Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R23: Focus Session: Theory and Simulation for Information Storage Applications |
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Sponsoring Units: GMAG FIAP Chair: Andreas Berger, Hitachi Global Storage Technologies Room: Baltimore Convention Center 320 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R23.00001: Nanomagnetic Simulations of Recording Media Invited Speaker: The optimization of ultra-high density recording systems requires the joint simulation of the recording head, the data layer and the soft underlayer. This talk presents micromagnetic simulations of recording processes in perpendicular and patterned media. The numerical technique for the multiscale simulation combines the finite element method with an accelerated boundary method. The use of hierarchical matrices and FFT methods significantly speeds up the computation time for the magnetostatic interactions between the head and the data layer. In addition to dynamic magnetization processes, energy barriers of recording media are calculated using a nudged elastic band method. The rise time of the write field was found to depend significantly on vortex motion in the pole tip of the head. The shortest field rise time was obtained for intermediate values of the Gilbert damping constant. Under the influence of the write field, magnetization reversal in composite perpendicular media occurs by the nucleation and expansion of reversed domains. However, thermally activated switching in composite media shows reversal by quasi-uniform rotation. Therefore it is possible to keep a high energy barrier while reducing the switching field in composite media, where a soft magnetic layer is exchange coupled to the hard magnetic layer. The energy barrier of composite media was calculated as a function of the applied field. The results show that the extrapolation of barriers measured at high fields underestimates the zero-field energy barrier. The calculated energy barrier of an island of a patterned media is smaller than the anisotropy constant times the island volume. This result shows that thermally activated magnetization reversal in patterned islands is non-uniform. The energy barrier of a square island with a size of only 20 nm was found to be 17{\%} lower than the barrier expected for uniform rotation. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R23.00002: Magnetization Dynamics in Ultrahigh-Density Magnetic Recording R. Skomski, J. Zhou, D. J. Sellmyer Thermally activated magnetization reversal is a key consideration in the development of magnetic recording materials with ultrahigh densities. We consider the onset of magnetization reversal (nucleation) and describe the magnetization by a Langevin model, where the magnetization dynamics is realized by random thermal forces. The exchange, anisotropy, and Zeeman energies are expanded into powers of a small perpendicular magnetization component, and the dynamics reduces to a time-dependent superposition of normal modes. In the Stoner-Wohlfarth (SW) model, the approach reproduces the Arrhenius-N\'{e}el-Brown law $\tau$ = $\tau$$_{o}$ exp(E$_{a} $$\backslash$k$_{B}$T) with an approximate energy barrier E$_ {a} $ and a particle-size dependent constant $\tau$$_{o}$. The same is true for the micromagnetic approach, where the local micromagnetic parameters such as K$_{1}$(\textbf{r}) = $<$K$_{1} $(\textbf{r})$>$ give rise to nonuniform magnetization modes in inhomogeneous and interacting particles. However, both the coercivity H$_{c}$ and the energy barrier E$_{a}$ are smaller than the SW predictions. A further reduction of H$_{c}$ and E$_ {a}$ is obtained by taking into account local anisotropy fluctuations of the type $<$K$_{1}$(\textbf{r})$^{2}$$>$ - K$_ {1}$(\textbf{r})$^{2}$. This reduction corresponds to fluctuating energy barriers, and establishes a particle-shape and materials-dependent upper limit to energy barriers in very small particles. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R23.00003: Experimental determination of switching field distributions in perpendicular and patterned recording media Andreas Berger, Byron Lengsfield, Yoshihiro Ikeda, Olav Hellwig, Eric Fullerton We have developed and applied a magnetometry method for the measurement of intrinsic switching field distributions in magnetic recording media [1]. The method is based upon a comparison between the major loop and a set of recoil loops, which start at a certain distance $\Delta $M away from saturation. Hereby, we measure the applied field difference $\Delta $H between the recoil loops and the major loop at identical M-values. By simultaneously analyzing complete $\Delta $H(M, $\Delta $M)-data sets, we gain a refined measure of the entire switching field distribution D(H$_{S})$. We studied the reliability of this method by means of micromagnetic modeling and find it to be robust and precise in reproducing the input parameters. In particular, the method is rather insensitive to inter-granular exchange coupling up to a certain threshold value. Also, a self-consistency check of the methodology was developed. An existing polar-MOKE setup was adapted to allow for $\Delta $H(M, $\Delta $M)-data acquisition, which enables a non-destructive sample characterization that is also compatible with the disk shape of recording media. Measurements on perpendicular and patterned recording media were made and results were compared to recording performance tests. [1] A. Berger, B. Lengsfield, Y. Ikeda, Y. H. Xu, and E. E. Fullerton, IEEE Trans. MAG \textbf{41}, 3178 (2005) [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R23.00004: Simulations Of Nanomagnet Clusters With Perpendicular Uniaxial Anisotropy For Multilevel Data Storage Qijun Xiao, Robert Krotkov, Mark Tuominen Digital data storage technology generally relies on a binary storage paradigm. In this work we explore a different scheme that exploits the stepwise, \textit{multilevel} total magnetization of a small cluster of interacting nanomagnets. The magnetization of a cluster can be resolved more easily than that of a single nanomagnet, due to the larger lateral size. Micromagnetic simulations, based on the Landau-Lifshitz-Gilbert (LLG) equation with parameters representative of Co$_{3}$Pt, reveal that magnetostatic interactions within a cluster produce a rich multilevel magnetic response, each level providing a stable remanent magnetization state. This work describes simulations used to investigate a multilevel data storage unit based on a hexagonal cluster of interacting uniaxial single domain nanomagnets. The accessibility and stability of the discrete magnetization states are studied. The switching properties of the nanomagnet clusters can be tuned by modifying the geometry, providing the ability to engineer desirable magnetic properties. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R23.00005: Magnetic properties of nanosized systems, discrete and continuum approaches Patricio Vargas, David Laroze In this work we present results obtained with a scaled Monte Carlo technique in two systems: The first one is the system proposed at the NIST web page and known as the $\mu $MAg Standard Problem No 3. There, the system is a perfect cube made of a magnetic material, with uniaxial anisotropy and the problem is to find the critical edge length at which the magnetic vortex structure and the most uniform arrangement of magnetization (the so-called flower state) are of equal energy. The second system is a study of the reversion process in an ideally soft magnetic nanowire of cylindrical shape, (diameter of 60nm and 1$\mu $m length) at room temperature. By calculating magnetic energies, using continuum and exact methods, in a simple one dimensional discrete system, we show that the usual scaling technique used in some micromagnetic models breaks down at the nanometer scale. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R23.00006: Micromagnetism in the ultrathin limit Danilo Pescia, Oliver Portmann, Matthias Buess, Alessandro Vindigni, Andreas Vaterlaus, Christian Horst Back We derive some results concerning the static and dynamic micromagnetic behavior of magnetic elements in the ultrathin limit. In this limit, a most remarkable, shape-independent, logarithmic correction of the magnetostatic energy appears, produced by the long-range character of the dipolar interaction. Apparently unrelated phenomena such as the pinning of the precessional motion at some of the boundaries of a thin magnetic element, the non-pinning at other boundaries and a multi-to-single-domain transition in ultrathin elements with perpendicular magnetization have a common origin: the logarithmic correction. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R23.00007: Magnetic properties of coupled Gd/Pd/Ni thin films Ilir Zoto, Gary Mankey Transition metal-rare earth bilayers might allow magnetizations higher than that available from transition metal if the strong exchange interactions of the transition metal layer could be used to raise the Curie temperature of an adjacent, high moment rare earth layer. Literature suggested for a ferromagnetic interaction in the Ni-Gd bulk alloys and an increased moment at room temperature when Gd layer is deposited onto thin Co and permalloy films. Recently, we found that Ni/Gd bilayers couple antiferromagnetically at the interface. Adding a spacer layer could change the coupling behaviour. Here, a study of a Gd(15nm)/Pd(tnm)/Ni(10nm) trilayer is presented with t varying from 0.5 to 2.5nm. The hysteresis loops were measured with VSM in the temperature range 5-250K. A Stoner-Wolfarth model simulates the hysteresis loops through the minimization of the energy of the following expression: \[ \begin{array}{l} E=-M_1 Ht_1 \cos (\theta _1 )-M_2 Ht_2 \cos (\theta _2 )+K_1 t_1 \sin ^2(\theta _1 )+K_2 t_2 \sin ^2(\theta _2 ) \\ +J_1 \cos (\theta _1 -\theta _2 )+J_2 \cos ^2(\theta _1 -\theta _2 ) \\ \end{array} \] where J$_{1}$ and J$_{2}$ are the bilinear and biquadratic coupling constants. The simulated loops are in good agreement with the experimental results and confirm the antiferromagnetic coupling of Ni and Gd layers for t$<$2.5nm and the disappearance of the coupling for t$>$2.5nm. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R23.00008: Influence of the antiferromagnet spin structure on exchange bias. R. Morales, Zhi-Pan Li, J. M. Alameda, Ivan K. Schuller The exchange bias phenomenon (EB) is known by a shift of the hysteresis loop along the magnetic field axis ($H_{EB})$. In ferromagnet/antiferromagnet (FM/AF) thin films EB originates from the exchange interaction between FM and AF spins at the interface. Therefore, some theoretical models consider the EB as a purely interfacial phenomenon, but some experimental results are incompatible with these models. In this work we investigate the influence of an internal AF spin structure on EB. FM1/AF/FM2 trilayers have been prepared with disimilar FM1 and FM2 coercivities, so the sample can be cooled down below the AF N\'{e}el temperature with parallel or antiparallel FM magnetizations. It has been observed that $H_{EB}$ changes drastically from the parallel to the antiparallel cooling configuration for AF thicknesses $t_{AF} \quad <$200nm while it hardly varies for $t_{AF} \quad >$300nm. This result demonstrates that the internal spin structure of the AF is a key point to the understanding of EB. Work supported by US-DOE and European Marie-Curie-OIF. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R23.00009: First-principles study on the magnetic and structural properties of Fe-Co alloys Dangxin Wu, Ping Liu, Qiming Zhang, Ruqian Wu Fe$_{x}$Co$_{1-x}$ alloys with different compositions are investigated using first-principles methods, FLAPW and PAW, based on density functional theory. The structural geometries of the alloys were optimized at a given composition by PAW method implemented in VASP. The initial structures were fcc and hcp for Co-rich systems, and bcc for Fe-rich systems. The formation energies were then calculated and the stability of the alloys was studied. For those systems with lower symmetries and favorable formation energies, the magnetic properties, such as the saturated magnetization and the magneto-crystalline anisotropy, were carefully studied using the FLAPW method. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R23.00010: Magnetic depth profile of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ / YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ trilayers S.G.E. te Velthuis, A. Hoffmann, V. Pe\~{n}a, D. Arias, C. Leon, J.L. Martinez, J. Santamaria, M.R. Fitzsimmons, B.J. Kirby, M. Varela Recent experiments showed magnetoresistance in excess of 1000\% in epitaxial tri- layers containing highly spin polarized La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) manganite and high Tc superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [1]. This large magnetoresistance originates from spin imbalance due to the injection of spin polarized carriers. In a series of trilayers, with varying LCMO layer thickness, polarized neutron reflectometry has determined the detailed magnetization depth profile. For trilayers exhibiting a strong magnetoresistance, differences in the magnetization profiles of the two LCMO layers is observed, providing a mechanism for antiferromagnetic alignment during the magnetization reversal process. Obtained details of the magnetization at the interfaces lead to a better understanding of these variations. [1] V. Pe\~{n}a {\it et al.}, Phys. Rev. Lett {\bf 94} (2005) 057002. [Preview Abstract] |
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