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 tim