Giant unidirectional spin Hall magnetoresistance in topological insulator – ferromagnetic semiconductor heterostructures

The unidirectional spin Hall magnetoresistance (USMR) is one of the most complex spin-dependent transport phenomena in ferromagnet/nonmagnet bilayers, which involves spin injection and accumulation due to the spin Hall effect, spin-dependent scattering, and magnon scattering at the interface or in the bulk of the ferromagnet. While USMR in metallic bilayers has been studied extensively in very recent years, its magnitude (∼10⁻⁵) is too small for practical applications. Here, we demonstrate a giant USMR effect in a heterostructure of BiSb topological insulator – GaMnAs ferromagnetic semiconductors. We obtained a large USMR ratio of 1.1% and found that this giant USMR is governed not by the giant magnetoresistancelike spin-dependent scattering but by magnon emission/absorption and strong spin-disorder scattering in the GaMnAs layer. Our results provide new insights into the complex physics of USMR, as well as a strategy for enhancing its magnitude for device applications.