Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y22: Switching, Torques and Spin Transport in InsulatorsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Vivek Amin, NIST -Natl Inst of Stds & Tech Room: LACC 402A |
Friday, March 9, 2018 11:15AM - 11:51AM |
Y22.00001: Switching, Torques, and Spin Transport in Magnetic Insulators with Perpendicular Anisotropy Invited Speaker: Can Onur Avci Magnetic insulators (MIs), especially iron-based garnets, possess remarkable properties such as ultralow damping and long magnon decay lengths, which can provide significant advantages for practical applications with respect to their metallic magnetic counterparts. Recently, robust perpendicular magnetic anisotropy is obtained in ferrimagnetic films of thulium, europium, and terbium iron garnet (TmIG, EuIG and TbIG) with high structural quality down to a thickness of 5.6 nm with saturation magnetization close to bulk [1]. By using the spin Hall effect in platinum we have demonstrated efficient spin current injection through the TmIG/Pt interface, which we quantified by the spin Hall magnetoresistance and harmonic Hall effect measurements [2,3]. We then achieved deterministic spin-orbit torque-driven magnetization switching of TmIG(~10 nm)/Pt bilayer both with quasi-dc (5 ms) as well as pulsed currents down to 2 ns width[3,4]. The switching current density is found to be of the order of ~107 (~108) A/cm2 using dc (pulsed) current, comparable to reported values for Pt/Co[5]. We reveal that the threshold switching current strongly depends on the absence or presence of an initially reversed domain in the structure implying a reversal mediated by efficient current-driven domain wall motion. Based on this hypothesis we estimate the current-driven domain wall velocity ~400 m/s per j=108A/cm2, exceeding the most efficient domain wall velocities reported in metallic ferromagnets[6]. These results suggest the utility of PMA rare earth garnets and pave the road towards ultralow dissipation spintronic devices based on MIs. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y22.00002: Investigation of Optically Generated Spin-Currents in Pt/magnetic Insulator Bi-layered Structures Subash Kattel, Joseph Murphy, David Ellsworth, Peng Li, Tao Liu, Mingzhong Wu, William Rice Pure spin current, which is important for novel spintronic devices, can be produced by spin Seebeck effect or by spin pumping. Recently, a new method was introduced for generating pure spin current directly by optically exciting a nanometer-thick Pt on YIG. Initial measurements using a broadband light source suggested that photogenerated electrons and holes generates a voltage in the Pt that is in proximity to YIG. Here we report the observation of a near-constant, optically created voltage due to spin current over a wide spectral range from 350 - 1600 nm. Despite strong spin-orbit coupling in Pt, we show that this signal is insensitive to the optical polarization and only depends on the optical power. Temperature, magnetic field, and time-dependent measurement are consistent with both the spin Seebeck and photo-spin-voltaic effects. These measurements show that pure spin currents can be optically generated in Pt/YIG heterostructures over a broad spectral range, which suggests their potential for implementation into a broadband photo-detection devices. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y22.00003: Joule Heating Effect on Field-Free Spin-Orbit Torque Magnetization Switching in Exchange Biased Systems Seyed Armin Razavi, Guoqiang Yu, Di Wu, Yong-Chang Lau, Ching-Tzu Chen, Zongzhi Zhang, J M Coey, Plamen Stamenov, Pedram Khalili Amiri, Kang L Wang Deterministic spin-orbit torque (SOT) switching of perpendicular magnetization is usually achieved with an external magnetic field, not desired for practical applications. However, it has been shown that field-free switching can be realized using the in-plane exchange bias (EB) at an antiferromagnet/ferromagnet (AFM/FM) interface. We study Pt/CoFe/IrMn system and achieve field-free switching. Then, we quantitatively investigate the Joule heating effect in AFM/FM structures, and show that applying high current densities for switching may raise the device’s temperature close to the blocking temperature. Consequently, if not properly designed, the in-plane EB may be lost after several switching cycles, and degrade the field-free switching. Next, to enable readout using magnetic tunnel junctions, we study IrMn/CoFeB/MgO structure, where IrMn (an AFM) provides both the EB and SOT. We show that field-free switching can be realized in this structure and we discuss the performance and heating effects, providing important insights for practical field-free SOT switching in AFM/FM systems. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y22.00004: Length scale of spin-orbit torques and efficient switching in magnetic insulators Qiming Shao, Chi Tang, Guoqiang Yu, Peng Zhang, Aryan Navabi, Cheng Zheng, Pramey Upadhyaya, Hao Wu, Yawen Liu, Xiufeng Han, Yaroslav Tserkovnyak, Jing Shi, Kang Wang Magnetic insulators (MIs) attract tremendous interest for spintronic applications due to low damping and absence of Ohmic loss. The efficient manipulation of magnetization in MIs via current-induced spin-orbit torques (SOTs) promises future development of low-power spintronics based on MIs. Here, we report a systematic study of MI layer thickness dependent SOTs and current-driven switching behaviors by using tungsten/thulium iron garnet (W/TmIG) bilayers. Our results reveal a characteristic increase of the damping SOT efficiency with the TmIG thickness, which is linked to the exchange coupling strength at the interface. The characteristic length of TmIG is around 8 nm, which is much larger than the case of around 1 nm in ferromagnetic metals. We then demonstrate the current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up to 15 nm, where the switching direction is opposite to that of Pt/TmIG. The switching current density is significantly lower than that of Pt/TmIG, which is attributed to a larger spin Hall angle and stronger thermal effect. Our results about length scale of SOTs and efficient switching in MIs shed light on the future development of low power MI-based spintronics. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y22.00005: Spin Orbit Torques in Disordered Multiband Electron Systems: Boltzmann Regime and Beyond Cong Xiao, Qian Niu We study spin-orbit torques (SOT) in nondegenerate multiband electron systems. In the weak-disorder limit, a semiclassical Boltzmann approach equivalent to the microscopic Kohn-Luttinger linear-response approach is formulated. This semiclassical framework accounts for the interband-coherence effects induced by both the electric field and static impurity scattering, as well as the recently highlighted coherent skew scatterig. Using the two-dimensional Rashba ferromagnet as a model system, we show that the antidamping-like SOT arising from disorder-induced interband-coherence effects is very sensitive to the structure of disorder potential in the internal space. On the other hand, we show that the Boltzmann theory produces the same results as Kubo diagrammatic calculations only in the weak-disorder limit. Our analysis indicates that rich behaviors of various nonequilibrium phenomena beyond the Boltzmann theory may also be present even within the good metal limit in multiband systems where the Fermi energy is not the unique intrinsic energy scale. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y22.00006: Spin-orbit torque magnetometry for as-prepared films by wide-field magneto-optical Kerr effect Tsung-Yu Tsai, Tian-Yue Chen, Hsin-I Chan, Chun-Ting Wu, Chi-Feng Pai Magneto-optical Kerr effect (MOKE) is an efficient tool to probe surface magnetization in thin film samples. Here we present a wide-field MOKE technique that adopts a Köhler illumination scheme to characterize the current-induced spin-orbit torques (SOTs) in micron-sized and more importantly, unpatterned/as-prepared magnetic heterostructures with perpendicular magnetic anisotropy. Through a current-induced hysteresis loop shift analysis, we quantify the SOT efficiency of both patterned and unpatterned Ta-based heterostructures to be ~-0.08, which is fairly close to previously reported values obtained by various electrical means. The proposed wide-field MOKE approach therefore provides an instant and direct characterization of SOT, without the need of any further processing on the as-prepared film stacks and further interpretation on the measured signals. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y22.00007: Gradient Expansion Formalism for Generic Spin Torques Atsuo Shitade Spin torques have been studied in the field of spintronics since the celebrated discovery of the current-induced magnetization reversal by the spin transfer torque (STT). In addition to the STT, the so-called β-term arises from spin relaxation. In the presence of spin-orbit interactions (SOIs), the spin-orbit torque arises even without magnetic textures. In real materials, both magnetic textures and SOIs exist, and hence a systematic formalism for spin torques is desired. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y22.00008: First-principles calculation of the intrinsic spin-orbit torque in a heavy-metal/ferromagnet bilayer Kirill Belashchenko, Alexey Kovalev, Mark van Schilfgaarde We describe the implementation of spin-torque calculations using the Landauer-Büttiker non-equilibrium Green’s function (NEGF) formalism within the tight-binding LMTO method [1]. This approach is applied to study the intrinsic spin torque in a ballistic Co/Pt bilayer. We consider the contributions from both the Fermi sea and the bias window and analyze the numerical convergence of the results. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y22.00009: Enhanced Signal-to-noise Ratio in a Magneto-optic Kerr Effect Based Spin-orbit Torque Magnetometer Wenrui Wang, Xin Fan, David Cahill, Virginia Lorenz Current-induced spin-orbit torques have drawn extensive attention in the last few decades, due to their potential application in next-generation spintronic devices. Magneto-optic Kerr effect (MOKE) based magnetometry with balanced detection has been used as a convenient and sensitive tool in spin-orbit torque studies. Although the technique has demonstrated high sensitivity, further improvement of the signal-to-noise ratio is mainly limited by the detector shot noise (0.5√(10×Power/mW) µV/√Hz), and maximum optical power (1 mW) tolerated by the detector. We report a simple “unbalanced” detection method that realizes one order-of-magnitude signal-to-noise ratio improvement, utilizing only one additional neutral-density filter. With its ultrahigh (<100 nrad) sensitivity and enhanced signal-to-noise ratio, we demonstrate fast and precise spin-orbit torque vector measurement using polar and quadratic MOKE, on a Permalloy/Pt bilayer sample with no lithography or processing performed. The results show good consistency with balanced detection results. This improved system can be a powerful tool in both fundamental spin-orbit torque studies and application-related characterization. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y22.00010: Spin Torque Efficiency and Spin Transport In β-W and β-W(Ox) Thin Films Ryan Tapping, Yongxi Ou, Shengjie Shi, Lijun Zhu, Daniel Ralph, Robert Buhrman Tungsten, in its high resistivity, metastable, A15 form (β-W), has the largest known damping-like spin torque efficiency (|ξDL|≥0.3) of the elemental metals, whereas the low resistivity, stable, body-centered cubic structure (α-W) has |ξDL| of less than 0.05. We find that slow (≈0.02 nm/sec) sputter deposition of W onto amorphous surfaces, and polycrystalline surfaces with other than the bcc structure, generally results in β-W films, although the β-W phase is only stable to high temperature (≥400 °C) annealing for film thicknesses of less than 5 nm. Previous work has suggested that sputtering W in a partial atmosphere of O2 can promote β-W growth and lead to even higher values of |ξDL|. To investigate this approach further, we have sputtered thin film bilayer stacks of β-W/Fe60Co20B20 onto silicon wafers, varying the O2 concentration during the deposition of the W, and determined ξDL by ST-FMR measurements. In our case, |ξDL| steadily decreases from over 0.30 for W films sputtered without O2 to less than 0.10 for films sputtered with the highest O2 to Ar ratio (4.0%). We will also report on the interfacial spin transparency of various β-W/FM interfaces and on the spin diffusion length in β-W, which are key parameters for optimization of spin-orbit torque devices. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y22.00011: Time-Resolved Quadratic Magneto-Optical Kerr Effect for Studies of In-plane Magnetic Dynamics Kexin Yang, Arun Ramanathan, Jingwen Yu, Daniel Shoemaker, David Cahill We present an approach for the measurement of in-plane magnetic dynamics with time-resolved quadratic magneto-optical Kerr effect (TR-QMOKE). Because the QMOKE signal is proportional to the square of the magnetic moment, TR-QMOKE can be applied to both ferromagnetic and antiferromagnetic materials with in-plane magnetic moment (e.g., Mn2As, Mn3B4). The measurement is done with an ultrafast laser based pump-probe system, in which the dynamics of the second order term of dielectric constant is measured. We measured the demagnetization of Co thin film with in-plane anisotropy with perpendicularly incident laser beam. The transient rotation signal is on the order of a few micro-radians, approximately an order of magnitude smaller than the transient polar MOKE signal of a CoPt multilayer with perpendicular anisotropy. We extract the demagnetization signal by separating the precession contribution to the polarization-dependent raw data using semi-phenomenological fitting. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y22.00012: Spin current generation in spinel-ferrite/Cu/Pt heterostructures Jacob Wisser, Hyung-Min Jeon, Ramesh Budhani, Matthew Gray, Satoru Emori, Brandon Howe, Yuri Suzuki Understanding spin-pumping at the interface between low-damping magnetic oxides and normal metals with large spin-orbit coupling (e.g. Pt) is crucial to the development of "pure" spin current spintronic devices. We demonstrate that the spinel-ferrite (Ni,Zn)(Al,Fe)2O4 is a low loss spin current source on par with Y3Fe5O12 (YIG). Broadband ferromagnetic resonance (FMR) spectroscopy shows a low Gilbert damping parameter of α ≈ 0.003 for 15-nm thick single-layer NZAFO. In NZAFO/Pt bilayers the damping parameter increases to α ≈ 0.009 accompanied by a voltage peak coinciding with FMR, suggesting significant spin pumping from NZAFO to Pt with a spin-mixing conductance of ~2×1014 Ω-1m-1. To investigate the possible contribution of proximity induced magnetism (PIM) in the Pt layer, we insert Cu spacer layers at the NZAFO/Pt interface. Similar damping enhancement and FMR-driven voltage are detected in NZAFO/Cu/Pt, and angular dependent magnetoresistance measurements show an order of magnitude larger spin-Hall magnetoresistance compared to PIM-induced anisotropic magnetoresistance. Our results demonstrate that the observed FMR-driven voltage is dominated by spin pumping from the ferrite layer through the copper into the platinum layer, with a minimal contribution from PIM in the Pt layer. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y22.00013: Thermal transport through junctions of critical spin chains Sonja Fischer, Lars Fritz We investigate the steady-state energy transport through a system of two coupled 1D spin chains at different temperatures. In the case of two identical spin chains we compare our results to the known expression JE(TL,TR) = cπ/12 (TL2-TR2) for gapless chains with a conformal charge c. In addition we also consider a system of two different spin chains, where the situation is less clear. |
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