Unveiling a Hidden\nOrder Transition at the Interface\nof an Exchange-Spring-Coupled Ferromagnet/Antiferromagnet Bilayer

Study of the exchange\nbias (<i>H</i><sub>ex</sub>) switching\nin a ferromagnet (FM)/antiferromagnet (AFM) bilayer driven by spin–orbit\ntorque (SOT) has currently been an explicit research field for various\nSOT applications, but a complete understanding of AFM and its interaction\nwith SOT at an FM/AFM interface remains elusive. This study reports\nan SOT-driven hidden order transition behind the <i>H</i><sub>ex</sub> in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer along\nwith the interaction between Co magnetization (with perpendicular\nanisotropy) and IrMn spins. The exchange coupling across the Co/IrMn\ninterface facilitates a long-range spin order in the form of an exchange-spring\nconfiguration, resulting in two distinct orders in IrMn: a canting-induced\nFM order (m⇀) responsible for hosting <i>H</i><sub>ex</sub> and a hidden\nNéel order (n⇀) representing the AFM nature. The SOT generated\nin Pt appeared to disturb the order in IrMn when it was first applied\nto the device, leading to the reduction in m⇀ along the perpendicular direction and an\nincrease in the component of n⇀ along the current axis; therefore, a diminished <i>H</i><sub>ex</sub> along with a developed planar anisotropic\nmagnetoresistance (MR) can be observed. The coupled transitions of m⇀ and n⇀ appear to be irreversibly driven by SOT\nand independent of current polarity at the first SOT measurement.\nSubsequent SOT measurements reveal a reversible switching of m⇀, manifested by the reversible <i>H</i><sub>ex</sub> switching\nwhile n⇀ remains unaltered along the current axis,\nas indicated by the unchanged anisotropic MR. This work unveils a\nhidden Néel order transition at a fully dynamic FM/AFM interface.