Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D32: Focus Session: Spin Transfer Torque I |
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Sponsoring Units: GMAG DMP FIAP Chair: Bill Rippard, National Institute of Standards and Technology, Boulder Room: Morial Convention Center 225 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D32.00001: Switching behavior of a Stoner-Wohlfarth particle subjected to spin-torque effect Huy Pham, Dorin Cimpoesu, Alexandru Stancu, Leonard Spinu The concept of the ``spin-transfer torque'' proposed by Slonczewski and Berger offers a new way of controlling the magnetization reversal in ferromagnetic multilayer systems, which replaces the conventional method utilizing magnetic field. The novel technology is expected to reduce the switching time of magnetization as well as to increase the recording density of the magnetoresistive random access memories. In this paper the switching properties of a Stoner-Wohlfarth magnetic particles, subject to a continuous or short magnetic field pulses, and to a short current pulse are presented. The theoretical investigation of precessional motion is described by using phenomenological modified Landau-Lifschitz-Gilbert equation with a spin-transfer torque term included. The switching under the influence of spin transfer torque is discussed as a function of the applied field strength and direction, and also as a function of the length of the current pulse. The main goal is to determine the parameters of field pulse for that the fast and stable switching can be achieved. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D32.00002: Dynamical Coupling of Nanomagnets due to Spin Transfer Sergei Urazhdin, Weng-Lee Lim, Nicholas Anthony, Andrew Higgins Spin transfer devices typically incorporate a thick magnet polarizing the electric current, and a thin layer driven by spin torque. However, spin torque acting on both layers is significant in devices with comparable thickness of magnetic layers. Moreover, dynamics of one of the magnetic layers results in oscillations of the polarization of the current flowing through the other layer, which can lead to dynamical coupling between them. We discuss results of simulations and measurements, demonstrating several consequences of such dynamical coupling. First, the dynamics of both layers are always simultaneously excited by the current. Second, the critical current for the onset of magnetic dynamics is scaled by the ratio of the thicknesses of the magnetic layers, diverging when the two are the same. This behavior is caused by the coupled precession of two magnetic layers reducing the efficiency of spin transfer. Below the critical current, a hysteretic regime is found in which a dynamical state and a static parallel configurations are possible. This regime may explain the 1/f noise and broad precession peaks that are often observed in the spectra of current-induced excitations. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D32.00003: Magnetic propeller driven by spin transfer Weng Lee Lim, Nicholas Anthony, Andrew Higgins, Sergei Urazhdin Spin-transfer devices usually contain two magnetic layers in which the thicker layer polarizes the electron current and the thinner layer experiences dynamics due to spin transfer. However, both magnetic layers can polarize current and experience simultaneous dynamics when the thickness of magnetic layers is similar (symmetric nanopillars). We investigated current-driven magnetization switching in \textit{symmetric} nanopillars with structure Ni$_{80}$Fe$_{20}$= Py(4nm)/Cu(3.5nm)/Py(4nm). Time-resolved measurements of resistance for both directions of current and magnetic field showed reversible switching of magnetization between parallel (P) and anti-parallel (AP) states with unusual dependence on the current. We observed that the dwell times displayed two different dependences on the current $I$ for different values of applied field $H$. At large $H$, the dwell time in the P state $t_{P}$ decreases with increasing $I$ while the dwell time in the AP state $t_{AP}$ increases, similarly to asymmetric devices. However, at small $H$, both $t_{P}$ and $t_{AP}$ decrease with increasing $I.$ We explain this unusual behavior by a thermal activation model involving four-cycle sequential reversal of two magnetic layers. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D32.00004: Coherence of spin-torque microwave oscillators Invited Speaker: Recently discovered effect of microwave generation in current-driven magnetic nano-structures caused by the spin-transfer torque opens a possibility for the development of a new class of tunable microwave auto-oscillators. The spin-torque oscillators (STO) are strongly nonlinear as their frequency $\omega (P)$ and total (positive plus negative) damping $\Gamma (P)$ are dependent on the oscillation power $P$ . We developed a theory of the generation linewidth of a \textit{nonlinear} auto-oscillator, and showed that the nonlinear frequency shift (characterized by the coefficient$N=d\omega /dP)$ leads to an effective increase of the phase noise. In a strongly supercritical regime, when the oscillation energy $E(P)$ is much larger than the thermal energy $k_{B}T$, the generation linewidth of a STO can be written as $\Delta \omega =\Delta \omega _0 [1+(N/\Gamma _{eff} )^2]$, where $\Delta \omega _0 =\Gamma (0)[k_B T/E(P)]$ is the oscillator linewidth without account for the nonlinear frequency shift and $\Gamma _{eff} =d\Gamma /dP$ is the effective nonlinear damping of the oscillator. Our theory explains the following features of the STO linewidth observed in experiment: (i) general linewidth narrowing with the increases in the bias current $I$ and the oscillation energy $E(P)$; (ii) presence of a minimum in the linewidth dependence on the orientation of the external bias magnetic field; (iii) linear dependence of the linewidth on the absolute temperature. Our theory also demonstrates that in the array of $n$ phase-locked STO the generation linewidth decreases linearly with the increase on the number of oscillators $n$, while the generated power $P $ increases as $n^2$. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D32.00005: Magnetization excitations in magnetic nanopillars induced by a d.c. spin polarized current Nikoleta Theodoropoulou, Amit Sharma, William Pratt Jr., Jack Bass We have measured spin-transfer-torque driven magnetization dynamics at 293K in Py(24nm)/Cu(10)/Py(6) magnetic nanopillars, with the top Py(6) magnetic layer and part of the Cu layer shaped into a 140x70 nm$^{2}$ ellipse, and the rest left extended. Among the more interesting results are sharp peaks at zero applied field when a large enough negative d.c. current, I, is applied. The oscillations extend from 0.6 to 4 GHz. They disappear when a magnetic field, $\mu _{0}$H, larger than 10 mT is applied in the plane of the layers, but persist up to 0.2 T when $\mu _{0}$H is applied perpendicular to this plane. The peaks persist up to 5-9 times the switching current and appear to be current-hysteretic. At $\mu _{0}$H =0, the frequency of the oscillations increases with I ($\sim $40MHz/mA). Except for the direction of I, these observations seem to be consistent with the vortex dynamics reported by the NIST and Cornell groups. If time permits, we will present results on magnetic nanopillars where the Py(24) layer has been replaced by NiCr, which inverts the spin asymmetry. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D32.00006: Resonant Spin-Transfer-Driven Switching of Magnetic Spin Valves Assisted by Microwave Current Pulses Yong-Tao Cui, Jack C. Sankey, Chen Wang, Kiran V. Thadani, Zhi-Pan Li, Robert A. Buhrman, Daniel C. Ralph Spin transfer torque from an electrical current can reverse the magnetization in a nanomagnet. We show experimentally that applying a microwave-frequency current pulse in addition to a DC pulse can improve switching characteristics at low temperature by exciting a nanomagnet resonantly at its precession frequency. We study spin valve nanopillars with the structure IrMn(8 nm)/permalloy(4 nm)/Cu(8 nm)/permalloy(4 nm) where exchange bias causes an initial offset angle of $\sim $45 degrees between the permalloy magnetizations. We apply nanosecond-scale microwave-frequency current pulses prior to completing the switching with a DC current pulse. We find that the probability of successful switching has a resonant dependence on frequency, and it also depends on the phase of the microwaves at the moment when the DC pulse is applied. With a microwave pulse, the DC pulse length required for switching is shorter and has a narrower distribution compared to switching driven by a DC pulse alone. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D32.00007: Phenomenological model for size-dependent switching behavior in spin transfer torque devices Venkatesh Chembrolu, Yves Acremann, John Paul Strachan, Xiaowei Yu, Ashwin Tulapurkar, Jordan Katine, Mathew Carey, Tolek Tyliszczak, Joachim Stohr Recent results based on time resolved x-ray imaging of magnetization dynamics in nano-magnetic devices have shown size dependent trends in the switching behavior. Samples with a lateral dimension of 100x180nm show a vortex-driven switching mechanism, whereas smaller samples with a lateral dimension of 110x150nm do not switch by a vortex. Further studies have shown that when a non-zero angle in introduced between the fixed and the free layers, vortex-driven switching becomes manifest in samples with smaller dimensions also. Here, we would like to present a phenomenological model based on linearlized LLG equations to explain the various regimes of observed switching behaviors. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D32.00008: Linewidths of Spin-Torque-Driven Nanomagnetic Oscillators as a Function of Field Angle and Temperature Kiran V. Thadani, Z.-P. Li, O. Ozatay, J.C. Sankey, I.N. Krivorotov, Y.-T. Cui, R.A. Buhrman, D.C. Ralph, G. Finocchio In a magnetic multilayer device, spin-transfer torque from a direct current can excite steady-state magnetic oscillations. We observe that the linewidths of the oscillations decrease dramatically as an applied magnetic field is rotated away from the magnetic easy axis towards the in-plane hard axis. Micromagnetic simulations show that the spatial coherence of the oscillations improves greatly as the field is rotated, and their amplitude increases, making them less susceptible to thermal fluctuations. We report the temperature dependence of the linewidths for the field directions giving the minimum linewidths. It has been suggested previously that the linewidths are dominated by fluctuations of the precession angle of the nonlinear oscillator [1,2]. We analyze the mechanisms governing our linewidths by comparing them to micromagnetic modeling. [1] J. C. Sankey et al., Phys. Rev. B 72, 224427 (2005). [2] J.-V. Kim et al., cond-mat/0703317. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D32.00009: Enhancement in spin-torque efficiency by nonuniform spin current generated within a tapered nanopillar spin valve P.M. Braganca, O. Ozatay, A.G.F. Garcia, O.J. Lee, D.C. Ralph, R.A. Buhrman When modeling spin torque related phenomena, it is generally assumed that the polarization of the incident current is spatially uniform and invariant in time across the surface of the free layer nanomagnet. This is not necessarily the case for a relatively thick, low saturation magnetization reference layer that is patterned as part of a nanopillar structure, where the role of the reference layer on the spin torque dynamics of the system is considerably more complex. Here, we discuss the results of spin-torque micromagnetic simulations, confirmed by both dc and short-pulse switching measurements of nanopillar spin valve structures, which reveal that the use of this type of reference layer can result in non-uniform polarization of the current that impinges onto the free layer. This effect can enhance magnetic reversal in the nanosecond-switching regime over the case of a fixed and uniformly magnetized reference layer, substantially reducing the current amplitude required for magnetic reversal with a given ns pulse-width. We will discuss these results, which differ substantially from descriptions provided using macrospin approximations, and describe a nanopillar spin-torque device configuration that simulations and experiments indicate could be quite effective in reducing the spin torque switching current for MRAM applications. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D32.00010: Theory of Ferromagnetic Resonance in Perpendicularly Magnetized Nanodiscs; Excitation by Injected AC Current Rodrigo Arias, Douglas Mills Recent experiments explore the ferromagnetic resonance (FR) response of nanodiscs incorporated into nanopillars, where a DC spin torque current has a small AC component superimposed. For such a circular perpendicularly magnetized disc, we develop the theory of the FR response via AC current. Earlier we discussed the vortex state induced by the DC Oersted field in such a sample, and the nature of the spin waves in the presence of the vortex$^{2}$. The present study explores the linear response of the disc, when a small AC current is superimposed on the DC current. A Green's function approach allows us to describe the linear response of the system. We argue that the AC component of the Oersted field is responsible for spin wave excitation; the modes excited thus differ from those observed in ferromagnetic resonance studies via microwaves. We shall present calculations which explore the spectrum and eigenvectors of modes excited by modulation of the DC current, their width as a function of DC current, and their intensity. $^{2}$R. E. Arias and D. L. Mills, Phys. Rev. B\textbf{75}, 214404 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D32.00011: Quantum transport in Spin Torque Transfer Devices Sayeef Salahuddin, Deepanjan Datta, Prabhakar Srivastava, Supriyo Datta We present a simulation of tunneling based Spin Torque Transfer (STT) devices using the Non Equilibrium Greens Function (NEGF) formalism in the ballistic regime. Our method is based on effective mass treatment of the magnetic contacts and tunneling oxide, including the effect of transverse modes in the transverse direction. We show that it is possible to achieve a quantitative agreement with experiments for both the tunneling magneto resistance (TMR) and the amplitude of the switching current with the same set of device parameters [1]. We shall talk about some implications of these results in the context of improving the device performance. We shall also briefly discuss how the nature of the torque may change if there is spin flip scattering. [1] S.Salahuddin, Deepanjan Datta, Prabhakar Srivastava and Supriyo Datta, proceedings of International Electron Devices Meeting (IEDM), 2007. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D32.00012: Spin Pumping of Current in Non-Uniform Conducting Magnets Wayne Saslow Using irreversible thermodynamics we show that current-induced spin transfer torque within a magnetic domain implies spin pumping of current within that domain. This has experimental implications for samples both with conducting leads and that are electrically isolated. These results are obtained by deriving the dynamical equations for two models of non-uniform conducting magnets: (1) a generic conducting magnet, with net conduction electron density $n$ and net magnetization $\vec{M}$; and (2) a two-band magnet, with up and down spins each providing conduction and magnetism. For both models, in regions where the equilibrium magnetization is non-uniform, voltage gradients can drive adiabatic and non-adiabatic bulk spin torques. Onsager relations then ensure that magnetic torques likewise drive adiabatic and non-adiabatic currents -- what we call bulk spin pumping. For a given amount of adiabatic and non-adiabatic spin torque, the two models yield similar but distinct results for the bulk spin pumping, thus distinguishing the two models. As for recent spin-Berry phase work, we find that within a domain wall the ratio of the effective emf to the magnetic field is approximately given by $P(2\mu_{B}/e)$, where $P$ is the spin polarization. The adiabatic spin torque and spin pumping terms are shown to be dissipative. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D32.00013: First principles calculation of the spin transfer torques Ke Xia, Yuan Xu, Shuai Wang A first principles method was developed to calculate the spin transfer torques in the noncollinear magnetized system. We found that the behavior of spin torques in a spin valve depends on the materials(Co, Ni and Ni$_{80}$Fe$_{20}$ were taken into account). The formulism is also applied to the anti- ferromagnetic domain wall. It is found that the spin torques could be exerted over a long range in the anti-ferromagnetic materials and move the domain wall away, which can detected by measureing the resistance of an antiferromagnetic point contact. [Preview Abstract] |
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