Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S47: Spin Phenomena in Topological InsulatorsFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Cuneyt Sahin, Univ of Iowa Room: 710/712 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S47.00001: Topological Insulator/Magnetic Insulating Oxide: A Platform for Efficient Spin Current Transport Invited Speaker: Peng Li Topological insulators (TIs) hold great promise for spintronic devices due to their large charge-to-spin conversion efficiency. It has been demonstrated that a TI can induce a spin-orbit torque to switch the magnetization of a magnetic metal. However, it is unclear if this is due to the topological surface state (TSS) because the electrons from the magnetic metal can suppress TSS. Here we discuss experiments that identified bona fide surface state-induced spin-orbit torques in topological insulator/magnetic oxide bilayers. In Bi2Se3/BaFe12O19, a large spin-orbit torque from Bi2Se3 switched the magnetization of BaFe12O19. When the magnetization was switched by a magnetic field, a current in Bi2Se3 can reduce the switching field by about 4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature. When BaFe12O19 is replaced with Mg(Al,Fe)2O4, efficient spin pumping in the bulk-dominated regime was found at room temperature. These results highlight the promise of topological insulator/ferromagnetic insulating oxide bilayers as a platform for studying topological surface states in the context of spin-to-charge interconversion. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S47.00002: Nonlinear Hall effect in topological insulator-ferromagnet heterostructures Yang Wang, Yong Wang, Tao Wang, Yu-Sheng Ou, Yi Ji, Matthew F Doty, Branislav Nikolic, Stephanie Ann Law, John Q Xiao Topological insulators (TIs) are a new class of materials which have insulating bulk bands and gapless helical surface states. The unique spin-momentum locking property leads to large charge-to-spin conversion efficiency which can be used to manipulate the magnetization in an adjacent ferromagnetic layer via spin-orbit torques (SOTs). However, more rich phenomena can happen in topological insulator/ferromagnet bilayers. Here, we report current-nonlinear Hall effect in Bi2Se3/CoFeB heterostructures measured by second harmonic Hall voltage method, which is commonly used to extract SOTs in TI/FM bilayers. By field dependence analysis and magneto-optical Kerr effect spin-orbit torque measurement, the origin of the second harmonic Hall voltage is mainly attributed to asymmetric magnon scattering mechanism. We further show that the second harmonic Hall resistance might be linked to quantum interference effect in TIs. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S47.00003: Nonlinear planar Hall effect Pan He, Shulei Zhang, Dapeng Zhu, Shuyuan Shi, Olle Heinonen, Giovanni Vignale, Hyunsoo Yang An intriguing property of a three-dimensional topological insulator (TI) is the existence of surface states with spin-momentum locking. We report the discovery of a new type of Hall effect in a TI Bi2Se3 film [1]. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall and anisotropic magnetoresistance. This novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry-breaking, which also exists in other non-centrosymmetric materials [e.g., WTe2 and the 2DEG on the SrTiO3(001) surface] with a large span of magnitude. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S47.00004: Nonlinear Hall signatures of ferroelectric switching in atomically thin WTe2 Benjamin T. Zhou, Cheng-Ping Zhang, Xuejian Gao, Kaifei Kang, Kin Fai Mak, Kam Tuen Law Recent experiments have observed nonlinear Hall effects(NLHEs) in atomically thin WTe2, an ultrathin ferroelectric material which may enable miniaturizing ferroelectric memories. However, signatures of ferroelectricity in NLHEs have remained elusive. In this work, we demonstrate that the direction and magnitude of nonlinear Hall currents can be highly sensitive to the ferroelectric polarization in WTe2. In particular, there exists a wide experimental regime in which different polarization states are associated with opposite Hall current directions. We point out that such strong NLHE signatures of ferroelectricity originate from polarity-dependent Berry curvature dipoles, which arises from the broken mirror symmetry under strong out-of-plane displacement fields near polarity-switching points. Our work establishes the polarity-dependent NLHE as a novel scheme for fast and nondestructive reading in WTe2-based ferroelectric memory devices. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S47.00005: Magnetization control of ferromagnetic materials based on topological insulator Sb2Te3 sputtered thin films Sofia Ferreira Teixeira, Alexander Vanstone, Ana L. Pires, Will R Branford, João P. Araújo, Lesley Cohen, André M. Pereira Topological Insulators (TI) are a recent category of materials. 3D TIs host a protected metallic state on their surface while having an insulating bulk. The state appears in the form of a spin-textured Dirac cone. Due to these properties, several applications are foreseen. However, a better fundamental understanding of the relationship between the TI surface and bulk states is required. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S47.00006: Persistence of spin memory in the insulating state of the crystalline phase-change material SnSb2Te4 revealed by magnetotransport measurements Nicholas Breznay, Johannes Reindl, Hanno Volker, Matthias Wuttig Understanding the driving mechanisms for quantum materials, whether strongly correlated, strongly disordered, or characterized by nontrivial band topology, often derives from precise studies of electronic lifetimes. Sufficiently long electronic spin and phase-coherence lifetimes result in weak localization or (in the presence of strong spin-orbit coupling) antilocalization, effects that can be used to spectroscopically probe complex materials. Here we identify a distinct, complementary phenomenon that can also be used to probe electronic lifetimes in strongly disordered materials [1]. We find an unexpected spin sensitive hopping conductivity in the phase change material SnSb2Te4 that can be tuned with disorder. An isotropic magnetoconductance arises from disruption of spin correlations that inhibit hopping transport, the recently described ‘spin memory’ effect [2], whose occurrence signals that the spin plays a previously overlooked role in the disorder-driven transition between weak and strong localization in spin-orbital materials. [1] J. Reindl, H. Volker, N. Breznay, M. Wuttig, npj Qauntum Materials (2019). [2] O. Agam, I. Aleinder, B. Spivak PRB 89, 100201(R) (2014). |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S47.00007: Observation of magnetoresistance effects in a heterostructure of Bi2Se3 and a ferromanget with perpendicular magnetic anisotropy Junseok Oh, Vincent Humbert, Joseph Sklenar, Bora Basa, Matthew Gilbert, Nadya Mason Topological insulators (TIs) have insulating bulk states and topologically protected gapless boundary states that exhibit spin momentum locking and prohibited backscattering. Such unusual properties of topological surface states (SSs) in three dimensional TIs make the material much favorable for spintronic applications. However, the properties of these SSs can be largely modified in the presence of ferromagnetism in out-of-plane direction which breaks the time reversal symmetry. We report magnetoresistance (MR) signals in a heterostructure of 3D TI Bi2Se3 and Co/Pt multilayer with perpendicular magnetic anisotropy. In particular, the abrupt switching between the high and low MR states was observed for temperature lower than 1K and for field along in-plane and out-of-plane directions. This MR is incompatible with the magnetization behavior of the ferromagnetic layer and thus attributed to the magnetic ordering in the TI. Our observations indicate the possible existence of proximity magnetized SSs in Bi2Se3 with out-of-plane spin component. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S47.00008: Bi-linear magnetoresistance in surface states of topological insulator: the role of Rashba-Edelstein effect and relaxation processes Anna Dyrdal, Jozef Barnas, Albert Fert Magnetoresistance effects scaling linearly with both external electric and magnetic fields are currently of great interest. These phenomena may be observed even in a uniform layer of material with strong spin-orbit coupling (SOC). A recent theoretical description of this bi-linear magnetoresistance (BMR) assumes a hexagonal warping of the band structure, which enables electron backscattering [1]. However, the experimental data on the topological insulator α-Sn(001) (a material without hexagonal symmetry) indicate that BMR can also exist in materials without hexagonal warping. Thus, one may expect another mechanisms contributing to the BMR in systems with isotropic energy spectra. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S47.00009: Nonreciprocal second harmonic generation in CuB2O4 Shingo Toyoda, Manfred Fiebig, Taka-hisa Arima, Yoshinori Tokura, Naoki Ogawa In a matter with broken time-reversal and space-inversion symmetries, optical responses can differ for the photons propagating in the opposite directions. The so-called nonreciprocal optical effect has been observed ranging from GHz to X-ray. Especially, CuB2O4 is known to show huge nonreciprocal effects for the linear optical response, such as absorption, luminescence and refraction. In this study, we have investigated nonreciprocity of second harmonic generation. It is demonstrated that such gigantic nonreciprocal effects can show up for not only linear optical responses but also for nonlinear optical responses. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S47.00010: Magnetic torque of holmium antimonide topological semimetal Narayan Poudel, M. Mofazzel Hosen, Dariusz Kaczorowski, Madhab Neupane, Krzysztof Gofryk The family of compounds AX, where A = Lanthanides or Actinides and X = pnictides, have drawn a great scientific interest recently due to their unusual magneto-transport properties and potential non-trivial nature of the topological ground state. HoSb in one of the candidates of this family that shows antiferromagnetic transition at 5.7 K. Our recent magneto-transport and ARPES studies indicate the presence of the Rashba-type surface-states and non-trivial semimetallic characteristics in this material. In non-trivial magnetic semimetals it is important to understand the interplay between topology and magnetism. In HoSb, however, this behavior is still not well-understood. In order to account for this issue, we have initiated detailed studies of magnetic torque of HoSb along the three crystallographic directions, and in the antiferromagnetic and paramagnetic phases. We show that in this cubic system the torque exhibits large magnetic anisotropy in antiferromagnetic phase and an angle dependence indicates the presence of complex magnetic structure below the Neel temperature. We will discuss implications of these studies in the context of the presence of non-trivial topological state in this material. |
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S47.00011: Large Spin-to-Charge Conversion Induced by Hybrid Rashba- and Surface-State in Topological Insulator Heterostructures Rui Sun, Shijia Yang, Yang Xu, Eric Vetter, Dali Sun, li na, Lei Su, Yan Li, Yang Li, Zizhao Gong, Zongkai Xie, Qeemat Gul, Wei He, Xiangqun Zhang, Zhaohua Cheng Efficient spin-to-charge conversion at room temperature is of great value in spintronics applications. Recently, Topological insulators (TIs) have emerged as some of the most efficient spin-to-charge convertors because of their correlated spin-momentum locking at helical Dirac surface states. While endeavors have been made to pursue large “charge-to-spin” conversions in novel TI materials using spin-torque-transfer geometries, the reciprocal process “spinto-charge” conversion, characterized by the inverse Edelstein effect length (λIEE) in the prototypical TI material (Bi2Se3), remains moderate. Here, we demonstrate that, by incorporating a “second” spin-splitting band, namely, a Rashba interface formed by inserting a bismuth interlayer between the ferromagnet and the Bi2Se3 (i.e., ferromagnet/Bi/Bi2Se3 heterostructure), λIEE shows a pronounced increase (up to 280 pm) compared with that in pure TIs. We found that λIEE alters as a function of bismuth interlayer thickness, suggesting a new degree of freedom to manipulate λIEE by engineering the interplay of Rashba and Dirac surface states. Our finding launches a new route for designing TI- and Rashba-type quantum materials for next-generation spintronic applications. |
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