Jian Shen Oak Ridge National Laboratory
Zvonimir Bandic Hitachi Research
Shouheng Sun Brown University
Jing Shi University of California
K1: Magnetic Thin Films and Multilayers
Monday PM, December 01, 2008
Room 206 (Hynes)
9:30 AM - **K1.1
Ferromagnetic-Superconducting Hybrid Systems.
Samuel Bader 1 Show Abstract
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
10:00 AM - **K1.2
Anomalous Hall Effect in Ultra-thin Iron Films.
Xiaofeng Jin 1 , Yuan Tian 1 Show Abstract
1 physics, Fudan University, Shanghai China
Hall effect in magnets includes two parts, a normal Hall effect and an anomalous Hall effect (AHE) depending on magnetization. It is a long debate about the mechanism of AHE. One kind of explanation is based on scattering of carriers, including skew-scattering and side-jump mechanism. The other one, intrinsic mechanism, can be understood in the language of Berry phase in magnets and can be calculated quantitatively. Iron is a typical system to study AHE. Previous work measured the anomalous Hall resistivity in pure iron and iron alloys, which is inconclusive because both the band structure and electron scattering change upon alloying. Our experiments were performed in ultra-thin iron films (1-20 nanometers), in the range where the dimension effect induces change in band structure. By measuring Hall resistivity of different temperature and thickness, contributions from extrinsic scattering is found to be temperature-independent. Moreover, the part of anomalous Hall resistivity which depends quadratically on longitudinal resistivity is found to decrease below 4 nm. This can be understood by considering the dimension effect on intrinsic AHE.
10:30 AM - K1.3
Temperature Dependence of the Training Effect in Exchange Coupled all Ferromagnetic Bilayers.
Srinivas Polisetty 1 , Sarbeswar Sahoo 1 , Andreas Berger 2 , Isabel Delgadillo-Holtfort 3 , Christian Binek 1 Show Abstract
1 Physics & Astronomy, University of Nebraska, Lincoln, Lincoln, Nebraska, United States, 2 , CIC nanoGUNE Consolider, Donostia Spain, 3 Instituto de Física, Universidad de Guanajuato, León Mexico
The temperature dependence of the training effect is studied in an all ferromagnetic bilayer heterostructure. The sample under investigation consists of a magnetically soft CoCr pinned layer of 3 nm thickness which is in proximity of a magnetically hard CoPtCrB pinning layer of 15 nm thickness. Antiferromagnetic exchange coupling between the soft and the hard layer takes place via a diamagnetic Ru interlayer of 0.7 nm. This interlayer exchange shifts the hysteresis loops of the soft layer along the magnetic field axis by the bias field μ0HB. This loop shift is in far-reaching analogy to the exchange bias of conventional antiferromagnetic/ferromagnetic heterostructures. Our all ferromagnetic bilayer system possesses, however, inimitable advantages over the conventional exchange bias systems. For instance, the pinning layer magnetization can be isothermally tuned by applying an external magnetic field allowing unique experimental access to the change in its magnetization state. An applied magnetic set field can induce a ferromagnetic domain state in the hard layer. This domain state can experience an aging effect very similar to the training behavior of a pinning layer in a conventional exchange bias system. In general, training can be observed when the spin structure of the pinning layer is initially out of equilibrium and approaches the equilibrium spin configuration triggered via subsequent reversals of the pinned magnetization. Isothermal initialization of a non equilibrium spin configuration in a ferromagnetic pinning layer is achieved by partial demagnetization of the hard layer. Next, the training effect is measured at various temperatures 300K≤T≤395K by consecutively cycled hysteresis loops of the soft layer using SQUID magnetometry. Training behavior is quantified by the μ0HB vs n dependence where n is the number of consecutively cycled soft layer hysteresis loops. In addition to the experimental investigation we derive a theoretical description of the temperature dependence of the training effect. Particular emphasis lies on the understanding of the rate of change of μ0HB vs n which hitherto entered the theory as a free fitting parameter only. Our theory is based on a discretized Landau-Khalatnikov dynamical equation. A Landau-type free energy enters the dynamic equation as an expression for the energy landscape of the pinning layer magnetization away from equilibrium state. The theory is in excellent agreement with our experimental data.
This work is supported by NSF through Grant No. DMR-0547887, the Nebraska Research Initiative (NRI), and by the MRSEC Program of the NSF through Grant No. DMR-0213808.
10:45 AM - K1.4
Interface Characterization of [Pt/Co]-IrMn Multilayers by Tomographic Atom Probe : Influence of a Pt spacer between [Pt/Co]n and IrMn layers
Rodrigue Larde 1 , Amjaad Zarefy 1 , Luc Lechevallier 1 , Jean-Marie Le Breton 1 , Bernard Rodmacq 2 , Bernard Dieny 2 Show Abstract
1 , Groupe de Physique des Matériaux UMR CNRS 6634, St Etienne du rouvray France, 2 , SPINTEC, URA 2512 CNRS/CEA, Grenoble France
11:30 AM - **K1.5
An Element-specific Study of Magnetic Multilayers using X-ray Magnetic Dichroism.
Ziqiang Qiu 1 Show Abstract
1 , UC-Berkeley, Berkeley, California, United States
Interesting magnetic properties in magnetic multilayers often stem from the magnetic interaction between different magnetic layers. An element-specific magnetic measurement is crucial to identify the effect of the magnetic interaction on the new magnetic properties. The recent development of the x-ray magnetic circular/linear dichroism (XMCD, XMLD) makes it possible to do element-specific measurement for ferromagnetic/antiferromagnetic materials with monolayer sensitivity. In particular, the combination of the XMCD/XMLD with electron microscopy allows element-specific magnetic domain imaging. In this talk, I will present some recent results from our group to illustrate the application of the XMCD/XMLD on magnetic multilayers.
12:00 PM - **K1.6
Perpendicular Magnetic Anisotropy Driven by Antiferromagnetic Layer.
Minn-Tsong Lin 1 2 Show Abstract
1 Department of Physics, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
A novel perpendicular anisotropy and new kind of spin-reorientation transition were found in ferromagnetic/antiferromagnetic bilayer system. The perpendicular anisotropy was found to be driven by an antiferromagnetic layer with suitable control of the thickness. This bilayer system can be used as a new building block for designing nanomagnetic device or storage media with perpendicular magnetization.In details, Fe/Mn bilayers grown on Cu3Au(100) [1,2] were investigated. With increasing Mn thickness, the magnetic easy axis of 6 ML Fe/n ML Mn was observed to switch from in-plane to perpendicular direction by magneto-optical kerr effect (MOKE) and photo emission electron microscopy (PEEM) in application of synchrotron radiation. The PEEM provides the technique for element-specific magnetic domain imaging with help of magnetic circular dicroism effect and by selecting various absorption edges of different elements. The magnetic domain of antiferromagnetic Mn layer was shown to be antiparallel to the Fe domain in both regions of perpendicular and in-plane magnetization. The perpendicular magnetization is accompanied with the enhanced coercivity, being proposed to be attributed to the exchange coupling from Fe/Mn interface, which overcomes the in-plane anisotropy of the magnetic thin films. A detailed phase diagram at variation of thickness and temperature is given for optimizing material parameters and estimation of the effective perpendicular anisotropy as well.*E-mail: email@example.com W. C. Lin,T. Y. Chen, L. C. Lin, B. Y. Wang, Y. W. Liao, K.-J. Song, and Minn-Tsong Lin, Phys. Rev. B 75, 054419 (2007) W. C. Lin, B.Y. Wang, T.Y. Chen, L.C. Lin, Y.W. Liao, W. Pan, N.Y. Jih, and K. J. Song, and Minn-Tsong Lin, Appl. Phys. Lett. 90, 052502 (2007).
12:30 PM - K1.7
Structure and Magnetism of Two-dimensional Nanoscale Fe-Co Alloys.
Stefanos Tzivanakis 1 , Alejandro Diaz Ortiz 1 , Helmut Dosch 1 Show Abstract
1 Low-Dimensional and Metastable Materials, Max Planck Institute for Metals Research, Stuttgart, BW, Germany
The interplay between structure, magnetism, ordering, and dimensionality in alloys of iron and cobalt is investigated using quantum mechanical and high-throughput calculations. First-principles density-functional calculations of Fe-rich are used to bridge the atomic with the thermodynamic limit via cluster expansions of the relevant physical quantities. The parameterized Hamiltonian is used then to exhaustively sample the configurational space for the ground states of the system. Our results show that dimensionally effects fundamentally change the magnetic transitions between the saturated and unsaturated alloys as the concentration is varied, although the strongly ordering character of the alloys is preserved. The implications thereof and the effect of the substrate on the self-organized growth of Fe-Co alloys are discussed at the end.
12:45 PM - K1.8
MgO(111) Surface Reconstructions Effects on Structural and Magnetic Properties of Fe and Hematite Films.
Vlado Lazarov 1 , Marija Gajdardziska-Josifovska 2 , Roger Ward 4 , Scott Chambers 3 , Michael Weinert 2 , Saua Cheung 3 Show Abstract
1 Materials, University of Oxford, Oxford United Kingdom, 2 Physics, University of Wisconsin Milwaukee, Milwaukee, Wisconsin, United States, 4 Physics, University of Oxford, Oxford United Kingdom, 3 , Pacific Norhtwest National Labaratory, Richland, Washington, United States
Magnetic properties of thin films depend on their interface with the substrate, crystalline structure, and film morphology. The effects of polar surface reconstructions on the film growth mode, phase composition, surface and interface structure, and magnetic properties are explored for epitaxial growth of Fe and hematite films on MgO(111)-(√3×√3)R30° and (2×2) reconstructed surfaces.Surface reconstruction of MgO(111) influences significantly the growth of Fe thin films and consequently their crystalline structure. The Fe films grown on(√3×√3)R30° are magnetically softer and show larger magnetic anisotropy compared to the films grown on MgO(111) (1×1) surface. Besides Kurdumov-Sachs and Nishiyama-Wasserman standard crystallographic orientations for bcc/fcc systems the Fe films show also a new crystallographic orientation, Fe(111)//MgO(1-10), not previously reported for the Fe/MgO system. Fe grains in the new orientation significantly change the magnetic properties of the films grown on reconstructed MgO(111) surface due to additional magnetic anisotropy axis when compared to the Fe films grown on MgO (1×1) surface. Growth of alpha-Fe2O3(0001) on the (√3×√3)R30° and (2×2) reconstructed MgO(111) surfaces results in formation of a self-organized Fe3O4(111) interfacial nano-buffer that persists after growth. The interface magnetite-like phase is absent from the hematite films formed on hydrogen-stabilized unreconstructed MgO(111)-(1×1) under equivalent conditions. This study shows that surface reconstructions offers additional opportunities for electronic and magnetic device engineering when combined with strain, spin, and band-gap engineering.
K2: Magnetic Nanoparticles I
Monday PM, December 01, 2008
Room 206 (Hynes)
2:30 PM - **K2.1
Physically Fabricated Synthetic Magnetic Nanoparticles with Tunable Magnetic Properties.
Shan Wang 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Chemically synthesized superparamagnetic nanoparticles are widely used in biology and medicine for applications that include biomolecule purifications and cell separations, magnetic resonance imaging (MRI) contrast agents, and biomagnetic sensing. Magnetic nanoparticles with high moments are often desired to produce large signals for biomagnetic sensing or to avoid restrictive requirements for high magnetic field gradients in separations. Increasing the size of single-grain superparamagnetic particles is not a viable route because these particles become coercive and, consequently, spontaneously aggregate at sizes above the superparamagnetic limit (ca. 12nm for Fe). One solution is to incorporate numerous magnetic nanoparticles into larger composites using matrices comprised of dextran or silica. However, there are still limitations associated with controlling the monodispersity, magnetic response, and variations in the number and size of the embedded nanoparticles. Here we report the fabrication of strongly magnetic, zero remanence, monodisperse nanoparticles with tailorable magnetic properties using nanoimprint lithography (NIL) and ferromagnetic multilayer structures, followed by release and stabilization of nanoparticles in solution. Interlayer magnetic interactions are exploited to achieve zero remanence in these synthetic antiferromagnetic (SAF) nanoparticles , which possess magnetic moments well above those typical of superparamagnetic nanoparticles. As an example, we demonstrate magnetic nanoparticles with adjustable saturation fields that are desired for multiplex magnetic labeling in biodetection, which is akin to multicolor fluorescent labeling. Their high magnetic moments afford great ease for magnetic manipulation in solutions with only modest field gradients, which is highly desirable for magnetic sorting. They can also have tunable optical properties, and can be functionalized with fluorescent dyes. This fabrication technique is ideal for producing multimodal nanoparticles by exploiting layers with unique magnetic, optical, radioactive, or electronic properties. This work is supported by National Cancer Institute Contract 1U54CA119367-01 through the Center for Cancer Nanotechnology Excellence focused on Therapeutic Response (CCNE-TR) at Stanford University.W. Hu, R.J. Wilson, A. Koh, A. Fu, A.Z. Faranesh, C.M. Earhart, S.J. Osterfeld, S.-J. Han, L. Xu, S. Guccione, R. Sinclair, and S.X. Wang, “High-moment antiferromagnetic nanoparticles with tunable magnetic properties,” Advanced Materials, 20, 1479–1483, 2008.
3:00 PM - K2.2
Composition, Size, Structure, and Magnetic Characterization of Iron-Platinum Nano-particles.
Nirav Parekh 1 , Gregory Young 1 , Maninder Kaur 2 , Kiumars Parvin 2 , Spencer Wong 3 , David Bruck 3 , Abhishek Singh 1 Show Abstract
1 Chemical and Materials Engineering, San Jose State University, San Jose, California, United States, 2 Department of Physics and Astronomy, San Jose State University, San Jose, California, United States, 3 Department of Biological Sciences, San Jose State University, San Jose, California, United States
Iron-platinum (FePt) nano-particles were synthesized using a hot metal salt reduction reaction with iron chloride and platinum acetylacetonate as the precursor salts. The synthesis route was based upon work done previously by Sun et Al.. The size and composition of the FePt nano-particles were controlled by varying the surfactant to reagent molar ratio and refluxing time during synthesis. The size of FePt nano-particles synthesized ranged from 4.18 nm to 5.2 nm. The composition of the nano-particles ranged from Fe68Pt32 to Fe55Pt45. The size was shown to increase as the surfactant to reagent molar ratio was changed from 1:1 to 1:6. The platinum composition increased from 32±2 % to 45 ±2 % with an increase in the refluxing time from 30 minutes to 60 minutes. The X-ray analysis and magnetization measurements showed that as synthesized particles have fcc crystal structure and are superparamagnetic at room temperature. A blocking temperature of 38 K was measured for 5.2 nm fcc Fe55Pt45 nano-particles. Upon annealing, the fcc Fe55Pt45 nano-particles transform to a fct crystal structure with a measured room temperature coercive field of 15000 Oe.
3:15 PM - K2.3
Elaboration of Magnetic Nanodots by Nanosphere Lithography : a Computerized Method for Order Quantification.
Pierre Colson 1 , Rudi Cloots 1 2 , Catherine Henrist 1 2 Show Abstract
1 Chemistry of Inorganic Materials, University of Liege, Sart Tilman Belgium, 2 Centre for Applied Technology in Microscopy, University of Liege, Sart Tilman Belgium
3:30 PM - K2.4
Self-assembly of Magnetic Nanoparticles on Silica Template via a Dewetting Process using a Pulsed Laser.
Yong Jun Oh 1 , Caroline Ross 2 , Seok Hun Hwang 1 , Jung Hwan Kim 1 , Carl Thompson 2 , Yeon Sik Jung 2 Show Abstract
1 Materials Science and Engineering, Hanbat University, Daejon Korea (the Republic of), 2 Materials Science and Engineering , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Metallic nanoparticle arrays have recently attracted considerable interest as they can be utilized in a wide range of applications such as patterned magnetic recording media and catalyst arrays for the growth of nanotubes. One of the key challenges in the fabrication of periodic nanoparticle arrays with large areas is the development of a simple, low-cost self-assembling fabrication technique. In this study, we have deposited metallic thin films on topographically patterned SiO2–coated substrates and created metal nanoparticle arrays by liquid-state dewetting using direct pulsed laser irradiation. We have examined the effects of the laser energy, the initial film thickness, and the topography of the substrate on the particle morphology and long-range ordering. The templates used in this study consisted of 200 nm period square arrays of inverted pyramidal pits made in (100) silicon wafers. The structures were fabricated using laser interference lithography, which utilized two exposures that were perpendicular to each other to obtain a square array of holes in the top negative-resist layer. Thermal oxide was grown on all the Si substrates. Co and Ni films with a thickness of 5~15 nm were deposited onto the templates by pulsed laser deposition using a focused beam generated by a Nd:YAG pulsed laser, which operated at its fourth harmonic of 266 nm with a pulse width of ~5 ns and a repetition rate of 10 Hz. Following the deposition, the film was directly irradiated in an ambient of forming gas (Ar+4%H2) for different numbers of shots in order to induce dewetting of the films. The energy density of the laser beam used for the dewetting process ranged from 40 to 150 mJ/cm2. Above the critical energy density value of 80 mJ/cm2, the metal films were completely dewetted and the agglomerated particles were periodically positioned in the inverted pyramidal pits. Thus, by using an appropriate combination of pit dimensions, film thickness, and laser energy density, it is possible to fabricate large-area periodic arrays of metal nanoparticles on topographic templates. The crystallographic and magnetic properties of the nanoparticle arrays were examined using an x-ray pole figure, a transmission electron microscope (TEM), and a vibrating sample magnetometer (VSM). Results are compared with those derived from thermal annealing of similar films at 700 – 900 degree C. In conclusion, it can be stated that the dewetting method using pulsed laser irradiation has the potential to develop well-ordered arrays of metallic nanoparticles, which may be suitably utilized in patterned media and other applications.
4:30 PM - K2.6
Wrap-bake-peel Process for Nanostructural Transformation from Akagenite Nanorods to Biocompatible Iron Oxide Nanocapsules.
Yuanzhe Piao 1 , Jaeyun Kim 1 , Hyon Bin Na 1 , Taeghwan Hyeon 1 Show Abstract
1 School of chemical and biological engineering, Seoul National University, Seoul Korea (the Republic of)
4:45 PM - K2.7
Size-dependent Magnetization in Nanomagnets of the Ferromagnetic Elements from ns Pulsed Laser-induced Self Organization.
Hare Krishna 1 2 , N. Shirato 3 , Z. Nussinov 1 2 , A. Gangopadhyay 1 2 , R. Kalyanaraman 3 4 Show Abstract
1 Department of Physics, Washington University in St. Louis, St. Louis, Missouri, United States, 2 Center for Materials Innovation, Washington University in St. Louis, St. Louis, Missouri, United States, 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 4 Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee, United States
Recently we have shown that nanosecond (ns) pulsed laser-induced self organization can produce hemispherical Co nanoparticles with size-dependent magnetic anisotropy [J. Appl. Phys. 103, 073902, 2008]. In this work we present the magnetization behavior for the ferromagnetic elements Fe, Ni (and Co). The laser-induced self-organization approach was used to create arrays of hemispherical polycrystalline nanomagnets of Fe, Co, and Ni on SiO2 substrates. Magnetic properties of these nanomagnets were studied using magnetic force microscopy (MFM), Surface Magneto-Optic Kerr Effect (SMOKE) and hysteresis loop measurements as a function of particle size. The results show that the single domain nanomagnets demonstrate a size dependent magnetic orientation for all the ferromagnets. The smaller particles tend to align in-plane, while the magnetization direction approaches out-of-plane with increase in particle size. This unusual size-dependent behavior is not consistent with shape anisotropy predictions, which suggest in-plane magnetic orientation for hemispherical shaped magnets. The reason for this unusual behavior has been attributed to the significant residual tensile strain present in the nanoparticles due to the extremely fast heating/cooling rates (~1010 K/s) under ns laser processing, a large thermal expansion mismatch between the metals and SiO2 substrates, and the negative magnetostrictive coefficients for polycrystalline 3d-elemental ferromagnets. This work provides several opportunities to control the magnetization direction of these hemispherical nanomagnets thus making them potential candidates for applications requiring thermally stable magnetic anisotropy, such as in magnetic recording.The authors acknowledge support by the CMI and NSF.
5:00 PM - K2.8
Fabrication and Process Simulation of Permalloy Nanoparticles by Pulse Laser Ablation.
Ruqiang Bao 1 , Shannon Johnson 1 , Anthony Caruso 2 , Yong Huang 3 , Douglas Chrisey 1 Show Abstract
1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, University of Missouri-Kansas City, Kansas City, Missouri, United States, 3 Department of Mechanical Engineering, Clemson University, Clemson, South Carolina, United States
Permalloy (Ni80Fe20) nanoparticles from ones to hundreds of nanometers have successfully been fabricated by pulse laser ablation due to the impact of 248 nm excimer laser pulses on the surface of permalloy pellets in air and collected in sodium dodecylsufate (SDS) in water opposite to the target. This novel fabrication technique and size selective separation provides an opportunity to determine the transition from bulk soft magnet to superparamagnetic as the size / shape (possible anisotropy at nanosize) are reduced / changed. Permalloy was chosen due to its high *bulk* magnetic permeability, low coercivity, significant anisotropic magnetoresistance and near zero magnetostriction, which are critical for industrial applications. The experimental data show that the particle size was mainly controlled by the laser energy and spot size while the shape depends on complex dynamics as the ablatant makes contact with the water + SDS. Scheer analysis was used to determine the nanoparticle size, which was further confirmed by SEM and TEM. The fabrication process was numerically simulated by studying the whole process as laser energy absorption, plume generation, and nanoparticle formation. The process simulation expected that the formed nanoparticle size and its distribution can be controlled by selecting the combination of laser fluence, laser spot size, and target material properties. We expect to make further refinements in this process based on these results.
5:15 PM - K2.9
Synthesis and Characterization of Bimetallic Pd-Co Nano Magnetic Materials in Mesoporous Silica.
Debasish Kuila 1 , Balaji Tatineni 1 , Dhananjay Kumar 2 , Joslyn Perkins 1 , Sergey Yarmolenko 2 Show Abstract
1 Department of Chemistry, North Carolina A&T State University, Greensboro, North Carolina, United States, 2 Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, North Carolina, United States
The synthesis of mesoporous materials containing bimetallic nano-scale metals is an exciting area of research as these materials display unique magnetic, optical, catalytic and electronic properties due to quantum-size effect of the individual metal particles. We have developed one-pot hydrothermal procedure to synthesize mesoporous nano silica (MCM-41) encapsulated Pd-Co nanoparticles using cetyl trimethyl ammonium bromide (CTAB), tetramethoxy silane (TMOS), ammonia and water in the presence of Pd(NO3)2 and CoCl2 as precursors. The synthesized materials are calcined to remove the CTAB surfactant. The novel silica materials were characterized using high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), FT-IR, SEM, BET surface area and magnetic measurements. The bimetallic Pd-Co nanoparticles, 1 to 3 nm, are uniformly distributed on the inside of the pore walls of silica and they do not block the pores. The low angle XRD data are indicative of well defined pore geometry. The STEM data suggest that the size of mesoporous silica varies from 100 to 300 nm. The magnetic measurements of reduced Pd-Co particles show a typical ferromagnetic behavior up to 20 K and exhibit superparamagnetic properties above 30 K. The blocking temperature (~30 K) of our Pd-Co bimetallic nanoparticles in nanoporous silica is much lower that synthesized by sol-gel procedure. The unique properties of the bimetallic nanoparticles may find potential applications in ultra high density magnetic recording media, super-high-density memory storage, and LCD displays.
Jian Shen Oak Ridge National Laboratory
Zvonimir Bandic Hitachi Research
Shouheng Sun Brown University
Jing Shi University of California
K3: Magnetic Nanoparticles II
Tuesday AM, December 02, 2008
Room 206 (Hynes)
9:30 AM - **K3.1
Magnetic Nanostructures using Block Copolymer Self-assembly.
Caroline Ross 1 Show Abstract
1 , MIT, Cambridge MA 02139, Massachusetts, United States
Block copolymers, which microphase-separate into ordered periodic nanoscale structures, provide a path to accomplish large-area patterning of arrays of dots or lines with periodicity on a scale of about 10 – 100 nm. In this presentation we will describe the block copolymer lithography process and the magnetic properties of nanostructures made using this method, including CoCrPt dot arrays, Co/Cu/NiFe multilayer antidot arrays, and Co ring structures. We will describe the selection of block copolymer chemistry and processing methods, including substrate functionalization. Long-range order can be imposed on the self-assembled block copolymer microdomains using topographical features such as trenches or small pillars made using optical or electron-beam lithography, giving well-ordered arrangements of nanoscale features. Applications in patterned magnetic media and ring-shaped magnetoelectronic memory devices will be discussed.
10:00 AM - K3.2
Formation and Size Control of Magnetic Nanoparticles in Polymeric Matrix.
Kensuke Akamatsu 1 , Satoshi Adachi 1 , Takaaki Tsuruoka 1 , Shingo Ikeda 1 , Hidemi Nawafune 1 , Satoshi Tomita 2 Show Abstract
1 , Faculty of Science and Engineering, Konan University, Kobe Japan, 2 , Nara Institute of Science and Technology, Ikoma Japan
Preparation of polymer nanocomposites containing magnetic nanoparticles and control of their size have been investigated. Dibinylbenzene-based polymer matrices with carboxylic acid groups were used as precursors for doping of Ni ions through ion exchange reaction. We have prepared the ion-doped precursors with different ion loadings which were annealed in hydrogen atmosphere to form polymer composites containing Ni nanoparticles. The advantage of this process is the uniform distribution of the metal source and the structural homogeneity of the precursors, which provides a resultant homogeneous dispersion of nanoparticles in polymer matrix. We demonstrate the formation process of Ni nanoparticles in polymer microspheres and structural change of polymer matrix upon reaction by several vibrational and microscopic studies, and show that initial Ni ion loading determines final nanoparticle size ranging from 3 to 10 nm, which are homogeneously distributed in whole of the microspheres. The resultant nanocomposites showed superparamagnetic behavior, depending on the annealing conditions. The heat-treatment in hydrogen atmosphere at fixed temperature causes decomposition of polymer matrix via catalytic reaction of Ni nanoparticles, resulting in the gradual increase of concentration of Ni naoparticles in the matrices and thus the decrease of average interparticle distance. Such the control over the parameters in the magnetic nanocomposites not only is an important for fabricating high performance nanodevices with desired functions but also should lead to fundamental understanding of the relationship of the magnetic properties to the composite microstructures.
10:15 AM - K3.3
Designing Mixed Oxides Magnetic Nanoparticles for Sensing Applications.
Monica Sorescu 1 , Lucian Diamandescu 2 , Adelina Tomescu 2 , Sean Krupa 1 Show Abstract
1 Physics, Duquesne University, Pittsburgh, Pennsylvania, United States, 2 Materials Science, National Institute for Materials Physics, Bucharest Romania
Zirconium-doped hematite nanoparticles of the type xZrO2-(1-x)alpha-Fe2O3 (x=0-1) were obtained using mechanochemical activation and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. XRD patterns yielded the dependence of lattice parameters and particle size as a function of ball milling time for each value of the molar concentration x. The particle dimension reached the value of 9 nm after 12 hours of ball milling.For x=0.1, the Mossbauer spectra were fitted with one, two, three or four sextets, corresponding to Zr ions substituting Fe ions in the hematite structure. For x=0.5, Mossbauer spectra fitting required the addition of a quadrupole-split doublet, representing Fe substituting Zr in the ZrO2 lattice. This transition can be in principle evidenced by recording the recoilless fraction (f) as a function of the ball milling time, provided there is a precise enough method to do so. We applied the dual absorber method developed by us for the precise determination of the recoilless fraction using a second absorber (for instance, a stainless steel foil) and a single room temperature Mossbauer spectroscopy measurement. Indeed, we observed the occurrence of a maximum in the values of the recoilless fraction for t=8 hours of milling for x=0.1 and t=4 hours of milling for x=0.5, followed by a further decrease of the f factor due to the appearance of nanoparticles in the system. It is for the first time that the recoilless fraction is studied as function of the particle size for two different values of the molar concentration.The same investigation procedure was applied to other related systems: xSnO2-(1-x)alpha-Fe2O3, xIn2O3-(1-x)alpha-Fe2O3, xZnO-(1-x)alpha-Fe2O3 and xCeO2-(1-x)alpha-Fe2O3. For these systems, the gas sensing properties in both CH4 and CO were measured at different temperatures, humidity and gas concentration. It was found that the sensitivity to methane and carbon monoxide is influenced by temperature. The compound with Sn at x=0.1 was demonstrated to exhibit a reduced sensitivity to water vapors. A model based on the simultaneous evaluation of the electrical resistance and workfunction was elaborated to describe the sensitivity for the tin-containing system.
10:30 AM - K3.4
Magnetic, Electrical and Magnetotransport Properties of SiO2 coated Fe3O4 Nanoparticle Compacts.
K. Mohan Kant 1 2 3 , K. Sethupathi 1 3 , Mamidanna Rao 1 2 3 Show Abstract
1 Physics, Indian Institute of Technology, Chennai, TamilNadu, India, 2 Materials Science Research Centre, Indian Institute of Technology, Chennai, Tamilnadu, India, 3 Nanofucntional Materials Technology Centre, Indian Institute of Technology, Chennai, Tamilnadu, India
10:45 AM - K3.5
Anomalous Magnetism and Exchange Bias in Coupled Au-Fe3O4 Nanoparticles.
Natalie Frey 1 , Manh-Huong Phan 1 , Sanyadanam Srinath 3 , Chao Wang 2 , Shouheng Sun 2 , Hariharan Srikanth 1 Show Abstract
1 Department of Physics, University of South Florida, Tampa, Florida, United States, 3 School of Physics, University of Hyderabad, Hyderabad India, 2 Department of Chemistry, Brown University, Providence, Rhode Island, United States
Anomalous magnetic response is observed in coupled Au-Fe3
nanoparticles consisting of Fe3
particles (mean size ~9 nm) grown epitaxially on one or multiple facets of polyhedral Au seed particles (mean size ~8 nm) forming dumbbell- or flower-shaped clusters. Temperature and field-dependent magnetization, magnetic training and memory effect studies show that the Au interfaces influence the alignment of surface spins distinct from the core spins in the ferrite nanoparticles. This leads to onset of a strong exchange bias (EB) effect at characteristic temperature (~50K) well below the blocking temperature (~90K). We show that the strength of the EB can be correlated with the amount of interfacial coupling between the Au and Fe3
particles, as the flower nanoparticles (EB ~ 1000 Oe) show a much stronger exchange bias than the dumbbell particles (EB ~ 150 Oe). For comparison, we also show that there is no EB present in traditional spherical Fe3
nanoparticles of the same size. Other anomalous features such as negative values of the low temperature zero-field cooled (ZFC) magnetization, cusps in the field-cooled (FC) magnetization associated with a surface spin glass characteristic and difference in magnetic relaxation below the EB onset temperature, are observed. The field and temperature dependence of the cusp in the FC magnetization follows de Almeida-Thouless scaling confirming the role of surface spin freezing. Frequency-dependent AC susceptibility in flower-shaped coupled Au-Fe3
nanoparticles cannot be fit by Neel-Arrhenius or Vogel-Fulcher models pointing to complex magnetic interactions in the system.
Work at USF supported by DOE through BES grant DE-FG02-07ER46438. Work at Brown supported by NSF through grant DMR-0606264.
11:30 AM - K3.6
Spontaneous Phase Separation of Exchange Biased Nanoparticle–Monolith Composites.
Daniel Shoemaker 1 , Madeleine Grossman 1 , Ram Seshadri 1 Show Abstract
1 Materials, UC Santa Barbara, Santa Barbara, California, United States
We are developing methods to spontaneously convert homogeneous, single-phase oxides into magnetically functional nanocomposites. Traditional ceramic processing is used to create dense NixMn3-xO4 (0.15 ≤ x ≤ 0.60), which is transformed by hydrogen reduction into ∼ 20 nm ferromagnetic Ni nanoparticles embedded within a porous antiferromagnetic MnO monolith. Cross-sectional focused ion beam micrographs reveal that approximately 80% of the Ni nanoparticles are completely encased in MnO, with the remainder attached to the monolith surface or intragranular pores. Exchange bias fields of HE ≈ 40 Oe - 100 Oe at 5 K persist up to the Néel temperature of MnO at 119 K, and coercivity ranges from HC ≈ 400 Oe - 1300 Oe, depending on Ni content and reduction conditions. The threefold increases of HE and HC imply that we can tailor the exchange bias properties by controlling either the Ni nanoparticle size or the volume of MnO between them. These adjustments are facilitated by the wide solubility range of Ni or other magnetic transition metals in Mn-based spinels. Additionally, MnO grain size and orientation can be controlled because they remain unchanged during the reduction process. Our reduction process affords an elegant route to controlling magnetic, electric, or elastic responses at the interface between nanoscale precipitates in metal–oxide or oxide–oxide composites.
11:45 AM - K3.7
Reversal of Flux Closure States in Cobalt Nanoparticle Rings with Coaxial Magnetic Pulses.
Jie Liu 1 2 , Takeshi Kasama 3 4 , Rafal Dunin-Borkowski 4 5 , Michael Scheinfein 6 , Steven Tripp 1 2 , Alexander Wei 1 2 Show Abstract
1 Department of Chemistry, Purdue University, West Lafayette, Indiana, United States, 2 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States, 3 Frontier Research System, Institute of Chemical and Physical Research , Hatoyama, Saitama, Japan, 4 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, United Kingdom, 5 Center for Electron Nanoscopy, Technical University of Denmark, Kongens, Lyngby, Denmark, 6 Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada
Thermoremanent Co nanoparticles (25-30 nm) were synthesized by the thermal decomposition of Co2(CO)8 in the presence of calixarene-derived surfactants . They self-assemble into ring structures comprising 5-11 particles when dispersed in a toluene solution with C11 resorcinarene as surfactant. Electron holography reveals that these Co nanoparticle rings support well-defined flux closure (FC) states with clockwise (CW) and counterclockwise (CCW) polarization at room temperature . The FC polarizations can be switched upon applying coaxial magnetic pulses (Hz) in alternating directions. Micro-magnetics simulation of the novel FC switching mechanism has been conducted, revealing a metastable “double-vortex” domain structure at intermediate field strengths . Larger (macrocyclic) nanoparticle rings also have FC states that can be switched by Hz pulses, but can support an intermediate dipolar (“onion”) state as well.
12:00 PM - K3.8
Magnifying Magnetomechanical Phenomena in Magnetizable Nanoarchitectures.
Debra Rolison 1 , Jeffrey Long 1 , Katherine Pettigrew 1 , Everett Carpenter 2 , Rhonda Stroud 3 Show Abstract
1 Surface Chemistry Branch, Naval Research Laboratory, Washington, District of Columbia, United States, 2 Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia, United States, 3 Materials and Sensors Branch, Naval Research Laboratory, Washington, District of Columbia, United States
Aerogels and ambigels are sol-gel-derived ultraporous, aperiodic nanoarchitectures that unite high surface area for heterogeneous reactions with a continuous, porous network. The vast open, interconnected space characteristic of a building, represented by the interpenetrating nanoscopic pore network (3D plumbing), ensures rapid diffusional flux of molecular and nanoscopic reactants and analytes throughout the interior. The surface area is expressed by "walls" that are defined by the nanoscopic, covalently bonded, one-dimensional solid network of the gel—and because the walls are erected by sol-gel chemistry, the architecture is readily scaled from nanometer to meter length scales. Combining the 1D-interconnected nanoscopic solid network with the 3D-interconnected nanoscopic pore network creates nanoarchitectures that yield higher (and sometimes new) performance relative to compositionally related nanoparticles [1,2]. We prepare crystalline, superparamagnetic nanoarchitectures of magnetite (Fe3O4) and maghemite (γ-Fe2O3) [3,4], and oxygen-stable manganese (MnFe2O4)  and nickel ferrite (NiFe2O4)  using epoxide-driven sol–gel chemistry and select temperature–atmosphere treatments. X-ray diffraction, transmission electron microscopy, magnetic analyses, porosimetry, thermal analysis, and Raman spectroscopy are employed to study the materials in detail. In the vicinity of their blocking temperature, the single-phase, single magnetic domain nickel ferrite aerogels exhibit reversible magnetocompliant behavior that lowers pore volume (by >20%) or shifts pore size (by nanometers) in the presence of applied magnetic fields (gradient and uniform N/S, respectively). The large magnetomechanical effects we observe are a function of the magnitude of the saturation magnetization of the nanomaterials coupled to the flexibility characteristic of such underdense nanoarchitectures.  D.R. Rolison, Science 299 (2003) 1698. D.R. Rolison, J.W. Long, Acc. Chem. Res. 40 (2007) 854. J.W. Long, M.S. Logan, C.P. Rhodes, E.E. Carpenter, R.M. Stroud, and D.R. Rolison, J. Am. Chem. Soc. 126 (2004) 16879. E.E. Carpenter, J.W. Long, D.R. Rolison, M.S. Logan, R.M. Stroud, M.S. Logan, L. Theil Kuhn, B. Rosendahl Hansen, S. Mørup, J. Appl. Phys. 99 (2006) 08N711-1. J.W. Long, M.S. Logan, E.E. Carpenter, R.M. Stroud, D.R. Rolison, J. Non-Cryst. Solids 350 (2004) 182. K.A. Pettigrew, J.W. Long, E.E. Carpenter, C.C. Baker, J.C. Lytle, C.N. Chervin, M.S. Logan, R.M. Stroud, and D.R. Rolison, ACS Nano 2 (2008) 784.
12:15 PM - K3.9
Thermal Stability of Magnetite (Fe3O4) Nanoparticles.
Boon Hoong Ong 1 , Nisha Kumari Devaraj 1 , Mitsunori Matsumoto 1 , Mustaffa Haji Abdullah 2 Show Abstract
1 Faculty of Engineering, Multimedia University, Cyberjaya, Selangor, Malaysia, 2 Faculty of Science and Technology, National University of Malaysia, Bangi, Selangor, Malaysia
12:30 PM - K3.10
Optically Active Water Soluble Iron Oxide Nanoparticles Synthesized by Surface Modification of Magnetite Exhibiting Arsenic Removal Characteristics.
Christopher Jones 1 , J. Mayo 1 , Adina Boyd 1 , Cafer Yavuz 1 , Vicki Colvin 1 Show Abstract
1 , Rice University, Houston, Texas, United States
12:45 PM - K3.11
Superparamagnetic Colloidal Nanocrystal Clusters: Synthesis and Self-Assembly for Tunable Optical Devices.
Jianping Ge 1 , Yadong Yin 1 Show Abstract
1 Chemistry, University of California, Riverside, California, United States
We report here the rational synthesis, surface modification, self-assembly and field-responsive optical diffractions of monodisperse colloidal nanocrystal clusters (CNCs) of magnetite with tunable sizes from ~30 to ~180 nm. These CNCs are prepared through a high-temperature hydrolysis process using polyelectrolyte as a surfactant. Each cluster is a three-dimensional aggregate of many single magnetite crystallites of ~10 nm, thus retaining the superparamagnetic properties at room temperature. The CNCs show strong responses to external magnetic field due to their much higher magnetization per particle than that of individual magnetite nanodots. The application of external magnetic fields induces magnetic attractive forces between the neighboring particles which balance the repulsive interactions including electrostatic and solvation forces. The colloidal particles can therefore assemble into ordered structures with periodicity comparable to the wavelength of light, and show brilliant colors in various solvents. Such diffraction is magnetically tunable in the entire visible spectrum, and the optical response to the external magnetic field is rapid and fully reversible. Finally, we will discuss our recent efforts in fabricating optical devices using these interesting magnetically responsive materials.
K4: Patterned Thin Films
Tuesday PM, December 02, 2008
Room 206 (Hynes)
2:30 PM - **K4.1
Tuning the Magnetic Properties of Bit Patterned Media Fabricated by Blanket Deposition of Perpendicular Anisotropy Multilayers onto Pre-patterned Substrates.
Olav Hellwig 1 Show Abstract
1 Research, Hitachi GST, San Jose, California, United States
In general, there are two fabrication approaches for Bit-Patterned Media (BPM). In one approach, substrates are patterned prior to any magnetic media deposition, and then the magnetic thin film is blanket deposited on these pre-patterned substrates. The major advantage of this technique is that the pattern fabrication process and the magnetic recording media can be fabricated and optimized independently from each other, and etching of magnetic material is avoided. In the other approach, the magnetic media is deposited as a continuous thin film and then patterned into discrete islands. The major advantage of this approach is that there is no magnetic material left between the islands or in the etched trenches. An additional advantage is that the magnetic film is grown on a flat substrate, so overgrowth and curvature effects that may disturb the magnetic film growth on pre-patterned substrates are avoided. However, at competitive densities of 500 Gbit/in2 and beyond, impact on and damage of the magnetic properties during patterning with either ion beam milling or reactive ion etching are still existing issues [1-4].In my talk, I will focus on the blanket deposition onto pre-patterned substrates using perpendicular anisotropy multilayer materials. I will present techniques on how to avoid magnetic trenches and demonstrate the realization of tight magnetic Switching Field Distributions (SFD) on polymer rectified/interpolated pre-patterned substrates with long range order defined by e-beam lithography assistance. Media concepts such as exchange spring or laminated media will be discussed, and the importance of controlling dipolar interactions will be highlighted . B. D. Terris and T. Thomson, J. Phys. D 38, R199 (2005). A. Moser, K. Takano, D. T. Margulies, M. Albrecht, Y. Sonobe, Y. Ikeda, S. H. Sun, and E. E. Fullerton, J. Phys. D 35, R157 (2002). H. J. Richter, A. Y. Dobin, R. T. Lynch, D. Weller, R. M. Brockie, O. Heinonen, K. Z. Gao, J. Xue, R. J. M. van de Veerdonk, P. Asselin, and M. F. Erden, Appl. Phys. Lett. 88, 222512 (2006). J. Moritz, B. Dieny, J. P. Nozieres, R. J. M. van de Veerdonk, T. M. Crawford, D. Weller, and S. Landis, Appl. Phys. Lett. 86, 063512 (2005). O. Hellwig, A. Berger, T. Thomson, E. Dobisz , H. Yang, Z. Bandic, D. Kercher and E. E. Fullerton, Appl. Phys. Lett. 90, 162516 (2007).
3:00 PM - K4.2
Periodically-Aligned Arrays of Ferromagnetic Nanoring and Nanofin via Electroless Plating on Macroscopic Scale.
Shigenori Fujikawa 1 , Kentaro Miyoshi 2 , Toyoki Kunitake 2 Show Abstract
1 Innovative Nanopatterning Lab/ Nanocompartment Engineering Lab., RIKEN, Wako, Saitama, Japan, 2 Innovative Nanopatterning Lab, RIKEN, Wako, Saitama, Japan
The nanostructured ferromagnetic materials have widely been extensively investigated because of their variety of potential applications, such as memory devices, data storages, magnetic sensors, and other devices. Although electron beam lithography is commonly used for fabrication of patterned-magnetic nanostructures, this process is a low throughput process and does not have large-scale processability. Therefore, development of a facile and cost efficient process with highly tunable in shape and size is strongly required desired.Here, we report that fabrication of the periodically-patterned ferromagnetic Co and Ni nanorings and nanofins by means of the specially developed electroless plating combined with photo lithography and plasma etching. The dimensions of these nanostructures can be precisely and easily adjusted by varying process conditions and template shape. The magnetic hysteresis loops of the fabricated arrays exhibit the expected geometrically induced magnetic anisotropy. The Co nanostructures exhibited characteristic magnetization behaviors expected from its unique 3D structures. Especially in the Co rings, stepwise magnetization reversal process through flux closure and onion states was clearly observed. This process enables us to fabricate long ferromagnetic nanowires over 10 mm length and periodically ordered ferromagnetic nanostructures over centimeter-sized regions with high throughput.
3:15 PM - K4.3
Magnetic Domain Walls in Permalloy Nanostructures Made by FIB.
Anna Vila 1 , Antonis Olziersky 1 , Miquel Rubio-Roy 2 , Enric Bertran 2 , Josep Fontcuberta 3 Show Abstract
1 Electronics, University of Barcelona, Barcelona Spain, 2 Applied Phyics and Optics, University of Barcelona, Barcelona Spain, 3 , Institute of Materials Science of Barcelona - ICMAB, Barcelona Spain
Permalloy is well known as a nickel iron magnetic alloy with high magnetic permeability, low coercivity, near zero magnetostriction and significant anisotropic magnetoresistance. With these properties, it has been proposed as an useful material for magnetic tunel junctions. Moreover, experiments by several groups have observed the contribution of domain walls to the resistance of a ferromagnet. Some of these experiments have shown that in narrow Ni wires the resistance of a domain wall can be negative. This work presents the fabrication with Focused Ion Beam (FIB) of narrow permalloy wires and their magnetic characterization. Domain wall pinning is visible, and the associated resistance measured.
3:30 PM - K4.4
High Throughput Fabrication of Epitaxial Oxide Nano-dot and Nano-heterojunction Arrays Composed of Ferromagnetic Spinel Fe Oxide by Nanoimprint Lithography with Mo Lift-off Technique.
Hidekazu Tanaka 1 , Satoru Yamanaka 1 , Tomoji Kawai 1 Show Abstract
1 ISIR-Sanken, Osaka University, Osaka Japan
Nanoimprint lithography (NIL) is an emerging technology that enables high throughput and low cost fabrication of nanostructures. Large-area nano-dot arrays of ferromagnetic Fe2.5Mn0.5O4 (FMO)  and FMO/semiconductive NiO nano-heterostructure, with dot size as small as 100 nm, were successfully fabricated by a combination with dry deposition based on the Pulsed Laser Deposition (PLD) and the Mo lift-off based on NIL without any post crystallization process. In this process, a patterned mold (porous type with depth ~ 200 nm and the lateral sizes of the pores ranging from 100 nm to 3 μm.) is pressed onto a bilayer (NXR-3030/ NXR-2030) polymer resist, and finally the mold pattern was transferred to Mo nano-mask. Magnetic oxides were deposited onto the Mo mask by PLD at a substrate temperature as high as 340°C, and then the Mo layer was lifted-off using H2O2 solution, leading to highly integrated magnetic oxide nano-dot arrays. The three-fold symmetric pattern by the X-ray diffraction pole figure analysis for FMO (220) evidences the high quality epitaxial relationship between FMO nano-dots and the Al2O3 (0001) single crystal substrate without Anti Phase Boundary (APB), although epitaxial spinel oxide films have usually six-fold symmetry due to existence of APB.The resulting FMO nano-dot array showed room temperature ferromagnetic properties with their coercive magnetic field identical with that of continuous thin film, measured by the Magneto Optic Kerr Effect (MOKE: λ=670nm).Taking advantage of direct epitaxial growth by dry deposition, the epitaxial FMO/NiO p-n magnetic heterojunction diode were also fabricated as highly integrated nano-device array. The estimated density for nano-magnetic oxide arrays reaches up to 16G bit/inch2.References  N. Suzuki, H. Tanaka, T. Kawai, Small (2008) in press,  J. Takaobushi, S. Ueda, H. Tanaka, T. Kawai et al, Phys. Rev. B, 76 (2007) 205108
4:15 PM - **K4.5
Nanoimprint for Device Application.
Akihiro Miyauchi 1 Show Abstract
1 , Materials Research Laboratory, Hitachi Ltd., Hitachi Japan
4:45 PM - K4.6
Laser Patterned Ferromagnetic Nanostructures in Hydrogenated Ga1-xMnxAs.
Rouin Farshchi 1 3 , David Hwang 2 , Nipun Misra 2 , Rajesh Chopdekar 1 , Paul Ashby 3 , Costas Grigoropoulos 2 3 , Yuri Suzuki 1 , Oscar Dubon 1 3 Show Abstract
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 3 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Mechanical Engineering, UC Berkeley, Berkeley, California, United States
Diluted magnetic semiconductors (DMSs) are potential candidates for use in spintronics due to their intertwined electronic and magnetic properties, as well as their potential for injecting spin-polarized currents into semiconductors. Hydrogenation of DMSs, most notably Ga1-xMnxAs, has proven to be an effective method to reversibly tune the ferromagnetism. Manganese-hydrogen complex formation leads to suppression of ferromagnetism in Ga1-xMnxAs while subsequent thermal annealing can dissociate the complexes and restore ferromagnetism. Therefore, by controlling hydrogen incorporation in DMSs with high spatial resolution, one may fabricate planar spintronic nanostructures to allow spin-polarized currents to flow through ferromagnetic and non-ferromagnetic regions while remaining in an isostructural medium.We present direct writing of ferromagnetic nanostructures in a hydrogenated Ga1-xMnxAs film by local dissociation of Mn-H complexes using tightly focused laser irradiation. Such local laser annealing leads to the formation of electrically and ferromagnetically active structures embedded uniformly within the non-active film, and the magnetic properties of these structures can be tuned by controlling the laser parameters. Using nanosecond laser activation under near-field configurations, we have realized features as small as 150 nm , and preliminary experiments using femtosecond laser activation show potential for achieving sub-100 nm resolution. Dot, line, ring, and Hall-bar structures have been fabricated using femtosecond laser irradiation and have been imaged using conductance atomic force microscopy. Magnetotransport measurements on both nanosecond and femtosecond laser activated Ga0.96Mn0.04As structures demonstrate significant recovery of hole-mediated ferromagnetism while conductance imaging shows that the laser irradiation has a minimal visible effect on the surface topography of the film. The ideal structural interface between laser activated (ferromagnetic semiconducting) and neighboring hydrogenated (paramagnetic semiconducting) regions is very promising for achieving efficient spin-injection, and nanoscale resolution of laser activated features may accommodate for the short spin-diffusion lengths of holes in GaAs, potentially allowing for direct laser writing of planar all-semiconductor spintronic devices. R. Farshchi, O. D. Dubon, D. J. Hwang, N. Misra, C. P. Grigoropoulos, and P. D. Ashby, Appl. Phys. Lett. 92, 012517 (2008).
Jian Shen Oak Ridge National Laboratory
Zvonimir Bandic Hitachi Research
Shouheng Sun Brown University
Jing Shi University of California
K5: Correlated Materials
Wednesday AM, December 03, 2008
Room 206 (Hynes)
9:30 AM - **K5.1
Detecting and Optimizing Magnetic Interfacial Properties using Magnetization-Sensitive Optical Second Harmonic Generation (M-SHG).
Harry Tom 1 Show Abstract
1 Physics and Astronomy, University of California, Riverside, Riverside , California, United States
We have used a combination of magnetization-sensitive second-harmonic generation (M-SHG) and linear magneto-optical Kerr effect (MOKE) to study interfacial and bulk magnetization in Fe/MgO(001) heterostructures grown by molecular beam epitaxy and La1-xSrxMnO3/SrTiO3 and related oxide heterostructures on SrTiO3 (001) and (110) grown by laser molecular beam epitaxy. Both material systems are candidates for magnetic tunnel junction and spin-valve applications due to the high transmission of spin-polarized electrons in MgO and the high spin-polarization in LSMO, respectively. The Fe/MgO interface is ferromagnetic, has different magnetic anisotropy from the Fe film, and the in-plane easy-axes are observed to be rotated from those of the Fe film by ~1 degree. The LSMO/STO interface is ferromagnetic, has a lower Curie temperature than the bulk LSMO layer, and has a different Curie temperature when grown on the (001) and (110) oriented substrates. In collaboration with R. Kawakami and J. Shi who grow and make magnetic tunnel junctions from these material systems, we are engineering and optimizing interfacial magnetic properties for spintronics applications. Acknowledgements: funding from DMEA-CNN thru the UCR-CNSE, NSF-DMR-0706681, and NSF-EECS-0802214.
10:00 AM - K5.2
Simplifying Complexity in Electronic Phase Separated Manganites.
Thomas Ward 1 2 , Shuhua Liang 1 2 , Kenji Fuchigami 1 2 , Lifeng Yin 2 , Elbio Dagotto 1 2 , Ward Plummer 1 , Jian Shen 1 2 Show Abstract
1 Physics, University of Tennessee, Knoxville, Tennessee, United States, 2 Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
We show that by combining the electronic complexity of manganites with nanoscale confinement new electronic properties emerge due to the fine balance between electron spin, charge, and lattice pressure. These complex interactions between electrons and atoms in oxides such as the manganites and cuprates form the basis of intriguing properties like superconductivity and colossal magnetoresistance. Until recently, these properties have only been studied in macroscopic bulk systems and thin films. We have fabricated single crystal wires of electronically phase separated manganites that are comparable in size to the domains of the electronic phases in the material. Transport measurements on simple confined structures reveal new properties such ultrasharp steps in resistivity, a reemergent metal-insulator transition and time dependent resistivity jumping that are unseen in larger samples. Further, these properties are tunable through doping and strain. This ability to control key elements of the underlying complexity and observe the resulting changes in the emergent behavior help answer questions about the fundamental physics that rule them and could help unlock these systems for future applications.
10:15 AM - K5.3
Microstructural, Electrical-, Magneto-transport Properties of Grain Size Modulated Nanocrystalline Nd0.67Sr0.33MnO3 CMR Manganites.
Sanjoy Paul 1 , Tapan Nath 2 Show Abstract
1 Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal, India, 2 Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal, India
Manganese oxides (Perovskite structure, having form ABO3, where, A (trivalent rare earth element, La, Nd, Pr etc. and divalent alkali earth element, Sr, Ca, Ba etc.), B is the Mn)are important materials from few past decades. It opened an exciting research in field of new generation digital recording, magnetic read write heads, magnetic sensors etc. In this work we have investigated the effect of grain size in Nd0.67Sr0.33MnO3 on Microstructural, electrical-, magneto-transport and magnetic behaviors for magnetic sensor application. Three nanocrystalline powders of Nd0.67Sr0.33MnO3 are synthesized through chemical route “Pyrophoric Reaction Process” and calcined for 5 hrs at temperatures 6500C, 7500C, and 8500C. XRD patterns indicate that all the synthesized powders have pseudo-cubic perovskite structure without any secondary impurity phase. By Debye Scherrer formula calculated crystallites size for three Nd0.67Sr0.33MnO3 powder(~ 30 nm, 40 nm and 54 nm for calcination temperatures 6500C, 7500C, 8500C respectively). TEM micrographs show that the average particle sizes are in nanometric regime (φ ~ 30-50 nm). In AC susceptibility and resistivity studies we observed that there is an almost constant Curie temperature (TC) and gradual decrease of metal-insulator transition temperature (TP) (from 200 K to 129 K). We observed enhancement of %MR at 78 K for Nd0.67Sr0.33MnO3 (~ 24 %) more than that of La0.7Sr0.3MnO3 (~21%), both calcinated at 8500C. Interestingly we got very small values of positive magneto-resistance (~ 2.5 %) for all samples due to grain boundary effect. Within the magnetic field range 0 to 1000 Oe magneto-resistances initially increases up to a positive values and then decreases very fast due to spin polarized tunneling suitable for magnetic sensor. Variable range hopping transport comes to explain this transport at low temperature. Phenomenological demonstration with the help of core-shell structure of nanometric grains with the grain size as the parameter can explain the above experimental achievement. Also effects such as (a) breaking of Mn-O-Mn paths at the grain surface, (b) deviation of stoichiometric composition at the grain surface, (c) termination of the crystal structure at the grain boundaries, and (d) dislocation of at the grain boundaries etc contributes on the effect of enhancement of %MR. H. Y. Hwang, S-W. Cheong, N. P. Ong and B. Batlogg, Phys. Rev. Lett. 77, 2041 (1996). P. Roychaudhury, T. K. Nath, A. K. Nigam and R. Pinto, J. Appl. Phys. 84, 2048 (1998). C. Zener, Phys. Rev. 82, (1951) 403 Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, edited by C. N. Rao and B. Baveau (World Scientific, Singapore, 1998) A. P. Ramirez, J. Phys.: Condens. Matter, 9 (1997) 8171 Gupta S, Ranjit R, Mitra C, Raychaudhuri P and Pinto R Appl. Phys. Lett. 78, 362 (2001)
10:30 AM - K5.4
A Study of the Relation of Magnetism and the Metal-Insulator Transition in SrRuO3 as a Function of Thickness.
Wolter Siemons 1 2 , Jing Xia 2 , Gertjan Koster 1 , Dave Blank 1 , Malcolm Beasley 2 , Aharon Kapitulnik 2 Show Abstract
1 TNW, University of Twente, Enschede Netherlands, 2 GLAM, Stanford University, Stanford, California, United States
Thin films of the perovskite SrRuO3 have attracted considerable interest due to their low room temperature resistivity and small lattice mismatch with a range of functional oxide materials. In addition they exhibit “bad metal” behavior, which is one of the unsolved problems of contemporary condensed matter physics, show signs of electron correlation in the material, and are ferromagnetic below a temperature of 160 K. For these reasons thin films of SrRuO3 are of great current interest, both from the materials science and the physics point of view.Indeed, recent studies from several groups emphasized the interplay between itineracy and ferromagnetism in thin films of SrRuO3. In particular, Toyota et al. presented results on the thickness dependence of metallicity and ferromagnetism in SrRuO3 films concluding that a metal-insulator transition (MIT) accompanied by the disappearance of ferromagnetism occurs in these films at a critical film thickness of 4 to 5 monolayers (ML). The observation was supported by photoemission measurements showing that below that critical thickness no finite density of states is found at the Fermi level, and by resistivity measurements showing insulating behavior below that thickness.While Toyota et al.  presented a complete set of measurements with a definite conclusion, the quality of the samples near the critical thickness suggests that much of the behavior observed is due to the island-like growth and coalescing of three-dimensional patches. Thus, a complementary study, in which the films grow layer-by-layer for at least the first 10 layers, is needed to establish the relation between ferromagnetism and itineracy in ultrathin films of SrRuO3. In addition, a more direct probe of ferromagnetism is needed to establish the nature of the magnetic moment, its strength and anisotropy, as well as its strength below the critical thickness.In this presentation we present new results on the MIT in ultrathin SrRuO3 films and the magnetic properties of such films. We show that in homogeneous films of SrRuO3 a MIT occurs at a critical thickness below 4 monolayers (ML), below which ferromagnetism is undetectable using an ultra-sensitive SAGNAC magneto-optic interferometer . While TC drops rapidly below ~10 ML, the size of the moment remains unchanged from its 1.6 μB in thick films. Examination of the transport properties of the measured films shows an increase in the sheet resistance with decreasing thickness. At 4 ML the extrapolated low-temperature sheet resistance is ~7 kΩ (about a quarter of the two-dimensional quantum of resistance), jumping to several megaohm just below the transition. D. Toyota, I. Ohkubo, H. Kumigashira, M. Oshima, T. Ohnishi, M. Lippmaa, M. Kawasaki, and H. Koinuma, Journal of Applied Physics 99, 08N505 (2006). J. Xia, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, Applied Physics Letters 89, 062508 (2006).
11:15 AM - K5.5
Field Sensitivity Studies of PLD Grown Manganite based Nanostructured p-n Junction Devices.
Deelip Kuberkar 1 , Prashant Vachhani 2 , Jaysukh Markna 3 , Dattatray Phase 4 , Ramjanay Choudhary 5 Show Abstract
1 Department of Physics, Saurashtra University, Rajkot, Gujarat, India, 2 Department of Physics, Saurashtra University, Rajkot, Gujarat, India, 3 Department of Physics, Saurashtra University, Rajkot, Gujarat, India, 4 Thin Film Deposition Lab, UGC-DAE CSR, Indore, Madhya Pradesh, India, 5 Thin Film Deposition Lab, UGC-DAE CSR, Indore, Madhya Pradesh, India
11:30 AM - K5.6
Local Crystal Structure Modifications in Pulsed Laser Deposited Colossal Magnetoresistive Oxide Thin Films.
Mehmet Sahiner 1 , Wiqar Shah 1 , Marc Aranguren 1 , Jeffrey Serfass 1 , Joseph Woicik 2 Show Abstract
1 Physics Dept., Seton Hall University, South Orange, New Jersey, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
The local structure around the manganese atom is probed by extended x-ray absorption spectroscopy (EXAFS) measurements in pulsed laser deposited thin films of La1-xCaxMnO3 (x=0.12-0.53). The thin films were deposited on various single crystal oxide substrates and also on Si(100) and Ge(100) semiconductor substrates at various growth conditions. The effect of the lattice parameters and crystal structure of the substrate on the local structural modifications around Mn atoms were probed by EXAFS. The local electronic structural responses, such as the Mn d-orbital occupancy and the orbital symmetries of the doped holes, to the systematic Ca substitutions were monitored by x-ray absorption near-edge spectroscopy (XANES) measurements. All the x-ray absorption experiments were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. By detailed EXAFS theoretical modeling for the possible local structures and the least square fitting to the EXAFS data using these models, the overall substrate and the Ca concentration effects will be discussed. This work is supported by NSF Award #:DMI-0420952 and Research Corporation Award #:CC6405
11:45 AM - K5.7
Flexomagnetic Effect in Antiferromagnetic Antiperovskites.
Renat Sabirianov 1 , Pavel Lukashev 1 Show Abstract
1 Physics, University of Nebraska, Omaha, Omaha, Nebraska, United States
We present the observation of appearance of net magnetization in antiferromagnetic antiperovskites, such as Mn3GaN, due to the gradient of applied strain, i.e. flexomagnetic effect. To simulate the gradient of the strain we build 40-atom Mn24Ga8N8 supercell (4 unit cells in x-direction, 2 unit cells in z-direction and 1 unit cell in y-direction) and introduced small atomic shifts of up to +/-1% of the lattice constant one end in such a way that 4 domains of the flexed crystal appear. This results in the symmetry breaking of the triangular (R3m) magnetic ground state of Mn atoms and appearance of net magnetization in the (0,-1,1) direction of original cubic lattice. We also studied the magnetization as a function of flexion. The dependence is of parabolic nature and the magnetization reaches its maximum of about 0.8 µB at the maximum strength of the gradient of 0.005 which we considered. Larger values of the flexion lead to the larger rotations of LMMs from their equilibrium positions as well as change in its magnitude. Nanostructured materials have usually large gradient of stresses due to the surface tension or interface bonding. Thus, we expect flexomagnetic effect to be important in these materials. The control of such gradients provides unique opportunity to tailor magnetic properties of nanostructures.
12:00 PM - K5.8
Ferromagnetism in Disordered Dilute Magnetic Semicondustors Applied to GaMnAs.
Antonio Ferreira da Silva 1 , Rogerio da Silva Neves 1 , Ram Kishore 2 Show Abstract
1 Instituto de Física, Universidade Federal da Bahia, Salvador, Bahia, Brazil, 2 LAS, Instituto Nacional de Pesquisas Espaciais -INPE, São José dos Campos, São Paulo, Brazil
The discovery of the ferromagnetic transition in temperature TC Ga1-xMnxAs exceeding 100K, is much of interest to investigate the physical properties of dilute magnetic semiconductor (DMS) in view of the potential application in the spintronic devices . In our study, we consider that the ions Mn+2 impurities replacing the Ga sites in GaAs semiconductor have a null local moment angular, momentum for spin S = 5/2 and holes moderately bounded. Based on the mean field approach we studied the ferromagnetism phenomenon in this semiconductor, considering the holes in an impurity band and your exchange interaction with the spin of Mn. We calculate the spontaneous magnetization and evaluate the critical temperature TC in function the coupling constant of magnetization, the impurity concentration and compensation. In order to calculate the mean configuration of spins of impurities Mn+2 we used a formalism proposed for spatial disorder . The best results, showed for intermediate and high concentrations (metallic phase systems), agree well with experimental results and other theoretical results , indicating a behavior character ferromagnetic phases. Although there is a great uncertainty in determining the exact value of the coupling magnetic constant, we may consider that our method is very useful in understanding the physical properties this material.  A. H. Macdonald, P. Schiffer and N. Samarth, Nature Materials, 4, 195 (2005).  A. Ferreira da Silva, R. Kishore and I. C. da Cunha Lima, Phys. Rev. B 23, 4035 (1981). M. Berciu and R. N. Bhatt, Phys. Rev. B 69, 045202 (2004). A. Chattopadhyay, S. Das Sarma, and A. J. Millis, Phys. Rev. Lett. 87, 227202 (2001).
12:15 PM - K5.9
Anisotropic Magnetoresistance and Magnetic Anisotropic Studies of Micrometer Constricted (Ga,Mn)As Devices.
Uzma Rana 1 Show Abstract
1 , University of Cambridge, Cambridge United Kingdom
K6: Spintronic Applications
Wednesday PM, December 03, 2008
Room 206 (Hynes)
2:30 PM - **K6.1
Magnetic Nanowires and Rings with Tailored Spin-Wave Properties.
Jan Podbielski 1 , Bernhard Botters 2 , Fabian Giesen 1 , Sebastian Neusser 2 , Jesco Topp 1 , Detlef Heitmann 1 , Dirk Grundler 2 Show Abstract
1 Institut fuer Angewandte Physik, Universitaet Hamburg, Hamburg Germany, 2 Physik-Department, Technische Universitaet Muenchen, Garching b. Muenchen Germany
Spin dynamics in magnetic nanomaterials currently attracts considerable interest. It concerns basic phenomena and research ranging from spin-wave confinement effects observed in magnetic wires to the emerging field of magnonics where one explores magnon wave guides, spin-wave logic and magnonic crystals. We have performed experiments on permalloy nanowires and rings. Nanorings constitute an intriguing prototype system to investigate, both, nanoscale magnetism and spin dynamics properties on a profound level. [1,2] By varying the diameter and position of the central hole we tailored the magnetic configurations and controlled deterministically the circulation direction of the so-called vortex state. [3-5] Broadband spin-wave spectroscopy using an optimized coplanar waveguide technique revealed a rich spectrum of characteristic spin-wave eigenmodes. These were hysteretic and reflected confinement [2,4], localization  and interference effects  depending on the magnetic history. By increasing the microwave excitation power we observed non-linear phenomena. Due to large-angle spin precession microwave-assisted switching between vortex and onion states occurred.  In straight nanowires we recently found an additional mechanism for spin-wave confinement using a zig-zag-shaped magnetization configuration.  This opens further perspectives in spin-dynamics research. Financial support by SFB 668 and the Excellence Cluster "Nanosystems Initiative Munich (NIM)" is gratefully acknowledged.References: J. Podbielski, F. Giesen, M. Berginski, N. Hoyer, and D. Grundler, Superlattices and Microstructures 37, 341 (2005). F. Giesen, J. Podbielski, T. Korn, M. Steiner, A. van Staa, and D. Grundler, Appl. Phys. Lett. 86, 112510 (2005). F. Giesen, J. Podbielski, B. Botters, and D. Grundler, Phys. Rev. B 75, 184428 (2007). D. Grundler, F. Giesen, and J. Podbielski, Physics in Canada 63, 63 (2007). F. Giesen, J. Podbielski, and D. Grundler, Phys. Rev. B 76, 014431 (2007). J. Podbielski, F. Giesen, and D. Grundler, Phys. Rev. Lett. 96, 167207 (2006). J. Podbielski, D. Heitmann, and D. Grundler, Phys. Rev. Lett. 99, 207202 (2007). J. Topp, J. Podbielski, D. Heitmann, and D. Grundler, submitted.
3:00 PM - **K6.2
Spin Transfer Torque in Nanoscale Magnetic Devices.
Dan Ralph 1 Show Abstract
1 Physics Department, Cornell University, Ithaca, New York, United States
I will discuss experiments which probe the spin-transfer torque effect in magnetic devices, whereby the transfer of angular momentum from a spin-polarized current can be used to apply a torque to a nanoscale magnet, as an alternative to using applied magnetic fields. Spin-torque-driven ferromagnetic resonance (ST-FMR) experiments on individual nanoscale devices allow direct measurements of both the strength and direction of the spin transfer torque and also the magnetic damping. In MgO-based magnetic tunnel junctions, we find that the strength of the spin-torque component in the plane defined by the magnetizations of the two electrodes is in quantitative agreement with theory, and there is also a significant out-of-plane component of torque whose bias dependence is quadratic. In all-metal spin valve devices, no bias-dependent out-of-plane spin torque is measured; this component is smaller than 1% of the in-plane torque. The damping measured by ST-FMR at room temperature is in good agreement with values for bulk films. However at cryogenic temperatures the damping in nanoscale devices can increase above the bulk value due to the effects of oxidation at the device sidewalls. I will also report recent results related to improving the performance of magnetic memory elements and microwave-frequency oscillators in which spin torque is used to drive the magnetic dynamics. This work was done in collaboration with the Cornell Nanomagnetics Group, Jonathan Sun (IBM), John Slonczewski (IBM), and Jordan Katine (Hitachi GST).
3:30 PM - K6.3
The Effect of Magnetic Coupling on Electronic Transport in a Single Redox Molecule.
Brian Premerlani 1 , G. Qian 1 , K. Lewis 1 Show Abstract
1 Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
We determine how coupling between an external magnetic field and self-assembled molecules govern the molecules' transport properties. The molecule, diphenylporphine, contains a macrocycle ring that theoretically supports ring-currents in the presence of a magnetic field perpendicular to the plane of the molecule. It is expected that such currents will affect the conductance properties of the molecules. In particular we focus on how a magnetic field will affect the molecules’ conformation and conformational transitions, as well as the magnetic anisotropy. However, to explore these and other possibilities one must be able to isolate a single redox molecule and apply a controlled magnetic field. Many commercially available STM systems contain components responsive to magnetic fields, and so are not suitable for such experiments. We propose to use a commercial scanning tunneling microscope (STM) capable of isolating a single molecule via a break-junction technique. The STM will be modified to support a magnetic field. Advantages of using STM are that molecules can be imaged to verify conformal transitions and one can have active control over the number of molecules contacted. Experiments will be conducted in solution at room temperature. We will use this preliminary setup to study diphenylporphine. It is a redox molecule with two conducting states associated with different conformations. The states will be accessed through an electrochemical gate. During the experiment the molecules are in solution and exposed to the STM tip and a gold substrate. The magnetic field will be applied in plane with the substrate, perpendicular to the direction of tunneling current via an electromagnet. Under different conformal states and with various field strengths and directions, current-voltage (I-V) and dI/dV measurements will show the conductance properties. By controlling the direction of the magnetic field, anisotropic spin effects on electron transport will also be studied. If the molecules studied here demonstrate favorable or unique electron transport properties in response to magnetic fields (i.e. increased conductance as a result of excitation via ring-currents), it can be useful for the fabrication of novel components for molecular computation. Additionally, because of the molecules’ ability to remain in one conducting state for long periods of time (on the order of minutes), characterization of the different states can help create a foundation for the development of single-molecule data storage.
3:45 PM - K6.4
Magnetoresistance and Spin Transfer Torque in Arrays of Co/Cu Multilayer Nanowires.
Xiaobo Huang 1 , Liwen Tan 1 , Bethanie Stadler 1 Show Abstract
1 , U Minnesota, Minneapolis , Minnesota, United States
Multilayered Co/Cu nanowires with diameters as small as 10nm have been fabricated in arrays using anodic aluminum oxide templates. As nanowire size decreases, the magnetic properties of Co/Cu nanowires will be affected by boundary scattering, quantum effects and magneto-hydrodynamics during the deposition process. However, smaller diameter nanowires are good candidates for fundamental research of the electron spin transfer, current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) sensors and high density magnetic recording media. The anodic aluminum oxide (AAO) templates used here were formed using a two-step anodization process, and the pore diameters could be varied from 10-60nm by changing the anodization conditions. As the diameter of the AAO nanopores decreased, the distance between the nanopores also decreased. Therefore, once the nanowires are grown in the pores, the magnetostatic energy between the wires and the layers became complicated. The magnetic properties were measured using vibrating sample magnetometry, and the easy axis of each Co layer changed with variations of either the Co thickness itself or of the Cu interlayers. This was the result of the competition between magnetostatic energy and demagnetization energy. When the thickness of Cu layers was less than 3 nm, the easy axis of each Co layer was parallel to nanowires. When the thickness of Cu layer was larger than 7 nm, the easy axis of each Co layer was perpendicular to nanowires. Magnetoresistance (MR) was measured using a physical properties measurement system (PPMS), and the largest MR (11%) was found in Co (27 nm)/Cu (5 nm) multilayer nanowires with 10 nm diameter (applied field perpendicular to nanowires axis) and 10% (applied field perpendicular to nanowires axis) at ambient temperature. The actual MR (= deltaR/R) of the multilayers can be calculated as 33% by subtracting the resistance of the Cu leads on either side of the multilayers from the denominator. Shorter wires are currently under construction to avoid this effect. Finally, spin transfer torque was measured in the samples using a head-tester. The resulting curves were parabolic from heating due to high current densities. For the 10nm diameter nanowires, these curves were overlaid with small jumps (~2%) that were attributed to individual layers flipping and larger jumps (13.8%) that can be modeled as collective flips between interacting Co layers. For larger diameter nanowires, the individual flips were not obvious, and the curves were similar to typical STT results. The ability to construct large area arrays of these small STT structures makes these first results exciting for eventual use in MRAM.
4:30 PM - K6.5
Magnetic Field and Current Induced Remanent State Change in a Layered Pseudo Spin-valve Ring.
Chung Hee Nam 1 , Bryan G Ng 1 , Fernando J Castano 1 , Caroline A Ross 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
4:45 PM - K6.6
δ-Manganese Gallium on Gallium Nitride: a Magnetically Tunable Spintronic System.
Kangkang Wang 1 , Abhijit Chinchore 1 , Wenzhi Lin 1 , Jeongihm Pak 1 , Arthur Smith 1 , Kai Sun 2 Show Abstract
1 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Ferromagnetic metal/semiconductor layered structures are of great interest due to their potential for novel spintronics applications such as blue spin light-emitting diodes. It has been reported that ferromagnetic δ-MnGa can be grown epitaxially on top of Ga-polar GaN(0001) with controllable magnetism by adjusting slightly the Mn:Ga atom concentration. Here we are interested in further exploring substrate and polarity dependencies of the magnetic tunability. We have performed a comparative growth study of MnGa grown on various substrates, including N-polar and Ga-polar w-GaN(0001) and also cubic ScN(001). Growth experiments are carried out in a UHV chamber equipped with Ga, Sc, and Mn effusion cells and an rf (N2)-plasma source. Samples are characterized by in-situ reflection high energy electron diffraction and scanning tunneling microscopy, and by ex-situ atomic force microscopy, high-resolution transmission electron microscopy, and x-ray diffraction. For GaN, Ga-polar w-GaN surface promotes quicker MnGa-GaN interface formation compared to N-polar w-GaN and eventually leads to a better crystalline MnGa film. In either case, a (111)-oriented MnGa film results. Different interface bonding configurations as well as different surface chemical environments could be responsible for the difference observed. For ScN(001), growth results in the same phase of MnGa but with (001)-orientation and with a strong 3x ordering along  which may be determined by the Mn concentration. This work is supported by the National Science Foundation. S.A.Wolf et al, Science 294, 1488 (2001). E.Lu et al, Phys. Rev. Lett. 97, 146101 (2006).
5:00 PM - K6.7
Magnetic Nanocomposite Films with Tunable Permeability forLeft-Handed Metamaterials.
Satoshi Tomita 1 , Chiharu Mitsumata 2 , Kensuke Akamatsu 3 4 , Hidemi Nawafune 3 Show Abstract
1 , Nara Institute of Science and Technology, Ikoma, Nara, Japan, 2 , Hitachi Metals Ltd., Kumagaya, Saitama, Japan, 3 , Konan University, Kobe, Hyogo, Japan, 4 , Japan Science and Technology Agency, Kawaguchi, Saitama, Japan
We have experimentally succeeded in embedding metallic Ni nanodots into polymer thin films in a controlled manner. A significant feature was a decrease in the interdot distance with maintaining a fixed diameter at the level of several nanometers. This was not achieved through particle manipulation, but by shrinking the polymer matrices. We have constructed a numerical model, in which magnetic dipole interactions between the dots are considered explicitly, and carried out micromagnetic calculations. Magnetic permeability of the nanocomposite films around the electron magnetic resonance was evaluated. The negative permeability was obtained in the region below the resonance. The value of permeability was strongly influenced by the volume fraction of Ni dots. As the volume fraction increased, the negative peak of permeability became small. The lower limit of volume fraction for the negative permeability was 3.5 % for Ni dots having an average diameter of 8 nm. The negative permeability was also affected by the dispersion of a dot diameter. As deviations of diameter increased, the negative peak of permeability drastically shrunk. In comparison with the positive peak of permeability, the negative permeability was unstable. However, the region of applied field, where the permeability was negative, became broader with increasing the deviation. This may open the door to broad band left-handed metamaterials in microwave regions.
5:15 PM - K6.8
Visible-Light Scattering on Submicron-Scale Hollow Spheres of Magnetite and Titania.
Kunio Awaga 1 , Quan-Lin Ye 1 , Yoshihiko Kondo 1 , Hirofumi Yoshikawa 1 , Tatsuo Mori 1 , Shunji Bandow 2 Show Abstract
1 , Nagoya University, Nagoya Japan, 2 , Meijo University, Nagoya Japan
The preparation of sub-micron hollow spheres is a promising approach towards advanced magnetic and/or optical materials. We prepared magnetite hollow spheres with a diameter of 650 nm and a shell thickness of 40 nm, using a template method. The obtained particles exhibited a green color, in contrast to the black color of the dense particles with a similar size. This green color was interpreted in terms of the Mie scattering on the inhomogeneous, low-density structure of the magnetite hollow spheres. Thin films of the hollow spheres were found to be magnetically isotropic, reflecting the isotropic structure of the individual particles.We also prepared submicron-scale spherical hollow particles of anatase TiO2, with a diameter of 490 nm and a shell thickness of 30 nm. The dye-sensitized solar cell (DSC) using an electrode of the present TiO2 hollow spheres, exhibited a rather low per-area efficiency (1.26%) due to a low area density of TiO2 on the electrode, but the per-weight efficiency was 2.5 times higher than those of the conventional DSCs of TiO2.
5:30 PM - K6.9
Magnetocaloric Properties of Co/Cr Superlattices.
Tathagata Mukherjee 1 , Sarbeswar Sahoo 1 , Ralph Skomski 1 , David Sellmyer 1 , Christian Binek 1 Show Abstract
1 Physics & Astronomy, University of Nebraska, Lincoln, Lincoln, Nebraska, United States
Tailored magnetocaloric functionality is studied in magnetic superlattices of ultrathin Co and Cr films. Ultrathin Co/Cr heterostructures are grown by Molecular Beam Epitaxy at a base pressure below 1x10-10 mbar. Our experiments aim to realize artificial antiferromagnetic multilayers with tailored transition temperatures and maximized isothermal entropy changes. Recently, antiferromagnetic bulk materials have shown promising entropy changes when field-induced metamagnetic transitions were involved. However, bulk materials may suffer from thermodynamic constraints which limit the flexibility of tailoring magnetocaloric properties. In addition, current bulk systems with giant magnetocaloric effects require magnetic first-order phase transitions which are typically accompanied by structural transitions. Cycling through the latter gives rise to material fatigue and limits the operation frequency due to enhanced magnetization relaxation times. We intend to overcome both issues avoiding structural transitions. Our experiments focus is on Co/Cr superlattices, taking advantage of thickness-controlled Curie temperatures of the ferromagnetic Co constituents. Moreover, Cr is a prototypical spacer material for the realization of antiferromagnetic interlayer coupling. The coupling strength is an oscillating function of the Cr thickness and can be experimentally tailored. The growth of ultrathin multilayers which brings the TC of individual Co films down to room temperature is a challenge because imperfect layer-by-layer growth and intermixing at the Co/Cr interfaces can hinder the evolution of a well-defined periodic superstructure. Small-angle X-ray diffraction of the superlattices shows a peak indicating that Co/Cr periodicity has been achieved. Here we present magnetic and magnetocaloric data of a series of multilayer structures with small but important variations of the nominal Co film thickness. These samples have been characterized with the help of SQUID magnetometry. Antiferromagnetic interlayer coupling is evidenced from isothermal magnetization hysteresis loops. The exchange coupling constants are used to model the temperature dependence of the magnetization of the sample in the presence of applied magnetic fields. The temperature dependence of the isothermal magnetic entropy is obtained from magnetization isotherms and alternatively from a detailed study of M vs. T using a dense grid of applied magnetic fields in accordance with the integrated Maxwell relation. Estimating the heat capacity of the superlattice allows calculating the adiabatic temperature change in the vicinity of room temperature.
Financial support by NCESR, Teledyne Isco Inc, NSF through Career DMR-0547887, MRSEC DMR-0213808, NRI and Nebraska EPSCoR is gratefully acknowledged, as is the support of R. Skomski and D.J. Sellmyer by the U.S. DOE.
K7: Poster Session
Thursday AM, December 04, 2008
Exhibition Hall D (Hynes)
9:00 PM - K7.1
Oxa-azaacenes: Structures with Triplet Ground States and Promising Candidates for Purely Organic High-spin Systems.
Shadi Amiri 1 , Peter Schreiner 1 Show Abstract
1 Organische Chemie Institut, Justus-Liebig-Universität Giessen, Giessen, Hessen, Germany
Paramagnetic single molecules represent attractive building blocks for the development of ferro- and ferrimagnetic materials. Towards the goal of preparing stable, neutral open-shell systems at first we investigated the electronic structures of a novel series of p-phenyl substituted 3,5,7,9-hexaazaacridines and 3,5,7,9-hexaazaanthracenes derivatives, which have azaacene as their basic structures.The effects of substitution on the molecular electronic properties was probed computationally [B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p)]. While the experimentally prepared structures already have small (about –25 kcal/mol) singlet–triplet energy gaps, systems with even smaller (–8 kcal/mol) singlet–triplet energy separations can be realized through systematic variation of the substituent numbers, types, and patterns. Hexaazaanthracenes show generally smaller singlet–triplet energy gaps than hexaazaacridines. The larger ΔEST of the latter is in line with their somewhat more twisted topology, greater total dipole moment (more zwitterionic character), greater dipole moment of the first excited state relative to the ground state, and a larger λmax in the UV absorption spectra. Nitrogen-bonded substituents show larger effects than those bonded to carbon, owing to the general molecular orbital structure of these systems that favors the involvement of π-delocalization via the nitrogen atoms. The ΔEST’s in N-bonded pull-pull electronic systems are smaller than in pull-push and push-push models. As conclusion, the Nitrogen-bonded σ- and π-acceptor substituents that cause positive inductive and mesomeric effects as well as carbon-bonded σ-donor substituents make substituted hexaazaanthracenes materials with nearly degenerate singlet-triplet energy separations.Based on the results for studied azaacenes, in the next step we could design the basic structure oxa-azaacenes with two zigzag-edged graphene ribbons joined by long CC cross-linked together, for that the triplet ground state is about 25 kcal/mol more stable than singlet closed-sell state. This triplet ground state could be obtained by breaking the aromaticity through using single CC bonds at the termini sides of the both out rings. P. Langer, S. Amiri, A. Bodtke, N. N. R. Saleh, K. Weisz, H. Gorls, P. R. Schreiner, J. Org. Chem. 2008, ASAP Article. DOI: 10.1021/jo8005123
9:00 PM - K7.10
Structural and Magnetic Properties of B-doped FePt Thin Films.
Haibao Zhao 1 , Hao Wang 1 , Das Anirban 2 , Racine Mike 2 , Imakawa Makoto 2 , Jian-Ping Wang 1 Show Abstract
1 Electrical & Computer Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 2 Thin Film Materials Division, Heraeus Inc., Chandler, Arizona, United States
9:00 PM - K7.11
The Effect of Carbon Black Additions on Electromagnetic Wave Absorption of Fe73Si16B7Nb3Cu1 Base P/M Sheets.
Sun-I Kim 1 , Keun Yong Sohn 1 , Won-Wook Park 1 Show Abstract
1 nano engineering , inje university, Gimhae, Gyeongnam, Korea (the Republic of)
9:00 PM - K7.12
Electromagnetic Wave Absorption Properties of Fe-based Nanocrystalline P/M Sheets Mixed with Carbon Black and BaTiO3 Additives.
Mi-Rae Kim 1 , Sun-I Kim 1 , Keon Sang Cho 1 , Su Jung Woo 1 , Keun Yong Sohn 1 , Won Wook Park 1 Show Abstract
1 Department of Nano System Engineering, Inje University , Republic of Korea Korea (the Republic of)
9:00 PM - K7.13
Synthesis and Magnetic Properties of Nanocomposite Magnets Self-organized from Fe-B-Nd-Nb Metallic Glasses.
Ryuji Tamura 1 2 , Shin-ichi Kobayashi 1 , Tomokazu Fukuzaki 2 Show Abstract
1 Dept. of Mater. Sci. & Tech., Tokyo University of Science, Chiba Japan, 2 Polyscale Technology Research Center, Tokyo University of Science, Chiba Japan
Recently, high glass forming ability with a critical diameter of 4 mm has been reported in Fe-B-Nd-Nb system by Zhang et al. In addition, they have obtained an extremely high coercivity, as high as 1100 kA/m,after self-organization of uniformily distributed particles of Nd2Fe14B hard phase on a nano scale. However, the remanence of the nanocomposite is not high enough (0.44 T) resulting in a moderate value (33 kJ/m3) of the energy product, mainly because of the poor Fe concentration, i.e., 64.3 at.%Fe. In this paper, the glass forming ability of the Fe-B-Nd-Nb systems has been investigated in an Fe-rich region in order to raise the Fe concentration of the metallic glass. The magnetic properties of nanocomposite magnets self-organized from metallic glasses have been systematically studied in order to optimize the energy product (BH)max by improving the remanence as well as by tuning the grain size on a nano scale. Zhang et al., Scripta Mater. 56 (2007) 943
9:00 PM - K7.14
Spin Polarised Study of Size-Controlled Nanoclusters.
Charles Woffinden 1 , Andrew Pratt 1 , Steve Tear 1 , Chris Binns 2 Show Abstract
1 Department of Physics, University of York, York United Kingdom, 2 Department of Physics and Astronomy, University of Leicester, Leicester United Kingdom
Metastable de-excitation spectroscopy (MDS) is a technique that probes the surface electronic structure of materials using metastable atoms to produce an electron energy spectrum characteristic of the surface density of states . As the metastable atoms do not penetrate into the structure, only surface states are probed. In spin polarised MDS (SPMDS) these metastable atoms are spin polarised, which enables the magnetic properties of a surface to be probed by comparing the electron emission yield for opposite spin states. This yields a value for the spin asymmetry of the surface. SPMDS is designed to provide surface magnetic information with no contribution from bulk effects.In magnetic nanoclusters, where the surface-to-volume ratio is large, surface magnetic effects are expected to dominate . SPMDS with its surface sensitivity is ideally placed to identify the contribution from the surface to the magnetic properties of clusters. It will also provide us with an idea of the spin polarisation of the nanocluster surface.Clusters under investigation are both single elemental clusters and binary clusters consisting of a ferromagnetic core with an antiferromagnetic surrounding shell. Preliminary results for SPMDS of mass-selected, magnetic clusters will be presented. Y. Harada, S. Masuda and H. Ozaki, Chem. Rev. 97, 1897 (1997) C. Binns, J. Nanosci. Nanotechno. 1, 243 (2001)
9:00 PM - K7.15
One pot Synthesis L10 FePt and L10 CoPt Nanostructures and their Assembly.
Bratindranath Mukherjee 1 , Ravishankar Narayanan 1 Show Abstract
1 Materials Research Centre, Indian Institute of Science, Bangalore India
Synthesis of hard magnets like FePt and CoPt nanostructures with controlled shape and atomic composition in L10 phase and their assembly in proper substrates is of tremendous interest due to its potentialapplication in high density magnetic storage and sensitive magnetotransport devices. In this work we synthesized nanodots, nanowires and rod shaped FePt, CoPt nanostructures in L10 phase in a single step using awell established chemistry based on high temperature reduction of theprecursors and coated them in-situ on a variety of substrates like FTO glass and silicon. Microstructural and magnetic characterization has been donusing high resolution electron microscopy (HREM) and SQUID respectively.
9:00 PM - K7.16
Perpendicular Magnetic Anisotropy of Ni81Fe19/Fe50Pt50 Exchange Spring Films.
Nyun Jong Lee 1 , Jae Young Ahn 1 , Sun Hee Kim 1 , Anny Michel 2 , Guy Schmerber 3 , Tae Hee Kim 1 Show Abstract
1 Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 Physics, Université de Poitiers, Futuroscope-Chasseneuil France, 3 GEMME, Institut de Physique et Chimie des Matériaux de Strasbourg, Strasbourg France
FePt is a leading candidate for future extremely high density magnetic recording media, due to its very high values of KU. However, this very high anisotropy gives films with very high coercivity, too high for reliable recording using conventional write heads. Hard magnetic/soft magnetic bilayers could be an excellent solution to tailor their magnetic properties according to the specific requirements, thereby enabling efficient recording. In this work, we investigated the thickness-dependent magnetic response of exchange-coupled Ni81Fe19/Fe50Pt50 bilayers. The hard magnet films, such as FePt, were prepared beyond the soft magnetic permalloy layer, Ni81Fe19 (Py), of a material having an fcc crystalline structure by using UHV-MBE deposition technique. In order to have the fcc Py films, the epitaxial MgO(001) film was grown as a buffer layer beyond the bare Si(100) substrate. Structural analysis by x-ray diffractometry and transmission electron microscope (TEM) showed the fairly well-crystallized face-centered-tetragonal FePt films beyond the Py film. The out-of-plane coercivity of the FePt films decreased dramatically as the soft Py film becomes thicker. Room-temperature magnetoresistance measurements reveal that perpendicular magnetic anisotropy exists for these films. Our results could open intriguing possibility for media applications for high density magnetic recording. This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Republic of Korea and supported by KOSEF grant R01-2006-000-11227-0.*correspondence author: firstname.lastname@example.org
9:00 PM - K7.17
Magnetic Properties on Nanocrystalline BaFe12O19 Ferrite.
Hua Yang 1 , Yaowen Li 1 Show Abstract
1 , Jilin University, Changchun China
9:00 PM - K7.2
Magnetic and Transpot Properties of Ni Doped Pr0.5ca0.5mn1-Xnixo3-Δ Manganites.
Liora Damari 1 , Joshua Pelleg 1 , Gad Gorodetsky 2 , Charles Koren 2 , Vladimir Markovich 2 , Alex Shames 2 , Xiadong Wu 2 , Dmitry Mogilyansky 3 , Ivan Fita 4 Show Abstract
1 Materials Engineering , Ben Gurion University of the Negev, Beer Sheva Israel, 2 Department of Physics, Ben Gurion University of the Negev, Beer Sheva Israel, 3 Applied Research Institute, Ben Gurion University of the Negev, Beer Sheva Israel, 4 Low Temperature Magnetism, Donetsk Institute for Physics & Technology, Donetsk Ukraine
9:00 PM - K7.20
Magnetic Properties and Thermal Stability of SmCo/Fe Nanocomposite Powders Produced by Ball Milling.
Chuanbing Rong 1 , Yuzi Liu 1 , Matthew J Kramer 2 , J.Ping Liu 1 Show Abstract
1 Department of Physics, University of Texas at Arlington, Arlington, Texas, United States, 2 Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, Ames, Iowa, United States
Mechanically alloyed hard/soft nanocomposite magnetic materials have attracted great attention in the past decades.[1,2] However, the relation between the magnetic properties, microstructure and phase structure needs to be further studied. In this work, powder mixtures of SmCo5 + x a-Fe (x=0-30 wt%) were mechanically ball milled for 2-10 hours followed by post-annealing at different temperatures. The crystallization process and thermal stability of the as-milled powders were studied using a differential scanning calorimetry device. The structure was determined by X-ray diffraction, transmission electron microscopy with element mapping and energy dispersive spectrum (EDS). The magnetic properties were measured by a SQUID magnetometer. Nanocrystalline SmCo5 and SmCo5/Fe powders with grain size around 10-15 nm were obtained by optimized annealing. For SmCo5 single phase, it was found that high-temperature annealing led to the decomposition of SmCo5 phase into Sm2Co7 phase and an unidentified phase. For the SmCo5/Fe two-phase powders, high-temperature annealing led to the formation of Sm2(CoFe)17 phase due to the atomic diffusion between the SmCo phase and Fe grains. The decomposition and atomic diffusion were more pronounced for powders milled for longer time. EDS element mapping analysis revealed that the Sm distribution is not homogenous after high temperature annealing, which may be attributed to the high-temperature decomposition. Henkel plots confirmed the decrease of exchange coupling strength with increasing annealing temperature and ball-milling time. The magnetic properties of the ball-milled powders were then optimized based on the structural and phase analyses. The maximum (BH)max about 17.3 MGOe was obtained for the powders with 20-25% a-Fe by ball milling for 4 hours followed by 550 oC annealing for 30 minute..J. Wecker, M. Katter, and L. Schultz, J. Appl. Phys. 69, 6058 (1991);.J. Zhang, S.Y. Zhang, H.W. Zhang, and B.G. Shen., J. Appl. Phys. 89, 5601 (2001)
9:00 PM - K7.21
Theory and Modeling of Anhysteretic Magnetostructural Response of Compositionally Heterogeneous Magnetic Alloys and Giant Magnetostriction.
Yong Ni 1 , Armen Khachaturyan 1 Show Abstract
1 Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
9:00 PM - K7.22
Molecular Beam Epitaxy Integration of Barium Hexaferrite on Wide Bandgap Semiconductors
Zhuhua Cai 1 , Trevor Goodrich 1 , Zhaohui Chen 2 , Vince Harris 2 , Katherine Ziemer 1 Show Abstract
1 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Department of Electrical and Computer Engineering, Northestern University, Boston, Massachusetts, United States
Barium hexaferrite (BaM) is ideal for microwave device applications because of its high resistivity and particularly large uniaxial magnetocrystalline anisotropy (17 kOe). Integration of such ferrite microwave devices with semiconductor platforms would allow for reduced volume and enhanced power management in phased array radar electronics and wireless technologies. Initial attempts to optimize BaM films directly deposited on 6H-SiC by pulsed laser deposition (PLD) produced aligned but textured films with saturation magnetization (MS) ~4.3 kOe and ferromagnetic resonance (FMR) linewidth >1000 Oe. A 10% increase in saturation MS and a 20-fold decrease in FMR linewidth have been demonstrated through different levels of controlled interface engineering between the 6H-SiC and the BaM film. BaM films with MS >4.4 kOe and FMR linewidths <100 Oe have been deposited by PLD on a 10nm single crystalline MgO template grown by molecular beam epitaxy (MBE) on 6H-SiC. Since the improvement in magnetic properties of BaM films is attributed to the initial stages of BaM film growth on the atomically smooth MBE-grown MgO, MBE deposition of high quality BaM has the potential to be an ideal seed layer for thick BaM film deposition by PLD or liquid phase epitaxy. BaM was grown by MBE using an oxygen plasma source, a Ba effusion cell, and a Fe effusion cell at substrate temperature ranging from 300 to 800 °C. A O/Fe flux ratio below 20 could not fully oxidize metal iron to ferric iron (Fe3+), resulting in magnetite phase (Fe3O4) in the BaM film. However, a flux ratio of O/Fe greater than 100 led to a random oriented film without anisotropy. A growth temperature above 750 °C contributed to an ordered, single crystalline BaM film, but enabled Si diffusion. A 10 nm MBE-grown MgO interlayer between the BaM and SiC prevented Si diffusion at 750 °C. High quality BaM films with strong c-axis aligned normal to the substrate and low coercivity (200 Oe) was achieved at 750 °C and O/Fe flux ratio of 40 on the 10nm MgO interlayer. In-situ x-ray photoelectron spectroscopy and reflection high-energy electron diffraction showed a stoichiometric Fe/Ba ratio and an o