] R. Streubel, et al. Appl Phys Lett (2015) accepted
[3] M.-Y.Im, et al. Nature Communication 5 5620 (2014)
[4] H. Jung, et al., ACS Nano 6 3712 (2012)
[5] M.J. Robertson, et al., J Appl Phys 117 17D145 (2015)
[6] R. Streubel, et al. Nature Communication 6 7612 (2015)

Recently a scanning transmission x-ray microscopy (STXM) setup has been combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics in the GHz regime [1] enabeling spatially resolved x-ray-detected ferromagnetic resonance (XFMR) studies on magnetic micro- and nanostructures. Compared to other spatially resolved detection schemes based on conventional ferromagnetic resonance (FMR) [2] the novel STXM-XFMR setup features element-selective detection as well as a high temporal and spatial resolution of down to 18 ps and 35 nm, respectively [1]. Here we will briefly present the detection scheme which allows us to probe the high-frequency transverse component of the precessing magnetization down to precession angles of 0.1° [1]. In addition, first results derived for coupled magnetic microstripes and heterostructures will be discussed and compared to conventional spatially resolved FMR [3]. We are able to directly image the magnetic excitations and identify the contributions of the respective constituents to complex conventional FMR spectra. The relative phases between microwave excitation and standing spinwave can furthermore serve to separate groundstate from coupling modes.
[1] S. Bonetti et al., Rev. Sci. Instrum. 86 (2015) 093703
[2] R. Meckenstock, Rev. Sci. Instrum. 79 (2008) 041101
[3] T. Schaffers et al. (in preparation, 2015)

Apparent diamagnetic response has been reported for various materials that possess both ferromagnetic (FM) and antiferromagnetic (AFM) components of the spin ordering, e. g. [1, 2]. However, origin of the anomalously large diamagnetic moment is not yet clarified. In the present work we report the “superdiamagnetic” response measured for single-phase pyrite FeS₂, NiS₂ and CuCl powders. Magnetization measurements revealed a giant diamagnetic response in zero-field-cooled (zfc) regime below the transition temperature T_c = 60 K (FeS₂), 30 K (NiS₂) and 20 K (CuCl). The field-cooled (fc) magnetization M_fc(T) revealed a characteristic for FM materials behaviour for T