Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E21: Current-induced Spins, Spin-orbit Torques and Magnetoresistance in Topological InsulatorsFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Connie Li, Naval Research Lab Room: LACC 309 |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E21.00001: Optical detection of current-generated spin in topological insulators by magneto optical Kerr effect Connie Li, Olaf van 't Erve, Chenhui Yan, Lian Li, Berry Jonker The Dirac surface states of topological insulators (TIs) exhibit spin-momentum locking, where spin is locked at right angles to momentum, and hence an unpolarized charge current creates a net spin polarization. This polarization has been measured electrically using FM contacts [1]. Here we optically detect the current generated spin using magneto optic Kerr effect (MOKE). A thin layer of Al is deposited onto the TI films to enable the measurement of predominantly spins on the top surface, as Al is a good spin diffusion layer and optically opaque. Modulating the direction of the charge current through the TI and hence the direction of the spin, we observe corresponding response in the MOKE signal. This response is linear with current, exhibits a maximum at an intermediate Al thickness of ~20 nm, and exhibits a temperature dependence similar to that of electrical measurements. Furthermore, MOKE detection of magnetization switching of an adjacent ferromagnet from the current-generated spin in TI surface states via spin orbit torque will also be discussed at the meeting. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E21.00002: Spin Pumping in Transferred Topological Insulator on Ferrimagnetic Insulator and Metal/ Ferrimagnetic Insulator C. C. Tseng, Yu Chi Liu, C. C. Chen, Shang Rong Yang, Yu-Ting Fanchiang, C. K. Cheng, Shang-Fan Lee, Jauyn Grace Lin, Minghwei Hong, Jueinai Kwo Efficient spin-to-charge conversion is expected in topological insulators (TIs) due to the large spin-orbit coupling (SOC) and the spin-momentum locking in the surface state. However, obtaining high quality TI films directly grown on various substrates such as ferrimagnetic insulator of yttrium iron garnets (YIG) is challenging, especially for novel phenomena that demand sharp interfaces. In this work we performed thorough study on exfoliated Bi2Se3 films transferred on YIG and Ag/YIG films. We demonstrated that in-plane ordered triangular domains and excellent crystallinity attained in MBE growth of Bi2Se3 on sapphire were preserved after the transfer process, as confirmed by AFM, XRD, and ARPES. Ferromagnetic resonance on the transferred Bi2Se3/YIG did not display the substantially enhanced damping constant, as was found in Bi2Se3/YIG by direct film growth. Spin pumping measurement was employed to investigate the spin-charge conversion from inverse Rashba-Edelstein effect or inverse spin Hall effect in transferred Bi2Se3/YIG and transferred Bi2Se3/Ag/YIG with varying Ag thickness. Our transfer technique has circumvented the difficulties of growing high quality Bi2Se3 on various substrates, providing a new route to fabricate TI-based heterostructures. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E21.00003: Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature Chi Tang, Qi Song, Cui-Zu Chang, Yuichi Ohnuma, Mamoru Matsuo, Yawen Liu, Wei Yuan, Yunyan Yao, Jagadeesh Moodera, Sadamichi Maekawa, Wei Han, Jing Shi The unique spin-momentum locking of the surface Dirac electrons in TI has been shown to be responsible for highly efficient spin-charge-conversion in ferromagnet (FM)/TI heterostructures. Here, we report the strongly modulated spin dynamics in yttrium iron garnet (YIG) due to the spin-momentum locked Dirac surface states in TI via ferromagnetic resonance. When the Bi-concentration x is systematically tuned in the 5 nm thick (BixSb1-x)2Te3 TI layer, the relative weight of the Dirac surface states varies. The Gilbert damping constant of the processing magnetization in YIG increases by one order of magnitude when the chemical potential of TI is close to the Dirac point. Remarkably, the magnetic anisotropy of YIG is also dramatically altered to favor in-plane orientation. The modification of the spin dynamics in YIG by the Dirac surface states at room temperature is much stronger than that in YIG-heavy metal heterostructures, which provides a superb knob for controlling ultrafast magnetization responses in potential nanoscale spintronic devices. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E21.00004: Observation of the inverse spin Hall effect in the topological crystalline insulator SnTe using spin pumping Shinobu Ohya, Akiyori Yamamoto, Tomonari Yamaguchi, Ryo Ishikawa, Ryota Akiyama, Le Duc Anh, Shobhit Goel, Yuki Wakabayashi, Shinji Kuroda, Masaaki Tanaka Topological crystalline insulator SnTe is a promising material for future spintronics applications because of the strong spin-orbit coupling and surface states protected by the mirror symmetry of the crystal. Here, using a high-quality epitaxial (001)-oriented Fe/SnTe/CdTe/ZnTe heterostructure grown on GaAs [1,2], we have successfully observed the inverse spin Hall effect in SnTe induced by spin pumping, which was confirmed by detailed analyses of the dependence of the electromotive force on the microwave power, magnetic-field angle, and temperature. By a rough estimation, a relatively large spin Hall angle of ∼0.01 is obtained for bulk SnTe at room temperature. This large value may be partially caused by the surface states. Our result suggests that SnTe can be used for efficient spin-charge current conversion [3]. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E21.00005: Proximity Band Structure and Spin Textures on Both Sides of Topological-Insulator/Ferromagnetic-Metal Interface and Their Charge Transport Probes Juan Marmolejo-Tejada, Kapildeb Dolui, Predrag Lazic, Po-Hao Chang, Søren Smidstrup, Daniele Stradi, Kurt Stokbro, Branislav Nikolic The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin textures around their interfaces. Here we combine density functional theory with Green’s function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level EF0, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from Co or Cu layers, a few tenths of an electron-volt below EF0 for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. We predict that out-of-plane tunneling anisotropic magnetoresistance in Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at EF0 .[Nano Lett. 17, 5626 (2017)] |
Tuesday, March 6, 2018 9:00AM - 9:36AM |
E21.00006: Magnetic Switching with Topological Insulator and Compensated Ferrimagnet Invited Speaker: Luqiao Liu New materials and physics mechanisms are required to realize next generation spintronic devices with fast speed and low power consumption. In this talk, I will focus on two novel material systems which can provide superior performances for magnetic switching. First of all, it is known that with the application of surface charge current, one can generate spin accumulation on topological insulators, which can be further utilized to manipulate magnetic moments via the spin-orbit torques. Through experiments, we show that magnetic switchings can be realized in a topological insulator/magnet bilayer at room temperature. This represents a major advance compared with previous works where flipping of magnetic moments was only realized at liquid helium temperature. The large effective spin Hall angle provide a definitive proof on the high spin-orbit torque efficiency from topological insulators. The large reduction in switching energy makes topological insulators outstanding candidates to realize high-efficiency magnetic switching devices . Secondly, antiferromagnetically coupled material can exhibit fast dynamics as well as robust protection against external magnetic fields, which can enable spintronic devices with fast speed and high density. In this talk, I will discuss our recent study on rare earth based ferrimagnetic alloys which has antiferromagnetically coupled sublattices, net zero magnetic moment and electrically controllable magnetic state. Particularly, I will show that the spin orbit torque provides an efficient writing mechanism for these materials even at the compesnation point. Moreover, to illustrate the speed advantage, we further carry out current induced domain wall motion experiment, where the highest domain wall mobility is obtained in samples with compensated angular momentum. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E21.00007: Room temperature spin-orbit torque switching induced by a topological insulator Jiahao Han, Anthony Richardella, Saima Siddiqui, Joseph Finley, Nitin Samarth, Luqiao Liu The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. Here we report spin-orbit torque (SOT) switching in a TI/ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The low switching current density (~ 3 × 106 A/cm2) provides definitive proof of the high SOT efficiency from TI. The SOT efficiency is measured by the current-induced shift of the Hall resistance-vs-magnetic field hysteresis loops, which is consistent with the model of the current-induced Néel-type domain wall expansion. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Furthermore, we calculate the power consumption for switching a ferromagnetic layer with TI, Pt, and Ta, and find that the magnetization switching with TI presents much higher energy efficiency than the heavy metals. Our results demonstrate the robustness of TI as an SOT switching material and provide an avenue towards applicable TI-based spintronic devices. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E21.00008: Current-driven magnetization reversal in a Rashba bulk ferromagnet Ryutaro Yoshimi, Kenji Yasuda, Atsushi Tsukazaki, Kei Takahashi, Masashi Kawasaki, Yoshinori Tokura The Rashba effect is the spin band splitting due to broken inversion symmetry through spin-orbit coupling, typically observed at surface and interface. Recently, some noncentrosymmetryic crystals have been found to have the bulk Rashba bands that are larger than surface/interface ones. In Rashba systems the charge current generates the net spin accumulation, which is known as Rashba-Edelstein effect (REE), leading to the electrical manipulation of the magnetization. Although such a current-driven magnetization manipulation has so far been studied for surface or interface Rashba systems, REE in bulk Rashba bands may be efficient because the local magnetic moment is in contact with the accumulated spin moment of the conduction spin. Here, we observed current-driven magnetization reversal in Ge1-xMnxTe thin film, which is a bulk ferromagnetic Rashba semiconductor. The switching ratio of anomalous Hall resistivity does not decrease with the film thickness, indicating the spin accumulation stems not from the interface or surface but bulk Rashba spin splitting bands. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E21.00009: Spin Orbit Torque-Driven Magnetization Switching in Two-Dimensional Weyl Semimetal WTe2/Py SHUYUAN SHI, SHIHENG LIANG, YI Wang, QISHENG Wang, Hyunsoo Yang In recent years, two-dimensional (2D) materials have been proposed as potential spin current sources for spin orbit torque (SOT)-driven magnetization switching. In particular, 1T’-WTe2 is a new class of 2D Weyl semimetals, which possesses topologically-protected spin-momentum locking both in bulk and surface states as well as a good conductivity. Such properties make it a promising candidate as a spin current source. To date, however, SOT-driven magnetization switching has never been reported in either 2D materials or Weyl semimetals. Here, we extract the spin orbit torque efficiency (0.16) of WTe2 thin films using a WTe2 (10 nm)/Py (6 nm) heterostructure by the spin torque ferromagnetic resonance technique. Further, using MOKE imaging, we demonstrate SOT-induced magnetization switching in WTe2/Py at room temperature. This work provides a new route for efficient spin current generation and manipulation of the magnetization. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E21.00010: Surface State Magnetoresistance in Proximity Magnetized Topological Insulators Yingjie Zhang, Joseph Sklenar, Matthias Benjamin Jungfleisch, Axel Hoffmann, Peter Schiffer, Nadya Mason Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, which can be utilized for nano-electronic, spintronic, and quantum computing applications. While tremendous effort has been devoted to studying (quantum) anomalous Hall effects in magnetic proximity-coupled TIs, the inherent magnetoresistance (MR) properties of TI surface states remain largely unexplored. Here we present a strategy to fabricate TI-magnetic insulator (MI) heterostructure devices with high-quality interfaces, and experimentally measure the MR at various temperatures. We observe a plethora of hysteretic MR features including sharp, negative and positive MR switching, and broad, positive MR bumps, which we attribute to domain wall switching and out-of-plane anisotropic MR in the surface state, respectively. The out-of-plane magnetic anisotropy leads to gapping in the TI surface state, confirmed by an analytical diffusive transport model. These results provide insights into the fundamental mechanisms of magnetic proximity and spin exchange interactions in topological systems. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E21.00011: Proximity-Induced Tunneling Anisotropic Magnetoresistance: Massive vs. Massless States Chenghao Shen, Timothy Leeney, Alex Matos Abiague, Benedikt Scharf, Jong E Han, Igor Zutic We study the tunneling anisotropic magnetoresistance (TAMR) in the system of a magnetic barrier on top of two-dimensional electron gas (2DEG) with proximity-induced exchange splitting and Rashba spin-orbit coupling. Both numerical simulations and complementary model calculations show that TAMR is less than 2% in most of the cases. However, a relatively large TAMR (~10%) occurs at certain gate voltages and magnetizations. By analyzing Fermi contours in the barrier and lead, we give an explanation on the origin of this large TAMR. A comparison between TAMR in 2DEG and topological insulators (TI) is given, which can be used to experimentally distinguish signals from trivial and nontrivial states in TI. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E21.00012: Unidirectional magnetoresistance in a nonmagnetic topological insulator with hexagonal warping in the surface states. Pan He, Shulei Zhang, Dapeng Zhu, Yang Liu, YI Wang, JIAWEI YU, Giovanni Vignale, Hyunsoo Yang Unidirectional magnetoresistance (UMR) was recently observed in various bilayers consisting of a paramagnetic and a ferromagnetic material, arising from the interplay of spin accumulation and spin dependent scattering. In this work [1], we report the emergence of a novel UMR effect in a non-magnetic topological insulator (TI) layer, which scales linearly with the applied electric and magnetic fields. We show theoretically that this UMR originates from the conversion of a nonlinear spin current in the hexagonally warped surface states to a nonlinear charge current. Remarkably, with an out-of-plane magnetic field, the UMR also depends on the orientation of the current with respect to the crystallographic axes, exhibiting a three-fold rotational symmetry around the axis perpendicular to the [111] surface of the TI layer. This unique feature can be used to map the spin texture of the TI surface states based on the angular dependence of the UMR, alternative to ARPES. |
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