Session F43: Topological Spintronics

Topological Insulator Surface States for Spintronics

Spintronics is the study of the mutual influence between charge transport in conductors and charge neutral spin degrees of freedom, especially the collective spin degrees of freedom of magnetically ordered systems. The effects of spintronics all require coupling between spin and orbital degrees of freedom. Systems containing thin-films of three dimensional topological insulators (TIs) are in many cases ideal hosts for spintronics studies becasue of the strong spin-orbit coupling present in the protected TI surface states, which can also serve as an isolated two-dimensional charge transport channel. I will discuss theories of the closely related anomalous Hall, spin Hall, spin-orbit torque, magneto-resistance, and spin-pumping effects in isolated TI thin films, and in TI thin films proximity coupled to conducting and insulating ferromagnets and antiferromagnets. My talk will focus on discussing possible applications of topological insulators in thin-film devices for electrically controlled magnetism, and emphasize the separate roles played by top and bottom TI surfaces.   

Magnetic Topological Insulator Heterostructures: From Axion Physics to the Topological Hall Effect

Topological insulator is a material in which the interior is insulating but wherein the electrons travel without resistance along surface/edge conducting channels. The nontrivial Dirac surface/edge states of a TI are induced by intrinsic strong spin-orbit coupling and protected by time-reversal symmetry (TRS). Breaking the TRS of a TI with magnetic doping can lead to many exotic topological quantum phenomena such as the quantum anomalous Hall (QAH) effect [1]. The QAH effect has been experimentally demonstrated in magnetic TI thin films, specifically Cr- and/or V- doped (Bi,Sb)₂Te₃ [2]. Recently, we fabricated magnetic TI based sandwich heterostructures wherein two TI films doped with the same or different magnetic ions are sandwiched by an undoped TI layer. The undoped TI layer serves as a spacer to decouple the exchange interaction between the two magnetic TI layers, thus the magnetization in each magnetic TI layer can be treated independently. In heterostructures with different magnetic TI layers, we realized a zero Hall conductance plateau in the antiparallel magnetization alignment region, which is a realization of the axion insulator state. This observation provides a promising platform for the exploration of axion electrodynamic physics [3]. In heterostructures with the same magnetic doping, we observed a crossover from the QAH to topological Hall (TH) effects. The combination of QAH and TH effects in a single sample facilitates the study of the interplay of momentum-space and real-space Berry curvatures in magneto-transport phenomena [4].
[1] R. Yu et al, Science 329, 61 (2010).
[2] C. Z. Chang et al., Science 340, 167 (2013); Nat. Mater. 14, 473 (2015).
[3] D. Xiao et al., PRL 120, 056801 (2018).
[4] J. Jiang et al, submitted (2018).   

Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature

In heterostruvtures of magnetic insulator/topological insulator or MI/TI, a large enhanced spin Seebeck effect was attributed to the spin-momentum locking of the TI surface. In this work, we demonstrate dramatically modified spin dynamics of MI by the spin-momentum locked Dirac surface states of the adjacent TI. As the Bi-concentration x is systematically tuned in 5 nm thick (Bi_xSb_1-x)₂Te₃ TI film, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10 nm thick Y₃Fe₅O₁₂ MI film in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi-concentration dependence. These extraordinary effects of the Dirac surface states along with the enhanced spin Seebeck effect distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer, which can be harnessed for spintronic applications.
  

Discovery of quantum confinement effect in sputtered topological insultor films and observation of room-temperature high spin–orbit torque

In this talk, we will report and discuss the experimental demonstration and first-principles calcuation of quantum confinement in topological insulator. First, we report Bi_xSe_(1-x) films with high SOT at RT grown onto thermally oxidized silicon substrates by magnetron sputtering, which is a semiconductor industry compatible process. The dc planar Hall and ST-FMR methods were used for the characterization of SOT in Bi_xSe_(1-x)/CoFeB heterostructures with in-plane CoFeB. At RT, the of the sputtered Bi_xSe_(1-x) film is up to two orders of magnitude larger than that of HMs. Notably, we developed perpendicular CoFeB multilayers on Bi_xSe_(1-x) films, and we demonstrated switching of the magnetization using SOT arising from the Bi_xSe_(1-x) with very low switching current density in bilayers at RT. Second, we will report our discovery of the quantum confinement effect in topological insulators. The sputter deposited Bi_xSe_(1-x) has granular structure with grain size as small as approximately 6 nm. Electronic band-structure analysis indicates that the reduced dimensionality and quantum confinement strongly influences the spintronic properties of the TI. Our theory identifies the presence of lowly dispersive surface bands with large charge-to-spin conversion efficiency in nanoscale grains, which might explain the experimentally observed enhancement in the figure-of-merit. The demonstrated , ease of growth of the films on a silicon substrate, and successful growth and switching of perpendicular CoFeB multilayers on Bi_xSe_(1-x) film provide an avenue for the use of Bi_xSe_(1-x) as a spin-density generator in SOT-based memory and logic devices.

Mahendra DC, et al, "Room-temperature high spin–orbit torque due to quantum confinement in sputtered Bi_xSe_(1–x) films" Nature Materials. 17, 800-807 (2018); DOI: 10.1038/s41563-018-0136-z   

Exploring Non-Trivial Nature of Topological Surface States in SmB6

Being the first rare earth mixed valence system and the first Kondo insulator, SmB₆ may very likely be the first topological Kondo insulator too. Recent studies have confirmed the existence of topological surface states (TSS) and surface chiral spin textures in single-crystal SmB₆ bulk materials, but the non-trivial nature and depth scale of the TSS remain unresolved. In this talk I will report our recent work on the use of spin pumping to study the non-trivial feature of the TSS in SmB₆ [1]. Our experiments used SmB₆ thin films grown on magnetic insulator Y₃Fe₅O₁₂ (YIG) bulk materials. Upon the excitation of magnetization precession in the YIG, a pure spin current is generated that flows from the YIG into the SmB₆ film and produces, via spin-orbit coupling, a lateral electrical voltage in the SmB₆ film. The spin-pumping voltage signal becomes considerably stronger as the temperature decreases from 150 K to 10 K. Such an enhancement most likely originates from the spin-momentum locking feature of the TSS of the SmB₆ film and may thereby serve as a strong evidence for the nontrivial nature of the TSS. The voltage data also show a unique film thickness dependence that suggests a TSS depth of about 32 nm for the TSS. The data also indicate a spin diffusion length of about 39 nm in the SmB₆ film. The spin-pumping results are supported by temperature-dependent transport measurements and theoretical analyses using a tight binding model.

[1] "Nontrivial nature and penetration depth of topological surface states in SmB6 thin films” Tao Liu, Yufan Li, Lei Gu, Junjia Ding, Houchen Chang, P. A. Praveen Janantha, Boris Kalinikos, Valentyn Novosad, Axel Hoffmann, Ruqian Wu, C. L. Chien, and Mingzhong Wu, Physical Review Letters 120, 207206 (2018).