Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L32: Focus Session: Spin Transfer Torque II |
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Sponsoring Units: GMAG DMP FIAP Chair: Jordan Katine, Hitachi Room: Morial Convention Center 225 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L32.00001: Time-Resolved X-ray Microscopy for Direct Observation of Spin-Torque and Oersted-Field Driven Vortex Gyration M. Bolte, G. Meier, L. Bocklage, A. Drews, B. Krueger, T. Tyliszczak, A. Vansteenkiste, B. Van Waeyenberge, K.-W. Chou, H. Stoll Due to their symmetry, magnetic vortices are ideal candidates for studying the influence of the spin-transfer torque on the local magnetization. The out-of-plane magnetization of the vortex, the vortex core, can be excited to gyrate around its equilibrium position by in-plane magnetic fields or spin-polarized currents. Here we present results from time-resolved X-ray microscopy on permalloy squares with a vortex in the center. Spin-polarized currents with densities of $4.7~-~12~\cdot $~10$^{10}$~A/m$^2$ are laterally driven through the permalloy sample, and the gyration is imaged for different phases of the ac-excitation. The results are compared to micromagnetic simulations, to good agreement. For vortices having opposite chirality a chirality-dependent phase shift of $40^{\circ}$ is observed that is attributed to Oersted fields from the spin-polarized current. An analytical model estimates corresponding field strengths of $40~\mu$T. This study confirms our assumption that Oersted fields from spin-polarized currents cannot be neglected in spin-torque experiments and shows the sensitivity of the measurement technique. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L32.00002: Time-resolved X-ray Imaging of Spin-Torque-Induced Vortex Oscillation Xiaowei Yu, Vlad Pribiag, Yves Acremann, Venkatesh Chembrolu, Ashwin Tulapurkar, Tolek Tyliszczak, Zhipan Li, Robert Buhrman , Joachim Stohr, Hans Siegmann Recent transport measurements demonstrated a persistent oscillation of a magnetic vortex isolated in a nanoscale spin valve structure driven by a d. c. spin-polarized current [1]. The magnetic information is inferred from the giant magnetoresistance (GMR) signal, which depends on the relative average magnetization of the two magnetic layers. Here, we report spatially resolved measurements of the vortex oscillation driven by spin-transfer torque by using advanced x-ray imaging technique. The microwave-frequency vortex oscillation is synchronized to the fast x-ray pulses. Motion images with 70ps time resolution and 30nm spatial resolution reveal the complicated dynamics underlying the previously observed oscillating GMR signal. [1] V. S. Pribiag, I. N. Krivorotov, G. D. Fuchs, P. M. Braganca, O. Ozatay, J. C. Sankey, D. C. Ralph, and R. A. Buhrman, Nature Physics 3, 498 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L32.00003: Spin-current induced magnetic excitations in single magnetic layer nanopillars Bernd Beschoten, Nicolas M\"usgens, Mark Weidenbach, Eva Maynicke, Coen Smits, Matthias B\"uckins, Joachim Mayer, Gernot G\"untherodt We investigate current-induced spin-wave excitations in Cu/Co/Cu single magnetic layer nanopillar devices with asymmetric Cu leads by means of transport and microwave probes at room temperature. The thin film stack is deposited by MBE in prefabricated nanostencil masks with lateral dimensions below 100 nm. At high current densities, we observe narrow excitations (bandwidth $\sim $ 100MHz) and higher harmonics for magnetic fields perpendicular to the layers. The frequency increases with increasing current and magnetic field, which indicates an out-of plane precessional mode as found in bilayer systems (e.g., Kiselev et al., PRL 93, 036601(2004)). Furthermore, we observe frequency jumps as a function of both current and magnetic field, which might originate from transitions between different localized nonlinear spin-wave modes. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L32.00004: Large angle out of plane steady precession induced by spin-transfer with perpendicular to plane polarizer Invited Speaker: The dynamics of a ferromagnetic system is characterized by a conservative precession torque, as well as a non-conservative damping torque. The damping torque is responsible for the realignment of the magnetisation with the equilibrium direction after excitation. Recently it has been shown that the damping torque can be compensated by a spin transfer torque that is due to the interaction between a spin polarized current and the local magnetization. This additional spin transfer torque can lead to auto-oscillations of the magnetization close to constant energy trajectories. The potential exploitation of such large angle auto-oscillations for tuneable microwave devices is currently driving many research efforts. For an in-plane magnetized thin film with uniaxial anisotropy, two types of constant energy trajectories exist which are commonly called in-plane precession (IPP), where the magnetization oscillates around the in-plane energy minimum, and out of plane precession (OPP) where the magnetization oscillates around the out of plane energy maximum [1]. IPP and OPP oscillations differ substantially in their dependence of frequency and amplitude as a function of current and/or applied bias field. In many experiments so far, IPP precessions have been obtained at the threshold current using in-plane magnetized spin valve structures. However, from an applications point of view it will be of interest to excite OPP oscillations since they will lead to a larger output signal than IPP oscillations. Here, we will present experimental evidence of large angle OPP oscillations using a spin torque oscillator that contains a perpendicularly magnetized polarizing layer and an in-plane magnetized analyzing layer in addition to a planar free layer [2]. We will show that OPP oscillations are induced at the threshold current for moderate current densities of 9x10$^{6}$ A/cm$^{2}$. The experimental current-field state diagram as well as the dependence of the frequency vs. current and applied bias field is in good qualitative agreement with macrospin and micromagnetic simulations. Furthermore, due to the planar analyzer, there exist IPP oscillations which allow a direct comparison of the OPP and IPP precession amplitudes. [1] A. N. Slavin, V. S. Tyberkevich Phys. Rev. B 72, 94428 (2005) [2] Houssameddine et al, Nature Materials 6, 441 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L32.00005: Current-Induced Magnetization Switching with a Spin-Polarized Scanning Tunneling Microscope Matthias Bode, Stefan Krause, Luis Berbil-Bautista, Gabriela Herzog, Roland Wiesendanger The understanding of current-induced magnetization switching is in the focus of many ongoing investigations since switching the magnetization by the injection of a spin-polarized current rather than by magnetic fields may open the gateway for new data storage technologies at much higher bit densities. We show how individual superparamagnetic Fe nanoislands with typical sizes of 100 atoms can be addressed and locally switched using a magnetic scanning probe tip. We demonstrate current-induced magnetization reversal across a vacuum barrier and combine it with the ultimate resolution of spin-polarized scanning tunneling microscopy. This technique allows us to clearly separate and quantify three fundamental contributions that are involved in magnetization switching, i.e.\ current-induced spin torque, heating the island by the tunneling current, and Oersted field effects. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L32.00006: Current-Induced Magnetoresistance in Antiferromagnetic Spin Valves Z. Wei, A. Sharma, J. Bass, M. Tsoi Influence of the magnetic state of a ferromagnet (F) on its electronic transport properties has been found in various phenomena, including giant magnetoresistance (GMR) in magnetic multilayers, where the relative orientation of the magnetic moments of F-layers affects the current flow. MacDonald and co-workers recently predicted that a corresponding effect - antiferromagnetic GMR (AGMR) - should exist in structures where F-layers are replaced by antiferromagnets (AFM). To test this prediction, we measured the (closely) current-perpendicular to plane (CPP) magnetoresistance (MR) of three types of AFM spin-valve multilayers: (I) AFM/N/AFM, (II) F/AFM/N/AFM, and (III) F/AFM/N/AFM/F, with a non-magnetic (N) layer between the two AFM layers. We saw no MR in samples of type I or II at any current density j, or of type III when j was small. But large enough j $\sim $ 10$^{13}$ A/m$^{2}$ applied to type III multilayers gave small positive MRs (largest resistance at high field). As these MRs are inverted from the usual GMR associated with the F-layers, they must be due to the AFM layers, and thus be an AGMR. We will describe how this AGMR varied with applied current j and AFM layer thickness [arXiv:0711.0059]. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L32.00007: RF Assisted Spin Transfer Switching in Nanopillar Spin-Valves S.H. Florez, J.A. Katine, M. Carey, L. Folks, O. Ozatay, B.D. Terris We study at low temperature spin transfer torque (STT) driven free-layer magnetization reversal in current perpendicular to plane (CPP) spin valves with in-plane magnetization. The precessional frequencies of the direct current driven pre-switching modes were measured. Based on this characterization we compare the pre-switching and switching behavior, when driven by direct currents only and in the presence of an additional rf current bias. We find interesting rf induced dynamics such as frequency locking as well as effects on the critical switching boundary. These effects appear for applied frequencies close to the dc-only driven pre-switching resonance frequencies. In particular, we observe a reduction in the critical current for switching when applying rf with frequencies slightly below this range. Macrospin simulations (using Slonczewski STT) reproduce well our experimental data and serve as a basis for the development of a phenomenological model that describes the observed behavior. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L32.00008: Noise Reduction and Other Effects of Rare-Earth Doping in Magnetic Spin-Transfer Systems Eric Ryan, O. Ozatay, P. M. Braganca, N. C. Emley, D. C. Ralph, R. A. Buhrman, J. A. Katine At sufficiently large current densities, spin-torque can excite high frequency dynamics in magnetic trilayers. While this effect can lead to interesting new applications and phenomena, it also introduces detrimental effects, such as creating high frequency noise in next generation GMR read heads. Light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase the damping parameter $\alpha $ by several orders of magnitude [1], which should suppress spin-torque effects. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.004 um$^{2}$ Py/Cu/Py nanopillar spin valves with 0 and 2{\%} Tb in the free layer. We will present data from these devices showing that critical switching current, coercive field, and high frequency noise suppression are all increased in the presences of terbium, and that these effects have a strong and extended temperature dependence. Proper choice of materials can lead to even larger effects at room temperature and beyond, an important regime for technological applications. We will explore the mechanism of the enhanced damping effects as a function of temperature in terbium-doped CoFe and NiFe films and devices. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L32.00009: Spin-torque-driven ferromagnetic resonance of Co/Cu/NiCo spin valves Wenyu Chen, Gregoire de Loubens, Jean-Marc Beaujour, Andrew Kent, Jonathan Sun Spin-torque-driven ferromagnetic resonance is a quantitative tool for studying spin-transfer interactions in nanojunctions that enables tests of microscopic models of spin transport [1]. Using this method we have studied Co/Cu/NiCo spin values, in which the NiCo free layer has perpendicular magnetic anisotropy. Perpendicular field swept resonance lines were measured under low amplitude GHz current excitation. The resonance field and linewidth were measured as a function of rf frequency and dc current bias, from which magnetic anisotropy constants and damping parameters were determined [2]. The magnitude of spin transfer torque, d$\tau $/dI, was estimated from both the zero dc bias resonance amplitude and from the change of the resonance linewidth with dc current. These two sets of results are in agreement with each other, and show a sinusoidal dependence of the torque on the angle between the Co and NiCo layer magnetizations in the range studied, 60$^{o}$ to 80$^{o}$. The resulting torque magnitude will be discussed in the context of theoretical models of spin transfer in metallic structures. [1] J. C. Sankey et al., Nature Physics, doi:10.1038/nphys783 [2] W. Chen et al., arXiv/0711.0405 [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L32.00010: Coherent control of magnetic moment dynamics and switching via spin momentum transfer L. Ye, S. Garzon, T.M. Crawford, R.A. Webb, M. Covington, S. Kaka We have measured the switching probability of CoFe/Cu/CoFe nanopillars driven by shaped current waveforms consisting of two $\sim $30ps FWHM pulses with adjustable amplitudes and delay. We observe oscillations in the switching probability as the delay is varied over the timescale of a free precession cycle, demonstrating large sensitivity to precise pulse timing. We also observe a non-monotonic increase in the switching probability as the amplitudes of the two pulses are simultaneously increased, showing that employing larger current pulses does not necessarily increase switching probabilities. Our data shows that two pulses with precisely adjusted amplitudes and delay can switch a nanopillar device with higher probability than a single pulse with equivalent total power, and that $\sim $100{\%} switching probability can be obtained even with short ($\sim $30ps FWHM) pulses. Our results suggest a new set of techniques for studying coherent time-domain magnetic moment dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L32.00011: Magnetic Dynamics of Single-Domain Planar Spin-Transfer Devices Ya. B. Bazaliy We study spin-transfer devices with dynamic magnets characterized by large easy-plane anisotropy. This situation is standard for planar devices where it arises due to the shape anisotropy. Dominating easy-plane anisotropy keeps the motion of the magnetic moment close to the easy plane, with small out-of-plane deviations. As a result, it is possible to approximately describe magnetization vector by the in-plane angle and derive an effective one dimensional equation for that angle in the absence [1] and in the presence [2] of spin-transfer torques. Effective description maps a spin-transfer device problem onto a problem of an ``effective particle'' moving in external potential with variable friction coefficient. The advantage of such a description is that the motion of the effective particle can be qualitatively understood by applying the usual energy conservation and dissipation arguments. We show how the effective description produces analytic results for current induced precession states [3] and predicts unconventional ``stabilization by repulsion'' of static states [2]. // [1] C. J. Garcia-Cervera, Weinan E, J. Appl. Phys. 90, 370 (2001). [2] Ya. B. Bazaliy, Phys. Rev. B 76, 140402(R) (2007). [3] Ya. B. Bazaliy, arXiv:0705.0508, to be published in Appl. Phys. Lett. (2007). [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L32.00012: Electron transport driven by nonequilibrium magnetic textures Yaroslav Tserkovnyak, Matthew Mecklenburg Spin-polarized electron transport driven by inhomogeneous magnetic dynamics is discussed in the limit of large exchange coupling. Electron spins rigidly following the time-dependent magnetic profile experience spin-dependent fictitious electric and magnetic fields. We show that the electric field acquires important corrections due to spin dephasing, when one relaxes the spin-projection approximation. Furthermore, spin-flip scattering between the spin bands need to be taken into account in order to calculate voltages and spin accumulations induced by the magnetic dynamics. A phenomenological approach based on the Onsager reciprocity principle is developed, which allows us to capture the effect of spin dephasing and make connection to the well studied problem of current-driven magnetic dynamics. In addition, we relate and generalize the results that recently appeared in literature. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L32.00013: Spin-Diffusion Lengths in Ag(4{\%}Sn) and Cu(2{\%}Ge) alloys Amit Sharma, Brandon Richard, Quinton Fowler, Reza Loloee, William Pratt Jr., Jack Bass Alloying Ag with a little Sn, or Cu with a little Ge, greatly increases elastic scattering of electrons---i.e., greatly decreases the electron mean-free-path (mfp), but does not produce much spin-flipping---i.e., leaves the electron spin-diffusion length, $l_{, }$relatively long. Thus, dilute AgSn and CuGe alloys were used to study effects of changing the mfp on current-perpendicular-to-plane (CPP) magnetoresistance [1] and current-induced magnetization switching (CIMS) [2], while leaving spin-flipping weak. Published transport data in dilute AgSn and CuGe alloys give only lower bounds for $l$ [3-5]. We find $l$ = 34 $\pm $ 4 nm for Ag(4{\%}Sn) and $l$ = 125 $\pm $ 10 nm for Cu(2{\%}Ge). [1] K. Eid et al., J. Magn. Magn. Mat. \textbf{224}, L205 (2001). [2] N. Theodoropoulou et al., Phys. Rev. B (rapid comm.) in press. [3] S.-F.Lee et al., J. Magn. Magn. Mat. \textbf{118}, L1 (993). [4] J. Bass et al, Mat. Sci. and Eng. \textbf{B31}, 77 (1995). [5] J. Bass and W.P. Pratt Jr., J. Phys. Cond. Matt. \textbf{19}, 183201 (2007). [Preview Abstract] |
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