Magnetic freezing transition in a CoO/Permalloy bilayer revealed by transverse ac susceptibility.

Highlights • Variable-temperature, variable-frequency susceptibility of a thin-film F/AF bilayer. • Unidirectional anisotropy is significantly smaller than the exchange bias. • Exchange bias caused by the different reversal mechanisms between two states. • Magnetic viscosity of AF increases by 4 orders of magnitude over 20 K. • Results indicate that the Heisenberg domain state of AF is a magnetic glass. Abstract We utilize variable-temperature, variable-frequency magneto-optical transverse magnetic susceptibility technique to study the static and dynamical magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements demonstrate that in the studied system, the directional asymmetry of the hysteresis loop is associated mainly with the difference in the reversal mechanisms between the two reversed states of magnetization stabilized by the exchange-induced uniaxial anisotropy. The latter is found to be much larger than the exchange-induced unidirectional anisotropy of the ferromagnet. We also observe an abrupt variation of the frequency-dependent imaginary part of ac susceptibility near the exchange bias blocking temperature, consistent with the magnetic freezing transition inferred from the previous time-domain studies of magnetic aging in similar systems. The developed measurement approach enables precise characterization of the dynamical and static characteristics of thin-film magnetic heterostructures that can find applications in reconfigurable magnonic and neuromorphic circuits. [ABSTRACT FROM AUTHOR]