Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B8: Focus Session: Spin Transfer Torque |
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Sponsoring Units: GMAG Chair: Olle Heinonen, Argonne National Laboratory Room: 104 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B8.00001: Mode coupling in spin-torque oscillators: first-principle derivation Olle Heinonen, Yan Zhou, Dong Li A number of recent experimental works have shown that the dynamics of a single spin torque oscillator can exhibit complex behavior that stems from interactions between two or more modes of the oscillator. Examples are observed mode-hopping or mode coexistence$^{\mathrm{1-3}}$. There has been some initial work indicating how the theory for a single-mode (macro-spin) spin torque oscillator should be generalized to include several modes and the interactions between them. In the present work, we derive such a theory starting with the Landau-Lifshitz-Gilbert equation for magnetization dynamics. We compare our results with the single-mode theory, and show how the coupled-mode theory is a natural extension of the single-mode. Argonne National Laboratory is a US DOE Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. References: [1] S. Bonetti, V. Tiberkevich, G. Consolo et al., \textit{Phys. Rev. Lett. }\textbf{105}, 217204 (2010). [2] P. Muduli et al., \textit{Phys. Rev. Lett}. \textbf{108}, 207203 (2012). [3] R. Dumas, E. Iacocca, S. Bonetti et al., \textit{Phys. Rev. Lett.} \textbf{110}, 257202 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B8.00002: Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques Andrew Smith, Zheng Duan, Liu Yang, Brian Youngblood, Ilya Krivorotov We report microwave signal emission from a spin torque oscillator driven by spin orbit torques in a 17 um long Py(5 nm)/Pt(5 nm) ferromagnetic nanowire with an 1.8 um long active region. The emitted signal arises from excitation of the bulk and edge spin wave eigenmodes of the nanowire and detected with anisotropic magnetoresistance. This type of self-oscillatory dynamics is qualitatively different from the previously reported self-localized nonlinear bullet mode excited by spin orbit torques in extended ferromagnetic films. The eigenmode self-oscillations in the nanowire geometry are enabled by geometric confinement suppressing nonlinear magnon scattering. Our work demonstrates feasibility of spin torque oscillators with a micrometer-scale active region. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B8.00003: Magnetization reversal in orthogonal spin transfer magnetic devices Georg Wolf, Andrew D. Kent, Bartek Kardasz, Mustafa Pinarbasi Orthogonal spin transfer (OST) magnetic devices have distinct magnetization dynamics and switching characteristics compared to conventional collinearly magnetized devices. A perpendicular magnetized layer provides a large initial spin torque on the free layer magnetization and thus initiates magnetization dynamics. In order to read out the information stored in the OST device, the free layer forms a magnetic tunnel junction with an in plane magnetized reference layer, which also exerts a spin torque on the free layer. The combination of those two spin torques leads to different switching dynamics of the free layer. Quasistatic and fast pulsed measurements have been conducted to explore the state diagram and magnetization dynamics of such devices. The absolute value of the switching current I$_s$ is in general smaller for the antiparallel (AP) to parallel (P) transition, due to the angular dependence of the reference layer torque. I$_s$ also has a weak field dependence for this transition, indicating that the reference layer torque governs this transition. On the other hand, the P to AP transition shows a stronger field dependence of I$_s$ and occurs for both current polarities. Both these features denote the influence of the spin-torque generated from the perpendicular polarizer. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B8.00004: Spin wave mode coexistence on the nanoscale: A consequence of the Oersted field induced asymmetric energy landscape Invited Speaker: Randy Dumas The emerging field of magnonics relies on the systematic generation, manipulation, and detection of spin waves (SWs). Nanocontact spin torque oscillators (NC-STOs) provide an ideal platform to study spin transfer torque induced SW emission [1, 2]. In analogy to two species competing for the same food supply it has been argued that only one SW mode can survive in the steady state [3]. However, as evidenced in many experiments clear signatures of mode-hopping are often observed [1, 4]. Here, we present a third possibility, namely that under the correct experimental conditions, mode \textit{coexistence }can be realized in NC-STOs [5]. Micromagnetic simulations reveal that the SW modes are spatially separated under the NC. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field in the vicinity of the NC and further promoted by SW localization. Finally, both simulation and experiment reveal a weak low frequency signal exactly at the difference of the mode frequencies, consistent with inter-modulation of two coexistent modes. \\[4pt] [1] S. Bonetti, V. Tiberkevich, G. Consolo, G. Finocchio, P. Muduli, F. Mancoff, A. Slavin, and J. {\AA}kerman, Phys. Rev. Lett. \textbf{105}, 217204 (2010). \\[0pt] [2] M. Madami, S. Bonetti, G. Consolo, S. Tacchi, G. Carlotti, G. Gubbiotti, F.B. Mancoff, M.A. Yar, and J. {\AA}kerman, Nature Nanotechnology \textbf{6}, 635 (2011). \\[0pt] [3] F. M. de Aguiar, A. Azevedo, and S. M. Rezende, Phys. Rev. B \textbf{75}, 132404 (2007).\\[0pt] [4] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. Lett. \textbf{108}, 207203 (2012).\\[0pt] [5] R.K. Dumas, E. Iacocca, S. Bonetti, S.R. Sani, S.M. Mohseni, A. Eklund, J. Persson, O. Heinonen, and Johan {\AA}kerman, Phys. Rev. Lett. \textbf{110}, 257202 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B8.00005: Spin-Torque Ferromagnetic Resonance in PMA Thin Film Structures Luis Vilela-Le\~ao, Chi-Feng Pai, Yongxi Ou, Yun Li, Daniel Ralph, Robert Buhrman Thin film systems with strong perpendicular magnetic anisotropy (PMA) are important for many spintronic device applications. For example for magnetic memory, strong PMA can enable ultra-high density storage, in combination with enhanced non-volatility and low write energy. Recently, some interesting phenomena, including the spin Hall effect and spin orbit fields, have been reported in normal metal/ferromagnetic (NM/FM) structures with PMA. These effects, which arise from spin-orbit coupling in the structure, convert charge current into torques (a field-like torque and/or a damping-like torque) on the magnetization of the FM that can be strong enough to switch the magnetization, generate persistent magnetic oscillation, and efficiently move domain walls. Here we show that spin-torque ferromagnetic resonance (ST-FMR) can be effectively employed to characterize the anisotropy, the spin-orbit torques, and the magnetic damping of a PMA structure. We will report on ST-FMR results from several PMA systems, including W/Hf(t)/CoFeB/MgO and W/CoFeB where we have found large values for both the first order and second order terms of the magnetic anisotropy and where we have measured intrinsic and extrinsic contributions to the effective magnetic damping. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B8.00006: Field-modulated spin torque ferromagnetic resonance: Characterization of spin wave eigenmodes in magnetic tunnel junctions Yu-Jin Chen, Alexandre Goncalves, Igor Barsukov, Liu Yang, Jordan Katine, Ilya Krivorotov A common technique for measurements of magnetic parameters in nanoscale magnetic tunnel junctions (MTJs) is spin torque-driven ferromagnetic resonance (ST-FMR) based on an amplitude-modulated microwave drive [1,2]. We demonstrate a technique of broadband ST-FMR based on magnetic field modulation for measurements of spin wave properties in magnetic nanostructures [3]. Application of the field-modulated ST-FMR technique to MTJs gives reliable information on magnetization dynamics for arbitrary magnetic state of the MTJ, including the case of collinear magnetizations. This configuration is difficult to measure in conventional ST-FMR due to the weak spin torque drive. The improved signal-to-noise ratio and improved sensitivity of field-modulated ST-FMR allow us to measure the entire spectrum of low-frequency standing spin waves. We find the magnetic field dependence of the measured spin wave eigenmodes to be in good agreement with micromagnetic simulation results, which allow us to identify the observed modes as the free layer eigenmodes and to determine their spatial profiles [3]. \\ 1. A. A. Tulapurkar et al., Nature 438, 339-342 (2005)\\ 2. J. C. Sankey et al., Phys. Rev. Lett. 96, 227601 (2006)\\ 3. A. M. Goncalves et al., Appl. Phys. Lett. 103, 172406 (2013) [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B8.00007: Control of Propagating Spin Waves via Spin Transfer Torque in a Metallic Bilayer Waveguide Kyongmo An, Daniel Birt, Chi-Feng Pai, Kevin Olsson, Daniel Ralph, Robert Buhrman, Xiaoqin Li We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin wave amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase locked nano-oscillators and hybrid information processing devices. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B8.00008: Experimental observation of exchange-mode spin-wave via domain wall annihilation Seonghoon Woo, Tristan Delaney, Geoffrey Beach Spin waves (SWs) in magnetic nanostructures have generated great interest recently, motivated by the possibility of high-speed, low-power magnonic devices applications. A number of micromagnetic researches, therefore, have been conducted, revealing the particular behaviors of SWs in nanostructured ferromagnets. However, SWs' short attenuation length prevents them from being observed and used experimentally. Generating large-amplitude exchange-mode SWs, which is thus indispensable for real device applications, are still challenging because their very short wavelengths cannot be directly excited. Here, we present the first experimental evidence of the exchange-mode SWs. Using micromagnetics, we firstly show that the annihilation of two DWs releases their exchange energy by a mean of localized SW burst, which has broad range band and intense amplitude. Another micromagnetic result also shows that the collision-induced SWs inside a nanowire can cause the depinning of a DW with an assisting magnetic field. By taking advantage of an anisotropic magneto-resistance (AMR) effect and relative electrical measurements, we observe the generation/annihilation of DWs and the contribution of generated SWs to the DW depinning process experimentally. The additional depinning field of $\sim$ 8 Oe caused by SWs can be readily achieved, enough to propagate a standstill DW in a well-defined pinning-free nanostructure. This work shows the first experimental observation of exchange-mode SWs and highlights a new route towards SW-integrated spintronic devices. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B8.00009: Onset and annihilation of droplet solitons in Spin Torque Nano-Oscillators with perpendicular magnetized free layers Ferran Macia, Dirk Backes, Andrew Kent Nanometer scale electrical contacts to ferromagnetic thin films (STNOs) can provide sufficient current densities to excite magnetization dynamics resulting in either localized or propagating short wavelength spin waves. These oscillations can be detected through the magnetoresistance because of the change in the relative orientation between the current polarization and the free layer magnetization. We have fabricated point contacts to continuous magnetic bilayers where the polarizer magnetic film has in-plane magnetic anisotropy and with free layers with different magnetic anisotropies ranging from in-plane to perpendicular magnetic anisotropy (PMA). Our measurements on STNOs with perpendicularly magnetized free layers indicate that over a region of magnetic field and current there is an onset of an excitation with characteristics consistent with the formation of a \textit{droplet soliton}. We have systematically studied the state diagram of these excitations that shows both their onset and annihilation. We also studied the onset and annihilation of droplet solitons in arrays of STNOs. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B8.00010: Macrospin model of spin-transfer oscillators: an energy space approach Daniele Pinna, Andrew Kent, Daniel Stein A direct current applied to a nanomagnet produces a spin-transfer torque that drives the magnetization out of equilibrium. In this talk we discuss an effective theory to characterize the magnetization dynamics by focusing on its diffusive evolution over the energy landscape. The procedure allows us to model macrospin behavior with a one dimensional stochastic differential equation. We model spin-transfer oscillators (STOs) with a spin-current at an angle to the easy plane of a biaxial magnet (i.e. have a component along the magnet's hard axis). We trace the properties of stable out-of-plane precessional states and discuss their hysteretic behavior on applied current. We discuss the structure of the expected linewidth and phase noise, along with how the oscillator frequency is expected to depend on applied current. Finally, contributions due to thermal noise will be outlined and some thermally activated properties described. D. Pinna, A. D. Kent, D. L. Stein, Phys. Rev. B 88, 104405 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B8.00011: State Diagram of Orthogonal Spin-Transfer Spin-Valve Devices Li Ye, Georg Wolf, Daniele Pinna, Andrew Kent Orthogonal spin transfer (OST) devices that incorporate an out-of-plane magnetized polarizing layer with an in-plane collinear spin valve are expected to exhibit ultrafast magnetization switching as well as large amplitude precessional modes. The current and field dependence of the switching thresholds are also distinct from the collinear spin-valves because of the combined effect from in-plane reference layer (RL) and polarizing layer (PL). Here we present an experimental investigation of complete current-field state diagrams, demonstrating reversal between parallel (P) and anti-parallel (AP) states and dynamic states of the free layer in both OST pseudo spin valve and spin valve devices, where in the latter a synthetic anti-ferromagnetic layer (SAF) is used as reference layer. Switching from AP (P) to P (AP) states is observed at both positive and negative current with a different field dependence of the critical current, reflecting spin polarization asymmetry between AP and P states and different RL and PL spin torque efficiencies. High frequency noise spectra have also been acquired providing evidence of out-of-plane precessional modes, where an intermediate resistance is seen in quasistatic measurements. Modeling of this orthogonal spin transfer system is also discussed. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B8.00012: Distribution of domain wall spin torque in magnetic metals and ferromagnetic semiconductors Elizabeth Golovatski The design and implementation of many spintronic devices[1] will be dependent on good models of spin torque and domain wall motion caused by coherent carrier transport[2]. We model spin torque in N\'eel walls[3] using a piecewise linear transfer-matrix method[4], and calculate the spin torque distribution[5] throughout the system. We examine the differences in spin torque for ferromagnetic semiconductors (where the Fermi energy is much less than the spin splitting) and magnetic metals (where the Fermi energy is much greater than the spin splitting). We find that the torque distribution is more asymmetric for adiabatic torques and more symmetric for non-adiabatic torques in a magnetic metal vs. a ferromagnetic semiconductor, leading to differences in velocities for two domain walls in close proximity. [1] S. Parkin et al., Science 320, 190 (2008) [2] M. Yamanouchi et al., Nature 428, 539 (2004) [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89, 098302 (2002) [4] E. Golovatski and M. Flatt\'e, Phys. Rev. B, 84, 115210 (2011) [5] J. Xiao et al., Phys. Rev. B, 73, 054428 (2006) [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B8.00013: Current-Induced Domain-Wall Motion in Perpendicularly Magnetized Magnetic Nanowires with Unflatted Surfaces Hirofumi Morise, Tsuyoshi Kondo, Shiho Nakamura We study the current-induced domain wall motion in perpendicularly magnetized magnetic nanowires with unflatted surfaces numerically. There, the axis of the anisotropy is assumed to be normal to the surfaces, that is, it is inclined continuously along the extended direction of the nanowire. The relationship between the current density and the velocities of domain walls are investigated by use of micromagnetics simulations. Comparing with the motion in a flat nanowire, the existence of an additional exchange energy due to the curvature significantly affects the motion of the domain walls. [Preview Abstract] |
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