Session J36: Spin-Orbitronics in Semiconductor, Topological, and 2D Materials I

Live

Over the past few years, nonreciprocal transport measurement has emerged as a probe of spin orbit interaction, spin texture, superconductivity and other fundamental properties of materials [1]. In this presentation, we report on the nonlinear Hall effect (NLHE) in Bi₂Se₃/CoFeB heterostructures measured by second harmonic voltage method. Magneto-optical Kerr effect and transport measurements point the origin of the NLHE to be the asymmetric magnon scattering mechanism [2]. Moreover, a linear relation between the nonlinear Hall resistance and the electron phase coherence length of Bi₂Se₃ is observed between 5 and 50 K. We propose a phenomenological model and suggest the enhancement of the NLHE below 50 K as evidence for strict one-dimensional antilocalization of electrons from spin disorder [3].

[1] Y. Tokura and N. Nagaosa, Nat. Commun. 9, 3740 (2018).
[2] K. Yasuda, et al., Phys. Rev. Lett. 119, 137204 (2017).
[3] Y. Wang, et al., Phys. Rev. B 102, 125430 (2020).   

Live

Spin-orbit torques with an out-of-plane spin-polarization component are forbidden for spin-current-generating materials with high crystal symmetry, but are permitted for materials with sufficiently low symmetry such as WTe₂ and MoTe₂. However, it remains unclear if symmetry-breaking in the bulk of the spin-current generator is crucial for such torques to exist or if symmetry breaking at the spin-source/magnet interface is sufficient. In addition, it is unclear whether the efficiencies of these torques depend on the spin-orbit coupling strength in the same way as expected for conventional Rashba-type spin-orbit torques. We report measurements of spin-orbit torques generated by ZrTe₃ and other low symmetry materials to understand the existence and strength of the out-of-plane anti-damping spin-orbit torques.   

Live

We theoretically show that the reduced symmetry of strong spin-orbit coupling materials such as MoTe₂ or WTe₂ enables a new form of oblique spin Hall effect (SHE), characterized by large and robust in-plane spin polarizations. Through quantum transport calculations on realistic device geometries, including disorder, we found long spin diffusion lengths (λ_s) and a gate tunable charge-to-spin interconversion efficiency with an upper value reaching θ_xy ∼ 80%. The SHE figure of merit λ_sθ_xy ∼ 1-50 nm, can significantly exceed values of conventional SHE materials, and stems from momentum invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour. Specific guidelines for unambiguous experimental confirmation are proposed, paving the way towards exploiting such phenomena in spintronic devices.   

Live

Current-induced spin-orbit torque (SOT) has been proposed as a promising candidate to control the magnetization of ferromagnetic materials. To realize the multifunctional spin logic and memory devices utilizing SOT, achieving the SOT manipulation is important. Nowadays, SOT switching can be manipulated by controlling the interfacial oxidization, but its efficiency is limited because it depends on the interface quality.

Here, we report a successful control of a full SOT switching via an external electric field in a single-crystalline ferromagnetic semiconductor (Ga,Mn)As with strong spin-orbit coupling^1,2. By applying the gate voltage, the switching current density can be solidly manipulated reversibly with a large manipulation ratio of 14.5%, which is ascribed to the successful modulation of the interfacial electric field. Our finding will advance the development of energy-efficient gate-controlled spin-orbit-torque devices.

1. M. Jiang et al., Nat. Commun. 10, 2590 (2019).
2. M. Jiang et al., to be published in Nat. Electron. (2020).   

Live

Spin-orbitronics, which takes advantage of spin-orbit coupling (SOC), has expanded the research objects of spintronics to nonmagnetic materials. Here, we explore the emerging nonlinear spintronic phenomena in the inversion-asymmetric nonmagnetic materials with SOC. The surface state of three-dimensional topological insulator (TI) owns helical spin textures with the spin and momentum perpendicularly locked. We reported the observation of a nonlinear magnetoresistance (called bilinear magneto-electric resistance, BMER) [1] and nonlinear Hall effect [2] in a prototypical TI Bi₂Se₃, which scale linearly with both the applied electric and magnetic fields. A close link between the BMER and the spin texture was established in TI surface states, which enables a novel transport probe of spin textures. We further extended the observation of BMER effect to the d-orbital two-dimensional electron gas (2DEG) at a SrTiO₃ (STO) (111) surface [3]. The BMER probes a three-fold out-of-plane spin texture, in addition to an in-plane one at the STO(111) surface 2DEG. This novel spin texture is in contrast to the conventional one induced by the Rashba effect. We recently reported the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe₂ [4]. Theoretical calculations revealed the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. This novel nonlinear effect originates from the conversion of a nonlinear spin current to a charge current. These findings open a new branch in spintronics, ' nonlinear spintronics', which discusses the nonlinear transport effects in spin-polarized nonmagnetic materials.
References:
[1] Pan He, et al. Nature Physics 14, 495 (2018)
[2] Pan He, et al. Phys. Rev. Lett. 123, 016801 (2019)
[3] Pan He, et al. Phys. Rev. Lett. 120, 266802 (2018)
[4] Pan He, et al. Nature commun., 10, 1290 (2019)   

Live

The nonlinear Hall effect is an unconventional response, in which a voltage can be driven by two perpendicular currents in the Hall-bar measurement. Unprecedented in the family of the Hall effects, it can survive time-reversal symmetry but is sensitive to the breaking of discrete and crystal symmetries. It is quantum by nature because of its deep connection with the Berry curvature.
However, a full quantum description is still absent. Here we construct a quantum theory of the nonlinear Hall effect by using the diagrammatic technique. Quite different from nonlinear optics, 92 of the total 100 diagrams account for the disorder scattering, which plays a decisive role in the electronic transport. We present both qualitative and quantitative differences between the quantum theory and the semiclassical theories. As an application, we propose a pure electric experiment to detect the Berry curvature distribution near the band edge. We discuss the symmetry of the nonlinear-response diagrams and predict a pure disorder-induced nonlinear Hall effect for point groups C₃, C_3h, C_3v, D_3h, D₃ in 2D, and T, T_d, C_3h, D_3h in 3D. This work will be helpful for explorations of the topological physics beyond the linear regime.

arXiv:2004.09742   

Live

Heterostructures based on 2D van der Waals magnetic materials can provide clean interfaces with reliable transparencies for spin currents. This opens new avenues for improved spintronics devices with highly efficient spin-to-charge and charge-to-spin transduction. In this work, we probe the charge-to-spin conversion in heterostructures of 2D magnetic insulators with heavy metals. We quantitatively show that in the case of Cr₂Ge₂Te₆/Pt, the interface transparency for spin currents is similar to conventional 3D metallic-ferromagnet/Pt heterostructures but is much higher than for oxide-ferrimagnet/Pt interfaces. We also report measurements of spin-to-charge conversion in vdW heterostructures of 2D magnets with transition metal dichalcogenides, thereby revealing the microscopic nature of spin-orbit coupling in these systems.   

Live

Spin-orbit torque (SOT) is the leading technology to achieve low-energy magnetization switching for next-generation nonvolatile memory and logic. In SOT, a charge current applied through a channel with strong spin-orbit coupling produces a transverse spin current that applies a torque to switch an adjacent ferromagnetic layer. In addition to transport measurements, optical measurements can provide unique insights into the SOT switching dynamics because light can directly couple to the magnetic state through magneto-optic Kerr effect (MOKE). In this talk, I will discuss recent progress in using magneto-optic Kerr effect (MOKE) microscopy technique to probe SOT manipulation of both conventional metallic ferromagnets and van der Waals magnets. I also will illustrate the pros and cons of optical readout for quantifying SOT efficiency, in comparison with standard transport measurements such as the second-harmonic Hall technique and spin-torque ferromagnetic resonance (ST-FMR).   

Live

We report spin-to-charge and charge-to-spin conversion at room temperature in heterostructures of the archetypal Dirac semimetal Cd₃As₂ and Ni₈₀Fe₂₀, detected by both spin torque ferromagnetic resonance and ferromagnetic driven spin pumping. Analysis of the symmetric and antisymmetric components to the mixing voltage in spin torque ferromagnetic resonance and the frequency and power dependence of the spin pumping signal show that the behavior of these processes is consistent with previously reported spin-to-charge interconversion mechanisms in heavy metals, topological insulators and Weyl semimetals. We find that the efficiency of these phenomena is comparable to that due to the spin Hall effect in heavy metals. Finally, we compare our results with first principles calculations.   

Live

It is recognized that in a class of antiferromagnets with parity-time symmetry the Néel vector can be switched by staggered (Néel) spin-orbit torque (NSOT) generated by the electric currents, and furthermore an additional nonsymmorphic crystalline symmetry enables the control of the Dirac quasiparticles via the direction of the Néel vector. However, the real-time dynamics of the magnetic and the microscopic electronic structures induced by the NSOT is still largely unknown from theoretical and experimental viewpoints. We propose a theoretical framework for the ultrafast dynamics in AFM Dirac semimetals, taking not only localized magnetic moments but also itinerant electrons into account. We show that the Néel vector is rotated in picosecond timescales by the current- and the terahertz-pulse-induced NSOT as well as a magnetic anisotropy, which accompanies the modulation of the mass of the Dirac quasiparticles. It is also found that this reorientation can be experimentally observed through the time-resolved quadratic magneto-optical Kerr effect.   

On Demand

Topological insulators (TIs) are the most popular spin-orbit torque (SOT) materials in spintronics society. TIs possess extremely high damping-like (DL) SOT efficiency due to the spin momentum locking from the topological surface state (TSS). However, most works prepared TIs are utilizing the molecular beam epitaxy, which is hard to employ in the industrial fabrication process. Therefore, integrating the industry-favored tool to prepare large SOT materials becomes a crucial issue.
In this work, we use conventional magnetron sputtering to deposit the non-epitaxial BiTe/ferromagnet heterostructures. The harmonic Hall voltage measurement and the hysteresis loop shift measurement are performed to characterize the DL-SOT efficiency. Even without the TSS, the DL-SOT efficiency of the non-epitaxial BiTe can reach values greater than 100% at room temperature. From the thickness dependence analysis, we conclude that the bulk SHE has a great contribution to the SOT [1]. Moreover, the current-induced SOT switching is demonstrated in these BiTe-based devices, which indicates the non-epitaxial chalcogenide materials are the potentially efficient SOT sources in future SOT magnetic memory devices.

[1] T.-Y. Chen, et al. ACS Appl. Mater. & Inter. 12, 7788-7794 (2020).