Topological spintronics*

Tetradymite narrow bandgap semiconductors (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and their alloys) are known to support topologically protected, two dimensional (2D) helical Dirac fermion surface states characterized by a spin‐texture in momentum space [1]. The 'spin‐momentum locking' of the 2D surface states in these three dimensional (3D) 'topological insulators' lends itself naturally to 'topological spintronics,' device applications that might exploit efficient spin‐charge interconversion. We present an introductory overview of the emergence of 'topological spintronics' [2] and then focus on recent experiments that probe spin‐charge interconversion at the interface between a 3D topological insulator and an insulating ferrimagnet [3], with a view toward understanding how the spin Hall conductivity in topological insulators varies with chemical potential. An important issue examined in this context is the relative contribution to spin‐charge conversion of the surface and bulk states, both of which have large spin‐orbit coupling in the tetradymites. Finally, we address emerging demonstrations of efficient spin‐orbit torque switching using topological insulator/ferromagnetic metal heterostructures [4]. 

1. J. P. Heremans, J. J. Cava, N. Samarth, Nature Reviews Materials2, 17049 (2017).
2. A. R. Mellnik et al., Nature 511, 449 (2014).
3. Hailong Wang et al., Phys. Rev. Lett. 117, 076601 (2016).
4. J. Han et al., Phys. Rev. Lett. 119, 077702 (2017).