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

Study of the exchange bias (Hex) 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 Hex 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 Hex and a hidden Néel 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 Hex 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 Hex switching while n⇀ remains unaltered along the current axis, as indicated by the unchanged anisotropic MR. This work unveils a hidden Néel order transition at a fully dynamic FM/AFM interface.