Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M29: Focus Session: Spin-Torque Oscillators |
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Sponsoring Units: GMAG DMP FIAP Chair: Igor Barsukov, University of California, Irvine Room: 206A |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M29.00001: Time domain mapping of spin torque oscillators dynamics Jieyi Zhang, Yujin Chen, Graham Rowlands, Ilya Krivorotov, Patrick Braganca, Jeff Childress, Bruce Gurney Time domain measurements of spin torque oscillators (STOs) offer important insights into the magnetization dynamics under the action of spin torque. We use the time domain data to map statistical distributions of the STO free layer trajectories and analyze them in the framework of the Fokker-Planck effective energy approach. We make time-resolved measurements of the microwave voltage signal generated by an STO based on a 90 nm circular nanopillar patterned from IrMn/ Co/CoFe/ CoFeGe/ CoFe/ Cu/ CoFe/ CoFeGe/ CoFe multilayer. Based on our time domain data and the angular dependence of giant magneto-resistance of the device, we map out dynamic trajectories of the free layer. We then apply the Fokker-Planck approach to calculate spin torque dependent STO effective energy E$_{\mathrm{eff}}$ from the experimentally measured statistical distributions of the free layer trajectories. We compare the measured E$_{\mathrm{eff}}$ to that calculated in the macrospin approximation and discuss how such a comparison can be used for quantification of non-linear damping in the high-amplitude regime of STO operation. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M29.00002: Tapered nanowire spin torque oscillator driven by spin orbit torques Liu Yang, Andrew Smith, Brian Youngblood, Zheng Duan, Ilya Krivorotov We report microwave signal emission from a spin torque oscillator driven by spin orbit torques in a tapered Pt(7nm)/Py(5nm) ferromagnetic nanowire with 2 um long active region. The tapered nanowire oscillator exhibits lower spectral linewidth (\textless 1 MHz) and higher integrated power (\textgreater 1 nW) compared to a spin torque oscillator based on a nanowire with spatially uniform width. The tapered nanowire oscillator has two distinct regimes of the microwave signal emission: a low-current, low-amplitude regime with a soft onset of the microwave emission and a higher-current regime with a hard onset of large-amplitude oscillations. The existence of the two regimes arises from spatially non-uniform effective damping in the tapered nanowire geometry. The non-uniformity of the effective damping results in nucleation of magnetization self-oscillations at the narrow end of the nanowire and subsequent steady growth of the self-oscillatory region with increasing current throughout the low-power regime. The sudden turn on of the high-power regime takes place at a critical current, for which spatially averaged effective damping of the nanowire changes sign from positive to negative. Our work paves the way towards high-power spectrally pure spin torque oscillators driven by spin orbit torques. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M29.00003: Spin Torque Oscillators with Highly Spin-Polarized Heusler Alloy Takeshi Seki, Tatsuya Yamamoto, Koki Takanashi An intriguing spin torque device is the spin torque oscillator (STO). An important issue for STOs from the viewpoint of practical use is to enhance the rf output power (Pout). Since Pout is roughly proportional to the square of the magnetoresistance (MR) ratio, a ferromagnetic material showing a large MR effect is a candidate for high-performance STO. In this study, we have developed high-power all-metal STOs with a full Heusler Co2Fe0.4Mn0.6Si (CFMS) showing a large MR effect. The present STOs consist of current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) stacks of CFMS \textbar Ag \textbar CFMS. A pillar-shaped STO showed clear auto-oscillation. The large Pout of 23.7 nW was obtained owing to its out-of-plane magnetization precession. Simultaneously, the oscillation linewidth showed the minimum value of 10 MHz. On the other hand, a point-contact-type STO showed auto-oscillation even at zero external magnetic field, and the oscillation mode was drastically changed as the bias current was increased. The large Pout for both STOs indicates the CPP-GMR devices with CFMS layers are promising for high performance STO. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M29.00004: Linewidth reduction in spin--torque oscillators by delayed self-injection Guru Bahadur Khalsa, Julie Grollier, Mark Stiles Spin-torque oscillators offer the possibility of tunable microwave frequency generation in a nanoscale device that is compatible with conventional technology. Hurdles to their regular use have been the large critical current needed to sustain oscillations and a spectral linewidth that is too large for industrial adoption. In this talk we describe our analytic/numerical investigation of linewidth reduction in spin-torque oscillators based on delayed self-injection of the output current. We discuss the general response of the model system for arbitrary delay time which can include: linewidth and frequency variation, the possibility of multiple stable frequencies, and mode-hopping. We present analytic results for the critical current and linewidth and show that this technique can be used for meaningful linewidth reduction with realistic system parameters. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M29.00005: Spin wave beam mediated synchronization of nano-contact spin torque oscillators Afshin Houshang, Ezio Iacocca, Philipp Durrenfeld, Sohrab Sani, Johan Akerman, Randy Dumas The synchronization of multiple nano-contact spin torque oscillators (NC-STOs) [1-3] is mediated by propagating spin waves (SWs). While it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs [4], its role in the synchronization behavior of multiple NCs has not been considered. We investigate the synchronization behavior in double NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Two NCs with nominal diameters of 100 nm and a center-to-center spacing of 300 nm are defined on top of an all metallic Co/Cu/NiFe pseudo spin valve. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. The vertical positioning of the NCs gives rise to a unique magnetic field landscape that acts to localize SWs in a region just outside one of the NCs, as confirmed by micromagnetic simulations. [1] S. Kaka, et al., Nature \textbf{437}, 389 (2005) [2] F.B. Mancoff, et al., Nature \textbf{437}, 393 (2005). [3] S.R. Sani, et al., Nat. Comm. \textbf{4}, 2731 (2013) [4] R.K. Dumas, et al., Phys. Rev. Lett. \textbf{110}, 257202 (2013) [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M29.00006: Directional spin wave emission by the spin transfer torque oscillator into a nanomagnonic waveguide Vladislav Demidov, Sergej Demokritov, Sergei Urazhdin Magnonics is based on signal transmission and processing by spin waves in a magnetic medium. Spin-torque nanooscillators (STNO) driven by dc electrical current can provide a local source of spin waves for nanomagnonics, but their spectral mismatch with the magnetic medium limits the spin wave emission efficiency. We have developed a nanomagnonic structure that combines a point-contact STNO with a dipolar field-induced nanowaveguide. We will describe our microfocus Brillouin light scattering microscopy measurements demonstrating efficient excitation of spin waves by the STNO and their directional propagation in the waveguide. Spectroscopic measurements and micromagnetic simulations indicate that efficient spectral matching between the waveguide and the STNO is achieved due the internal dipolar field of the nanopatterned waveguide. We show that the spin wave propagation length is increased compared to the extended films, due to their larger group velocity and the lack of wavefront spreading. Our results provide a simple and efficient route for the implementation of magnonic structures that integrate spin torque-based sources of spin waves and their processing via waveguiding structures. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M29.00007: Coherent magnetization oscillation induced by nonlocal spin injection Sergei Urazhdin, Andrei Zholud, Vladislav Demidov, Sergej Demokritov We experimentally demonstrate coherent magnetization oscillations induced by nonlocally injected pure spin current in a CoFe/Cu/Permalloy(Py) magnetic thin film heterostructure. The current injected into CoFe through a point contact is drained through the Cu spacer, while the spins diffusing from the CoFe/Cu interface to Py cause magnetic oscillations of the latter. Symmetry analysis and numeric calculations show that the current is negligible in the active region of the Py layer, and does not exceed 3 percent of the total current away from it. The oscillation frequency decreases with increasing current, and an additional spectral peak appears at large currents. Micro-focus Brillouin light scattering microscopy shows that that the size of the oscillation region is larger than expected from the usual self-localization mechanism of auto-oscillation. Micromagnetic simulations suggest that the oscillation mode is localized by an effective potential well formed due to the local reduction of magnetization by the effects of spin current, providing a route for the development of spin-torque nano-oscillators with controllable spatial oscillation characteristics. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M29.00008: Noise-enhanced synchronization of stochastic magnetic oscillators Julie Grollier, Alice Mizrahi, Nicolas Locatelli, Artur Accioly, Rie Matsumoto, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Vincent Cros, Luis Gustavo Pereira, Damien Querlioz, Joo-Von Kim Due to their small magnetic volume the magnetization dynamics of spin-torque nano-oscillators is very sensitive to thermal fluctuations. This results in a large phase noise detrimental to their phase-locking. Here, we pursue an original paradigm in which noise is advantageous for improving coherence and facilitating synchronized states. This builds upon recent work in which spin torque driven magnetic tunnel junctions have been shown to exhibit stochastic resonance. Our system comprises a magnetic tunnel junction with a super-paramagnetic free layer, whose magnetization dynamics is driven with spin torques through an external periodic current. We show synchronization of this stochastic oscillator at very low current densities (below 3 10$^6$ A/cm$^2$) and input frequencies lower than the natural mean frequency of the oscillator. We show that such injection-locking is robust and leads to a drastic reduction in phase diffusion of the oscillator. This system is promising for applications where low energy is crucial and thermal noise has to be leveraged, such as bioinspired computing. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M29.00009: Terahertz Nano-oscillator using Antiferromagnet Ran Cheng, Di Xiao, Arne Brataas We present an analytical study on the current-induced dynamics of collinear antiferromagnets (AFs), and find that a sufficiently large current gives rise to terahertz auto-oscillation of the staggered order. For easy-plane AFs such as NiO, the acoustic and optical modes are well separated in frequency, and none of them exhibit chirality of precession. However, with increasing current-induced torque, the two modes become degenerate at a threshold, after which the chirality of precession is acquired. At a second threshold, the degenerate mode is driven into auto-oscillation with elliptical precession where the semi-major axis is 45 degree with respect to the easy-plane. The terahertz nano-oscillator is illustrated by X/AF/X heterostructures where X can either be a heavy metal or topological insulator. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M29.00010: Nanoconstriction-based Spin Hall nanooscillator Andrei Zholud, Sergei Urazhdin, Vladislav Demidov, Sergej Demokritov We experimentally demonstrate coherent magnetization oscillations induced in a bow tie-shaped Pt/Permalloy nanoconstriction by pure spin current generated by the spin Hall effect. The devices generate microwave signals with a significant power and the spectral linewidth as low as 6.2 MHz at room temperature (RT). In contrast to the previously demonstrated spin Hall oscillators governed by the dynamical nonlinear self-localization mechanism, the localized oscillation mode is present in the thermal fluctuation spectrum even below the auto-oscillation onset and exhibits a significant redshift. These observations suggest that the localized auto-oscillation mode is formed due to the confinement in an effective potential well produced by a combination of the dipolar field of the magnetic nanoconstriction and the Oersted field of electrical current. The studied devices are characterized by a large oscillation area, minimizing the effects of thermal fluctuations and resulting in a narrow RT spectral linewidth without compromising the single-mode regime of autooscillation. Moreover, the simple planar structure of the proposed oscillators enables their cascading, which can be utilized to further improve the oscillation characteristics in mutually coupled devices. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M29.00011: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M29.00012: Polarizatoin rotation of microwaves in ferromagnets subjected to spin torque Yaroslaw Bazaliy It is well known [1] that microwaves propagating through a ferromagnetic material experience a resonance increase of polarization rotation (Faraday and Kerr effects) near the ferromagnetic resonance frequency. Here we study how this effect is modified in the presence of spin torques acting on magnetization. [1] C. L. Hogan, Rev. Mod. Phys. {\bf 25}, 253 (1953). [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M29.00013: Spin-transfer torque induced spin waves in antiferromagnetic insulators Matthew Daniels, Wei Guo, G. Malcolm Stocks, Di Xiao, Jiang Xiao We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M29.00014: Spin dynamics in patterned nanometer-thick yttrium iron garnet films Matthias Benjamin Jungfleisch, Wei Zhang, Wanjun Jiang, Stephen M. Wu, John E. Pearson, Anand Bhattacharya, Axel Hoffmann, Joseph Sklenar, John B. Ketterson, Houchen Chang, Mingzhong Wu We present experimental investigations on the propagation of spin-wave modes in micro-structured yttrium iron garnet (YIG) stripes.$^{\mathrm{1}}$ The stripes were patterned by photo-lithography from high-quality 40-nm-thick YIG films grown by sputtering.$^{\mathrm{2}}$ Magnetization dynamics is driven by the rf field of a shorted coplanar waveguide patterned onto the YIG stripes. The propagation of spin waves are detected by means of spatially-resolved Brillouin light scattering microscopy. The propagation distance of spin waves is determined in the linear regime, where an exponential decay of 10 $\mu $m is observed. The estimated Gilbert damping parameter extracted from the spin-wave decay length is 3 times larger than that obtained through ferromagnetic resonance measurements in unstructured films, which is possibly due to enhanced two-magnon scattering in the patterned films. Furthermore, studies on the spin dynamics driven by spin-torque ferromagnetic resonance in YIG/Pt bilayers and the corresponding spatially-resolved spin-wave distribution are presented. $^{\mathrm{1}}$M. B. Jungfleisch, \textit{et al}., J. Appl. Phys., in press. $^{\mathrm{2}}$H. Chang, \textit{et al}., IEEE Magn. Lett. \textbf{5}, 6700104 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M29.00015: Probing excitations in insulators by injecting spin currents Shubhayu Chatterjee, Subir Sachdev Observation of fractional excitations in insulating spin-systems has been a long-sought goal in physics. In spite of promising evidence for observation of spin liquids, the exact nature of possible ground states, and in particular, the presence of a spin-gap is still unclear. Most experiments till this point have focused on thermodynamic measurements. We suggest a transport measurement as an alternate window into the nature of excitations of insulating spin systems. We couple a metal with a non-equilibrium spin-accumulation to an equilibrium insulating spin-system [1], and develop a general formalism to compute the spin current. We use this to calculate the current into ordered antiferromagnets as well as spin liquids, and note salient features in the spin conductance. [1] Takei et. al. Phys. Rev. B 90, 094408 (2014) [Preview Abstract] |
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