Hyunsoo Yang
Layered topological materials such as topological insulators (TIs) and Weyl semimetals are a new class of quantum matters with large spin-orbit coupling, and probing the spin texture of these materials is of importance for functional devices. We reveal spin textures of such materials using the bilinear magneto-electric resistance (BMR), which depends on the relative orientation of the current with respect to crystallographic axes [1,2]. We also visualize current-induced spin accumulation in topological insulators using photocurrent mapping [3]. Topological surface states (TSS) dominated spin orbit torques are identified in Bi2Se3 [4], and magnetization switching at room temperature using Bi2Se3 as a spin current source is demonstrated [5]. Nevertheless, the resistive nature of TIs can cause serious current shunting issues, leading to a large power consumption. In order to tackle this issue, we propose two approaches.
Weyl semimetals have a larger conductivity compared to TIs and they can generate a strong spin current from their bulk states. The Td-phase Weyl semimetal WTe2 can be produced with high quality, simplifying interfacial studies and facilitating device applications. Utilizing the magneto-optical Kerr microscopy, we show the current-driven magnetization switching in WTe2/NiFe with a low current density and a low power [6].
The current shunting issue can be also overcome by the magnon-mediated spin torque, in which the angular momentum is carried by precessing spins rather than moving electrons. Magnon-torque-driven magnetization switching is demonstrated in the Bi2Se3/NiO/Py devices at room temperature [7]. By injecting the electric current to an adjacent Bi2Se3 layer, spin currents were converted to magnon torques through an antiferromagnetic insulator NiO. The presence of magnon torque is evident for larger values of the NiO-thickness where magnons are the only spin-angular-momentum carriers. The demonstration reveals that the magnon torque is sufficient to control the magnetization, which is comparable with previously observed electrical spin torque ratios of TIs [5].
Looking towards the future, we hope that these studies will spark more works on harnessing spin currents from topological materials and revealing interesting spin textures at topological material/magnet interfaces. All magnon-driven magnetization switching without involving electrical parts could be achieved in the near future. The results will invigorate magnon-based memory and logic devices, which is relevant to the energy-efficient control of spin devices.
[2] P. He et al., Nat. Comm. 10, 1290 (2019)
[3] Y. Liu et al., Nat. Comm. 9, 2492 (2018)
[4] Y. Wang et al., Phys. Rev. Lett. 114, 257202 (2015)
[5] Y. Wang et al., Nat. Comm. 8, 1364 (2017)
[6] S. Shi et al., Nat. Nano. 14, 945 (2019)
[7] Y. Wang et al., Science 366, 1125 (2019)