Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L37: Focus Session: Novel Magnetic Devices: Spin Torque II |
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Sponsoring Units: DMP GMAG Chair: Jonathan Sun, IBM T.J Watson Research Center Room: E147-E148 |
Tuesday, March 16, 2010 2:30PM - 3:06PM |
L37.00001: Temporal Coherence of MgO Based Magnetic Tunnel Junction Spin Torque Oscillators Invited Speaker: Ursula Ebels In magneto-resistive (MR) devices, spin momentum transfer can be used to induce large angle steady state magnetization oscillations that are converted into an AC output voltage. This oscillating output signal has been in most cases investigated in the frequency domain using a spectrum analyser. However, in the case of tunnel junction devices, it is often difficult to clearly distinguish the steady state excitations from thermally excited spin wave modes. In this presentation we investigate the oscillator output signal in the time domain using single shot time domain analysis for MgO based magnetic tunnel junction oscillators with RA values of 1 and 1.5 Ohmmicron2 and TMR values of 30 - 90 {\%} [1]. Single shot time domain studies provide important additional information [2], since they do not average over several signal traces and thus reveal the true transient character of the oscillations. Here we address several issues: (i) the definition of the critical current for a stationary steady state, (ii) `simultaneous' presence of two modes, (iii) the coherence time deduced from the correlation function of the signal, amplitude and phase, (iv) identification of frequency fluctuations and (iv) estimation of the intrinsic linewidth, upon suppressing `numerically' frequency fluctuations [3]. This is illustrated for two types of tunnel junction oscillators that differ in their magneto-resistance value and in their dynamic behaviour. The experimental results will be supported by numerical simulations that take various noise contributions into account. \\[4pt] [1] D. Houssameddine et al., Appl. Phys. Lett. \textbf{93}, 022505 (2008) \\[0pt] [2] I. N. Krivorotov et al., Science \textbf{307}, 228 (2005); I. N. Krivorotov, et al., Phys. Rev. B \textbf{77}, 054440 (2008). \\[0pt] [3] D. Houssameddine et al., Phys. Rev. Lett. \textbf{102}, 257202 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L37.00002: Phase locking of a spin-torque nano-oscillator to a strong microwave magnetic field Andrei Slavin, Vasyl Tyberkevych, Phil Tabor, Sergei Urazhdin Magnetization precession excited by spin transfer effect in a current-driven spin-torque nano-oscillator (STNO) can be phase-locked to an external microwave signal having frequency $f_{e}$ close to the frequency $f_{0}$ of the STNO precession. In previous studies, only the phase-locking of STNO to a microwave \textit{current} was observed. In this work we studied experimentally STNO phase-locking to a microwave \textit{magnetic field} having amplitude $h_{e}$ up to 20 Oe rms. We observed both main ($f_{e}$/$f_{0}$ = 1) and secondary ($f_{e}$/$f_{0}$ = 2) locking regimes. For sufficiently large driving field $h_{e}$ both regimes of the STO phase-locking become hysteretic, with the boundaries of the locking interval dependent on the direction of the sweep of the external frequency $f_{e}$. The bandwidth of the main ($f_{e}$/$f_{0}$ = 1) locking regime was larger than the bandwidth of the secondary ($f_{e}$/$f_{0}$ = 2) regime when the dc bias magnetic field was perpendicular to the microwave field $h_{e}$. In contrast, for parallel orientations of dc and microwave magnetic fields, the secondary synchronization regime was more pronounced. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L37.00003: Nonlinear Synchronization in Spin Torque Magnetic Nano-Oscillators Phillip Tabor, Sergei Urazhdin, Vasil Tiberkevich, Andrei Slavin The magnetic oscillations driven in nanomagnetic devices by spin transfer torque exhibit strongly nonlinear behaviors indicated by the large frequency shifts with the driving current. We demonstrate experimentally that the synchronization of magnetic oscillators with a microwave magnetic field exhibits several signatures of nonlinear behaviors. First, the synchronization occurs not only when the frequency fe of the driving signal is close to the frequency f0 of the oscillation, but also for several other integer and rational relations between the two frequencies. Second, the synchronization exhibits a hysteresis with respect to the driving frequency, i.e. the synchronization limit depends on whether the driving frequency is being increased or decreased. The latter behaviors also depend on the relative orientation of the microwave field with respect to the dc bias field. We show that the observed behaviors can be understood in terms of the nonlinear dynamical properties of the nano-oscillators. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L37.00004: Phase Diagram of Spin-Torque Oscillators with Dual Free Layer Graham Rowlands, Ilya Krivorotov We theoretically study properties of spin torque oscillators that consist of two free layers with easy-plane magnetic anisotropy sandwiched between two fixed layers with strong perpendicular magnetic anisotropy. This type of spin torque oscillator can generate large-amplitude microwave signals with the frequency that is the sum of the precession frequencies of the free layers. Using micromagnetic simulations, we map out the available dynamic and static phases on the current-damping phase diagram of the system. We find that current-induced hysteretic switching occurs between macrospin-like precessional states and static vortex states. Switching between these states takes place via transient vortex states, and we show that its hysteretic nature derives from the energy barrier to vortex creation and annihilation at the free layer edges. We employ Thiele's equations to describe the current-dependent trajectory of the vortex core in the transient regime, thereby developing a model for critical switching currents in this system. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L37.00005: Polarizer angular dependence of spin transfer oscillation in magnetic tunnel junction Yisong Zhang, Hui Zhao, Xuan Li, Andrew Lyle, Jian-Ping Wang MgO based magnetic tunnel junction (MTJ) spin transfer oscillators (STO) are characterized using spin-transfer ferromagnetic resonance (ST-FMR)[1] and spectrum analysis [2]. The spin transfer torque vector angular dependence has been measured by ST-FMR with an external field[3]. However, the angular dependence has not been measured with spectrum analysis. In this report, the angular dependence of spin torque oscillation behavior is systematically investigated in the frequency and time domain without an external field. The angle $\phi $ between the free-layer easy axis and the polarizer is geometrically controlled using e-beam lithography patterning of MgO based MTJ with about 120{\%} MR ratio, 8$\Omega \mu $m2 resistance-area product and 1 MA/cm2 critical switching current density. By analyzing the power spectrum density at different currents and $\phi $, we found that the range of the oscillation onset current increased by a factor of four with $\phi $ from 10\r{ } to 70\r{ }which agrees with numerical analysis. Additionally, the oscillation onset behavior is investigated by real time analysis. [1]J.C.Sanky, et al, Nat. Phys. 4, 67 (2008). [2] A.M.Deac, et al, Nat, Phys. 4, 803 (2008). [3]C.Wang, et al, Phys. Rev. B 79, 224416 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L37.00006: Stochastic Resonance Driven by Spin Torque in Nanoscale Magnetic Tunnel Junctions Xiao Cheng, Jordan Katine, Graham Rowlands, Jian Zhu, Carl Boone, Ilya Krivorotov Application of a microwave current to a nanoscale magnetic tunnel junction (MTJ) gives rise to a rectified voltage, V$_{r}$, generated by magnetization oscillations driven by spin transfer torque. We study the effect of direct current bias on these dynamics in nanoscale MTJs with superparamagnetic free layers. For certain combinations of the direct current and external magnetic field, we observe large (nearly three orders of magnitude) enhancement of V$_{r}$ compared to V$_{r}$ at zero direct current. For a 2-GHz alternating current of 0.04 mA rms amplitude, the rectified voltage reaches 55 mV. The large magnitude of the rectified voltage indicates that large-amplitude magnetization precession is excited by weak ac spin transfer torque in the presence of the direct current bias. The large enhancement of V$_{r}$ takes place only above a threshold temperature, which points to a stochastic character of the observed large-amplitude dynamics. The dependence of V$_{r}$ on temperature and current reveals that this new type of large-amplitude high-frequency dynamics is non-adiabatic stochastic resonance of magnetization excited by spin transfer torque. This new type of magnetic resonance may find use in sensitive microwave signal detectors and magnetic field sensors of nanoscale dimensions. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L37.00007: Asymmetric Voltage Dependence of Spin Torque in Magnetic Tunnel Junction Devices Deepanjan Datta, Behtash Behin-Aein, Sayeef Salahuddin, Supriyo Datta A key mystery in our current understanding of spin-transfer torque in Magnetic Tunnel Junction (MTJ) devices is its voltage asymmetry. Experimentally it is seen that the free ferromagnetic layer of a MTJ device experiences a larger torque when a negative (rather than positive) voltage is applied to the fixed layer. This remains a key unsettled issue since there is no intrinsic difference between two magnets that could cause the effect. Theoretical treatments are divided on this issue and no explanation is available at this time. In this paper, we provide a simple physical explanation, based on the polarization of both ferromagnetic contacts in the energy range of transport. Our explanation is justified on the basis of a Non-Equilibrium Green's Function (NEGF)-based model, which we also believe to be the first theoretical model that provides quantitative agreement with experimentally measured (1) differential resistances, (2) Magnetoresistance (MR), (3) In-plane torque and (4) Out-of-plane torque over a range of bias voltages. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:54PM |
L37.00008: Self-consistent calculations of transport and magnetization dynamics Invited Speaker: Kyung-Jin Lee In layered structures like spin-valves where the current flows perpendicular to the plane, the direction and magnitude of the spin transfer torque (STT) at a point r is decided by the spin accumulation (SA) and associated spin current at the same point r. Initial STT theories commonly assumed that the dependence of SA on magnetization (M) is local and thus essentially fixed by the local M at the same point r. However, its dependence on M is inherently nonlocal because of the 3-dimensional spin diffusion [1]. In other words, when the conduction electron arrives at a point r on the ferromagnet-normal metal interface, the reflected (transmitted) electron takes the spin direction anti-parallel (parallel) to the local M at the point r, diffuses along the interface, and then transfers its spin-angular momentum to another local M at a far away point from the r. That is, SA at a point r is affected by all local M's at other points. The local assumption becomes really invalid when M is inhomogeneous. Note that micromagnetic and time-resolved imaging studies [2] have revealed excitations of incoherent spin-waves and thus inhomogeneous M due to STT. In this situation, the effect of SA on M (=STT) and the nonlocal effect of M on the SA should be treated on an equal footing. The conventional treatments, which ignore the latter part, actually deal with only half of the relevant parts. Therefore, the self-consistent feedback between inhomogenous M and STT through the nonlocal effect should be considered. In this talk, we present self-consistent calculation results that consider the feedback, which allows us to understand peculiar spin-wave modes in a single ferromagnet and a spin-valve. If time is allowed, we extend our talk to other feedback mechanisms which result in the oscillatory STT due to ballistic spin transport [3] and the damping tensor due to the spin-motive force [4] in a very narrow magnetic domain wall. These works have been done in collaboration with Hyun-Woo Lee at POSTECH, Jung-Hwan Moon and Sang-Il Kim at Korea University.\\[4pt] [1] M. L. Polianski and P. W. Brouwer, Phys. Rev. Lett. 92, 026602 (2004); M. D. Stiles, J. Xiao and A. Zhangwill, Phys. Rev. B 69, 054408 (2004); A. Brataas, Y. Tserkovnyak and G. E. W. Bauer, Phys. Rev. B 73, 014408 (2006).\\[0pt] [2] K.-J. Lee et al. Nature Mat. 3, 877 (2004); K.-J. Lee and B. Dieny, Appl. Phys. Lett. 88, 132506 (2006); Y. Acremann et al. Phys. Rev. Lett. 96, 217201 (2006).\\[0pt] [3] J. Xiao, A. Zhangwill and M. D. Stiles, Phys. Rev. B 73, 054428 (2006).\\[0pt] [4] S. Zhang and S. S.-L. Zhang, Phys. Rev. Lett. 102, 086601 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L37.00009: Frequency Domain Studies of Current-Induced Magnetization Dynamics in Single Magnetic-Layer Nanopillars Gernot Guntherodt, Nicolas Musgens, Sarah Fahrendorf, Bernd Beschoten, Barbaros Oezyilmaz, Alexander Heiss, Joachim Mayer In spin-transfer torque studies on single ferromagnetic(FM)-layer nanopillars [1] the magnetization dynamics could be inferred only indirectly by changes in the differential resistance. Here we present the first proof of current-induced spin excitations in the frequency domain in asymmetric Cu/Co/Cu single FM-layer nanopillar devices. Circular shape (diameter $<$ 100 nm) and magnetic fields perpendicular to the Co layer are used. For negative current polarity only we observe spin wave excitations in the GHz regime with minimum linewidths of 4 MHz for 15-nm thick Co layers at room temperature. Low frequency modes (f $\sim $ 2 GHz), decreasing upon increasing the absolute current, are attributed to vortex core precessions. High frequency modes (f $\sim $ 10 GHz), increasing with absolute current, are assigned to transverse spin waves. Frequency jumps indicate transitions between localized modes. - [1] B. Oezyilmaz et al., Phys. Rev. Lett. 93, 176604 (2004). [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L37.00010: Bias-voltage dependence of perpendicular spin transfer torque in asymmetric MgO-based magnetic tunnel junctions S.-C. Oh, S.-Y. Park, A. Manchon, M. Chshiev, J.-H. Han, H.-W. Lee, J.-E. Lee, K.-T. Nam, Y. Jo, Y.-C. Kong, B. Dieny, K.-J. Lee It has been demonstrated [1] that the magnetic tunnel junction (MTJ) has a sizable perpendicular spin-transfer torque (p-STT), which could substantially affect current-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of p-STT is quadratic [2], it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence [3]. In this talk, we present experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs [4]. The linear contribution is significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the p-STT, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena. [1] I. Theodonis et al. PRL 97, 237208 (2006); C. Heiliger {\&} M. D. Stiles, PRL 100, 186805 (2008). [2] J. C. Sankey et al. Nature Phys. 4, 67 (2008); H. Kubota et al., ibid 4, 37 (2008). [3] J. Xiao, G. E. W. Bauer {\&} A. Brataas, PRB 77, 224419 (2008). [4] S.-C. Oh et al. Nature Phys., published online (2009). [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L37.00011: Perpendicular spin torque in circularly exchange-biased trilayer structures Olle Heinonen Over the past few years, it has become clear that both in-plane and perpendicular spin torques are important for the magnetization dynamics in magnetic tunnel junctions. However, at the present the perpendicular spin torque, in particular its dependence on bias voltage, is not well understood. I will here show that both sign and magnitude of the perpendicular spin torque in magnetic tunnel junctions can be determined as a function of bias voltage by measuring the lowest eigenfrequency of a circularly exchange-biased system. A simple model allows for a qualitative and quantitative analysis. [Preview Abstract] |
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