Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P47: Spin Transport and TopologyFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Eric Edwards, NIST Boulder Room: 394 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P47.00001: Chirality-induced anomalous Hall effect: Bridging a gap between weak and strong coupling Kazuki Nakazawa, Hiroshi Kohno A non-coplanar spin configuration, which has finite spin chirality, is known to cause anomalous Hall effect (AHE) [1]. Usually it is discussed in terms of a fictitious magnetic field due to Berry phase, which is appropriate when the s-d exchange coupling between conduction electrons and magnetization is strong (strong coupling regime). On the other hand, in the case of weak coupling, it cannot be described by the Berry phase since the adiabatic condition fails [2]. In this study, we examined the chirality-driven AHE in the weak coupling regime by two methods; 1) treating the exchange coupling perturbatively, 2) employing gauge field method [3]. It is known that the adiabatic component (diagonal part in spin space) of the SU(2) gauge field contributes dominantly in the strong coupling regime. We investigated whether or not the two results coincide, and whether one can describe the weak and strong coupling regimes in a single framework. We find that the adiabatic and non-adiabatic components are both important in the weak coupling regime. [1] J. Ye {\it et. al.}, Phys. Rev. Lett. \textbf{83}, 3737 (1999). [2] K. S. Denisov {\it et. al.}, Phys. Rev. Lett. \textbf{117}, 027202 (2016). [3] G. Tatara, H. Kohno and J. Shibata, Phys. Rep. \textbf{468}, 213-301 (2008). [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P47.00002: Anomalous Hall effect of the magnon by the interaction with phonons Ryuji Takahashi, Naoto Nagaosa We report a theoretical study of the Berry curvature in the magnon-phonon hybrid system. We first discuss the hybridization between the magnon and acoustic phonons via the dipolar interaction. Next, the effective model of the magnon-phonon hybrid system is studied in the presence of both of the spin-orbit and dipolar interactions, and we show anisotropic texture of the Berry curvature and its divergence with and without gap-closing. We give realistic evaluations of the consequent anomalous velocity for yttrium iron garnet. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P47.00003: Local injection of pure spin current generates electric current vortices Yaroslaw Bazaliy, Revaz Ramazashvili We show that local injection of pure spin current into an electrically disconnected ferromagnetic--normal-metal sandwich induces electric currents, that run along closed loops inside the device, and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.(Preprint at arXiv:1607.06385) [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:42PM |
P47.00004: Uncovering Berry: The Role of Topology in the Anomalous Hall Effect of Amorphous Ferromagnetic Fe-Si and Antiferromagnetic Mn$_{\mathrm{3}}$Ge Invited Speaker: Julie Karel The consideration of topology has recently led to the emergence of new exotic physics; topological insulators and Weyl semimetals are two classes of materials where unique properties arise due to the topology of the electronic band structure. Since its discovery, the Berry phase has helped explain diverse phenomena in condensed matter physics, with perhaps one of the most important breakthroughs being the reinterpretation of the intrinsic contribution to the anomalous Hall effect (AHE) in ferromagnets in terms of a Berry phase curvature in momentum space.$^{\mathrm{\thinspace }}$ Recent theoretical predictions have further suggested that the Berry phase can also give rise to an AHE in antiferromagnets with non-collinear spin structures. This talk will examine the role of the Berry phase on the AHE in both amorphous ferromagnets and non-collinear antiferromagnets. It will be shown that the anomalous Hall conductivity, when suitably normalized by magnetization and number of charge carriers, is independent of the longitudinal conductivity in a series of amorphous Fe-Si thin films. This observation suggests a primary dependence on an intrinsic mechanism, which is remarkable because it indicates a local atomic level description of a Berry phase, resulting in an intrinsic AHE in a system that lacks lattice periodicity. The second part of the talk will discuss the emergence of the AHE in the non-collinear antiferromagnet, Mn$_{\mathrm{3}}$Ge. The AHE in ferromagnets generally scales with the magnetization, meaning that an antiferromagnet with no net magnetization should not exhibit an AHE. It will be shown that not only does Mn$_{\mathrm{3}}$Ge exhibit an AHE but one that is comparable to that of ferromagnetic metals. Theoretical calculations will demonstrate that this effect originates from a non-vanishing Berry curvature, arising from the chiral spin structure. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P47.00005: Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers. Gregory M. Stiehl, David MacNeill, Marcos H. D. Guimaraes, Robert A. Buhrman, Jiwoong Park, Daniel C. Ralph In experiments performed to date, spin-orbit torques have an important limitation -- the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin-orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that one can change the allowed symmetries of spin-orbit torques in spin-source/ferromagnet bilayer devices by using a spin-source material with low crystalline symmetry, such as the transition-metal dichalcogenide WTe2. Consistent with the symmetries of the WTe2 surface, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis [1]. Controlling spin-orbit torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies. [1] D. MacNeill et al., Nat. Phys. (2016), doi:10.1038/nphys3933. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P47.00006: Spin Transport and Accumulation in 2D Weyl Fermion System Tze Tzen Ong, Naoto Nagaosa In this work, we study the spin Hall effect and Rashba-Edelstein effect of a 2D Weyl fermion system in the clean limit using the Kubo formalism. The spin Hall current arises from two mechanisms, skew scattering off the non-magnetic impurities and an intrinsic spin-torque contribution from time-evolution of the spin-dipole moment. The spin-torque current is is found to be dominant, and the spin Hall conductivity $\sigma^{x}_{xy}$ is given in terms of the transport scattering rate, and a skew scattering rate. The spin-conversion efficiency for the SHE and the Rashba-Edelstein effect are summarized in the spin Hall angle, $\theta^{SH} = \big( \frac{e}{c} \big)^{-1} \alpha^{SH}$, and the spin-accumulation angle, $\theta^{SA} = \big( \frac{e v_F}{c}\big )^{-1} \alpha^{SA}$, with $\alpha^{SH} \approx \frac{1}{2}$ and $\alpha^{SA} = 1$ being universal constants. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P47.00007: Strong intrinsic spin Hall effect in the TaAs family of Weyl semimetals Yan Sun, Yang Zhang, Claudia Felser, Binghai Yan Since their birth topological insulators have been expected to be ideal spintronic materials due to the spin currents carried by the surface states with spin-momentum locking . However, the bulk doping problem still remains to be an obstacle that hinders such application. In this work, we predict that a newly discovered family of topological materials, the Weyl semimetals, exhibits large intrinsic spin Hall effects that can be utilized to generate and detect spin currents. Our ab-initio calculations reveal large spin Hall conductivity that is comparable to that of 4d and 5d transition metals. The spin Hall effect originates intrinsically from the bulk band structure of Weyl semimetals that exhibits large Berry curvature and spin-orbit coupling, naturally avoiding the bulk carrier problem in the topological insulators. Our work not only paves a way to employ Weyl semimetals in spintronics, but also proposes a new guideline to search for spin Hall effect materials in various topological materials. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:54PM |
P47.00008: Charge-spin conversion at interfaces with spin splitting Invited Speaker: kouta kondou The two-dimensional electronic systems with band-splitting like the surface state of a topological insulator (TI) and Rashba interface provide unique opportunities for spintronics applications. The spin-momentum locking in the surface state offers a possibility of a highly efficient charge-spin current (C-S) interconversion compared with ordinary spin Hall effect in paramagnetic metals. The interfacial C-S conversion mechanism has been proposed by Edelstein in 1990 [V. M. Edelstein, Solid State Commun. 73, 233--235 (1990).]. For Ag/Bi Rashba interface, it was recently observed [J.C. Rojas Sánchez et al, Nat. Commun. 4, 2944 (2013).]. For the further development of interfacial spin current devices, it is important to quantitatively evaluate the conversion efficiency between charge and spin current. Firstly, we investigated the C-S conversion in surface state of topological insulators, which are appropriate materials for efficient interfacial C-S conversion. We prepared the TI ((Bi1-xSbx)2Te3: BST)/non-magnetic metal (Cu)/ferromagnetic-metal (NiFe) tri-layer films. By applying spectral analysis based on spin-torque ferromagnetic resonance (ST-FMR) to BST/Cu/NiFe tri-layer films, we succeeded in determining the C-S conversion efficiency of a surface state of TIs, whose Fermi level was varied by tuning the Sb composition. We found that in bulk insulating conditions the interface C-S conversion effect via Dirac surface state is evaluated as nearly constant large values [K. Kondou et al., Nat. phys, (2016) doi:10.1038/nphys3833]. Secondary, we measured S-C conversion effect in non-magnetic metal/oxide interface. We found the S-C conversion phenomenon in non-magnetic metal (Cu)/oxide (Bi2O3) interface, which may be caused by Rashba effect like Ag/Bi interface [K. Karube et al., Appl. Phys. Exp. 9, 033001 (2016).]. Interestingly, the amplitude of S-C conversion efficiency strongly depends on Cu layer thickness. These findings have important implications for development of future spintronic devise using the interface spin conversion effect. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P47.00009: Formalism of Spin and Energy Conductivity in Magnetic Insulators Yurika Kubo, Susumu Kurihara One of the main aspects for spintronics is manipulating spin currents. However, theoretical studies on spin and energy transports in magnetic insulators are rather restricted to systems where spins and fields are collinear [1]. To open the possibility for realization of spintronics, we need a theoretical basis. We define spin current operators [2] and energy current operators in magnetic insulators, which are applicable for systems with conserved magnetization. Then, we develop formalism of the spin and energy conductivity, using linear response theory [3]. Calculated integrated intensities of conductivity are shown to satisfy f-sum rules. This strongly indicates reliability of our formalism [2, 4]. [1]M. Sentef et al., Phys. Rev. B $\bf 75$, 214403 (2007), F. Heidrich-Meisner, et al., Phys. Rev. B $\bf 71$, 184415 (2005). [2] Y. Kubo and S. Kurihara, J. Phys. Soc. Jpn. $\bf 82$ , 113601 (2013). [3] G. D.Mahan: Many-Particle Physics (Plenum Press, New York, 2000) 3rd ed., p. 160. [4] Y. Kubo and S. Kurihara in preparation. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P47.00010: Nature of Dynamic Magnetoelectric Coupling and Prediction of Room-Temperature Electromagnon Excitation PanShuo Wang, Z. L. Li, J. H. Yang, C. L. Jia, H. J. Xiang The dynamical magnetoelectric excitation i.e. electromagnon is essential not only for the intrinsic physical mechanism of magnetoelectric coupling but also to realize application in emerging fields such as magnonics. Here, on the basis of group theory and the general polarization model, we develop an approach to understand the electromagnon excitation in magnetoelectric systems. Both multiferroicity and electromagnon in delafossite CuFeO2 are revealed to originate from the general spin-current model, rather than the spin-dependent d-p hybridization model. Our model is general since it is independent on the specific spin Hamiltonian. Based on this dynamic magnetoelectric coupling model, we predict that the magnon in antiferromagnetic Cr2O3 (TN $=$ 307 K) and ferrimagnetic yttrium iron garnet (Tc $=$ 550 K) could be excited by the electric component of light at room temperature. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P47.00011: Abstract Withdrawn |
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