Theory of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

Recently, there has been increasing interest in the study of nonlinear magnetotransport phenomena in systems with broken inversion and time-reversal symmetries. A notable example of such effects is the unidirectional magnetoresistance (UMR), in which spin-orbit coupling (SOC) and spin-asymmetry in electron scattering play key roles. A UMR effect was recently detected in an antiferromagnetic FeRh/Pt bilayer, despite the lack of intrinsic spin-dependent scattering, when the Neel vector is parallel to the applied current but perpendicular to the external magnetic field. More interestingly, the UMR evolves nonlinearly with the increasing magnetic field and undergoes a sign change even when the direction of the field remains the same. Based on a tight-binding model that takes into account the interfacial SOC and the exchange coupling between electron spins and the sublattice magnetization, we theoretically examine the physical origin of the unexpected UMR effect, and unveil the connection between the UMR and antiferromagnetic ordering. We also argue that the characteristic dependence of the UMR on the magnetic field strength, with a sign change in particular, is unique to antiferromagnetic systems, hence may be regarded as a magnetotransport signature of antiferromagnetic ordering.