Unveiling a Hidden Order Transition at the Interface of an Exchange-Spring-Coupled Ferromagnet/Antiferromagnet Bilayer

Abstract

Study of the exchange bias (H ex ) switching in a ferromagnet (FM)/ antiferromagnet (AFM) bilayer driven by spin−orbit torque (SOT) has currently been an explicit research field for various SOT applications, but a complete understanding of AFM and its interaction with SOT at an FM/AFM interface remains elusive. This study reports an SOT-driven hidden order transition behind the H ex in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer along with the interaction between Co magnetization (with perpendicular anisotropy) and IrMn spins. The exchange coupling across the Co/IrMn interface facilitates a long-range spin order in the form of an exchange-spring configuration, resulting in two distinct orders in IrMn: a canting-induced FM order (m) responsible for hosting H ex and a hidden Neél order (n) representing the AFM nature. The SOT generated in Pt appeared to disturb the order in IrMn when it was first applied to the device, leading to the reduction in m along the perpendicular direction and an increase in the component of n along the current axis; therefore, a diminished H ex along with a developed planar anisotropic magnetoresistance (MR) can be observed. The coupled transitions of m and n appear to be irreversibly driven by SOT and independent of current polarity at the first SOT measurement. Subsequent SOT measurements reveal a reversible switching of m, manifested by the reversible H ex switching while n remains unaltered along the current axis, as indicated by the unchanged anisotropic MR. This work unveils a hidden Neél order transition at a fully dynamic FM/AFM interface.