Unconventional Spin Hall Magnetoresistance in Noncollinear Antiferromagnet/Heavy-Metal Stacks

We study the spin Hall magnetoresistance (SMR) in noncollinear antiferromagnet Mn_{3}Sn/heavy-metal stacks. The measured SMR exhibits peculiar magnetic field angle and magnitude dependence that sharply deviates from the conventional SMR theory based on the dampinglike spin-transfer torque. An alternative model based on a coherent fieldlike torque reproduces the observations well. Our work reveals a previously unrecognized mechanism of interfacial exchange that indicates a precession of the conduction-electron spins in the collective local exchange fields of the noncollinear antiferromagnetic order. The unraveled physics is essential to understanding and controlling spin transport in unconventional magnetic materials.