Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X29: Focus Session: Current Induced Dynamics in Magnetic Tunnel Junctions and Spin Waves |
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Sponsoring Units: GMAG DMP FIAP Chair: Ioan Tudosa, University of California, San Diego Room: 333 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X29.00001: Influence of electron-magnon scattering on spin transfer torque in magnetic tunnel junctions Aurelien Manchon, Shufeng Zhang Manipulating the magnetization direction using spin transfer torque in magnetic tunnel junctions (MTJs) has been one of the most important challenges in spin electronics for the past five years. Elastic tunneling theories show that the torque possesses two components, one being mainly linear in bias voltage whereas the other shows a quadratic bias dependence. These theoretical results have been recently confirmed by ``spin-diode'' experiments [1]. However, the validity of the elastic tunneling has been questioned by two experimental studies [2]. These studies show that the bias dependence of the out-of-plane torque can be dramatically different from the elastic quadratic dependence. Using the Transfer Hamiltonian formalism, we study the influence of interfacial electron-magnon scattering on the bias dependence of the spin transfer torque. We show that this mechanism can strongly affect the bias dependence of the spin transfer torque, in agreement with the recent experimental studies [2]. [1] J. C. Sankey et al., Nature Physics 4, 67 (2008); H. Kubota et al., Nature Physics 4, 37 (2008). [2] S. Petit, et al., Phys. Rev. Lett. 98, 077203 (2007); Z. Li, et al., Phys. Rev. Lett. 100, 246602 (2008). [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X29.00002: Macrospin model of incubation delay due to field-like spin transfer torque Samir Garzon, Yaroslaw Bazaliy, Richard A. Webb, Mark Covington, Shehzaad Kaka, Thomas M. Crawford While extensive measurements have tested the validity and limitations of the macrospin model with Slonczewski's spin transfer torque in metallic spin valves, recent experiments with magnetic tunnel junctions (MTJ's) have reported an additional ``field-like'' or ``perpendicular'' spin torque. The observed field-like torques generally agree with theoretical predictions, but some controversies remain: frequency domain measurements at low voltages and switching current measurements at large voltages report contradictory signs for the field-like term. Here we show that the absence of pre-switching oscillations (``incubation delay'') reported in magnetic tunnel junctions can be explained within the macrospin model by a sizable field-like component of the spin-transfer torque. Furthermore, we propose that measurements of the voltage dependence of the tunnel junction switching time in the presence of external easy axis magnetic fields can be used to determine the magnitude and voltage dependence of the field-like torque over a broad range of voltages. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X29.00003: Single-Shot Time Domain Studies of Spin-Torque-Driven Switching and Precession in Magnetic Tunnel Junctions Yong-Tao Cui, Robert A. Buhrman, Daniel C. Ralph, Daniele Mauri, Jordan A. Katine We present single-shot time domain resistance measurements of spin-transfer-driven dynamics in CoFeB/MgO/CoFeB tunnel junctions. In the regime of thermally-activated current-driven switching, we have sufficient sensitivity to resolve the pattern of resistance oscillations caused by the magnetic dynamics leading up to switching. When an in-plane hard-axis magnetic field is applied, within a short interval before the switching instant the resistance oscillations show a steadily-increasing amplitude, qualitatively consistent with expectations for large-angle precession in a simple macrospin model, although the oscillation amplitude can vary between individual switching events. Coherent large-angle oscillations are generally absent in the case of an applied field along the easy axis, which can be attributed to the differences in the precession axis and switching barriers as well as effects of thermal fluctuations. We will also report results of single-shot transport measurements in the regime of spin-torque-driven steady-state precession. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X29.00004: Bias and Angular Dependence of Spin-Transfer Torque in Magnetic Tunnel Junctions C. Wang, Y.-T. Cui, R.A. Buhrman, D.C. Ralph, J.Z. Sun We report measurements of the spin-transfer torque vector \textit{$\tau $} in MgO-based magnetic tunnel junctions (MTJs) by means of the spin-transfer-driven ferromagnetic resonance (ST-FMR) technique. We point out that for large applied DC biases, $I$, across the MTJ, for best accuracy one should correct for an artifact that arises because an applied microwave current changes the DC resistance of a MTJ slightly, which results in an extra DC voltage signal in the presence of a current bias. The correction depends strongly on the initial offset angle between the magnetizations of the two electrodes in the MTJ, and it can explain the very different results for the bias dependence of \textit{$\tau $} initially reported by two groups. After the correction is applied, we measure consistent values of \textit{$\tau $} ($I)$ over a wide range of offset angles and achieve sufficiently-improved precision to distinguish among competing theoretical predictions. We determine that the in-plane component of \textit{d$\tau $}/\textit{dV }for the MTJs we study has a weak but non-zero dependence on bias. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X29.00005: Effect of asymmetry on the bias dependence of spin torque in magnetic tunnel junctions N. Kioussis, Y. -H. Tang, Alan Kalitsov The switching of magnetic states in magnetic tunnel junctions (MTJ) by spin-polarized current via the spin torque has been the subject of intensive theoretical and experimental researches. One outstanding question which remains unresolved and controversial is the bias dependence of field-like spin torque, T$_{per}$, perpendicular to the plane of the two magnetizations. In this study, we show that bias behavior of T$_{per}$ can change dramatically with the asymmetry in the ferromagnetic electrodes from purely quadratic with negative curvature in agreement with Kubota's experimental results [1], to linear with sign reverse with bias in agreement of Li's observation [2], and finally to quadratic but with positive curvature in agreement with Sankey's experiments [3]. These results suggest that the asymmetry due to the amorphous alloys may cause the discrepancy in the bias dependence of T$_{per}$ in experimental findings [1-3]. Moreover, our results have important practical applications for MRAM devices, since the magnetic configurations of MTJ can be tuned by external bias and without the application of external magnetic field. This work is supported by NSF PREM Grant No. DMR-00116566. [1] J. Kubota et al., Nature Phys. \textbf{4}, 37 (2008) [2] Z. Li et al., Phys. Rev. Lett. \textbf{100}, 246602 (2008) [3] J. C. Sankey et al., Nature Phys. \textbf{4}, 67 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 4:06PM |
X29.00006: Voltage dependence properties of ballistic spin currents and spin transfer torques in magnetic tunnel junctions Invited Speaker: Interest in spintronics [1] has been strongly accentuated by the discovery of current induced magnetization switching caused by spin transfer torque (STT) [2]. Among the most favorable candidate systems for the realization of STT-based spintronic devices are epitaxial magnetic tunnel junctions (MTJ) [3]. Here we present a systematic study of voltage-induced STT in MTJs and provide an insight into the nature of its voltage behavior by investigating the properties of ballistic spin currents [4,5]. We demonstrate that the band filling has a dramatic impact on voltage dependence properties of both STT components, tunnel magnetoresistance (TMR) as well as on equilibrium interlayer exchange coupling [5]. Both in-plane (Slonczewski) and perpendicular-to-the-plane (field-like) STT components demonstrate a wide range of nontrivial behavior as a function of applied voltage [4,5]. The explanation is given in terms of the spin and charge current dependence on the interplay between evanescent states in the insulator and the Fermi surfaces of the ferromagnetic electrodes comprising the MTJ [5]. In particular we show that in ballistic regime the field-like torque is an even parity function of applied voltage while the parallel torque may exhibit a wide range of behavior [4,5]. Recent experiments [6] are in agreement with these predictions. Calculations are based on the non-equilibrium Green functions technique.\\[4pt] [1] A. Fert et al, \textit{Mat. Sci. Eng. B,} \textbf{84}, 1 (2001); S. A. Wolf, \textit{Science,} \textbf{294}, 1488 (2001)\\[0pt] [2] J. C. Slonczewski, \textit{J. Magn. Magn. Mat.} 159, L1 (1996); L. Berger, \textit{Phys. Rev. B} 54, 9353 (1996\\[0pt] [3] W. H. Butler et al, \textit{Phys. Rev. B,} \textbf{63}, 054416 (2001); J. Mathon and A. Umerski, \textit{Phys. Rev. B,} \textbf{63}, 220403(R) (2001)\\[0pt] [4] I. Theodonis et al, \textit{Phys. Rev. Lett.} \textbf{97}, 237205 (2006)\\[0pt] [5] M. Chshiev et al. \textit{IEEE Trans. Mag. }\textbf{44} (11) (2008);~A. Kalitsov et al., \textit{submitted}\\[0pt] [6] H. Kubota et al, \textit{Nature Physics} \textbf{4}, 37 (2008); J. C. Sankey et al, \textit{ibid.} \textbf{4}, 67 (2008); A. Deac et al, \textit{ibid.} \textbf{4}, 803 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X29.00007: Excitation of magnetization dynamics in patterned thin films using surface acoustic waves A. Baruth, S. Adenwalla The investigation of magnetization dynamics often involves the application of magnetic field or light pulses on very short time scales. Here we outline an alternative method that utilizes the changes in magnetic anisotropy associated with magneto-elastic strain. Surface Acoustic Wave (SAW) transducers are readily available at high frequencies ($>$10GHz), and provide an ideal method for straining thin film magnetic elements. SAWs propagate across a piezoelectric substrate, alternately compressing and expanding the surface with a wavelength and period that depends only on the propagation speed of the piezoelectric and the spacing of the interdigital transducer (IDT) that produces the SAW. Patterning thin film magnetic nanostructures at a spacing identical to the SAW wavelength ensures that all elements will be in phase as the SAW passes through. Passage of the SAW through a magnetic element leads to expansion and compression along the SAW propagation direction dynamically altering the easy axis of magnetization at ultra high frequencies. The subsequent dynamics can be probed using the Kerr effect. Using an IDT of 100 fingers operating at 87.2 MHz with realistic insertion losses, an array of 30nm thick, 10x20$\mu $m rectangular Co bars require voltages of $\sim $3.3V to fully switch the magnetization from the easy to hard axis without the application of an external field. Funded by NSF-MRSEC DMR-0820521. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X29.00008: Spin Wave Quantization by the Stripe Domain Structures in a Continuous Film Chun-Yeol You, Seung-Seok Ha, Jungbum Yoon, Sukmock Lee, Myung-Hwa Jung, Young Keun Kim Abnormal field dependence and dispersion relations of spin wave excitation spectra in the Brillouin light scattering were observed in a continuous CoFeSiB film. The observed spectra are similar to spin wave quantization in laterally-confined magnetic structures such as arrays of magnetic nanowires. In the array of nanowires, the propagating spin wave formed a standing wave due to the reflection from the geometrical confinement, boundaries of the nanowire, and the spin wave exciting modes are quantized. In our observation, the possible reflection source in the continuous CoFeSiB film is the regular domain boundaries, domain walls. As evidence, we observed very regular stripe domain structures by magnetic force microscopy. In the low field region (<1 kOe), the regular stripe domain patterns are formed and additional spin wave excitations are observed, while in the large field region, it behaves as usual continuous film. We believe that the regular domain wall acts as a scattering source of the spin wave, and it causes spin wave mode quantization. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X29.00009: Spin-Wave Resonance and Relaxation in CoFeB/Cr Superlattices Yu Gong, Zehra Cevher, Yuhang Ren, Hassan Imrane , Nian X. Sun We investigated the magnetic anisotropic properties and the spin wave relaxation in ten periods of CoFeB/Cr/CoFeB films grown on seed layers of Cu ($\sim $ 5 nm) with Co : Fe : B composition ratio as 2:2:1. The measurements were taken in samples with 50-angstrom layers of CoFeB using both the time-resolved pump-probe magneto-optical sampling and the ferromagnetic resonance techniques. The thickness of the Cr interlayers ranges from 4-angstrom to 40-angstrom for investigating the mechanisms of interlayer coupling and exchange interactions. Both the acoustic branch and the optical branch in spin wave resonance spectra are identified. We determine the magnetic anisotropic parameters by measuring spin wave frequency as a function of external magnetic field in the time domain and by orthogonally rotating the field aligned axis with respect to the spectral field in the frequency domain. Moreover, we estimate the intrinsic Gilbert damping for the in-plane magnetization orientation. When the interlayer coupling is weaker, the damping increases significantly. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X29.00010: Ferromagnetic resonance study of ion irradiated Co/Ni multilayers with perpendicular magnetic anisotropy J-M Beaujour, A. Kent, D. Ravelosona, E. Fullerton, Y. Samson, C. Beigne Ferromagnetic resonance (FMR) spectroscopy was used to investigate the effect of helium ion-irradiation on the magnetic properties and the magnetization dynamics of Co/Ni multilayer films. The anisotropy in these materials is associated with interfaces, which can be systematically disordered with light ion-irradiation without inducing major structural changes to the films. [Pd/Co]$\times 2|$[8\AA Ni/1.4\AA Co]$\times 3 |$Pd$|$Co$|$Pd$|$ have been exposed to He$^+$ irradiation with fluence up to $10^{15}$ ions/cm$^{2}$ [1]. FMR was conducted with a broad band coplanar waveguide up to 30 GHz. The resonance field and the FMR linewidth were determined as a function of frequency for dc magnetic fields in-plane, out-of-plane and for selected field angles. The perpendicular anisotropy decreases linearly with fluence, and at a particular fluence the direction of easy magnetization switches from perpendicular to in-plane. The Gilbert damping constant of the films irradiated at the higher and lower fluence is about the same: 0.03$\leq \alpha \leq$0.04. However, the linewidth exhibits a non-monotonic dependence on fluence, with a maximum at intermediate fluence. We will discuss this effect as well as possible explanations in terms of the changes in interface structure as a function of fluence. [1] Stanescu $et$ $al.$, J. Appl. Phys. (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X29.00011: A microstrip spin-wave amplifier Mingqiang Bao, Amber Chen, Alex Khitun, Kang Wang, Ajey Jacob To date, the spin-wave logic devices are seen promising for parallel data processing at high speed [Appl. Phys. Lett. 87, 153501 (2005), and Superlattices and Microstructures 38, 184 (2005)]. However, there is no power gain with the current spin-wave logic devices yet, and the spin-wave cannot propagate a long distance because of the spin-wave dumping effect. All those will prevent the spin-wave devices from real applications. Here we report a spin-wave amplifier with a power gain that is controlled by the pumping power level: At the pumping power of 16 dBm at 2.6 GHz, the power gain is 6.4 dB and the frequency is 1.3 GHz. The amplifier is made from a permalloy film with its thickness of 25 nm. The signal input, pump wave input, and signal output are simple microstrip lines, thus the device structure is simple. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X29.00012: Quantative characterization of a thin ferromagnetic film by pi-MFM and FMRFM Y. Obukhov, I.H. Lee, D.V. Pelekhov, E. Nazaretski, P. Banerjee, P.C. Hamme We present a theoretical analysis of two new methods for high resolution magnetic characterization of magnetic materials. These two, probe-induced (pi) Magnetic Force Microscopy and Ferromagnetic Resonance Force Microscopy are related in exploiting the modification of sample properties by the magnetic field of the probe to enable new imaging capabilities. Our analytic theory enables quantitative modeling of signals obtained in these two microscopy methods, and so allows us to extract parameters describing the magnetic properties of ferromagnetic films. We compare our theory with experimental data and find excellent agreement. More detailed experimental data will be presented in an accompanying talk. Our methodology allows detailed local characterization of ferromagnetic films in complementary MFM and MRFM experiments. [Preview Abstract] |
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