Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E18: Spin-Hall IIFocus Industry
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Sponsoring Units: GMAG DMP FIAP Chair: Se Kwon Kim, UC Los Angeles Room: 317 |
Tuesday, March 15, 2016 8:00AM - 8:36AM |
E18.00001: Role of transparency of platinum-ferromagnet interface in determining intrinsic magnitude of spin Hall effect Invited Speaker: Wei Han The spin Hall effect (SHE) converts charge current to pure spin currents in orthogonal directions in materials that have significant spin-orbit coupling. The efficiency of the conversion is described by the spin Hall Angle (SHA). The SHA can most readily be inferred by using the generated spin currents to excite or rotate the magnetization of ferromagnetic films or nano-elements via spin-transfer torques. Some of the largest spin torque derived spin Hall angles (ST-SHA) have been reported in platinum. In this talk, I will discuss that the transparency of the Pt-ferromagnet interface to the spin current plays a central role in determining the magnitude of the ST-SHA [1]. Using spin torque ferromagnetic resonance (ST-FMR) measurements, we measure a much larger ST-SHA in Pt/cobalt (0.11) compared to Pt/permalloy (0.05) bilayers when the interfaces are assumed to be completely transparent. Taking into account the transparency of these interfaces, as derived from spin--mixing conductances, we find that the intrinsic SHA in platinum has a much higher value of 0.19 \textpm 0.04 as compared to the ST-SHA. The importance of the interface transparency is further exemplified by the insertion of atomically thin magnetic layers at the Pt/permalloy interface that we show strongly modulates the magnitude of the ST-SHA. Improving the interface transparency can make the SHE more effective for spintronic applications and is critical to understanding the fundamental origin of the SHE. [1] W. Zhang*, Wei Han*, Xin Jiang, See-Hun Yang and Stuart S. P. Parkin, Nature Physics, 11, 496--502 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E18.00002: Multi-directional Spin Transport at Interfaces with Spin-Orbit Coupling Vivek Amin, Mark Stiles Spin transport remains poorly understood in multilayer systems with interfacial spin-orbit coupling. Currently, drift-diffusion models cannot accurately treat this phenomenon, since the important consequences of interfacial spin-orbit scattering remain uncharacterized in a systematic way. Here we present boundary conditions suitable for drift-diffusion models that capture the phenomenology of interfacial spin-orbit coupling. To access their viability we compare solutions of the drift-diffusion and Boltzmann equations in a Co/Pt bilayer, since the latter approach yields a momentum-dependent distribution function equipped to describe spin-orbit scattering. A key result is that in-plane electric fields create spin accumulations and spin currents polarized in all directions, which describes a generalization of the Rashba-Edelstein and spin Hall effects. In heavy metal/ferromagnet bilayers, this phenomenon provides a mechanism for the creation of damping-like and field-like torques; it also leads to possible reinterpretations of experiments in which interfacial torques are thought to be suppressed. We discuss the interpretation of experiments involving spin orbit torque, spin pumping/memory loss, the Rashba-Edelstein effect, and the spin Hall magnetoresistance. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E18.00003: Spin-Hall Non-Local Transport Mediated by a Magnetic Insulator Massoud Ramezani Masir, Hua Chen, Inti Sodemann, Allan. H. MacDonald Magnetic systems with easy-plane order support dissipationless spin supercurrents that can lead to non-local coupling between electrically separated conductors. Recently the electrical properties of a system containing two magnetic multilayer stacks with perpendicular magnetic anisotropy electrodes and a shared easy-plane magnetic layer have been discussed. In this research we discuss a closely related system in which the two conducting channels that are coupled by the easy-plane magnetic layer are co-planar thin film metals with large spin Hall effects. We theoretically explained the non-local relationship between the current-voltage relationships of two thin film metallic conductors. Coupling occurs because both conductors inject spins into the magnetic insulator and because this information is communicated between conductors via exchange interactions within the magnetic system. We investigate the non-local transport properties of the system in the macrospin and long thin nanomagnet limits, deriving conditions for the critical currents and using solutions to the Landau-Liftshitz-Gilbert equation to characterize the dynamic steady state case. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E18.00004: Nonlocal anomalous Hall effect Shulei Zhang, Giovanni Vignale Anomalous Hall effect (AHE) is a distinctive transport property of ferromagnetic metals arising from spin orbit coupling (SOC) in concert with spontaneous spin polarization. Nonetheless, recent experiments have shown that the effect also appears in a nonmagnetic metal in contact with a magnetic insulator. The main puzzle lies in the apparent absence of spin polarized electrons in the non-magnetic metal. Here, we theoretically demonstrate that the scattering of electrons from a rough metal-insulator interface is generally spin-dependent, which results in mutual conversion between spin and charge currents flowing in the plane of the layer. It is the current-carrying spin polarized electrons and the spin Hall effect in the bulk of the metal layer that conspire to generate the AH current. This novel AHE differs from the conventional one only in the spatial separation of the SOC and the magnetization, so we name it as nonlocal AHE. In contrast to other previously proposed mechanisms (e.g., spin Hall AHE and magnetic proximity effect (MPE)), the nonlocal AHE appears on the \textit{first order} of spin Hall angle and does \textit{not} rely on the induced moments in the metal layer, which make it experimentally detectable by contrasting the AH current directions of two layered structures such as Pt/Cu/YIG and $\beta $ -Ta/Cu/YIG (with a thin inserted Cu layer to eliminate the MPE). We predict that the directions of the AH currents in these two trilayers would be \textit{opposite} since the spin Hall angles of Pt and $\beta $ -Ta are of opposite signs. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E18.00005: Spin Hall effects from mesoscopic ferromagnetic NiFe thin films Chuan Qin, Shuhan Chen, Yunjiao Cai, Yi Ji The spin Hall effect (SHE) and inverse spin Hall effect (ISHE) have been explored primarily in nonmagnetic heavy metals such as Pt. In this work, we probe SHE/ISHE from mesoscopic ferromagnetic NiFe (Py) films in nonlocal lateral structures. The structure consists of a Py spin injector/detector (F1), a Cu channel, and a second Py stripe (F2) where SHE/ISHE occurs. Low-resistance AlOx layers are placed at all interfaces. For SHE, a charge current passes through F2, and a nonlocal voltage is detected between F1 and Cu. For ISHE, a charge current is injected from F1 into Cu, and the nonlocal voltage is measured between two ends of F2. The in-plane magnetic field is applied perpendicular to F1/F2 stripes. For both measurements, the nonlocal signal for large positive field is different from that of large negative field owing to the SHE/ISHE. Using a simple model, the apparent spin Hall angle (assuming long Py spin diffusion length) of Py is estimated to be 0.010 at 295K and 0.017 at 4.5K. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E18.00006: Demonstration of Kirchhoff's First Law for Pure Spin Currents Joseph Batley, M. C. Rosamond, M. Ali, E. H. Linfield, G. Burnell, B. J. Hickey In conventional electronics a fundamental component of circuit design is the principle of fan-out, which allows multiple operations to be performed in order to build up complex logical procedures. A fan-out device relies on the condition that electrical currents obey Kirchoff's laws and in order for spin-logic to be viable, the same must be shown for pure spin currents. Both fan-out and fan-in experiments have been performed to observe how spin currents behave in a multi-terminal circuit. The development of a 3-dimensional nonlocal IV and matrix fitting method provides information about each spin current, along with the thermal current generated at the injection point, and how they interact with each other. The fan-out geometry demonstrates that a pure spin current will divide between the different branches in a circuit, with a magnitude determined through the spin resistances of each arm. The fan-in measurements demonstrate that two pure spin currents will add and subtract with each other in a conventional manner expected from Kirchhoff's first law. These experiments have demonstrated the symmetry of pure spin currents with respect to the injection current and shown that they obey Kirchhoff's current law. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E18.00007: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E18.00008: Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets Koji Sato, Kei Yamamoto, Eiji Saitoh, Hiroshi Kohno An anomalous Hall effect is shown to arise from the exchange interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E18.00009: Strong Spin Hall effect in PtMn Yongxi Ou, Shengjie Shi, Daniel Ralph, Robert Buhrman Recent reports indicate that certain metallic antiferromagnets (AFM) can exhibit a significant spin Hall effect. Here we report a large damping-like spin torque efficiency ($\xi_{\mathrm{DL}})$ in PtMn/ferromagnet(FM) bilayer structures, determined from both FM-thickness-dependent spin-torque ferromagnetic resonance (ST-FMR), and harmonic response (HR) measurements of layers with perpendicular magnetic anisotropy (PMA). We find that $\xi _{\mathrm{DL}}$ can vary from \textless 0.1 to \textgreater 0.15, depending on the thickness of PtMn, the stacking order of the samples, and the choice of the FM material. The field-like spin torque efficiency ($\xi _{\mathrm{FL}})$ is also quite variable, 0\textless \textbar $\xi _{\mathrm{FL}}$\textbar \textless 0.5. The large broadening of the ST-FMR linewidth suggests extra spin attenuation at the AFM/FM interface that is possibly due to intermixing. The PtMn/FeCoB/MgO structures that exhibit PMA have a comparatively low switching current density and an unusual asymmetric switching phase diagram. These results indicate that AFM PtMn has significant potential both for advancing the understanding the physics of the spin Hall effect in Pt alloys, and for enabling new spintronics functionality. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E18.00010: Large anomalous Hall effect in Pt interfaced with perpendicular anisotropy ferrimagnetic insulator Chi Tang, Pathikumar Sellappan, Yawen Liu, Javier Garay, Jing Shi We demonstrate the strain induced perpendicular magnetic anisotropy (PMA) in a ferrimagnetic insulator (FMI), Tm$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (TIG) and the first observation of large anomalous Hall effect (AHE) in TIG/Pt bilayers. Atomically flat TIG films were deposited by a laser molecular beam epitaxy system on (111)-orientated substituted gadolinium gallium garnet substrates. The strength of PMA could be effectively tuned by controlling the oxygen pressure during deposition. Sharp squared anomalous Hall hysteresis loops were observed in bilayers of TIG/Pt over a range of thicknesses of Pt, with the maximum AHE conductivity reaching 1 S/cm at room temperature. The AHE vanishes when a 5 nm Cu layer was inserted between Pt and TIG, strongly indicating the proximity-induced ferromagnetism in Pt. The large AHE in the bilayer structures demonstrates a potential use of PMA-FMI related heterostructures in spintronics. [Preview Abstract] |
Tuesday, March 15, 2016 10:24AM - 11:00AM |
E18.00011: Direct measurement of spin accumulation in the Cu layer due to spin currents from Co Invited Speaker: Roopali Kukreja Spin transport is the key for reading or writing bits in spintronic devices by utilizing the Giant Magnetoresistance effect or the spin transfer torque effect. Spin currents have also been shown to play important role in the ultrafast manipulation of magnetization via all optical switching. Hence, detailed understanding of spin currents from ferromagnet to non-magnets is a crucial step in development of spintronic devices. However, directly observing these spin currents is extremely challenging due to magnetic moment injected into non-magnet being very small, less than 1/10000 of a regular ferromagnet. In this talk, I will present our recent measurements on the spin currents from a thin film Co ferromagnet into non-magnetic Cu metal in a nanopillar device. We have developed an extremely sensitive spectro-microscopy detection method based on element specific x-ray magnetic circular dichroism where current pulses driving the spin currents into the Cu layer are synchronized with the synchrotron x-ray photons. The sensitivity of this `lock-in' technique has allowed us to detect the extremely small transient Cu magnetization. We observe two spin currents induced effects in the Cu layer. The first effect is the transiently induced magnetization which occurs in bulk of the Cu layer due to spin accumulation and has a magnitude of 0.00003 $\mu $B per atom. The second effect occurs at the Co/Cu interface where we observe a 10{\%} increase or 0.004 $\mu $B per atom for the hybridized Cu atoms due to spin torque-alignment. [Preview Abstract] |
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