Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B39: Spin transport and Hall effectFocus
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Sponsoring Units: GMAG DMP Chair: Xiaoqin (Elaine) Li, Univ of Texas, Austin Room: BCEC 207 |
Monday, March 4, 2019 11:15AM - 11:51AM |
B39.00001: Spin-to-Charge Conversion in Bi Films and Bi/Ag Bilayers Invited Speaker: Di Yue Recently, there have been reports on the superior prowess of bismuth (Bi) in spin-to-charge conversion, including large spin Hall angle and long spin diffusion length, qualities that are highly favorable for exploring pure spin current phenomena and devices [1]. Bi/Ag bilayers have also been featured prominently in the inverse Rashba-Edelstein effect (IREE), a related effect offering strong spin-to-charge conversion at interfaces [2,3]. One notes that in most cases the spin injection has been accomplished by spin pumping from Py, a process well known to harbor a variety of parasitic effects due to metallic Py that complicate the voltage measurements of the inverse spin Hall effect (ISHE). |
Monday, March 4, 2019 11:51AM - 12:03PM |
B39.00002: Spin Hall Effect in NixCu1-x Alloys Mark Keller, Katy Gerace, Monika Arora, Justin Shaw, Thomas Silva We report a large spin Hall conductivity, σSH, in high resistivity, paramagnetic Ni60Cu40. The value is comparable to what we reported for Pt [1]. Broadband FMR measurements of ferromagnet/nonmagnet bilayers provide an accurate measure of σSH, and simultaneously the damping due to spin pumping, when the various contributions to the inductively detected signal are analyzed as described in [2]. These measurements do not require patterning of the bilayer, nor perpendicular anisotropy of the ferromagnet. In a series of samples with the layer structure substrate/Ta(3)/Py(3.5)/NixCu1-x(d)/Ta(3) (thickness in nm), we varied Ni fraction x for d = 10 nm and varied thickness d for x = 0.6, a composition with a magnetic ordering critical temperature TC = 140 K. Room temperature FMR measurements with a saturating out-of-plane field showed a substantial σSH for all compositions we deposited, 0.3 ≤ x ≤ 0.75, with a maximum at x = 0.7 where TC = 270 K. For the thickness series at x = 0.6, we use the approach detailed in [1] to extract a spin diffusion length of (7.8 ± 0.5) nm and a spin Hall ratio of 0.81 ± 0.14. Both values are about twice as large as we reported for Pt [1]. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B39.00003: Anomalous and spin Hall effect in tunnel junctions with spin orbit interaction Christian Ortiz Pauyac Considering a tight binding model in Keldysh formalism we describe the anomalous and spin Hall effect in tunnel junctions in the presence of Rashba spin orbit coupling. Two types of structures are considered, conventional magnetic tunnel junctions (normal metal - insulator - ferromagnet) and spin filter tunnel junctions (normal metal - magnetic insulator - ferromagnet). We show that the magnitudes of the anomalous and spin Hall effects are enhanced in the presence of magnetic insulators. In both types of structures the angular dependence exhibit a cosine and sine behavior. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B39.00004: Damping-like spin-orbit torque in Fe3GeTe2/Pt heterostructures Mohammed Alghamdi, Mark Lohmann, Junxue Li, Tang Su, Kenji Watanabe, Takashi Taniguchi, Palani Raja Jothi, Boniface Fowka, Jing Shi Van der Waals (vdW) layered ferromagnets have become a hot topic of research lately within the two-dimensional materials community due to the ability to study magnetic properties from a single atomic layer. Fe3GeTe2 (FGT) is one of very few known metallic members of the vdW ferromagnetic materials family. It has strong perpendicular magnetic anisotropy (PMA) and Curie temperature, TC ~ 220 K, which makes FGT a good candidate material to study the effect of spin-orbit torque (SOT) on the magnetization of 2D ferromagnet in FGT/heavy metal heterostructures. For this study we fabricated heterostructures of FGT/Pt with 5 nm of Pt sputtered onto the surface of ~ 15 nm – 50 nm flakes of FGT. We measured the first and second harmonic Hall responses to an applied magnetic field in the in-plane parallel and perpendicular to current directions. We also measured the angular dependence of the 2 ω signal with fixed in-plane fields larger than the field required to saturate the magnetization in the hard axis direction. Our analysis of the 2 ω signals show that damping-like SOT is at least an order of magnitude greater than the negligible field-like SOT and thus it is the dominant mechanism acting on the magnetization of FGT. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B39.00005: Observation of the nonlinear anomalous Hall effect in 2D WTe2 Kaifei Kang, Tingxin Li, Egon Sohn, Jie Shan, Kin Fai Mak The Hall effect has been observed in systems with broken time reversal symmetry, such as the ordinary Hall effect in solids under a magnetic field and the anomalous Hall effect (AHE) in magnetic materials. Theories have indicated that in nonmagnetic materials with certain low crystal symmetries, a nonlinear Hall effect can be observed under zero magnetic field, in which the Hall voltage scales quadratically to the longitudinal current. Here we report an observation of such effect in atomically thin Td WTe2 flakes, a semimetal with only one mirror plane along the crystal b axis. In our angle-resolved electrical measurements, we observed a sinusoidal angular dependence of the nonlinear Hall effect, in which the effect maximizes when the driven current is perpendicular to the mirror plane and minimizes when it aligns to the mirror plane. This effect can be described as an anomalous Hall effect with a current induced magnetization. We further studied the temperature and doping dependence of the nonlinear hall conductivity. The result suggests that both intrinsic Berry curvature dipole and extrinsic spin-dependent scatterings contribute to the observed nonlinear AHE. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B39.00006: Anomalous Hall effect in Pt/Cr2Ge2Te6 heterostructures Mark Lohmann, Tang Su, Mohammed Alghamdi, Ben Niu, Yusheng Hou, Mohammed Aldosary, Wenyu Xing, Jiangnan Zhong, Shuang Jia, Wei Han, Ruqian Wu, Yongtao Cui, Jing Shi Understanding the physical properties of 2D ferromagnets such as Cr2Ge2Te6 (CGT), amongst a plethora of others, has accelerated due to their properties in one to few layers as well as their ability to easily form heterostructures with other 2D materials. This feature is extremely attractive due to the possibility of manipulating interfacial interactions via proximity coupling. Here we demonstrate the anomalous Hall effect (AHE) in Pt thin films sputtered onto the surface of thin exfoliated flakes of CGT which must originate in Pt since CGT is essentially an insulator at low temperatures. The AHE loops persist up to TC ~ 60 K which matches well to the bulk value. Low-temperature magnetic force microscopy of Pt/CGT reveals magnetic field dependent domain structures consistent with the AHE behavior. The origin of the AHE in Pt/CGT heterostructures is explored through density functional theory calculations. The method used in this study is a powerful tool to understand magnetism in 2D ferromagnetic insulators and provides a new platform for the study of magnetic proximity exchange and spin-dependent transport phenomena. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B39.00007: Semiclassical theory of the spin transport of Bloch electrons Liang Dong, Qian Niu
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Monday, March 4, 2019 1:03PM - 1:15PM |
B39.00008: Two-band magnetoresistance in spin-filter CrVTiAl Gregory Stephen, Christopher Lane, Ioana G Buda, David E Graf, Stanislaw Kaprzyk, Bernardo Barbiellini, Arun Bansil, Donald E Heiman Thin films of spin-filter material CrVTiAl were grown via sputtering, and their magnetotransport properties have been studied up to B=35 T. The resistivity vs T shows 2-channel semiconducting behavior with a disordered gapless channel and an activated gapped channel. Magnetoresistance measurements provide values for the mobilities of the two conducting channels, 900 and 50 cm2/Vs. This leads to a ratio of 18:1 for the effective masses, which corresponds to that predicted by density functional theory. Hall and magnetization measurements reveal a small moment saturating near 15 T that is assigned to partial non-compensation of the sublattices. These results combined with previous studies [1,2] show CrVTiAl to be a compensated ferrimagnet with a spin-polarized electronic structure. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B39.00009: Intrinsic spin currents in ferromagnets Vivek Amin, Junwen Li, Mark Stiles, Paul Haney We present first principles calculations showing that an applied electric field generates intrinsic spin currents in ferromagnets that flow perpendicularly to the electric field. Reduced symmetry in ferromagnets with spin-orbit coupling enable a wide variety of such spin currents. However, we find the total spin current is approximately given by the sum of a magnetization-independent spin current with the symmetry of the spin Hall effect and a spin-polarized anomalous Hall current. We also present a simple tight-binding model that captures the relevant physics. Intrinsic spin currents are not subject to dephasing, enabling their spin polarizations to be misaligned with the magnetization, which is crucial to explain the magnetization-independent spin Hall effect. The spin Hall conductivity and spin anomalous Hall conductivities of transition metal ferromagnets can be comparable to Pt, opening new avenues for efficient spin current generation in spintronic devices. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B39.00010: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 1:39PM - 1:51PM |
B39.00011: Symmetric and anti-symmetric contributions in mixed spin-current effects in a NiO/Pt nanostructure F. L. A. Machado, P. R. T. Ribeiro, M. Gamino, A. Azevedo, Sergio M Rezende Spin-currents generated by thermal gradients are efficiently converted into charge-currents by the inverse spin-Hall effect in films of metals presenting strong spin-orbit coupling. The nature of the thermal induced effects depends on the relative orientation among the directions of the spin-current, the applied magnetic field (H), the thermal gradient and the electrical contacts in the metallic film. Mixings in the currents generated by different effects are expected to occur. In this work, the H-dependent anti-symmetric spin-Seebeck effect (SSE) was generated altogether with the symmetric planar Nernst effect in a NiO(100 nm)/Pt(6 nm) nanostructure grown on a 0.5 mm thick Si substrate. A sample holder adapted to a PPMS was used for measuring the voltage in the Pt-film for H in the range ±85 kOe and for temperatures (T) varying from 100 to 300 K. A simple procedure developed for separating the SSE from the planar Nernst effect yielded magnitudes for the SSE in the range ±30 pAcm/K for a temperature different of 10 K across the sample at 300 K. The magnitude of the SSE signal was found to vary with H and T in good agreement with a drift-diffusion magnonic theory. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B39.00012: Effects of oxidation on the spin-orbit torques and domain textures in ultrathin Pt/Co/AlOx heterostructures Junxiao Feng, Marvin Müller, Eva Grimaldi, Pietro Gambardella We investigated the correlation between structure, oxidation, and magnetic properties of Pt/Co/AlOx, Pt/CoOx/Co/AlOx and PtOx/Co/AlOx heterostructures. We find that the anomalous Hall resistance, magnetoresistance, magnetic anisotropy, and domain texture are affected in a systematic way by the degree of oxidation of the Co/AlOx, CoO/Co and PtOx/Co interfaces. Moreover, we find that oxidation also influences the spin-orbit torques as well as the current-induced domain-wall motion and switching. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B39.00013: Modeling electrical detection of current generated spin in topological insulator surface states Connie Li, Olaf M Van T Erve, Chenhui Yan, Lian Li, Berend Jonker Current generated spin in topological insulator (TI) surface states due to spin-momentum locking has been detected recently using FM/tunnel barrier contacts, where the projection of the TI spin onto the magnetization of the FM is measured as a voltage. However, opposing signs of the spin voltage have been reported, attributed to the coexistence of trivial 2DEG states on the TI surface that may exhibit opposite current-induced polarization. Models based on electrochemical potential have been presented to determine the sign of the spin voltage expected for the TI surface states, however neglecting critical experimental parameters. Here we present a Mott two-spin current resistor model which takes into account parameters such as spin-dependent interface resistances, and show that such inclusion can lead to a crossing of the voltage profiles for the spin-up and spin-down electrons within the channel, which can lead to measured spin voltages of either sign. These findings offer a resolution of the ongoing controversy regarding opposite signs of spin signal reported in the literature, and highlight the importance of including realistic experimental parameters in the model. |
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