Spin currents and the Dzyaloshinskii-Moriya interaction in Weyl semimetals

Spin currents and the Dzyaloshinskii-Moriya interaction in Weyl semimetals

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In recent years, topological materials such as topological insulators (TIs) and Weyl semimetals have emerged as leading contenders for generating and detecting spin currents. In terms of the spin current generation mechanism, topological materials distinguish themselves from extensively explored heavy metals, as the charge-spin conversion is enabled by their topological non-trivial band structures.

Prior to Weyl semimetals, the charge-spin conversion in TIs has been experimentally quantified and magnetization switching driven by the spin currents from TIs has been demonstrated in both in-plane and out-of-plane magnetized systems. It was shown that TIs could give rise to a larger spin current generation efficiency and smaller switching current densities compared to that of heavy metals. Nevertheless, the resistive nature of TIs can cause serious current shunting issues, leading to a large power consumption.

Weyl semimetals have emerged as potential alternatives to TIs. The nontrivial band structures of Weyl semimetals endow them with the potential for spin current generation. Unique advantages of Weyl semimetals over TIs are that Weyl semimetals have a much larger conductivity compared to TIs and they can generate a strong spin current from their bulk states. Therefore, exploring the possibility of the Weyl semimetals for next generation spintronic device applications is of great importance.

Other than spin current generation, interfacial Dzyaloshinskii-Moriya interaction (DMI) also plays a vital role at the interface between a magnetic and non-magnetic layer, especially when it comes to spin current driven magnetization switching. The emergence of DMI is a result of the combined effects of the structural inversion asymmetry as well as strong spin-orbit coupling. Therefore, the topological material/magnet heterostructure holds great promise for the advent of DMI.

In our recent work in Nature Nanotechnology, we performed magnetization switching and DMI measurement in Weyl semimetal WTe2/NiFe heterostructures. The Td-phase WTe2 was chosen not only because its Weyl semimetal nature, which endows the potential for an interfacial Rashba-like effect and an intrinsic spin Hall effect, but also the atomically flat surfaces which can be produced with high quality, simplifying interfacial studies and facilitating device applications. Utilizing the magneto-optical Kerr microscopy, we not only showed the current-driven magnetization switching in WTe2/NiFe with a low current density and a low power, but also quantified the interfacial DMI induced chiral domain wall tilting. Looking towards the future, we hope that this study will spark more works on harnessing spin currents from topological semimetals and revealing interesting spin textures at topological material/magnet interfaces.

Fig. (Left) The MOKE microscopy images show domain wall (DW) motion and tilting with an increasing amplitude of current pulses. (Right) Micromagnetic simulation results of DW tilting direction. The thick black arrows indicate the current directions. The small arrows in the track indicate the magnetization directions in the xy plane. The blue shading indicates the +y magnetization state and the red indicates the −y magnetization.

For more information, please refer to our recent publication in Nature Nano. (https://www.nature.com/articles/s41565-019-0525-8).

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