Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A18: Spin-Hall IFocus Industry
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Sponsoring Units: GMAG DMP FIAP Chair: Arne Brataas, Norwegian University Room: 317 |
Monday, March 14, 2016 8:00AM - 8:12AM |
A18.00001: In-plane current induced spin orbit effects in nanometer scale Hall bar of $\beta $-W/Ta/CoFeB/MgO/Ta multilayers Avyaya J. Narasimham, Yu-Ming Hung, Meng Zhu, Andrew D. Kent, Vincent P.LaBella The giant spin Hall effect (GSHE) is caused by spin orbit interactions in a semiconductor or metal that result in a spin current that is transverse to the charge current. Recent spin Hall effect studies in the beta phase metals Ta and W show that transverse spin currents are strong enough to switch an adjacent magnetic layer. Films with perpendicular magnetic anisotropy (PMA) can exhibit uniform magnetizations and higher thermal stability. Inserting a 1 nm Ta insert-layer between the CoFeB and W induces PMA which is confirmed by vibrating sample magnetometer and anomalous Hall voltage measurements. $\beta $-W(5)/Ta(1) channel and the adjacent CoFeB/MgO/Ta layers are patterned into a 100 nm wide Hall bar structures. Effect of in-plane current induced change in coercivity while sweeping in-plane magnetic field are studied. An empirical model to quantitatively understand the switching will be presented. [Preview Abstract] |
Monday, March 14, 2016 8:12AM - 8:24AM |
A18.00002: Spin Orbit Torque in TbCo Films with Bulk Perpendicular Magnetic Anisotropy Kohei Ueda, Maxwell Mann, Aik-Jun Tan, Geoffrey. S. D. Beach Spin-orbit torque (SOT) has generated considerable interest for manipulating magnetization in spintronic devices with ultra-low dissipation. Recent research has demonstrated that highly efficient magnetization control can be driven by current-induced SOT in ferromagnet/heavy metals bilayers with strong spin orbit coupling. However, most work on SOT has focused on ultra-thin magnetic films with interfacial perpendicular magnetic anisotropy (PMA), whereas future devices will require bulk PMA for sufficient thermal stability. Recently, Zhao et al reported SOT induced magnetization switching in a bulk PMA material; however, the films examined were still rather thin. Here we examine spin orbit torques in TbCo alloy films with bulk PMA, sandwiched between top and bottom Ta layers. By performing conventional harmonic and current-induced switching measurements, we quantified the current-induced effective fields generated by damping-like (DL) and field-like (FL) torques. The DL torque is much larger than FL torque, and corresponds to an effective spin Hall angle consistent with that of Ta. Owing to the relatively small saturation magnetized of these ferrimagnetic materials, the current-induced effective field is comparable to that observed in nm-thick Co films, despite the much larger film thicknesses used here. These results demonstrate ferromagnetic alloys with bulk PMA can be engineered to simultaneously provide thermal stability and efficient SOT switching. [Preview Abstract] |
Monday, March 14, 2016 8:24AM - 8:36AM |
A18.00003: Spin-orbit torque induced reversible coercivity change in Co/Pd multilayer thin films. Sandeep Kumar In this work we report reversible reduction in coercivity of Co/Pd multilayer thin films under high-density direct current biasing. We carried out in-situ focused magneto optic Kerr effect based hysteresis measurement while the specimen was under DC bias. The experiments show a reversible reduction in coercivity during the application of direct current. We propose this reduction occurs due to the spin-orbit torques (Rashba) generated at high current densities. Using an in-situ transmission electron microscope biasing experiment, we also showed the presence of dissymmetric lattice structure of Co/Pd multilayers. Our results suggest that the Rashba torque is the dominant spin-orbit torque since coercivity change is a bulk phenomenon as compared to spin Hall effect. [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 9:12AM |
A18.00004: Novel current driven domain wall dynamics in synthetic antiferromagnets Invited Speaker: See-Hun Yang It was reported [1,2] that the domain walls in nanowires can be moved efficiently by electrical currents by a new type of torque, chiral spin torque (CST), the combination of spin Hall effect and Dzyaloshinskii-Moriya interaction. Recently we domonstrated that ns-long current pulses can move domain walls at extraordinarily high speeds (up to $\sim $750 m s$-$1) in synthetic antiferromagnetic (SAF) nanowires that have almost zero net magnetization [3], which is much more efficient compared with similar nanowires in which the sub-layers are coupled ferromagnetically (SF). This high speed is found to be due to a new type of powerful torque, exchange coupling torque (ECT) that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers, showing that the ECT is effective only in SAF not in SF. Moreover, it is found that the dependence of the wall velocity on the magnetic field applied along the nanowire is non-monotonic. Most recently we predict an Walker-breakdown-like domain wall precession in SAF nanowires in the presence of in-plane field based on the model we develop, and this extraordinary precession has been observed [4]. In this talk I will discuss this in details by showing a unique characteristics of SAF sublayers' DW boost-and-drag mechanism along with CST and ECT. [1] Kwang-Su Ryu, Luc Thomas, See-Hun Yang, and Stuart Parkin, ``Chiral Spin Torque at Magnetic Domain Walls'', Nature Nanotechnology 8, 527-533 (2013). [2] Satoru Emori, Uwe Bauer, Sung-Min Ahn, Eduardo Martinez, and Geoffrey S. D. Beach, ``Current-driven dynamics of chiral ferromagnetic domain walls'', Nature Materials 12, 611-616 (2013). [3] See-Hun Yang, Kwang-Su Ryu, and Stuart Parkin, ``Domain-wall velocities of up to 750 m s$-$1 driven by exchange-coupling torque in synthetic antiferromagnets'', Nature Nanotechnology 10, 221-226 (2015). [4] See-Hun Yang, Chirag Garg, Paul Amari, Charles Rettner, and Stuart Parkin, in preparation. [5] Stuart Parkin and See-Hun Yang, ``Memory on the Racetrack'', Nature Nanotechnology 10, 195-198 (2015). [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A18.00005: Spin-Hall Switching of In-plane Exchange Biased Heterostructures Maxwell Mann, Geoffrey Beach The spin Hall effect (SHE) in heavy-metal/ferromagnet bilayers generates a pure transverse spin current from in-plane charge current, allowing for efficient switching of spintronic devices with perpendicular magnetic anisotropy [1,2,3,4]. Here, we demonstrate that an AFM deposited adjacent to the FM establishes a large in-plane exchange bias field, allowing operation at zero HIP. We sputtered Pt(3nm)/Co(0.9nm)/Ni80Co20O(tAF) stacks at room-temperature in an in-plane magnetic field of 3 kOe. The current-induced effective field was estimated in Hall cross devices by measuring the variation of the out-of-plane switching field as a function of JIP and HIP. The spin torque efficiency, dHSL/dJIP, is measured versus HIP for a sample with tAF$=$30 nm, and for a control in which NiCoO is replaced by TaOx. In the latter, dHSL/dJIP varied linearly with HIP. In the former, dHSL/dJIP~varied nonlinearly with HIP and exhibited an offset indicating nonzero spin torque efficiency with zero HIP. The magnitude of HEB was 600 Oe in-plane. [1] D'yakonov and Perel JETP Lett., 1971. [2] Hirsch, PRL 1999. [3] Kato et al. Science, 2004. [4] Liu et al. PRL 2012. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A18.00006: Few-nanosecond pulse switching with low write error for in-plane nanomagnets using the spin-Hall effect Sriharsha Aradhya, Graham Rowlands, Shengjie Shi, Junseok Oh, D. C. Ralph, Robert Buhrman Magnetic random access memory (MRAM) using spin transfer torques (STT) holds great promise for replacing existing best-in-class memory technologies in several application domains. Research on conventional two-terminal STT-MRAM thus far has revealed the existence of limitations that constrain switching reliability and speed for both in-plane and perpendicularly magnetized devices. Recently, spin torque arising from the giant spin-Hall effect in Ta, W and Pt has been shown to be an efficient mechanism to switch magnetic bits in a three-terminal geometry [1-3]. Here we report highly reliable, nanosecond timescale pulse switching of three-terminal devices with in-plane magnetized magnetic tunnel junctions. We obtain write error rates (WER) down to \textasciitilde 10$^{\mathrm{-5}}$ using pulses as short as 2 ns, in contrast to conventional in-plane STT-MRAM devices where write speeds were limited to a few tens of nanoseconds for comparable WER. Utilizing micro-magnetic simulations, we discuss the differences from conventional MRAM that allow for this unanticipated and significant performance improvement. Finally, we highlight the path towards practical application enabled by the ability to separately optimize the read and write pathways in three-terminal devices. [1] L. Liu et al., Science, 336, 2012; [2] C-F. Pai et al., APL, 101, 2012; [3] M-H. Nguyen et al., APL, 106, 2015. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A18.00007: Magnetization dynamics in LSMO/Pt nanowires in the presence of spin orbit torques Hankyu Lee, Igor Barsukov, Christopher Safranski, Alejandro Jara, Yu-Jin Chen, Adrian Swartz, Bongju Kim, Harold Hwang, Ilya Krivorotov La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO) possesses attractive magnetic properties for nanowire spin torque oscillators (STOs) driven by spin orbit torques: low magnetic damping, low saturation magnetization and high spin polarization. In this context, good understanding of magnetization dynamics in LSMO/Pt bilayer nanowires is important. Here, we report measurements of the spectral properties of spin-wave modes in LSMO/Pt nanowires magnetized along the two principal in-plane axes. In electrically-detected ferromagnetic resonance (FMR) we observe excitation of multiple spin wave modes, including non-aligned modes when the nanowire is magnetized perpendicular to its axis. Spectral linewidth of the FMR resonances gives quantitative information on the Gilbert damping parameter of the nanowire. In comparison to extended LSMO/Pt films, the magnetic damping in the nanowire is reduced due to the suppression of two-magnon scattering. We will present data on the effect of high bias current density applied to the wire on the frequency and linewidth of the observed spin wave resonances. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A18.00008: Study of spin orbit torque switching in ferrimagnetic Gd$_{x}($Fe$_{90}$Co$_{10})_{100-x}$ alloy Niklas Roschewsky, Tomoya Matsumura, Takeshi Kato, Satoshi Iwata, Suraj Cheema, James Clarkson, Sayeef Salahuddin Magnetization switching in ferromagnetic metals (FM) with spin-orbit torques (SOT) is a well established technique. The SOT originates from spin accumulation at the interface of the FM generated by the spin Hall effect in an adjacent heavy metal. Here we report measurements of SOT in the alloy Gd$_{x}($Fe$_{90}$Co$_{10})_{100-x}$, where the transition metal sub-lattice and the rare earth sub-lattice couple antiferromagnetically. By varying the composition $x$ of the alloy we can tune the total magnetization. Anormalous Hall effect measurements are conducted to study the effect of SOT on the Gd$_{x}($Fe$_{90}$Co$_{10})_{100-x}$ magnetization. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A18.00009: Nanowire spin Hall oscillators: width dependence Andrew Smith, Tobias Schneider, Liu Yang, Ilya Krivorotov We present experimental studies of auto-oscillatory magnetization dynamics in nanowire spin Hall oscillators (SHOs) as a function of the wire width ranging from 0.17 µm to 2 µm. These SHOs consist of long Pt(7 nm)/Py(5 nm)/AlOx(2 nm) wires on a sapphire substrate. Direct current generating anti-damping spin torque is applied to a section of the wire between two leads separated by a 2 µm gap, which defines the SHO active region.$^{1}$ All devices show onset of auto-oscillations at similar critical current densities. For the 0.17 µm and 0.34 µm wide nanowire SHOs, auto-oscillatory modes arising from the bulk and edge eigenmodes of the nanowire are clearly seen in the emission spectra. For SHO devices based on wider wires, the bulk auto-oscillatory modes dominate the emission spectrum due to the larger wire volume occupied by the bulk modes. Our work demonstrates robust operation of nanowire-based SHOs over a wide range of nanowire widths and presents an example of a spin torque oscillator with the active area extended into the µm$^{2}$ domain. [1] Zheng Duan et al, Nature Communications 5, 5616 (2014) [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A18.00010: Non-adiabatic spin-transfer torque independent of the spin relaxation rate Kyoung-Whan Kim, Kyung-Jin Lee, Hyun-Woo Lee, Mark Stiles Non-adiabatic spin-transfer torques play an important role in magnetization dynamics. For example, they determine current-induced magnetic domain wall velocity. A well-known mechanism for non-adiabatic spin-transfer torques arises from spin relaxation and is directly proportional to the spin relaxation rate. Here we report mechanism that is independent of the spin relaxation rate. This mechanism is related to the recently reported intrinsic damping-like spin-orbit torque, which is proportional to an electric field but is independent of the conductivity, and hence the scattering rate. Likewise, the mechanism we report is independent of the scattering rate. It originates from the effective spin-orbit coupling that arises in systems with magnetic textures as we previously reported for related processes. In this work, we demonstrate the existence of such a spin-transfer torque, which is a contribution to the non-adiabatic spin-transfer torque and is independent of scattering rates. We also demonstrate that the magnitude of this torque can be much larger than other mechanisms for non-adiabatic spin-transfer torques, and may be the dominant contribution in some systems. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A18.00011: Optical detection of spin Hall effect in metals Olaf Van T Erve, Aubrey Hanbicki, Connie Li, Berend Jonker Spin Hall effects in metals have been successfully measured using electrical methods such as nonlocal spin valve transport, ferromagnetic resonance or spin torque transfer experiments. These methods require complex processing techniques and measuring setups. Here we present room temperature measurements of the spin Hall effect in non-magnetic metals such as Pt and $\beta $-W using a standard bench top magneto-optic Kerr effect (MOKE) system. With this system, one can readily determine the angular dependence of the induced polarization on the bias current direction. When a bias current is applied, the spin Hall effect causes electrons of opposite spin to be scattered in opposite directions, resulting in a spin accumulation at the surface of the film. The MOKE signal tracks the applied square wave bias current with an amplitude and phase directly related to the spin Hall angle. Using this technique, we show that the spin-Hall angle of $\beta $-W is opposite in sign and significantly larger than that of Pt. In addition, we use this technique to detect spin diffusion from $\beta $-W into Al thin films, as well as spin diffusion from the topological surface states of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ into Al. We will also show direct modulation of the reflected light up to 100 kHz, using Bi doped Cu samples. This work was supported by internal programs at NRL. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A18.00012: Spin Hall magnetoresistance in ultra thin Pt/LSMO. Na LEI, Yu Bai, Zhao Ding, Jian Shao, Wengang Wei, Lifeng Yin, Yizheng Wu, Jian Shen Spin Hall magnetoresistance (SMR) in a non-/ ferro- magnetic (NM/FM) bilayer is an angular dependence of resistance of the NM layer on the magnetization of FM layer [1]. It provides an easy approach to the spin Hall effect in a simple bilayer system, however similar effects mixed in the system and might complicated the data analysis and interpretation. Here we present a case of ultra thin Pt/LSMO, in which LSMO (bellow 7 unit cells) layer is an insulating magnetic oxide with Curie temperature of 120K. Below 120K, our results clearly show the coexistence of the anisotropic magnetoresistance (AMR) and SMR effects. However, far away above Curie temperature, where the LSMO is paramagnetic, the magnetoresistance doesn't disappear but even increase, which is distinct from the case of Pt/YIG [2]. Here it is neither SMR nor AMR, and an additional mechanism is required. Anomalous Hall effect was also performed, which is consistent with SMR measurement. We propose some physical pictures which could attribute to this magnetoresistance in paramagnetic state. Reference: [1] H. Nakayama, M. Althammer, Y.-T. Chen, K. Uchida, et al., Phys. Rev. Lett. 110(20), 206601 (2013). [2] K. Uchida, Z. Qiu, T. Kikkawa, R. Iguchi, E. Saitoh, Appl. Phys. Lett. 106, 052405 (2015). [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A18.00013: Spin Transport in Ferromagnetic and Antiferromagnetic Insulators Shanshan Su, Gen Yin, Yizhou Liu, Jiadong Zang, Yafis Barlas, Roger Lake Recently, experiments of spin pumping have been done for system with antiferromagnetic oxides (AFMOs) as a spacer between YIG and Pt [1-3]. Observation of spin transport through the AFMO and the enhancement of spin pumping signal in the system due to the insertion of AFMO have been reported [1,2]. In this research, we model the spin transport in Pt/YIG/Pt and Pt/YIG/AFMO/Pt heterostructures using the Landau--Lifshitz--Gilbert equations coupled with the non-equilibrium Green's function equations. We show that a pure spin current generated at the first Rashba SOC electrode is carried by magnon through YIG, which can be converted back to spin pumping signal at the second electrode. The spin dynamical details at the heterostructure can determine the transport efficiency. The effect of different magnetization orientations and finite temperatures will be addressed. [1]C. Hahn et al., EPL 108, 57005 (2014) [2]H. Wang et al., Phys. Rev. Lett. 113, 097202 (2014) [3]H. Wang et al., Phys. Rev. B 91, 220419 (2015) [Preview Abstract] |
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