  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.
