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Giant unidirectional magnetoresistance in topological insulator -- ferromagnetic semiconductor heterostructures

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