Deviations from unidirectional anisotropy in layered exchange-bias systems due to breakdown of rigid spin rotations

The magnetic behavior of $\mathrm{Co}\mathrm{O}∕\mathrm{Co}$ bilayers has been investigated by magnetic hysteresis loop and torque measurements. The latter show a purely unidirectional anisotropy for small-angle rotations of the Co layer magnetization from the bias direction. For larger-angle rotations, high-order anisotropies have to be included. The deviation from a purely unidirectional anisotropy is attributed to a breakdown of rigid spin rotations in the CoO, which reveals spin-glass-like properties owing to a random magnetic anisotropy.