Jakub Zelezny
Spin currents are one of the key concepts of spintronics. In the past, two types of spin currents have been predominantly discussed and utilized: the spin-polarized current in ferromagnetic materials and the spin Hall effect. The spin-polarized current only exist in magnetic materials, has a non-relativistic origin and flows in the same direction as the charge current. In contrast, the spin Hall effect exists also in non-magnetic materials, has typically a relativistic origin and is transverse to the charge current. In recent years it has been discovered, however, that the phenomenology of spin currents is much richer. We have shown that the spin-polarized current can also exist in some antiferromagnetic materials and that a new type of spin Hall effect exists, which has origin in the magnetic order, and occurs in ferromagnetic and some antiferromagnetic materials [1]. This effect is now referred to as the magnetic spin Hall effect and has been recently experimentally demonstrated in non-collinear antiferromagnet Mn3Sn [2]. We have also shown that the conventional spin Hall effect can exist in some non-collinear magnetic systems even in absence of the relativistic spin-orbit interaction [3].
Furthermore, we have found that a non-relativistic magnetic spin Hall effect can exist in a collinear antiferromagnet [4]. Such system is distinct from systems where the transverse spin currents have been previously discussed because in the non-relativistic limit it conserves spin and thus it allows for spin-charge conversion in a spin-conserving system. Here we review the various types of spin currents that can occur in magnetic systems and give general conditions for their existence as well as a symmetry classification. In addition, we present calculations of these novel spin currents in various collinear and non-collinear antiferromagnets.
[2] M. Kimata et al., Nature 565, 627–630 (2019)
[3] Y. Zhang et al: New J. Phys. 20 ( 2018 )
[4] R. G. Hernández el., arXiv:2002.07073 (2020)