Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S41: Spin-Transfer and Spin-Orbit TorquesFocus
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Sponsoring Units: GMAG DMP Chair: Luqiao Liu, Massachusetts Institute of Technology Room: BCEC 209 |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S41.00001: First-principles calculation of spin-orbit torques in ferromagnet/heavy-metal bilayers Kirill Belashchenko, Alexey Kovalev, Mark van Schilfgaarde The spin-orbit torque in Co/Pt, Co/Pd, and Co/Au bilayers is calculated using a first-principles non-equilibrium Green's function formalism with an explicit treatment of disorder. The torque is formally split in the Fermi-sea and Fermi-surface contributions. The Fermi-sea term is important at low temperatures, but the Fermi-surface term dominates at room temperature. In addition to the usual damping-like and field-like terms, the odd torque contains a sizeable planar Hall-like term (mE)m×(z×m), which contributes to damping and has been observed experimentally. While the torques that contribute to damping are largely due to spin-orbit coupling on the heavy-metal atoms, the field-like torque does not require it. The dependence of the torque on the thicknesses of the layers is also examined. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S41.00002: Deterministic spin-orbit torque switching via structural engineering Tian-Yue Chen, Hsin-I Chan, Wei-Bang Liao, Chi-Feng Pai Current-induced spin-orbit torque (SOT) can be employed to control magnetization in magnetic random-access memory (MRAM) with perpendicular magnetic anisotropy (PMA), but typically requires an applied in-plane magnetic field. To eliminate the need of this applied field and achieve better on-chip memory designs, we deposite 4d transition metal Mo in a canted way as the SOT source. Although the spin-orbit interaction in Mo is weaker than in 5d metals such as Pt, W, or Ta, deterministic switching can still be achieved. This result suggests that growth configuration can play a more important role than material selection in some cases, which could really impact the engineering of next-generation field-free SOT-MRAM. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S41.00003: Spin-Orbit Torque Switching in Asymmetric Structures with Double Non-Magnetic Metal Layers Seyed Armin Razavi, Guoqiang Yu, Hao Wu, Qiming Shao, Kin Wong, Kang-Lung Wang Spin-orbit torque (SOT) switching of magnetization is a promising emerging technology for non-volatile memory and logic applications. However, deterministic switching with SOTs requires breaking of inversion symmetry, usually provided by an external magnetic field, which is not suitable for applications. It has been shown that structures with lateral asymmetry can eliminate the need for external field and realize field-free SOT switching [1]. In this work we study field-free SOT switching in asymmetric structures with double non-magnetic metal layers, where current-induced out-of-plane effective magnetic fields are created. We break the structural symmetry by inserting a wedge-shaped second metallic layer between the ferromagnet and the first heavy metal. We investigate the creation and origin of current-induced out-of-plane effective fields, HZeff, in various material systems (W/IrMn, W/Ta, Pt/IrMn, W/Ti), and we show the realization of field-free SOT switching in these structures. Our work provides a route for practical application of SOT devices and challenges the current understanding of the origins of SOTs in structures with lateral asymmetry. |
Thursday, March 7, 2019 11:51AM - 12:27PM |
S41.00004: Strong Spin-Orbit Torques, Tunable DMI, and Variable Interfacial Spin-Orbit Coupling in Pt-based Spin Hall Metal/Ferromagnet Systems Invited Speaker: Lijun Zhu Recent work finds that the spin Hall effect in Pt is dominated by the intrinsic Berry phase effect. The internal spin Hall ratio θSH should be enhanced by alloying Pt with a component that raises the resistivity ρ but does not materially reduce the spin Hall conductivity σSH. This talk will report on an extensive study of two fcc Pt alloys which establishes that for Au1-xPtx(Pd1-xPtx), at the optimal concentration x ≈ 0.25, anti-damping spin-orbit torque efficiency ξDL≈ 0.30 (0.26) for ρ = 83 (57.5) μΩ cm at 4 nm thickness [1,2]. This indicates θSH ≥ 0.58 (0.47) and σSH ≈ 0.7(1)×106 Ω-1m-1; considerably larger than predicted by recent first principles-calculations. Moreover, the DMI at the Pt alloy/FM interface is both strong and tunable by composition over a wide range. These results establish Au1-xPtx and Pd1-xPtx as the most energy-efficient spin current generators for spin-torque manipulation of metallic FM systems and for chiral spintronics applications. The effect of interfacial spin-orbit coupling (ISOC) on the spin transparency of the Pt(alloy)/FM interface has also been examined through controlled variation of its strength through thermal annealing, revealing that ISOC at HM/FM interfaces should be minimized via interfacial passivation to maximize ξDL [3,4]. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S41.00005: First Principles Study of Spin-Orbit Torque in Pt/Co and Pd/Co Bilayers Farzad Mahfouzi, Nicholas Kioussis Spin-orbit torque (SOT) induced by spin Hall and interfacial effects in heavy metal(HM)/ferromagnetic(FM) bilayers has recently been employed to switch the magnetization direction using in-plane current injection. In this work, using the Keldysh Green's function approach and first principles electronic structure calculations we determine the Field-Like (FL) and Damping-Like (DL) components of the SOT for the HM/Co (HM = Pt, Pd) bilayers. We investigate the effects of HM and FM thicknesses, srain, oxidation as well as rigid shift of the chemical potential on the SOTs. We compare the HM thickness dependence and also efect of oxigen in the FM with the experimental results where we see an overall good agreement. The dependence of the SOT on the position of the Fermi level suggests that the DL-SOT dominated by the Spin Hall effect of the bulk HM. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S41.00006: Excitation and Amplification of Propagating Spin Waves by Spin-Orbit Torque Boris Divinskiy, Vladislav Demidov, Sergej Demokritov, Sergei Urazhdin, Ryan Freeman Spin-orbit torque (SOT) produced by electric current is among the cornerstones of modern nanomagnetism. However, until now SOT could not be utilized for the generation of propagating spin waves for magnonic (spin wave-based) applications. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S41.00007: Controlling Interfacial Magnetic Anisotropy and Spin-Orbit Torques with Insulating CdWO4 Saba Karimeddiny, Gregory Stiehl, Robert Buhrman, Daniel Ralph We examine the effects of low crystal symmetry when metallic magnetic films are deposited on CdWO4, a readily-available, insulating substrate with strong spin-orbit coupling whose symmetries can be restricted to a single mirror plane if cleaved with the b-axis in-plane. We show that magnetic thin films interfaced with CdWO4 have strong magnetic anisotropy induced by the low-symmetry crystal structure. This anisotropy is strongly temperature-dependent, exceeding 200 Oe at low temperatures. In addition, measurements of spin-orbit torque in such bilayers exhibit torque components that are commensurate with the symmetry of CdWO4. This suggests an interfacial torque mechanism in which spin-polarized electrons flowing within the magnetic layer and scattering from the insulating CdWO4 can produce a spin-orbit torque acting back on the magnetic layer. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S41.00008: Demonstration of a micron-scale spin-orbit-torque emitter for coherent magnonics Michael Evelt, Lucile Soumah, Anatoliy Bronislavovich Rinkevich, Sergej Demokritov, Jamal Ben Youssef, Gregoire de Loubens, Olivier Klein, Paolo Bortolotti, Vincent Cros, Abdelmadjid Anane, Vladislav Demidov Using micro-focus Brillouin light scattering (BLS), we experimentally demonstrate generation of coherent propagating magnons in ultra-thin magnetic-insulator films by spin-orbit torque induced by dc electric current [1]. We show that this challenging task can be accomplished by utilizing magnetic-insulator films with large perpendicular magnetic anisotropy (PMA) possessing ultra-low Gilbert damping ( ≈ 8 10-4 for 20 nm thick Bismuth substituted YIG [2]). Fine tuning of the PMA allows to exactly cancel the dipolar field (the effective magnetization Meff ≈ 0). As a result, the usually observed non-linear shift of the auto-oscillation frequency is suppressed. Hence, the dominant mechanism for self-localization of the auto-oscillations is inhibited. We demonstrate simple and versatile spin-orbit torque devices, which can be used as highly efficient nanoscale sources of coherent propagating magnons for insulator-based spintronic applications. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S41.00009: Spin mixing conductance of ferromagnet/topological-insulator and ferromagnet/heavy-metal bilayers with strong interfacial spin-orbit coupling: A first-principles time-dependent quantum transport approach Kapildeb Dolui, Utkarsh Bajpai, Branislav Nikolic Spin pumping and related phenomena have been observed recently in heavy metals and topological insulators, where the spin-orbit coupling plays an essential role. Conventional spin mixing conductance, which governs the magnitude of pumped spin current, is not well-defined when spin-orbit coupling is present directly at the interface. Nevertheless, an effective one can be extracted as the pre-factor of spin current vs. precession cone angle dependence, where we compute spin current using density functional theory combined with Floquet-nonequilibrium-Green function formalism [Phys. Rev. B 85, 05446 (2012)]. In this work we compute the spin-mixing conductance for Co/Bi2Se3, Co/Pt and Co/W junctions. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S41.00010: Correlation between resonance mode crossing and spin torque switching probability in magnetic tunnel junctions Chris Safranski, Jonathan Sun We report on the effect of coupled free- and reference-layer dynamics on switching errors in a spin-torque switched magnetic tunnel junction. Perpendicularly magnetized magnetic tunnel junctions (MTJ) [1-3] are used for this study. The free layer switching speed and probability is related to magnetization dynamics as revealed by spin-torque driven, and thermal fluctuation-driven ferromagnetic resonance (STT- and Thermal-FMR) spectra. We investigate the probability in spin torque driven switching as a function of applied magnetic field, and compare with the STT- and thermal-FMR spectrum in the same field range. A nonmonotonic dependence of the switching probability on field and on bias-voltage is seen, correlating to STT-FMR mode-crossings between the free- and reference-layer spectra-lines. At field values corresponding to the intersections of free- and reference-layer spin wave modes, an increase of errors is observed in switching. Our results show that excitation of the reference layer’s dynamics can affect the MTJ’s net STT-switching probability. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S41.00011: Zero-Threshold Rectification Using Low-Barrier Magnets Shehrin Sayed, Kerem Y Camsari, Rafatul Faria, Supriyo Datta Stable magnets with energy barriers (~40-60 kBT) have been the center of focus for spintronics. Recently, low energy barrier magnets have attracted growing interest in the community for novel applications e.g. random number generation, stochastic oscillators, probabilistic computers, etc. Using experimentally benchmarked models [1,2], we show that the charge current induced spin voltage measurements, well-established for diverse materials with spin-orbit coupling, could be used for zero-threshold rectification when the stable magnet is replaced with a low barrier magnet either with an in-plane or perpendicular anisotropy. These experiments can be used to characterize such stochastic magnets and extract parameters that determine (i) spin-orbit torque induced magnetization pinning and (ii) the frequency band of rectification. We analytically determine this frequency band from angular momentum conservation principles. The proposed structure could find application as highly sensitive passive rf detectors and as energy harvesters from weak ambient sources where standard technologies may not operate. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S41.00012: Fermi surface of CrNb3S6: giant spin-orbit effect Tatsuya Shishidou, Michael Weinert Chiral magnets are of particular interest because of their exotic magnetic and transport properties. In the case of CrNb3S6, the ground-state magnetic configuration is a planar flat spiral with a periodicity of 48nm along the hexagonal c axis. Moderate external fields (H) alter the spin texture into the chiral soliton lattice (H⊥c) or the chiral conical phase (H ∥c). With increasing H, these phases continuously transform into forced ferromagnetic (FM) configurations that appear at H ~ 0.2 (⊥c) and 2 Tesla (∥c). Using a density functional theory approach, we investigate the FM state of CrNb3S6, paying particular attention to the spin-orbit coupling (SOC) and its effect on the electronic band structure and Fermi surface. Importantly, for some bands near the Fermi level, SOC introduces giant and peculiar first-order changes of the band energies when the magnetization is parallel to the c axis: the band dispersion appears to be shifted in k space. Consequently, the deformed Fermi surface has a highly asymmetric shape that could lead to nonreciprocal transport. Symmetry arguments and detailed analysis of SOC effects will be given. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S41.00013: Observations of large spin mixing conductance in poly(3-methylthiophene) polymer brushes/NiFe heterostructures Eric Vetter, Ian Vonwald, Dali Sun, Wei You Observations of large inverse spin Hall effect (ISHE) in a fullerene (C60) thin film – usually only appearing in metals and semiconductors with heavy elements - indicate that spin-orbit coupling in terms of spin Hall angles would be significantly influenced by chemical structure parameters, such as stacking and curvatures. Here, we report studies on the effective spin mixing conductance in ‘spuncast’ poly(3-hexylthiophene) (P3HT) and ‘polymer brush’ poly(3-methylthiophene) (P3MT) thin films using ferromagnetic resonance (FMR) and spin pumping techniques. P3HT and P3MT films were spuncast and self-assembly grown, respectively, onto ITO/glass substrates followed by NiFe. Frequency-dependent FMR and ISHE measurements were carried out on both films as a function of film thickness, from which the effective spin mixing conductances were obtained. We find that the P3MT films exhibit a large increase in the effective spin mixing conductance, in contrast to that in P3HT films. The large spin mixing conductance in the ‘polymer brush’ P3MT thin films is attributed to morphological differences as compared with the spuncast films. |
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