Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A15: Focus Session: Theory of Magnetization Dynamics |
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Sponsoring Units: DCOMP DMP GMAG Chair: Allan MacDonald, University Of Texas, Austin Room: Morial Convention Center 207 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A15.00001: Bias dependence of magnetic exchange coupling. Paul Haney, Mark Stiles, Christian Heiliger, Allan MacDonald An applied electrical bias can change the interlayer coupling in magnetic multilayers and magnetic tunnel junctions. The bias dependence of these changes is controversial; it is not clear whether the changes depend linearly or quadratically on the applied voltage. Motivated by this controversy, as well as proposals to exploit bias-dependent exchange coupling to accomplish current induced magnetic switching, we compute the bias-dependence of interlayer exchange coupling in magnetic multilayers and tunnel junctions. For simple tight-binding models, we derive expressions for this dependence, describe the special cases in which this dependence is particularly large, and derive the extent to which zero-bias expressions for interlayer coupling remain valid for biased systems. We also examine the related question of the bias-dependence of intralayer exchange interactions in a single ferromagnetic layer, and discuss experimental consequences of bias-modulated exchange stiffness, including induced changes in the Curie temperature and spin wave dispersion. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A15.00002: Spin current and rectification in quantum wires Feifei Li, Bernd Braunecker, Dima Feldman We study the spin and charge currents in a one-channel quantum wire with strong electron interactions in a uniform static magnetic field. We show that a dc-spin current can be generated by an ac voltage in the presence of an asymmetric potential barrier, e.g., two point scatterers of unequal strength. In an interval of voltages, the spin current increases with the decrease of the voltage bias as a negative power of the voltage. We find that the spin dc-current in units of $\hbar/2$ per second can greatly exceed the charge current in units of electron charge per second. Neither spin-polarized particle injection nor time-dependent magnetic fields are required for the generation of the spin current. \newline \newline [1] D. E. Feldman, S. Scheidl, and V. M. Vinokur, Phys. Rev. Lett. 94, 186809 (2005). \newline [2] Bernd Braunecker, D. E. Feldman, and Feifei Li, Phys. Rev. B 76, 085119 (2007). [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A15.00003: Vortices and Antivortices as Harmonic Oscillators B. Krueger, M. Bolte, A. Drews, U. Merkt, G. Meier, D. Pfannkuche In experiments the distinction between current-induced dynamics and the dynamics induced by the Oersted field of the current is still an open problem. Here we investigate the gyroscopic motion of current- and field-driven magnetic vortices/antivortices in micro- or nanostructured thin-film elements by analytical calculations and by numerical simulations. Starting from the micromagnetic equation of motion extended by the spin torque introduced by Zhang and Li, we derive an analytical expression for the current- and field-driven trajectory of the vortex/antivortex. The gyroscopic motion is well described by modeling the stray-field energy as a harmonic-oscillator potential. For small harmonic excitations the vortex/antivortex cores perform an elliptical rotation around their equilibrium positions. Our analytical model allows to calculate the amplitude and phase of the gyration. The phase of the rotation and the ratio between semi-axes are determined by the frequency and amplitudes of Oersted field and spin torque. The analytical result is compared to micromagnetic simulations with good accordance.[1] Even though the influence of weak magnetic fields on the vortex/antivortex trajectories is small, the phase of the rotation is significantly changed. Thus, the model can estimate the Oersted field's contribution in spin-torque experiments.[1] B. Krueger et al., Phys. Rev. B 76, December (2007). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A15.00004: Current-induced torques in magnetic textures and in antiferromagnets Invited Speaker: Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. Spin pumping is the opposite of spin transfer, namely the generation of spin currents by a time-dependent magnetization. In this talk I will discuss recent theoretical work aimed at understanding current-induced torques and spin pumping in situations that spin is not fully conserved, due to e.g., spin-orbit interactions, or when conservation of spin can not be used to infer order-parameter dynamics, as is the case in antiferromagnets. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A15.00005: Spin transfer and the role of spin-motive-forces for spin valves and domain walls Stewart Barnes The interaction of magnetic domains with electrical currents has potentially far reaching applications for spintronics. The requirements of energy conservation are reflected by spin- (smf) and electro-motive-forces (emf) [1] . For spin-valves and domain walls this smf redistributes the currents between the different possible conduction channels in a manner that significantly modifies the dynamics and introduces magnetic relaxation. Our Berry phase approach to domain walls [1,2] has been extended to spin-valves. The results are consistent with the requirements of angular momentum and energy conservation but differ in a number of important ways from those obtained when the Sloncewski torque transfer term is added to the Landau-Liftshitz equations with either Gilbert or Landau-Liftshitz relaxation [3].\break [1] S. E. Barnes and S. Maekawa: Phys. Rev. Lett. {\bf 98}, 246601 (2007)\break [2] S. E. Barnes and S. Maekawa: Phys. Rev. Lett. {\bf 95}, 107204 (2005).\break [3] See e.g., Concepts in Spin Electronics, ed. by S. Maekawa (Oxford Press, 2006). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A15.00006: Dynamic and temperature effects in spin-transfer switching Dorin Cimpoesu, Huy Pham, Alexandru Stancu, Leonard Spinu Recently, the current-induced spin-transfer torque has been proposed as a convenient writing process in high density magnetic random access memory. With increasing demand on the access time, the current pulse shape become important. Also, with memory area density increasing and the memory cell size further shrinking the study of thermal fluctuations in these magnetic structures becomes of extreme importance for their recording thermal stability. In this paper we have studied the dynamic switching in a spin-transfer memory and its dependence on thermal effects. The magnetic layers are assumed to be in the shape of ellipsoids, and each magnetic layer is assumed to be a single domain. The model is based on stochastic Landau-Lifshitz-Gilbert equation, which is numerically integrated, and the switching diagrams, as a function of current pulse amplitude and duration, are presented. Instead of a clear border between switching and non-switching areas we have a transition region, with a layer-like structure with switching/non-switching areas, where the final state is sensitive to current pulse amplitude and duration, to damping constant, and to thermal fluctuations. The extent of the instability region is increasing with the applied current sweep rate. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A15.00007: Frequency degeneracy in spin-torque induced precession Shuxia Wang, Pieter Visscher Magnetic precession in nanometer elements, first studied by Stoner and Wohlfarth in 1948, is central to the understanding of fast switching in magnetic information storage devices. Periodic orbits have recently gained more attention because they can be stabilized (and their frequencies measured) by spin torque techniques. We have observed a surprising and so far largely unexplained degeneracy in this system: when (as is often the case) there are two orbits with the same energy, even if their shapes and sizes are very different, their frequencies turn out to be the same. Although this is easy to show in the highest-symmetry (uniaxial) case, we find it is true far more generally -- for any quadratic energy function with arbitrary anisotropy tensor and arbitrary external magnetic field. We have calculated the frequencies for a random selection of anisotropy tensors and magnetic fields, will show examples of asymmetrical orbits, whose frequencies are equal within numerical accuracy ($\approx 10^{-6}$). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A15.00008: Evaluation of Gilbert damping in half metals Claudia K.A. Mewes, Chunsheng Liu, Mairbek Chshiev, Tim Mewes, William H. Butler According to Kambersk\'{y}'s spin torque correlation model of Gilbert damping [1,2], precessional damping in magnetic systems occurs through a combination of spin-flip exciations and orbital excitations. In half-metallic systems, Gilbert damping is expected to be reduced because of the absence of spin-flip scattering. This makes half-metals interesting potential candidates for information storage technologies especially for use in CPP/GMR read head devices and spin-torque MRAM. Using a combination of first principle calculations to predict the band structure for the half-metal of interest and an extended H\"{u}ckel tight binding model we calculate and discuss the Gilbert damping within the spin torque correlation model for different half-metallic structures, including the Heusler alloys Co$_{2}$MnSi, Co$_{2}$MnGe. [1] V. Kambersk\'{y}, Czech. J. Phys. B \textbf{26}, 1366 (1976). [2] B. Heinrich, D. Fraitov\'{a} and V. Kambersk\'{y}, Phys. Stat. Sol. \textbf{23}, 501 (1967). [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A15.00009: Evaluation of Gilbert damping in transition metals using tight binding schemes Chunsheng Liu, Claudia K.A. Mewes, Mairbek Chshiev, Tim Mewes, William H. Butler Recently first principle calculations of the damping in transition metals have reproduced the unusual temperature dependence observed experimentally [1, 2]. Here we present an alternative method to calculate the Gilbert damping within Kambersk\'{y}'s spin torque correlation model using a combination of first principle calculations and an extended H\"{u}ckel tight binding model. In our scheme we use ab initio calculations (VASP) including spin orbit coupling to obtain the band structure of the transition metal of interest. With the knowledge of the band structure we use a fitting procedure to construct an extended H\"{u}ckel tight binding model which then allows the evaluation of the Gilbert damping parameter. Because of the simplicity of our Hamiltonian, we can converge the integral over the Brillouin of the spin-orbit torque without extraordinary computational effort. We show that our results are in good agreement with the results obtained from previous calculations. [1] K. Gilmore, Y.U. Idzerda and M.D. Stiles, Phys. Rev. Let. \textbf{99}, 027204 (2007). [2] V. Kambersk\'{y}, Phys. Rev. B \textbf{76}, 134416 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A15.00010: Saturation of spin-polarized current in nanometer scale aluminum grains Chris Malec, Yaguang Wei, Dragomir Davidovic \\\\We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples ($\sim G\Omega$), spin-polarized current is carried only via the ground state and the low-lying excited states, leading to a saturation in spin polarized current with bias voltage. Both a qualitative argument based on relaxation rates, and a non-equilibrium transport model are developed and compared. In two samples, the spin-relaxation rate $T_1^{-1}$ for some of the low-lying excited states is comparable to the electron tunnelling rate: $T_1^{-1}\approx 1.5\cdot 10^6 s^{-1}$ and $10^7s^{-1}$, meaning the spin of an electron confined in a metallic grain is highly stable. The ratio of $T_1^{-1}$ to the electron-phonon relaxation rate is in quantitative agreement with the Elliot-Yafet scaling, an evidence that spin-relaxation in Al grains is driven by the spin-orbit interaction. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A15.00011: Kondo Resonance in the Presence of Spin-Polarized Currents Yunong Qi, Jian-Xin Zhu, Shufeng Zhang, Chin-Shen Ting We propose an improved method of the equation of motion approach to study the Kondo problem in spin-dependent non-equilibrium conditions. We found that the previously introduced additional renormalization for non-equilibrium Kondo effects is not required when we use a proper decoupling scheme. Our improved formulation is then applied to address the spin-split Kondo peaks when a spin current injects into a Kondo system. We believe that this work significantly advances our understanding of the non-equilibrium Kondo physics, and our predictions of the Kondo resonance are timely for the application of non-equilibrium spin-related phenomena. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A15.00012: Resolution-dependent mechanisms for bimodal switching time distributions in simulated Fe nanopillars S.H. Thompson, G. Brown, P.A. Rikvold, M.A. Novotny Numerical simulations of magnetization reversals of iron nanopillars in off-axis applied fields at different lattice resolutions reveal bimodal distributions in the switching times (first-passage times through $0$ of the longitudinal magnetization, $M_{\mathrm{z}}$). We show that the mechanisms responsible for these distributions are resolution-dependent. The highest-resolution model, in which the computational cell is smaller than the exchange length, is three-dimensional. Here, the bimodal distribution results from a reversal process in which the pillar sometimes avoids a metastable free-energy well. At medium resolution, the pillar is modeled as a $1$-D stack of spins. The bimodal distribution then reflects whether the reversal starts from one or both ends. Finally, for a low-resolution model in the form of a single spin with an anisotropic potential, the bimodal distribution is an artifact of the definition of a switching event: the result of the spin precessing close to $M_{\mathrm{z}}$~$=0$. While the zero- and one-dimensional models display bimodal switching-time distributions, the mechanisms are different than for the three-dimensional model. Only the latter captures the mechanism that is most interesting from an experimental and device-application point of view. [Preview Abstract] |
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