Exploring a hidden order transition behind exchange bias at ferromagnet/antiferromagnet interface and issue of interfacial dynamics

Chao-Yao Yang^1,2*, Chih-Hsiang Tseng³, Sheng-Huai Chen³, Chih-Wei Cheng¹, Chih-Huang Lai³

¹Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²Undergraduate Degree Program of Systems Engineering and Technology, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
³Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Chao-Yao Yang, email:cyyang8611@nycu.edu.tw

Study on the exchange bias (Hex) 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 Hex switching along with specific SOT applications. Hex is magnetically dominant but only reveals the exchange coupling at the FM/AFM interface rather than the bulk of the AFM. Therefore, the complete facet of AFM and its interaction with SOT in an exchange-coupled system is still not fully uncovered. This research reports a hidden AFM order transition driven by SOT in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer, behind the observation on Hex switching. The exchange coupling across Co/IrMn interface may allow the long-range spin order in the form of an exchange-spring configuration, which appears to soften IrMn spins to be switched with Co during the field and SOT reversal. Two kinds of order in IrMn have been found in the trilayer: one is responsible for the Hex and the other is responsible for the AFM nature of IrMn. Through a magnetoresistance (MR) technique, which is sensitive to the changes of in-plane spin components, an irreversible AFM order transition reorientated to the current axis was experimentally resolved after applying the SOT. On the counterpart, the interfacial IrMn spins responsible for Hex can be freely switched with Co, leading to the reversible Hex switching. The observation on the former transition was absent in the recent studies, but this was crucial to understand how a hidden AFM order byhind the Hex in the vicinity of FM/AFM interface communicates with SOT via the exchange-spring effect and can be probed by using MR technique. The results were supported by micromagnetic simulation and X-ray magnetic linear dichroism.

Keywords: Antiferromagnet, Spin-orbit torque, Magnetoresistance, X-ray magnetic linear dichroism