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

•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.