Revisiting a hidden order at an ferromagnet/antiferromagnet interface and its switching dynamics

Study on the exchange bias (H
ex
) switching in a ferromagnet (FM)/antiferromagnet (AFM) bilayer driven by the spin-orbit torque (SOT) has been an explicit research field and current focus has been devoted to determining the approaches to drive the H
ex
switching along with specific SOT applications. H
ex
is magnetically dominant but only reveals the partial facet of IrMn, which is only responsible for the ferromagnetic component at the interface of AFM. Therefore, the complete facet of AFM and its interaction with SOT at an FM/AFM interface is still not fully uncovered. This research reports an SOT-driven hidden order transition behind H
ex
in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer along with the issue of switching activity at Co/IrMn interface. The exchange coupling across Co/IrMn interface allows a long-range spin order in the form of an exchange-spring configuration, yielding two kinds of order in IrMn: a canting-induced ferromagnetic order for hosting H
ex
and a hidden Néel order for preserving the antiferromagnetic nature. The former appears to be strongly coupled to the magnetization state of Co during the SOT switching, so it can be reversibly switched with Co on the H
ex
switching. On the contrary, the hidden Néel order transition re-orientated to the current direction appears to be irreversibly driven by SOT and independent on the current polarity, which can reflect on the magnetoresistance (MR) measurement and X-ray magnetic linear dichroism (XMLD) spectroscopy. Finally, a micromagnetic simulation reveals the dynamic IrMn spins are mainly based on a canting type interface instead of an uncompensated type via an exchange-spring coupling with Co.