Abnormal field-like spin orbit torque in a synthetic antiferromagnet

Spin-orbit torque in synthetic antiferromagnets (SyAFs) has been studied extensively as a promising pathway for energy-efficient spintronic devices, but, to date, most studies have focused on the interaction of the SyAF with externally generated spin currents, such as those from the spin Hall effect in heavy metals. Here, we investigate internally generated spin torque in CoFeB/Ru/Ni SyAFs and its interaction with the magnetizations through harmonic resistance measurements. Through combined macrospin modeling and experimental investigations, we demonstrate that the second and third harmonic resistances are predominantly influenced by an unexpected effective field along the current direction, in a sharp contrast to conventional systems involving heavy metals. We attribute this abnormal effective field along the current direction to nonequilibrium spins with longitudinal polarization, induced either by a vertical component of magnetization at the Ni/Ru interface or by the reorientation of nonequilibrium spins through interfacial exchange coupling. Our findings offer insights into spin dynamics in SyAF structures and pave the way for advanced spintronic applications.