J. Ping Liu University of Texas-Arlington
Eric Fullerton University of California-San Diego
Oliver Gutfleisch IFW Dresden
Dave Sellmyer University of Nebraska-Lincoln
I1: Nanomagnetism, Theoretic Aspects
Monday PM, November 26, 2007
9:30 AM - I1.1
Domain Wall Formation and Domain Wall Resistivities in CocFe1-c and CocNi1-c.
Peter Weinberger 1 Show Abstract
1 , Center for Computational Materials Science, Vienna Austria
In using the fully relativistic versions of the Screened Korringa-Kohn-Rostoker method and of the Kubo-Greenwod equation equilibrium domain wall widths and corresponding domain wall resistivities are calculated for CocFe1-c and CocNi1-c making use of a multi-scale approach. It is found that in CocFe1-c the domain wall width becomes rather large at about c=0.4, but does not show an obvious singularity in the vicinity of the bcc to fcc phasetransition. In CocNi1-c the domain wall width varies much less with respect to the concentration. Furthermore, it is demonstrated that as compared to the homogeneous infinite systems the anisotropic magnetoresistance is reduced in the presence of a domain wall. This reduction is rather big for CocNi1-c, while for CocFe_1-c it is only of the order of 1 - 2%.
9:45 AM - I1.2
Magnetization Reversal in Small FePt:Fe Particles.
Ralph Skomski 1 , J. Liu 2 , Chuan-bing Rong 2 , David Sellmyer 1 Show Abstract
1 Nebraska Center for Materials and Nanoscience, and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska, United States, 2 Department of Physics, University of Texas, Arlington, Texas, United States
Using a salt-matrix annealing method, it has recently become possible to produce small Fe-Pt nanoparticles with a very narrow size distribution . The size of the particles, about 3 nm, and the high anisotropy of L10-ordered FePt make these particles the smallest ever-produced particles with stable room-temperature ferromagnetism. However, the magnetization reversal in these particles is little understood. The main reason is the involvement of several types of ordering, namely disordered, ordered, partially ordered, and coexistence of ordered and disordered phases. Furthermore, the anisotropy near the surface of Fe-Pt particles is known to differ from the bulk anisotropy. We present micromagnetic simulations for nanoparticles of varying size and structure. The considered geometries include core-shell particles with a shell of reduced anisotropy, hemispherical hard-soft composites, and coated hemispheres. The parameters varied are the anisotropy of the involved phases and the thickness of the shell. In some cases, different structures yield nearly identical hysteresis loops, so the understanding of the particles require additional structural and magnetic information, such as magnetic-viscosity measurements and high-resolution TEM. In fact, the most important parameter is the uniaxial anisotropy averaged over the particles, with relatively small corrections due to structural and magnetic inhomogenities. In our simulations, differences are also resolved by separately monitoring the magnetizations of the ordered and disordered phases, which makes these computer experiments a viable tool for the investigation of two- and multiphase nanostructures. — This research is supported at Nebraska by DOE and NCMN, and at Texas by DARPA/ARO and DoD/MURI. —  C. B. Rong, D. R. Li, V. Nandwana, N. Poudyal, Y. Ding, Z. L.. Wang, H. Zeng and J. P. Liu, Adv. Mater. 18, 2984 (2006).
10:00 AM - I1.3
Surface Induced Reduction of Magnetization in Nanoparticles with Competing Exchange Interactions.
Christopher Westman 1 , Joshua Koch 1 , Renat Sabirianov 1 , Hao Zeng 2 Show Abstract
1 Physics, University of Nebraska at Omaha, Omaha, Nebraska, United States, 2 Physics, University at Buffalo, the State University of New York, Buffalo, New York, United States
Magnetic properties of nanoparticles are a subject of active research because of their potential applications in magnetic memory, spintronics and biomedical fields. Magnetic nanoparticles frequently exhibit the reduction of saturation magnetization. A model based on competing exchange interactions is presented for the investigation of nanoparticle magnetization. The ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions contribute differently at the nanoparticle surface and interior, leading to reduced ferromagnetic order at the surface. This model predicts an unconventional temperature dependence of magnetization and a magnetically “dead layer” at finite temperatures. The competing exchange interactions are modeled with nearest neighbors being FM and second nearest neighbors AFM. We use Heisenberg model Hamiltonian and Monte Carlo method to find magnetization, susceptibility and related properties as function of temperature. This is confirmed by temperature dependent magnetization and Mössbauer measurements of FePt nanoparticles. The effects are sensitive to particle size and surface terminations.
10:15 AM - I1.4
Two Different Magnetization Reversal Modes in Iron Oxide Nanotubes of Systematically Varied Geometry.
Julien Bachmann 1 , Juan Escrig Murua 2 , Jing Jing 1 , Dora Altbir 2 , Ulrich Goesele 1 , Kornelius Nielsch 1 Show Abstract
1 Experimental Department II, Max Planck Institute of Microstructure Physics, Halle Germany, 2 Departamento de Fisica, Universidad de Santiago de Chile, Santiago Chile
Ordered arrays of Fe3O4 nanotubes have been prepared by combining a self-limited gas-solid chemical reaction (atomic layer deposition, or ALD) with the use of a structured substrate as a template, namely porous anodic alumina. With these tools, the length, diameter, and wall thickness of the tubes can be tuned accurately between 1 and 5 μm, between 40 and 160 nm, and between 1 and 40 nm, respectively. Such arrays give rise to a ferromagnetic response that strongly depends on geometry. Systematic variations of the tube wall thickness result in non-monotonic changes in coercive field. Given a constant set of the other structural parameters, a certain value of the wall thickness can be found which optimizes the quality of the magnetic response. Theoretical modeling of the magnetization reversal between the two magnetically saturated states is in quantitative agreement with the experimental data. It shows that for thin tubes the reversal occurs by propagation of a “vortex” domain boundary from one extremity of the tube to the other, while in thicker ones it is driven by propagation of a “transverse” domain boundary. The optimal wall thickness thus corresponds to the geometry of the crossover between the vortex and transverse modes of magnetization reversal. Finally, such a crossover is predicted to be a general phenomenon on similar size scales. Thus, it will play an important role for the design and optimization of ferromagnetic nanostructures to be developed into high-density data storage platforms.
10:30 AM - **I1.5
Theoretical Limit on the Minimal Switching Field and the Switching Current in Magnetization Reversal.
Xiangrong Wang 1 Show Abstract
1 , Hong Kong University of Science & Technology, Kowloon, Hong Kong, China
Efficient and controlled magnetization switching is one of the important issues in magnetic data storage. It is desirable that a bit has a large switching field during the memory state and a small switching field during the write operation. In terms of magnetization reversal of a magnetic nanostructure by either a magnetic field or a spin polarized electric current, important issues are to lower critical currents (fields) required to reverse a magnetization and to design a current (field) pulse such that the magnetization can be switched from one state to another fast. Many strategies have been proposed and examined both theoretically and experimentally with better and improved results. How much further improvement can one make? This question cannot be answered without knowing the theoretical limit of critical currents (fields) required to reverse a magnetization with an arbitrary polarized current (field). At the moment, other than comparing one reversal scheme with another, one has no objective criteria in evaluating infinite number of possible reversal schemes. In this talk, we shall present the theoretical limit of the minimal magnetization switching current (field) and the optimal current (field) pulse for the fastest reversal for Stoner particles. The results are based on the nonlinear dynamical Landau-Lifshitz-Gilbert equation, and they can be used as benchmarks to evaluate different reversal strategies besides other possible usages. References: 1) Z.Z. Sun and X.R. Wang, Phys. Rev. B 71, 174430 (2005).2) Z.Z. Sun and X.R. Wang, Phys. Rev. B 73, 092416 (2006).3) Z.Z. Sun and X.R. Wang, Phys. Rev. B 74, 132401 (2006).4) Z.Z. Sun and X.R. Wang, Phys. Rev. Lett. 97, 077205 (2006).5) X.R. Wang and Z.Z. Sun, Phys. Rev. Lett. 98, 077201 (2007).6) T. Moriyama, R. Cao, J.Q. Xiao, J. Lu, X.R. Wang, Q. Wen, and H.W. Zhang, Appl. Phys. Lett. 90, 152503 (2007)
I2: Magnetoresistance, Principles and Devices
Monday PM, November 26, 2007
11:30 AM - **I2.1
Magnetic Tunneling: Pitfalls, Temperature Dependence and Novel Phenomena.
Ivan Schuller 1 , Igor Roshchin 1 , Casey Miller 1 , Zhi-Pan Li 1 , Johan Akerman 2 Show Abstract
1 Physics Department, University of California - San Diego, La Jolla, California, United States, 2 Department of Microelectronics and Applied Physics, Royal Institute of Technology, Kista Sweden
Electronic tunneling in metal-insulator-metal junction has been an active field of research for many years. In the last 10 years these type of studies have been revitalized with the advent of principally magnetic tunneling and the development of novel devices which rely on these type of structures.I will describe a comprehensive series of experiments and numerical simulations to address importan issues magnetic tunnel junctions in which at least one of the electrodes is magnetic. The important issues that will be addressed include:1) How can tunneling be distinguished from pinhole conduction2) The impact of roughness on the extracted parameters3) What is the role of temperature4) When does the WKB approximation brake down5) What novel effects can be expected and are observed.Work supported by the US-DOE.
12:00 PM - I2.2
Large Current Suppression Induced by Magnetic Molecular Channels on the Exposed Sides of Magnetic Tunnel Junctions.
Pawan Tyagi 1 , Bruce Hinds 1 2 , Dongfeng Li 2 , Stephen Holmes 2 Show Abstract
1 Chemical and Materials Engg., University of Kentucky, Lexington, Kentucky, United States, 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Fabrication of electronic spin devices based on magnetic molecules coupled between magnetic metal electrodes is a difficult task due to the lack of a reliable fabrication process for molecular-scale electrodes. We have successfully fabricated magnetic molecular junctions (MMJ) by bridging organometallic magnetic molecules across the tunnel barrier of magnetic tunnel junctions (MTJ) [Ta/Co/NiFe/Al2O3(~2nm)/NiFe] on the exposed side edges of the pattern. The critical dimension of the molecular electrode is set by the thickness of the insulating Al2O3 layer. Surprisingly, molecule attachment on bare magnetic tunnel junction reduced its current from 1±0.5µA to 2±1nA (100mV bias and room temperature) due to changes in the magnetic ordering of the electrodes. This phenomenon was observed on over 105 devices. A large number of control experiments including using different bridging molecules, oxidation of metal electrodes, variation in the composition of magnetic electrodes and alternative preparation of the exposed edge MTJ were performed. In-plane magnetization and cooling to 77 K further decreased the MMJ current to 1-10pA level (100mV bias). Magnetic ordering of electrodes after molecule attachment is observed by magnetic force microscopy (MFM) suggesting that the device current reduction by 6 orders is due to the molecule producing strong antiferromagnetic coupling between the top and bottom magnetic electrodes. SQUID- magnetometer studies confirmed antiferromagnetically coupled layers. The central metal core of the molecule is tethered ~1.2nm from the ferromagnetic electrode by an alkane tunnel barrier that forces electron conduction through well defined molecular states of the core. Molecules directly assembled on individual magnetic electrodes (non-bridging) did not produce analogous changes in magnetic ordering. Magnetic measurements suggests that dramatic current reduction is due to the appearance of unusually high degree of spin polarization (>99.99%) in magnetic electrodes due to the molecule mediated strong antiferromagnetic coupling between two ferromagnetic electrodes of MMJ.
12:15 PM - I2.3
Nanoscaled Magnetoelectronical Sensors for Mechanical Measurements.
Eckhard Quandt 1 2 , Dirk Meyners 1 , Jochen Puchalla 2 Show Abstract
1 Faculty of Engineering, University of Kiel, Kiel Germany, 2 Smart Materials, caesar, Bonn Germany
Recently, highly sensitive strain gauges were developed, which are based on nanoscaled TMR (tunnel magnetoresistance) effects combined with the inverse magnetostriction. TMR structures generally possess a symmetrical characteristic which reflects the switching fields of the soft and hard layers, respectively. This characteristic can be changed by a stress field if the soft layer is replaced by a suitable magnetostrictive layer leading to a stress induced rotation of the magnetostrictive layer with respect to the reference layer.This approach illustrates an interesting, highly sensitive mechanism in order to detect mechanical variables with a high spatial resolution as well as an unrivaled high gauge factor. In addition, the feasibility of an integrated cost-effective fabrication using CMOS circuits and SMM membranes compatible is of great interest e.g. for the automotive industry.In this presentation, the basics, the fabrication and the features of these nanoscaled magnetoelectronical sensors will be discussed in view of an integrated pressure sensor for automotive applications.Grants by the German Federal Ministry of Education and Research, funding program Nanoelectronics (contract numbers 13N7943, 13N8492 13N9083) are gratefully acknowledged.
12:30 PM - **I2.4
MgO-based Magnetic Tunnel Junctions.
Michael Coey 1 , Gen Feng 1 , Jia-Feng Feng 1 , P. Stamenov 1 , K. Oguz 1 , S. van Dijken 1 Show Abstract
1 School of Physics / CRANN, University of Dublin, Trinity College, Dublin 2 Ireland
We have prepared a series of single and double barrier MgO tunnel junctions by sputtering. The ferromagnetic layers in the stack are produced in a Shamrock sputtering tool, and the MgO is prepared from a target-facing-target source in a second chamber without breaking the vacuum. A typical single-barrier stack is:Ta(5)/Ru(50)/Ta(5)/NiFe(5)/IrMn(10)/CoFe(2)/Ru(0.8)/CoFeB(t1)/MgO(t2)/CoFeB(3)/Ta(5)with 0.5 < t1 < 3 and 1.5 < t2 < 3 nm. Magnetic annealing in 800 mT at 250 – 400 °C increases the TMR of the as-deposited patterned structures dramatically, with magnetoresistance values of up to 280 % for t1 = 3 nm and t2 = 2.5 nm. A sign reversal of the TMR has been observed when t1 < 2 nm with inverted TMR as high as 75 % The origin of this effect is the imbalance of the synthetic antiferromagnet. The temperature and field dependence of the tunnel magnetoresistance and the tunneling anisotropic magnetoresistance will be discussed, and some aspects of their 1/f noise reviewed.Double-barrier tunnel junctions with IrMn-pinned artificial antiferromagnets top and bottom, and a central CoFeB free layer, exhibit TMR of more than 100 %. The bias dependence is asymmetric after low temperature magnetic anneals due to dissimilar CoFeB/MgO interfaces. The effect is reduced by half for a bias of 1.88V, and the largest output voltage of 0.62 V is obtained after annealing at 300 °C.
I3: Exchange Coupling & Exchange Bias
J. Ping Liu
Monday PM, November 26, 2007
2:30 PM - I3.1
Angular Dependence of Perpendicular Exchange Bias in FeMn/(FeNi/FeMn)n Multilayers.
Li Sun 1 , Hao Xing 1 Show Abstract
1 Mechanical Engineering, University of Houston, Houston, Texas, United States
Angular dependence of exchange bias has been extensively studied to reveal the field induced interactions between the FM and AFM layers. Most of the studies dealt with in-plane angular dependence of magnetization hysteresis for FM/AF layers with intrinsic shape anisotropy after longitudinal field-cool. Recent study showed that perpendicular exchange bias can be established in ferromagnetic (FM)/ antiferromagnetic (AF) multilayers with intrinsic in-plane ferromagnetic anisotropy through perpendicular field cool. The induced anisotropy along the magnetic hard axis processes the same interfacial nature as conventional exchange bias along the easy axis. Angular dependent investigation reveals that the competition between induced anisotropy and the intrinsic ferromagnetic shape anisotropy magnetic for different geometries greatly influence magnetization reversal behavior of exchange biased multilayers. An analytical model based on Stoner-Wohlfarth coherent rotation considering the competition between perpendicularly induced anisotropy and in-plane shape anisotropy has been developed.
2:45 PM - I3.2
Exchange Bias of Ni/FeF2 Bilayers as a Function of the Antiferromagnetic Thickness.
Rafael Morales 1 2 , Ivan Schuller 1 Show Abstract
1 Physics Department, University of California San Diego, La Jolla, California, United States, 2 Departamento de Fisica, Universidad de Oviedo, Oviedo Spain
3:00 PM - **I3.3
Factors Affecting Exchange Bias in Polycrystalline Metallic Thin Films.
Luis Fernandez-Outon 1 , Gonzalo Vallejo-Fernandez 1 , K. O'Grady 1 Show Abstract
1 Department of Physics, The University of York, York United Kingdom
Exchange bias occurs when a ferromagnetic (F) layer is in contact with an antiferromagnetic (AF) layer. When the system is field cooled (set) through the Néel temperature of the AF (TN) the AF layer causes a unidirectional anisotropy in the F leading to a shifted hysteresis loop along the field axis by an amount Hex . A universal theory to explain this effect has not been established. For all applications AFs are sputtered granular films with grain boundary effects and rough interfaces . It is well known that AFs and hence exchange bias are not stable over time and can be unstable over short times. In this work we present a detailed study of the exchange bias phenomenon in metallic polycrystalline systems. The use of careful measurement protocols where all the measurements are made at a thermal activation free temperature (TNA) makes it possible to distinguish between bulk and interface effects contributing to the loop shift. All the results are interpreted in terms of a granular model where the energy barrier to reversal within the AF is grain volume dependent. We will show how this affects setting in metallic AFs at T<TN. We have used this interpretation to explain the AF thickness dependence of the exchange field and the role of the AF grain size. The grain size was determined by TEM images and measuring over 500 grains for each sample. The grain size distribution within the AF has been shown to determine the distribution of energy barriers to reversal. The exchange field has been shown to both increase and decrease with the AF grain size depending on the system studied. We are able to explain both types of behaviour using a grain volume model. Samples with composition Si/Cu(10nm)/ CoFe(2.5nm)/IrMn(tAF)/Ta(10nm) have been prepared using a HiTUS sputtering system. The thickness of the AF layer tAF was varied between 3 and 12nm. Since the AF is set by a thermal activation process all large grains may not be set at T<TN. Small grains will be disordered by thermal energy above a given temperature known as TNA. Hence, the exchange field has been shown to be due to the stable and set fraction at finite temperatures. We have also observed the ordering of interface spins at low temperatures for both FeMn and IrMn AFs. We can order these spins via the setting field and in different orientations at the interfaces of a trilayer. Ordering of interfacial spins leads to an increase in Hex of up to 30%.The magnetic data and its interpretation will be presented in the full paper.  W.H. Meiklejohn, and C.P. Bean, Phys. Rev., 102 p.1413 (1956)  M.J. Carey, N. Smith, B.A. Gurney, J.R. Childress and T. Lin, J. Appl. Phys., 89 p.6579 (2001) J. Nogues, I.K. Schuller, J. Magn. Magn. Mat., 192 p.203 (1999)
3:30 PM - I3.4
Influence of Vicinal Steps and Interface Roughness on Uniaxial Anisotropy and Exchange Bias in Permalloy Thin Films.
Rantej Bali 1 , Mark Blamire 1 Show Abstract
1 Materials Science & Metallurgy, University of Cambridge, Cambridge United Kingdom
Vicinal steps have been shown to induce uniaxial anisotropy , enhancement in magneto-resistance (MR)  and two-staged switching  in ferromagnetic thin films. However, most of these studies are restricted to epitaxial films and have limited application. We stress upon the importance of vicinal steps by studying the combination of step-induced anisotropy and exchange bias in polycrystalline films. We have observed the directed rotation of ferromagnetic easy axis in sputter deposited permalloy films on high and low angle vicinal sapphire substrates. Variation of the squareness ratio in angular space in the presence of deposition field, uniaxial step-induced anisotropy and exchange bias can be modelled. Deposition of the antiferromagnet CoMn induced exchange bias which was controlled by field cooling through the blocking temperature of ~360 K. The angular distribution of exchange bias was found to be a function of orientation of vicinal steps, cooling field orientation and also vicinal step height and interface roughness. Interface roughness was controlled by laser-ablating Al2O3 onto the vicinal sapphire substrates prior to sputtering permalloy, and observing the oscillating intensity of the specular spot in reflection high energy electron diffraction (RHEED). The polycrystalline system presented here is commonly used in devices, and the realistic experimental conditions include step-induced anisotropy and exchange bias in the presence of interface roughness and is therefore of practical value. References Weber W. et al, Phys. Rev. Lett. 76 (11) 1940 (1996) Arora S.K., Phys. Rev. B 72 134404 (2005) Mireles, H. C. & Erskine, J. L., Phys. Rev. Lett. 87, 037201 (2001)
3:45 PM - I3.5
The Effect of Varying Crystallinity and Thickness of the Magnetic Hard Layer on the Exchange Coupling in Fe/CoPt Soft-hard Magnetic Bilayers.
Hiroyuki Oguchi 1 , Antonio Zambano 1 , Samuel Lofland 2 , Daniel Josell 3 , Ichiro Takeuchi 1 Show Abstract
1 Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Department of Physics and Astronomy, Rowan University, Glassboro, New Jersey, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Exchange-coupled hard/soft magnetic nanocomposites are being pursued for synthesis of future permanent magnets with high energy products. To gain a better understanding of the exchange coupling behavior between the soft and hard magnetic layers, we are using the high-throughput approach where a large number of samples are fabricated and measured simultaneously. Fe/CoPt magnetic bilayers are grown on MgO(110) substrates in a combinatorial electron-beam deposition chamber. The CoPt hard layer is grown on a Pt buffer layer at temperatures in the range of 600°C to 650°C with various post-annealing conditions. (100) and (112) oriented phases are observed by x-ray diffraction. The crystallinity of those phases changes depending on the growth conditions. On the hard layer, the polycrystalline Fe soft layer with a continuously varying thickness is deposited at room temperature. The magnetic hysteresis loop for each Fe thickness is measured using a magneto-optical Kerr effect (MOKE) system. Drastically changing hysteresis loops for CoPt hard layers with different crystallinities reveal that a very small magnetically soft region in the hard layer can affect the exchange coupling interaction with the soft layer. In order to find the optimum thickness of the hard layer, we have also fabricated Fe/CoPt bilayers thin-film libraries where in one direction the thickness of the soft layer is continuously changed and the thickness of the hard layer is changed in the other direction. Changing magnetic hysteresis loops show that optimum thickness of the hard layer for the exchange coupling is about 15 nm. This work is supported by ONR MURI N00014-05-1-0497.
4:30 PM - I3.6
Cooling Field Dependent Magnetic Depth Profiles in Exchange-coupled Superlattices.
M. Fitzsimmons 1 , C. Dufour 2 , K. Dumesnil 2 Show Abstract
1 LANSCE, LANL, Albuquerque, New Mexico, United States, 2 Laboratoire de Physique des Materiaux, Universite Henri Poincare Nancy I, Vandoeuvre les Nancy France
4:45 PM - **I3.7
High-throughput Investigation of Exchange-coupled Hard/soft Magnetic Bilayers.
Ichiro Takeuchi 1 Show Abstract
1 , University of Maryland, College Park, Maryland, United States
The high-throughput techniques allow simultaneous measurements of a large number of samples with continuously changing parameters. This talk will provide an overview of the high-throughput method as applied to investigation of exchange coupled hard/soft magnetic bilayer systems. By including composition variation as one of the parameters, the library approach can substantially increase the scope of the traditional gradient thickness method for rapidly mapping the dependence of physical properties on changing parameters. For instance, to elucidate the dependence of exchange coupling behavior of hard/soft magnetic bilayer systems on various micromagnetic constants, the coupling length (λ) and the nucleation field (HN) were systematically measured on five thin film libraries made of soft-magnetic/hard-magnetic bilayers . Hysteresis loops of CoPt/FexCo1-x (0≤x≤1), CoPt/Ni, CoPt/Fe, Sm2Co7/(Fe, Co or Ni), and Sm2Co7/Ni were measured using a magneto-optical Kerr effect (MOKE) set up. Some of the libraries were also measured using XMCD. We find that the dominant factors determining λ and HN are the hard layer magnetic constants and the saturation magnetization (M) of the soft layer. HN and λ display a direct correlation with the domain wall width of the hard layer and have an anticorrelation with M of the soft layers. They were found to not depend on exchange stiffness (A) and anisotropy (K) constants within the group of soft layer materials studied here. Comparison of the results with models will be presented. Recent results on the effect of varying interface conditions and interface layers as well as the effect of varying the hard layer crystallinity will be discussed. Implications of the present results on energy products of nanocomposite magnets will be discussed. The key contributors to the present work include A. Zambano, H. Ohguchi, S. E. Lofland, D. Josell and J. P. Liu. This work is supported by ONR MURI N00014-05-1-0497.  Zambano et al., Phys. Rev. B75, (2007) 144429.
5:15 PM - I3.8
Influence of Interfacial Non-magnetic Materials on Soft-hard Phase Interactions in Nanocomposite Magnets.
Antonio Zambano 1 , H. Oguchi 1 , I. Takuechi 1 , Y. Choi 2 , J. Liu 3 , S. Lofland 4 , D. Josell 5 , L. Bendersky 5 Show Abstract
1 Department of Materials Science and Engineering, and Center for Superconductivity Research, University of Maryland, College Park, Maryland, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Physics, University of Texas at Arlington, Arlington, Texas, United States, 4 Department of Physics, Rowan University, Glassboro, New Jersey, United States, 5 , NIST, Gaithersburg, Maryland, United States
The determination of the mechanisms that affect the exchange interaction is critical for the improvement of nanocomposite magnets. One of the possible effects comes from the presence of nonmagnetic regions among the hard and soft magnetic phases. One-dimensional bilayer systems are simple useful tools to establish the importance of those effects. We have used the high-throughput approach1 to systematically study thin soft-magnetic/non-magnetic/hard-magnetic trilayer systems. We have examined thickness gradient effects of nonmagnetic impurity layers, such as Cu and Ti, on the interaction between hard magnetic CoPt and a soft magnetic Fe layer. On single chips, multiple samples were grown by e-beam evaporation varying the impurity thickness from 0 to 50 Å typically. The magnetic hysteresis loop for each sample was rapidly measured using the magneto-optical Kerr effect (MOKE). Selected samples were also studied by X-ray magnetic circular dichroism (XMCD). The results indicate that there is an oscillatory behavior of the intensity of the exchange coupling interaction depending on the nonmagnetic layer thickness. We will discuss how the crystalline characteristics of the hard layer affect this behavior and, in some cases, give rise to a significant dipole interaction contribution. We will also talk about the general advantage of using the presence of nonmagnetic material to actually improve the hard/soft phase ferromagnetic interaction. This work is supported by ONR MURI N00014-05-1-0497.1. Zambano et al., Phys. Rev. B 75, 144429 (2007).
5:30 PM - I3.9
Micromagnetic Simulation of the Competition Between Exchange and Magnetostatic Interactions in Composite Magnets.
Chuanbing Rong 1 , J. Ping Liu 1 Show Abstract
1 Department of Physics, University of Texas at Arlington, Arlington, Texas, United States
Recently, it was reported that the anisotropic hot-pressed composite magnets, which contained coarse Fe grains with size larger than the critical dimension for exchange interaction, exhibited a single-phase demagnetization behavior with enhanced remanence Mr and maximum energy product (BH)max,[1-3] which was attributed to long-range magnetostatic (dipolar) interaction. In this work, we studied the size dependent magnetostatic and exchange interactions and their effect on the demagnetization behavior of the hard-soft composite magnets by micromagnetic finite-element simulation. The numerical results give a direct study on the distributions of the magnetostatic field and magnetizations, as well as the effect of exchange coupling and magnetostatic energies on the hysteresis loops. It was confirmed that the magnetostatic interaction improves the squareness of the hysteresis loop when the soft phase layer is perpendicular to the applied field. However, the improvement of (BH)max of the composite magnets comparison with single-phase can be obtained only when the size of soft-phase layer is small enough since the contribution of exchange coupling energy drops fast with increasing size while that of magnetostatic energy almost remains unchanged. This means that the control of the soft-phase dimension is still the key issue to achieve enhancement of the (BH)max of a composite magnets. M. Gabay, M. Marinescu, and G. C. Hadjipanayis, J. Appl. Phys. 99, 08B506 (2006). D. Lee, S. Bauser, A. Higgins, et al., J. Appl. Phys. 99, 08B516 (2006). A. M. Gabay, and G. C. Hadjipanayis, J. Appl. Phys., 101, 09K507 (2007).
5:45 PM - I3.10
Activation Energy for Crystallization in Nanocrystalline Exchange Coupled Alloys.
Matthew Willard 1 , Maria Daniil 1 , Juan Saavedra 2 1 Show Abstract
1 Multifunctional Materials Branch, U. S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 Department of Mechanical Engineering, University of Puerto Rico, Mayagüez Campus, Washington, Puerto Rico, United States
Nanocrystalline soft magnetic alloys have been studied for their excellent magnetic performance as core materials in inductor applications. These materials are processed using a rapid solidification technique, typically melt spinning, resulting in an amorphous alloys in a ribbon form. The rapidly solidified alloys are then isothermally annealed to improve the stability of the material by partially devitrifying the ribbon samples, resulting in improved magnetic performance. The reason for the performance improvement has been linked to the grain size and volume fraction of crystallites in the optimized microstructure. As a result, the processing conditions have a significant impact on the performance of the alloy with the annealing time and temperature being important factors. To examine the crystallization kinetics, constant heating rate experiments were performed using a TA Instruments SDT 2960 Differential Thermal Analysis system. The heating rates were varied between 2 and 85°C/min within the temperature range 50 to 900°C. In this study, the crystallization kinetics of alloys with composition (Fe, Co, Ni)-Zr-B-(Cu) are determined by Kissinger analysis. The thermally activated primary and secondary crystallization temperatures were observed for each sample at numerous heating rates to provide accurate activation energies. The trends of the activation energy for primary crystallization as a function of magnetic transition metal composition were examined in this study and compared to values in the literature. Alloys rich in Fe show activation energies between 3 and 3.5 eV/atom while alloys rich in Ni have substantially lower activation energies, near 2.1 to 2.5 eV/atom. The reduced activation energy likely results from the more active diffusion in Ni-based alloys and may be a reason for the deteriorated nanocrystalline alloy formation at these compositions.
I4: Poster Session
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - I4.1
Nanoconductive and Magnetic Properties of Nanostructured Iron Thin Films Prepared by Sputtering at Very Low Temperatures.
Carmen Munuera 1 , Felix Jimenez-Villacorta 2 1 , Carmen Ocal 3 , Carlos Prieto 1 Show Abstract
1 Instituto de Ciencia Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Madrid Spain, 2 Spline, ESRF, Grenoble France, 3 Instituto de Ciencia de Materiales de Barcelona, Consejo Superior de Investigaciones Científicas, Cerdanyola del Valles Spain
9:00 PM - I4.10
Enhanced Neel Temperature in Mn-ferrite Nanoparticles Linked to Growth Rate Induced Cation Inversion.
Aria Yang 1 , C. Chinnasamy 1 , Soack Yoon 1 , Jing Lou 1 , Zhuhua Cai 4 , Kailin Hsu 1 , Steve Oliver 3 , Shaheen Islam 2 , Carmine Vittoria 1 , Vincent Harris 1 Show Abstract
1 Electrical Engineering, Northeastern University, Boston, Massachusetts, United States, 4 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States, 3 , Gilford High School, Gilford, New Hampshire, United States, 2 , Virginia Union University, Richmond, Virginia, United States
The spinel ferrite is a big family of magnetic materials that is largely used in high frequency microwave devices due to their near insulating properties, high permeability, and moderate magnetization. Current interest has been to make nano sized ferrite particles to reduce their energy loss, and to study their size dependent electronic and magnetic properties. In the case of Mn-ferrite nanoparticles, size dependent magnetic properties have already been reported in detail a decade before. However, recently we found that the magnetic properties of MnFe2O4 nanoparticles are not directly related to the finite size scaling effect and size dependent cation inversion.1 We presented in this article the observed enhanced Néel temperature (TN) and quantitative inversion parameters results by fitting extended x-ray absorption fine structure analysis (EXAFS) data to theoretical standards following established EXAFS procedure with modified model exclusively for nanoparticles. Mn-ferrite (MnFe2O4) nanoparticles with diameters ranging from 4 nm to 50 nm were synthesized using a modified coprecipitation technique. The mixed metalchloride solutions were added to different concentrations of boiling NaOH solutions to control the particle growth rate and resulted in different particle sizes. Magnetization measurements indicated an increase in Néel temperature above the bulk equilibrium value (300°C) for larger particles having the higher growth rates. The ~4 nm MnFe2O4 particles showed a Néel temperature (TN) of about 320°C whereas the ~50 nm particles had a TN of about 400°C. Results of EXAFS indicated a corresponding systematic increase in cation inversion parameter, δ, described by the formula, (Mn1-δFe δ)tet[MnδFe2-δ]octO4. The enhanced Néel temperature is attributed to changes in the exchange energy resulting from cation inversion and is unrelated to particle size. Similar relationship between Néel temperature and inversion parameters were observed for pulsed laser deposited manganese ferrite films studied earlier.2 Low and high temperature magnetization were both measured from 5.5K to 723K for all the samples by the Lakeshore VSM system. Theoretical modeling based on the molecular field theory and the inversion parameters extracted from EXAFS analysis showed good agreement with the experimental magnetization (M) vs temperature (T) data sets. Reference:1 N. Ponpandian, A. Narayanasamy, C. N. Chinnasamy, and N. Sivakumar, J. –M. Greneche, K. Chattopadhyay, K. Shinoda, B. Jevadevan, and K. Tohji, Appl. Phys. Let., 86, 192510 (2005)2 X. Zuo, F. Yang, R. Mafhoum, R. Karim, A. Tebano, G. Balestrino, V. G. Harris, and C. Vittoria, IEEE Trans. Magn., 40, 2811 (2004)
9:00 PM - I4.11
Synthesis and Functionalization of FeCo Alloy Nanoparticles for Magnetic and Electronic Applications.
Q. Nguyen 1 , Chins Chinnasamy 1 , Soack Yoon 1 , Somu Sivasubramanian 2 , A. Baraskar 1 , Ahmed Busnanina 2 , Sanjeev Mukerjee 3 , Carmine Vittoria 1 , Vincent Harris 1 Show Abstract
1 Center for Microwave Magnetic Materials , Northeastern University, Boston, Massachusetts, United States, 2 Center for High- Rate Nanomanufacturing, Northeastern University, Boston, Massachusetts, United States, 3 Dept. of Chemistry and Chemical Biology , Northeastern University, Boston, Massachusetts, United States
Iron-cobalt based alloys exhibit particularly interesting magnetic properties, with high Curie temperatures, the highest saturation magnetizations, high permeability and low losses. Preparing the FeCo alloy nanoparticles with surface functionalization makes possible applications from biomedical to electronic applications. Here, the Fe100-xCox alloy nanoparticles have been prepared by using the modified low temperature chemical reduction technique. The X-ray diffraction pattern of the as-prepared particles clearly showed the formation of alloy nanoparticles and it was further confirmed by high resolution Transmission electron microscopic (TEM) analysis. The TEM analysis showed that the particle size was varied between 20 and 60 nm and nearly monodispersed (Fig. 1) The magnetic properties were measured using SQUID magnetometer at 300 K and as well as at 5 K. The maximum saturation magnetization of about 210 emu/g was achieved at room temperature for the 50 nm sized particles. The as-prepared nanoparticles were assembled and fixed on a substrate and aligned by using an external magnetic field. The microwave properties measured by in-plane ferromagnetic resonance at various frequencies indicate a minimum line width of ≈ 4500 Oe which is consistent with ferromagnetic nanoparticles (Fig. 2). For the surface functionalization, these Fe100-xCox alloy nanoparticles were coated with various kinds of capping ligands and its magnetic properties were studied. On the other hand, a few nanometer thicknesses of high resistive oxide particles were passivated on the surface of Fe100-xCox alloy nanoparticles to reduce the eddy current loss and its effect on the microwave properties were studied in detail.
9:00 PM - I4.12
Gregory Poole 1 , David Nikles 1 , Jin Mei Dong 1 , James Weston 1 Show Abstract
1 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama, United States
FeCoPt nanoparticles were prepared by four different procedures, either 1) the simultaneous diol reduction of platinum(II) acetylacetonate, cobalt(II) acetylacetonate and thermal decomposition of iron pentacarbonyl, 2) the superhydride reduction of iron(III) acetylacetonate, cobalt(II) acetylacetonate and platinum(II) acetylacetonate, the diol reduction of iron(III) acetylacetonate, cobalt(II) acetylacetonate and platinum(II) acetylacetonate, or 4) the hydrazine reduction of hexachloroplatinic acid, cobalt(II) acetate and iron(II) chloride. In each case the particles were superparamagnetic with a face-centered cubic structure. Heating under 5% hydrogen in argon transformed the particles to the ferromagnetic tetragonal phase giving films with high coercivity. Fits of the time dependence of the remanent coercivity to Sharrock’s law gave values of coercivity for FeCoPt (Ho = 5 to 7 kOe) films much lower than that for FePt films (Ho > 12 kOe). This indicates that substituting Co for Fe in FePt lowers the magnetic anisotropy.
9:00 PM - I4.13
Controlling Ruthenium Redox Chemistry: A Low Temperature Route to SrRuO3 Nanocubes.
Yolonda Smith 1 , Jennifer Noblitt 1 , Amy Prieto 1 Show Abstract
1 Chemistry Department, Colorado State University, Fort Collins, Colorado, United States
The strontium ruthenate family of compounds (including Srn+1RunO3n+1 and SrRuO3) exhibit a wide range of interesting electronic properties. Of particular interest is Sr2RuO4 (n=1), which is the only non-copper containing oxide to exhibit superconducting properties. Members of this family have been synthesized hydrothermally previously using fairly high temperatures and pressures. We are interested in exploring the synthesis of nanoparticles of members of the strontium ruthenate family by controlling the redox chemistry of Ru in solution. We will show that by tuning the oxidation state of Ru in solution, we can synthesize members of this family at much lower temperatures and pressures. This should allow for more flexibility in tuning the size and shape of the resulting particles using organic capping ligands. We have recently synthesized nanoscale cubes of SrRuO3, the parent compound of this family. Synthesis conditions such as pH, temperature, and reducing agents will be discussed, as well as preliminary magnetic measurements on these cubes.
9:00 PM - I4.14
Synthesis and Characterization of FePt/Fe3O4 Bimagnetic Nanoparticles.
Vikas Nandwana 1 , Kazuaki Yano 1 , J. Liu 1 Show Abstract
1 Physics, University of Texas at Arlington, Arlington, Texas, United States
Bimagnetic FePt/Fe3O4 nanoparticles are synthesized by means of high-temperature solution method by growing soft magnetic Fe3O4 phase on FePt nanoparticles. The soft phase can be coated or attached to the FePt phase in controlled manners. The size of the soft and hard phases can be tuned by changing reaction conditions. When the soft phases are coated on the hard phase particles, core/shell structured bimagnetic nanoparticles are formed; when the soft phases are attached to the hard phase, brick-like bimagnetic nanoparticles are formed. Magnetic properties of these nanoparticles are affected by dimensions of the soft and hard components due to the exchange coupling between them. Upon a reductive annealing, an assembly of the nanoparticles is transformed into a hard magnetic nanocomposite with enhanced energy product which is 36% higher than the FePt single phase.
9:00 PM - I4.15
Size and Property Control in the Synthesis of Magnetic Iron Nanoparticles.
Judith Lavin 1 , Dale Huber 1 , Todd Monson 1 , Eugene Venturini 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
We have synthesized a series of iron nanoparticles of varying sizes through the thermal decomposition of iron pentacarbonyl in dioctyl ether. The nanoparticles are stabilized by the presence of a weakly interacting β-diketone surfactant. This surfactant interacts strongly enough with iron to prevent agglomeration but does not induce oxidation. This allows us to synthesize very strongly magnetic iron nanoparticles, although they do display size-dependant magnetic moments. Iron nanoparticles with narrow polydispersity can be produced through this method only within a limited size range. Particles synthesized outside this size window tend to have high polydispersity and sometimes have bimodal distributions. The reasons for this limited range and the appearance of bimodal distributions will be discussed and hinge upon the issue of solubility. Evidence for the temporal separation of nucleation and growth in this relatively simple procedure will also be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - I4.16
Size Dependence of Magnetic Properties of Fe3O4 Particles in AC Magnetic Field.
Domingo Ferrer 1 , Erik Taylor 1 , Miguel Jose-Yacaman 1 Show Abstract
1 Chemical Engineering Department, The University of Texas at Austin, Austin, Texas, United States
Magnetic nanoparticles are an attractive agent for tumor hyperthermia therapy due to the possibility for external activation via applied AC magnetic field. It has also been proposed that these particles could be targeted using a powerful magnetic field enhancing the importance of the magnetic aspect of the particle. Yet, recent results show the use of Fe3O4 based magnetic heating is limited use for tumor lesions larger than 1 cm in diameter. It has been previously shown that other ferrite based magnetic nanoparticles may have a higher power loss than magnetite. One such material, MnFe2O4, is known but has been shown to be toxic to mammalian cells. Our proposed solution to this problem is to coat these particles with gold which is known to be biologically inert. Here, we present a consistent synthesis method for the production of XFe2O4 ferrite nanoparticles where X includes Mn, Fe, and other 2+ oxidation state metals. We have succeded in coating these magnetic nanostructures with gold in order to test the ability of these differing materials to cause localized heating with and without a gold coating.
9:00 PM - I4.17
Magnetic Field Assisted Fractionation of Non-Magnetic Colloids in Ferrofluid.
Derek Halverson 1 , Gennady Friedman 1 Show Abstract
1 ECE department, Drexel University, Philadelphia, Pennsylvania, United States
Creating monodisperse emulsions is an area of much interest with existing methods including drop break off  and entropic depletion forces . A new method is proposed to create monodisperse colloidal systems, including emulsions, nearly irrespective of their composition, by using ferrofluid and an external magnetic field. In this method ferrofluid, which is fluid containing ten nanometer superparamagnetic iron oxide nanoparticles, is added to the continuous phase of a non-magnetic, stabilized, polydisperse emulsion. A uniform magnetic field is then applied across the media. This causes an attractive force to occur between the non-magnetic emulsion droplets due to negative magnetophoresis. The combination of dispersion, double layer or steric, depletion, and magnetic potentials results in an energy minima at a small displacement for particles larger than a certain size which causes them to flocculate and allows them to be removed as a cream. Smaller particles remain in the continuous phase. In order to create a monodisperse system the field can be increased slightly after the cream has been removed. Only the monodisperse set of particles, which were just too small to floc at the lower field, appear in the newly formed cream, which can then be collected. The ferrofluid can be removed from the continuous phase after the separation using a commercially available magnetic rack, or other known methods. In this work oil in water microemulsions stabilized by sodium dodecylsulfate and polydisperse sets of electrostatically stabilized polystyrene particles are investigated experimentally and theoretically. The advantages of this method over drop break off are that it works regardless of the interactions that the different phases may have with surfaces, requires less specialized equipment, and can be scaled to high throughput operations. Depletion force based methods, which are the existing process that is the most similar to the proposed method, use extremely short ranged forces, requiring the separation to take place over the course of days as particles must diffuse into each other. The magnetic force, however, is longer ranged resulting in separations occurring in hours or less.P.B. Umbanhowar, V. Prasad, D.A. Weitz, ”Monodisperse emulsion generation via drop break off in a coflowing stream,“ Langmuir 2000, 16, 347J.Bibette, "Depletion interactions and fractionated crystallisation for polydisperse emulsion purification." J. Colloid Interface Sci. 147, 474–478 (1991).
9:00 PM - I4.18
Mechanical and Magnetic Properties Characterization of Highly-aligned Nickel Nanowires / Elastomer Composites.
Heather Denver 1 , Timothy Heiman 1 , Elizabeth Martin 2 , Amit Gupta 2 , Xueti Tang 3 , Mutsuhiro Shima 3 , Diana-Andra Borca-Tasciuc 1 Show Abstract
1 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Chemical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Materials Science Department, Rensselaer Polytechnic Institute, Troy, New York, United States
Applications that utilize magnetic polymer nanocomposites are currently emerging at a high rate. Examples include magnetic actuation in microelectromechanical systems (MEMS) and medical devices, thermal actuation through electromagnetic power harvesting, and magnetically actuated morphing structures. This work pursues the fabrication and characterization of nanocomposites based on polydimethylsiloxane (PDMS) with nickel (Ni) nanowires as fillers. An electrochemical deposition process was employed t