Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B19: Focus Session: Spin Transport & Magnetization Dynamics in Metals II |
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Sponsoring Units: GMAG DMP Chair: Hendrik Ohldag, SLAC National Accelerator Laboratory Room: D170 |
Monday, March 21, 2011 11:15AM - 11:51AM |
B19.00001: Effects of disorder on magnetic vortex dynamics Invited Speaker: Experimental measurements of domain wall propagation are typically interpreted by comparison to reduced models that ignore both the effects of disorder and the internal dynamics of the domain wall structure. Using micromagnetic simulations, first we study vortex wall propagation in magnetic nanowires induced by fields or currents in the presence of disorder. We show that the disorder leads to increases and decreases in the domain wall velocity depending on the conditions. These results can be understood in terms of an effective damping that increases as disorder increases. As a domain wall moves through disorder, internal degrees of freedom get excited, increasing the energy dissipation rate [1]. Next we study the effect of disorder on vortex gyration in a magnetic disc. A vortex gyrating in a magnetic disc has two regimes of motion in the presence of disorder. At large gyration amplitudes, the vortex core moves quasi-freely through the disorder potential. As the amplitude decreases, the core can become pinned at a particular point in the potential and precess with a significantly increased frequency. In the pinned regime, the amplitude of the gyration decreases more rapidly than it does at larger precession amplitudes in the quasi-free regime. In part, this decreased decay time is due to an increase in the effective damping constant and in part due to geometric distortion of the vortex. A simple model with a single pinning potential illustrates these two contributions [2]. \\[4pt] [1] Hongki Min, Robert D. McMichael, Michael J. Donahue, Jacques Miltat, and M. D. Stiles, Phys. Rev. Lett. {\bf 104}, 217201 (2010).\\[0pt] [2] Hongki Min, Robert D. McMichael, Jacques Miltat, and M. D. Stiles (unpublished). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B19.00002: Spin-torque-driven excitations in magnetic thin films C. Wang, H. Seinige, T. Staudacher, M. Tsoi Spin transfer torque (STT) refers to a novel method to control and manipulate magnetic moments using an electrical current. For the past decade it has proven to be a fascinating domain of research with a number of manifestations in various systems interesting both from fundamental science's point of view as well as for technological applications. In ferromagnetic/nonmagnetic (F/N) multilayers a dc electrical current can switch and/or drive its constituent F parts into high-frequency precession which is of interest for microwave and magnetic recording technologies. Interestingly, application of high-frequency currents can also drive the multilayer, e.g., into ferromagnetic resonance (STT-FMR) precession. In our experiments we use point contacts to inject high microwave currents into a variety of magnetic thin films including NiFe/Cu/NiFe/IrMn and NiFe/Cu/Co spin valves, and single ferromagnetic (NiFe or Co) films. The resulting magnetodynamics are detected electrically when a small rectified dc voltage appears across the contact at resonance. We find that in addition to a standard FMR, the microwave currents can excite other resonance modes in our point contacts. We study the behavior of the excitations as a function of applied magnetic field, dc bias current, and microwave frequency. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B19.00003: Minimization of Ohmic losses for domain wall motion in ferromagnetic nanowires Artem Abanov, Oleg Tretiakov, Yang Liu We study current-induced domain-wall motion in a narrow ferromagnetic wire. We propose a way to move domain walls with a resonant time-dependent current which dramatically decreases the Ohmic losses in the wire and allows driving of the domain wall with higher speed without burning the wire. For any domain wall velocity we find the time-dependence of the current needed to minimize the Ohmic losses. Below a critical domain-wall velocity specified by the parameters of the wire the minimal Ohmic losses are achieved by dc current. Furthermore, we identify the wire parameters for which the losses reduction from its dc value is the most dramatic. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B19.00004: Equilibration in All-Perpendicular Spin Valves Subject to Short Current Pulses * Daniel Bedau, Huanlong Liu, Jonathan Sun, Jordan Katine, Eric Fullerton, Stephane Mangin, Andrew Kent Our recent experiments have shown that all-perpendicular spin valves can be switched by short current pulses ($<$5 ns) [1]. In this limit we found that the switching probability only depends on the spin-angular momentum in the pulse [1]. However, such studies do not directly resolve the magnetization dynamics and relaxation. To study equilibration of spin valves driven out of equilibrium by short current pulse we have developed a pump-probe method, capable of 50 ps resolution. A probe pulse, a variable delay after the pump pulse, is used to determine the magnetization relaxation rate. When the delay between the pump and probe pulses is less than 1 ns the net switching probability differs from that at longer delays. An analysis of this difference shows that the free layer angular-momentum decays exponentially with time after the pump pulse. From these studies we obtain a lifetime, which we use to estimate the free layer damping. [1] Bedau et. al. Appl. Phys. Lett. 96, 022514 (2010) {\&} ArXiv:1009.5240 *supported by: USARO Grant No. W911NF0710643 [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B19.00005: Enhanced magnetization drift velocity and current polarization in (CoFe)$_{1-x}$Ge$_x$ alloys Robert McMichael, Meng Zhu, Brian Soe, Matt Carey, Stefan Maat, Jeff Childress We present measurements of current spin polarization and magnetization drift velocity in (CoFe)$_{1-x}$Ge$_x$ alloys $(x \leq 0 \leq 0.3)$, using a spin wave Doppler technique where spin wave transmission is measured between fixed-wavevector antennas coupled to current-carrying wires [1,2]. In a current density $J$, the transmission resonance frequency is shifted by $\Delta f = kv/2\pi$, where $v=Jg\mu_{\rm B}P/(2eM_{\rm s})$ is a magnetization drift velocity. Measurement of $\Delta f$ allows calculation of $v$ and current spin polarization $P$. With increasing Ge concentration, $v$ increases dramatically from (3.1 $\pm$ 0.2) m/s for CoFe to (8.2 $\pm$ 0.6) m/s for (CoFe)$_{0.7}$Ge$_{0.3}$ ($J= 10^{11}$ A/m$^2$). We attribute this increase in drift velocity primarily to decreased magnetization. The current polarization increases from 0.84 $\pm$ 0.04 for CoFe and reaches a maximum of 0.95 $\pm$ 0.05 at approximately 25\% Ge.\\[4pt] [1] V. Vlaminck and M. Bailleul, Science, 322, 410 (2008)\\[0pt] [2] M. Zhu, C. L. Dennis and R. D. McMichael, Phys. Rev. B. 81, 140407R (2010). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B19.00006: Planar spin-transfer device with dynamical polarizer and analizer Yaroslaw Bazaliy, Anton Kravchenko The behavior of the planar spin-transfer devices with monodomain magnetic layers can be described by the macrospin Landau-Lifshitz-Gilbert (LLG) equation with spin-transfer terms. The LLG description of a device with two layers is simplified after applying the overdamped, large easy-plane anisotropy approximation. A decrease of the magnetic layer thickness asymmetry creates a transition from the conventional polarizer-analizer (``fixed layer -- free layer'') operation regime to the regime of the nearly identical magnets. Here electric current leads to a ``Slonczewski windmill'' dynamic state, rather than producing the magnetic switching. The ``windmill'' precession state of a device with two free layers was investigated by numerical solution of the LLG equation. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B19.00007: Optimal field sweep rate in magnetic switching of a single-domain particle Shu Yan, Yaroslaw Bazaliy, Andrzej Stankiewicz The speed of magnetic switching is an important parameter of memory cells. We consider a magnetic moment with an easy axis anisotropy switched by an external field applied at a small angle to the axis. By solving the Landau-Lifshitz-Gilbert(LLG) equation numerically, it is found that the switching time of the magnet is not monotonically increasing with the field sweep rate of the applied field. The dependence has a minimum, i.e., there exists an optimal field sweep time. Analytic approximations are derived for the dependence of the switching time on the field sweep rate and for the value of the optimal field sweep time. Our results have important implications for the optimization of magnetic memory devices. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B19.00008: Ballistic (precessional) contribution to the conventional magnetic switching Andrzej Stankiewicz, Ya. B. Bazaliy, Shu Yan We consider a magnetic moment with an easy axis anisotropy energy, switched by an external field applied along the axis. Additional small constant bias field is applied perpendicular to the axis. It is found that the magnet's switching time is a non-monotonic function of the rate at which the field is swept from ``up'' to ``down''. Switching time exhibits a minimum at a particular optimal sweep time. This unusual behavior is explained by the admixture of a ballistic (precessional) rotation of the moment caused by the perpendicular bias field in the presence of a variable switching field. Analytic approximations are derived for the dependence of the switching time on the field sweep rate and for the value of the optimal field sweep time. The existence of the optimal field sweep time has important implications for the optimization of magnetic memory devices. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B19.00009: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 1:27PM - 1:39PM |
B19.00010: AC Magnetic Susceptibility and $\mu $SR Study of Spin Dynamics and the Onset of Magnetic Correlations in LiHo$_{x}$Y$_{1-x}$F$_{4}$ R.C. Johnson, B.Z. Malkin, A. Lascialfari, A. Amato, C. Baines, J.S. Lord, S.R. Giblin, B. Barbara, M.J. Graf The onset of correlation effects in the magnetic Ho$^{3+}$-subsystem in LiHo$_{x}$Y$_{1-x}$F$_{4}$ single crystals is studied by comparing measurements and simulations of the field and frequency dependent magnetic AC susceptibility at 1.8 K and field and temperature dependent muon depolarization rates ($\mu $SR) for the concentrations x=0.0017, 0.0085, 0.041 and 0.0855. Specific features in the field and frequency dependence of in-phase and out-of-phase susceptibilities, in particular, broadening of peaks (dips) in $\chi $' ($\chi $'') that indicate enhanced relaxation processes at field induced avoided level crossings, can be associated with x-dependent changes of cross relaxation rates and the phonon bottleneck effect in the spin-lattice relaxation. The observed peak in the measured temperature dependent muon relaxation rate appears to be related to a maximum at the frequency 60 cm$^{-1}$ in the acoustic phonon density of states. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B19.00011: Temperature dependence of the effective exchange and biquadratic coupling in ferromagnets: Calculation in the disordered local moment method Paul Larson, Kirill Belashchenko We have implemented the disordered local moment (CPA-DLM) method within the tight-binding linear muffin-tin orbital (LMTO) basis. This implementation self-consistently determines the angular distribution function of the generalized Heisenberg model and the angular-dependent local potentials in the symmetry-broken state; the CPA procedure involves numerical integration over the polar angle of the spin. We present benchmark calculations for several materials including Fe, Co, FePd, FePt, and CoPt. We further extract the temperature dependence of the effective exchange and biquadratic coupling parameters from the angular dependence of the single-site grand potential. We find that the effective exchange parameter in Fe is almost temperature-independent, while the biquadratic interaction is entirely negligible at all temperatures. In FePd the effective exchange varies noticeably as a function of temperature, while the biquadratic coupling is somewhat more pronounced but still relatively small. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B19.00012: Effect of occupation numbers on exchange coupling in low dimensional magnetic nanostructures D. Terrade, H. X. Yang, A. Kalitsov, L. Nistor, M. Chshiev, B. Dieny Interlayer exchange coupling (IEC) has been of great interest for spintronic community and has been shown directly related to equilibrium spin current (ESC). Here we present a study of the influence of the electronic occupation numbers on the angular dependence of the IEC in magnetic layered nanostructures with finite thickness ferromagnetic (FM) layers. The calculations were performed within the tight-binding model using the nonequilibrium Green function technique both within perturbation theory and exact diagonalization approaches. We found that the period of IEC oscillations as a function of FM layer thickness has nonmonotonic variation with electronic states occupation numbers (Fermi level position). In the limit of 2-site model it is found that perturbation theory fails to describe correctly exchange coupling angular dependence always giving sinusoidal behavior for the ESC while the exact solution alternates between sinusoidal and strongly nonsinusoidal behavior as a number of electrons in the system is varied. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B19.00013: Spin and phase coherence times in lithographically defined bismuth wires Martin Rudolph, J.J. Heremans We performed low temperature magnetotransport measurements on lithographically defined semimetal thin film bismuth wires and used the weak-antilocalization effect to determine spin and phase coherence times. Purpose-made Bi mesoscopic structures have not been extensively studied, yet are of interest due to the strong spin-orbit coupling in the material and its surface states. The spin and phase coherence times in mesoscopic Bi wires are here studied as function of temperature and wire width. The phase coherence time saturates at temperatures below 2 K, and appears limited by electron-phonon interactions above 2 K. The spin coherence time shows a dependence on width unexpected in Bi thin films. The spin coherence time increases as the width is reduced, similar to the dependence observed in wires fabricated on spin-orbit coupled semiconductor two-dimensional systems. The similarity may be an indication that the weak-antilocalization signature is dominated by two-dimensional strongly spin-orbit coupled Bi surface states (DOE DE-FG02-08ER46532). [Preview Abstract] |
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