Exchange bias without directional anisotropy in Permalloy/CoO bilayers

TL;DR

This study reveals that exchange bias in Permalloy/CoO bilayers can occur without directional anisotropy, due to spin flop asymmetry, opening new avenues for multistable nanomagnetic systems in neuromorphic computing.

Contribution

It demonstrates that exchange bias can exist without directional anisotropy in Py/CoO bilayers, highlighting the role of spin flop asymmetry in magnetic behavior.

Findings

Unidirectional anisotropy diminishes with increasing Py thickness.

Hysteresis loop asymmetry persists despite negligible unidirectional anisotropy.

Spin flop asymmetry influences magnetic reversal processes.

Abstract

We utilize transverse ac susceptibility measurements to characterize magnetic anisotropy in archetypal exchange-bias bilayers of ferromagnet Permalloy (Py) and antiferromagnet CoO. Unidirectional anisotropy is observed for thin Py, but becomes negligible at larger Py thicknesses, even though the directional asymmetry of the magnetic hysteresis loop remains significant. Additional magnetoelectronic measurements, magneto-optical imaging, as well as micromagnetic simulations show that these surprising behaviors are likely associated with asymmetry of spin flop distribution created in CoO during Py magnetization reversal, which facilitates the rotation of the latter back into its field-cooled direction. Our findings suggest new possibilities for efficient realization of multistable nanomagnetic systems for neuromorphic applications.