We predict a unidirectional magnetoresistance effect arising in a bilayer composed of a nonmagnetic metal and a ferromagnetic insulator, whereby both longitudinal and transverse resistances vary when the direction of the applied electric field is reversed or the magnetization of the ferromagnetic layer is rotated. In the presence of spin-orbit coupling, an electron wave incident on the interface of the bilayer undergoes a spin rotation and a momentum-dependent phase shift. Quantum interference between the incident and reflected waves furnishes the electron with an additional velocity that is even in the in-plane component of the electrons wavevector, giving rise to the unidirectional magnetoresistance - a nonlinear magnetotransport effect that is rooted in the wave nature of electrons.
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