A Spin‐Orbit Torque Ratchet at Ferromagnet/Antiferromagnet Interface via Exchange Spring

The antiferromagnet (AFM) and ferromagnet (FM) interface is a unique branch of magnetics of broad scientific interest. AFMs play an important role in spin‐orbit torque devices based on their ability to generate spin‐polarized current and exchange bias when combined with FMs. In this study, an interesting spin‐orbit torque (SOT) ratchet involving the exchange spring effect in an IrMn/CoFeB bilayer device with perpendicular anisotropy and exchange bias is developed. The combined use of electrical and spectroscopic analysis reveals that the exchange spring in IrMn/CoFeB bilayer yields unidirectional anisotropy, resulting in a collinear/orthogonal AFM/FM spin configuration at the interface upon switching CoFeB magnetization upward/downward. The ratcheting characteristics resulting from unidirectional anisotropy manifest in SOT switching. In this process, magnetization against the exchange spring features digital‐like switching with a sharp transition, whereas the reverse function is characteristic of analog switching with a gradual transition tail. The dual digital‐analog characteristics of the IrMn/CoFeB bilayer may be of benefit in neuromorphic and memory applications. A spin‐orbit torque ratchet is discovered in an anti‐ferromagnet (IrMn)/ferromagnet (CoFeB) bilayer device with exchange bias. An exchange spring effect is found to be responsible for the unidirectional anisotropy of CoFeB, which supports the multilevel cell feature for neuromorphic applications. This finding opens a new avenue for the advanced spin‐orbit torque (SOT) technology.