Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F29: Focus Session: Spin-Orbit Effects |
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Sponsoring Units: GMAG DMP FIAP Chair: Weigang Wang, University of Arizona Room: 206A |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F29.00001: Optical probe of spin-orbit fields in metallic magnetic structures Mohammad Montazeri, Pramey Upadhyaya, Guoqiang Yu, Kin L. Wong, Murong Lang, Yabin Fan, Pedram Khalili Amiri, Robert N. Schwartz, Kang L. Wang We report a novel self-consistent optical approach based on magneto-optical Kerr effect to directly and quantitatively probe the spin-orbit fields of magnetic devices with 1um diffraction limited spatial resolution. The optical probe is exemplified by investigating the spin-orbit fields in a magnetic stack of Ta(5 nm)/CoFeB(1.1 nm)/MgO(2.0 nm)/TaOx with enhanced perpendicular anisotropy. Both field-like and damping-like contributions were measured independently and their coefficients are quantified at $3.3\times 10^{-6}$ and $-2.0\times 10^{-6}Oe/A.cm^{-2}$ respectively. A detailed comparison with standard transport technique is presented in which a very good agreement were found. Our results establish the relevance of the optical methods for studying spin-orbit torque related physics. We acknowledge the support from the National Science Foundation (DMR-1411085) and the~FAME~Center, one of the six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F29.00002: Detection of inverse Rashba-Edelstein effect at Cu/Bi interface using lateral spin valves Miren Isasa, M. Carmen Mart\'Inez-Velarte, Estitxu Villamor, Luis Morell\'on, Jose M. De Teresa, Manuel R. Ibarra, Luis E. Hueso, Felix Casanova The spin-orbit coupling (SOC) can be exploited to generate and detect pure spin currents, which are key elements in the field of spintronics. One important example is the spin Hall effect. A novel SOC phenomenon, the inverse Rashba-Edelstein effect (IREE), is attracting a large interest. IREE arises from the Rashba coupling that appears at interfaces or surface states (SSs), leading to the conversion of a 3D spin current into a 2D charge current. An interesting system to study the IREE is thus the SS of a semimetal such as Bi. In this work [1], we study the spin-to-charge conversion in Bi using a device based on a lateral spin valve (LSV) geometry. We demonstrate a spin-to-charge current conversion in the LSV. The analysis of the obtained results leads us to argue that the spin-to-charge conversion occurs at the Cu/Bi interface, therefore detecting IREE. Moreover, we evaluate the IREE length, which characterizes the spin-to-charge conversion ratio, as a function of temperature. This ratio changes sign at a certain temperature threshold (125 K), in excellent agreement with the experimental observation of a change in the type of carriers that dominate the electronic transport in Bi. \\[4pt] [1] M. Isasa et al, arXiv: 1409.8540 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F29.00003: New mechanism of spin transfer torque and anisotropic magnetoresistance in a ferromagnetic metal with a spin-orbit coupled interface Shulei Zhang, Shufeng Zhang, Giovanni Vignale In a ferromagnetic metal layer (FML) with a spin-orbit-coupled interface, an in-plane electrical current can cause a perpendicular spin current injection from the interface and hence gives rise to spin transfer torque (STT) on the magnetization. The effect originates from spin rotation during electron scattering at the interface. We derive an analytical expression for the induced spin current and show that it is proportional to the strength of the interfacial spin orbit coupling (SOC) and to the spin polarization of the FML. Furthermore, the spin current is found to decay rapidly (over spin dephasing length) near the interface, leading to a STT that has both in-plane and out-of-plane components. We emphasize that our mechanism is completely based on the scattering of electrons in bulk Bloch states rather than on the properties of spin-orbit coupled surface states[1-4]. Another interesting consequence of the interfacial SOC is an anomalous anisotropic magnetoresistance. As long as the resistivity of the FML is spin dependent, the interfacial SOC will mix the two current channels, leading to an increase in resistivity even when the interface is perfectly flat. Moreover, the resulting AMR exhibits an unconventional angular dependence. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F29.00004: Magneto-ionic Control of Interfacial Magnetism Invited Speaker: Geoffrey Beach Interfaces in ferromagnetic heterostructures give rise to phenomena such as perpendicular magnetic anisotropy (PMA), spin-orbit torques (SOTs), and chiral exchange interactions that are the basis for emerging spintronics technologies. The ability to control these phenomena with a gate voltage would bring about new opportunities for realizing low-power memory and logic devices with field-effect operation. Here I describe a new approach to voltage control of magnetism based on solid-state electrochemical switching of the interfacial oxidation state at a ferromagnet/dielectric interface [1-4]. Interfacial PMA at ferromagnet/oxide interfaces derives from interfacial hybridization between the ferromagnetic 3d and oxygen 2p orbitals. By using GdOx as a gate oxide with high oxygen ion mobility [2,3], we show that O2- can be reversibly displaced at a Co/GdOx interface with a small gate voltage, leading to unprecedented large, non-volatile changes to interfacial PMA [3]. We demonstrate magneto-ionic coupling as a means to control domain wall propagation in magnetic nanowire conduits [1,2], toggle interfacial PMA by an amount approaching 1 erg/cm2 [3], and tune current-induced spin-orbit torques in ultrathin ferromagnetic films [4]. Finally, we show that by optimizing the gate oxide, the timescale for magnto-ionic switching can be reduced by at least 6 orders of magnitude. Progress and prospects for integrating magneto-ionic gates into prototype spintronic devices will be discussed. In collaboration with U. Bauer, A.J. Tan, S. Emori, S. Woo, P. Agrawal and H. L. Tuller, L. Yao and S. van Dijken. \\[4pt] [1] U. Bauer, S. Emori, and G. S. D. Beach, APL 100, 192408; ibid 101, 172403 (2012)\\[0pt] [2] U. Bauer, S. Emori, and G. S. D. Beach, Nat. Nano. 8, 411 (2013)\\[0pt] [3] U. Bauer, L. Yao, A. J. Tan, P. Agrawal, S. Emori, H. L. Tuller, S. van Dijken, and G. S. D. Beach, Nat. Mater. adv. online pub. doi:10.1038/nmat4134 (2014)\\[0pt] [4] S. Emori, U. Bauer, S. Woo, and G. S. D. Beach, APL 105, 222401 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F29.00005: Perpendicular Magnetization Switching via Current induced Spin-Orbit Torques on Flexible Substrate OukJae Lee, Long You, Jaewon Jang, Vivek Subramanian, Sayeef Salahuddin Implementation of perpendicularly magnetized thin films and of electrically functional devices on flexible substrates may offer new degree of freedom such as strain effect on the ultrathin magnetic films with a strong spin orbit coupling. Moreover the flexibility has advantages in applications with bendable, stretchable and/or mobile environment. In this talk we present the magnetic characteristics of ultrathin multilayers with a sufficient PMA that were grown on a flexible plastic substrate by dc/rf magnetron sputtering. In addition we fabricate cross-Hall bar devices and demonstrate fully deterministic magnetic reversal of perpendicularly magnetized square dots via in-plane dc and/or pulsed currents. We believe that integration of two emerging technologies promises new spintronic devices that can be utilized in arbitrary surface geometries and be worked in ultra small dimensions. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F29.00006: Microscopic calculation of Rashba spin-orbit torques Junji Fujimoto, Hiroshi Kohno We study current-induced spin-orbit toques (Rashba torques) in a two- dimensional Rashba ferromagnet, which may model the interface of the ferromagnetic and heavy paramagnetic metals. Using the linear response theory and the Green's function method and treating the nonmagnetic impurity scattering in the self-consistent Born approximation with ladder-type vertex corrections, we calculate the Rashba torques as functions of magnetization direction, strength of the spin-orbit coupling, and the chemical potential. It is found that the Rashba torques are independent of the direction of the magnetization for a parabolic dispersion in good metals, whereas they show a clear angular dependence for a tight-binding model. We will discuss our results comparing with other theoretical calculations [C. O. Pauyac, X. Wang, M. Chshiev and A. Manchon, Appl. Phys. Lett. {\bf 102}, 252403 (2013), K.-S. Lee, et al, arXiv:1409.5600] and the recent experiments. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F29.00007: Dzyaloshinsky-Moriya Interaction in a Two-Dimensional Electron Gas With Rashba Spin-Orbit Coupling Mohammad Mahdi Valizadeh, Sashi Satpathy There is considerable interest in the Rashba spin-orbit coupling due to its potential spintronics application. We study the interaction between two localized magnetic moments in the presence of the Rashba spin-orbit coupling and obtain expressions for the interaction, which contains the RKKY, Dzyaloshinsky-Moriya, as well as a tensor part, viz., $E=J\vec S_1\cdot \vec S_2+\vec D\cdot\vec S_1\times\vec S_2+\vec S_1 \cdot \Pi \cdot \vec S_2$. Explicit expressions are obtained for these terms for the two-dimensional electron gas and their oscillatory behavior is studied as a function of distance between the two localized moments. Results are compared with the earlier works in the literature. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F29.00008: Interlayer spin transfer torque excited by spin-orbit effects in ferromagnets Mark D. Stiles, Tomohiro Taniguchi, Julie Grollier While spin transfer torques generated by the spin Hall effect show the promise of effective switching in some devices, in others, the lack of control over the direction of the incident spins limits their efficiency. Here, we show that spin-orbit effects in ferromagnets, the anomalous Hall effect and the anisotropic magnetoresistance, allow greater control of the orientation of the incident spins. These spin-orbit effects cause in-plane electrical currents in one layer to inject spin currents, flowing perpendicularly to the electrical current, into another layer. The orientations of the flowing spins can be controlled through the orientation of magnetization in the ``fixed'' layer because transverse spin components dephase rapidly to zero and the spins become aligned with the magnetization. This control makes it possible to switch perpendicularly magnetized layers more easily. It also makes it possible to switch in-plane magnetized layers via propagation of transverse/vortex walls and can efficiently induce dynamics in coupled magnetic systems, e.g. coupled transverse domain walls. We calculate torques and critical currents for switching in CoFe/Cu/FePt structures, and domain wall velocities in Py/Co/Py structures. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F29.00009: Temperature dependence of current induced effective spin-orbit torques in perpendicular magnetic anisotropy systems Yongxi Ou, Chi-Feng Pai, Graham Rowlands, Junbo Park, Daniel Ralph, Robert Buhrman We report measurements of the temperature dependence (T $=$ 5-300 K) of current-induced spin-orbit torques for a variety of different heavy metal \textbar ferromagnet \textbar oxide (HM\textbar FM\textbar Oxide) multilayers with perpendicular magnetic anisotropy. Compared to the damping-like torque component, the field-like torque is much more sensitive to T, and generally, but not always, exhibits a quasi-linear variation with T. In some cases, this quasi-linear variation crosses zero, so that the field-like term reverses direction from being parallel to the Oersted field to being anti-parallel. Control experiments using a spacer material having a negligible spin Hall effect (SHE) show that both the field-like torque and the damping-like torque arise from the SHE in the HM layer. However, depending on the details of the FM-normal metal interfaces there is much greater variation in the strength and T behavior of the field-like torque, which points to a strong role for the interface in modifying the strength and direction of the field-like torque. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F29.00010: Anisotropic tunneling between spin-polarized tips and substrate with strong spin-orbit coupling Yonglong Xie, Sangjun Jeon, Ilya Drozdov, Jian Li, Andrei Bernevig, Ali Yazdani The ability to measure spin structure on the nanometer scale has attracted substantial interest for a long time. Spin-polarized scanning tunneling microscopy (SP-STM) is an excellent tool for studying fundamental aspect of magnetism at atomic scale. We combine a low temperature STM equipped with a vector magnet and a spin-polarizable tip, to probe superconductors with strong spin-orbit coupling such as Pb, which is emerging as a platform for engineering topological superconductivity [1]. We observe anisotropic tunneling conductance between tip and substrate as a function of the angle of applied in-plane magnetic field.~ This finding suggests that SP-STM may provide a tool to locally measure spin-orbit coupling, even in non-magnetic substrates. [1] S. Nadj-Perge, I.K. Drozdov, J. Li, H. Chen, S. Jeon, J. Seo, A.H. Macdonald, B.A. Bernevig, A. Yazdani, Science \textbf{346},602 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F29.00011: Enhanced Spin-Orbit Torques in Pt/Co/Ta Heterostructures Maxwell Mann, Seong-Hoon Woo, Lucas Caretta, Aik Jun Tan, Geoffrey Beach Current-induced torques in heavy-metal/ferromagnet/oxide (HM/FM/Ox) stacks attract attention for efficient magnetization switching and domain wall motion. [1-3] Spin-orbit torques (SOTs) arise by spin-Hall and Rashba effects at the HM/FM interface. [1-2] The oxide layer breaks inversion symmetry but typically does not actively contribute to SOTs. We measure SOTs in Co films sandwiched between Pt and Ta, metals with large, opposite spin Hall angles such that the Slonczewski-like torques (Hsl) at the top and bottom interfaces are anticipated to work in concert. [4] SOTs were characterized by harmonic measurements, and the contribution by the Ta layer was isolated by systematically varying its thickness. Increasing Ta thickness significantly enhances Hsl, giving an effective spin Hall angle of up to 34{\%}. A sizable field-like torque also increases with the addition of Ta. Current-induced switching measurements reveal a corresponding increase in switching efficiency, affirming that engineering both interfaces in trilayer structures significantly improves the SOTs. [4] [1] L. Liu et al, Science, 336, 555 (2012) [2] I. M. Miron et al, Nature, 476, 189 (2011) [3] S. Emori et al., Nat. Mater., 12, 611 (2013) [4] S.-H. Woo, et al., Appl. Phys. Lett. in press (2014) [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F29.00012: Spin-Orbit Torques in Ferromagnets and Antiferromagnets on the Surface of a Topological Insulator Aurelien Manchon It has been recently demonstrated that appropriately designed spin-orbit coupling can be used to generate spin torque in a single ferromagnet, without the need of an external polarizer [1,2,3]. In this work, I will first present recent theoretical investigations on the spin-orbit torque that emerges at the interface between a topological insulator and an insulating ferromagnet. This calculation shows that the symmetry of the intrinsic torque differs quite remarkably from the intrinsic torque obtained on the now standard magnetic Rashba 2-dimensional electron gas. I will then extend this exploration to the interface between a topological insulator and an insulating antiferromagnet. The analytical expressions obtained display remarkable differences from the previous case and allows for the coherent manipulation of the order parameter, as predicted in [4] in the case of an antiferromagnetic Rashba 2-dimensional electron gas. [1] A. Manchon and S. Zhang, Phys. Rev. B 78, 212405 (2008). [2] I. M. Miron et al., Nature Materials 9, 230 (2010). [3] A. Chernyshov et al., Nature Physic 5, 656 (2009). [4] Zelevny et al., Phys. Rev. Lett. 113, 157201 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F29.00013: Spin-orbit induced relaxation in combined molecular and spin dynamics simulations of BCC iron Dilina Perera, Markus Eisenbach, Don Nicholson, Junqi Yin, G. Malcolm Stocks, David P. Landau The combined molecular and spin dynamics (MD-SD) method has emerged as a powerful tool for integrating the effect of magnetism into the atomistic simulations of transition metals. The coupling between the atomic and spin degrees of freedom is established via a coordinate-dependent exchange interaction, which allows the dynamic exchange of energy between the lattice and spin subsystems; however such exchange-based coupling alone cannot facilitate the transfer of angular momentum between the two subsystems. This results in an unrealistic depiction of the spin-lattice relaxation process. To circumvent this drawback, we extend the conventional MD-SD approach by incorporating additional interaction terms that characterize spin-orbit coupling. These interactions are modeled in terms of the local magnetic anisotropies that arise as a consequence of the symmetry breaking due to lattice vibrations. Using MD-SD simulations, we investigate the effect of these terms on the spin-lattice relaxation in BCC iron. By coupling a conventional thermostat to the lattice subsystem, we show that this novel extension enables the exchange of angular momentum and leads to the mutual thermalization of both lattice and spin subsystems. [Preview Abstract] |
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