Generation of spin current is critical to developing technologies based on current-controlled magnetization via spin orbit torques. Recent studies reveal that, in addition to the 5d transition materials with strong spin orbit coupling, topological insulators, Weyl semimetals and van der Waal heterostructures may serve as an efficient source of spin current. It is the unique electronic structure of these materials, the large Berry phase in particular, that facilitates generation of spin current. We have studied the spin Hall effect in materials with strong spin orbit coupling, with particular emphasis on semimetals that possess Dirac-like electronic bands [1]. We find the spin Hall angle of BiSb alloy increases with increasing BiSb thickness until it reaches ~1.2. Surprisingly, the spin Hall angle increases with increasing temperature. These experimental results suggest strong contribution from Dirac electrons on the generation of spin current, the details of which will be discussed in the talk.

We have also studied metallic heterostructures with unique magnetic texture. In metallic multilayers with synthetic antiferromagnetic coupling, we find the spin orbit torque is larger by a factor of ~15 when the ferromagnetic layers are coupled antiferromagnetically compared to that with ferromagnetic coupling [2]. These results cannot be accounted for with the current understanding of spin orbit torque: possible origin of the effect will be discussed. We have also studied chiral magnetic texture in heavy metal/ferromagnetic metal bilayers. Interestingly, we find the Dzyalonshinski-Moriya interaction shows strong dependence on the current passed along the film plane of the bilayer [3]. The rate of increase of the DMI with the current is consistent with theoretical models based on a system with Rashba Hamiltonian. The implication of the results will be discussed.

Acknowledgement: CREST, JSPS Grant-in-Aid (16H03853, 15H05702), CSRN.

1.  Z. Chi, Y.-C. Lau, X. Xu, T. Ohkubo, K. Hono, and M. Hayashi, Science Advances 6, eaay2324 (2020).
2.  Y. Ishikuro, M. Kawaguchi, T. Taniguchi, and M. Hayashi, Phys. Rev. B 101, 014404 (2020).
3.  N. Kato, M. Kawaguchi, Y. C. Lau, T. Kikuchi, Y. Nakatani, and M. Hayashi, Phys. Rev. Lett. 122, 257205 (2019).