A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves.
The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry
complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization
decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains
coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the
ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with
a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding
minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the
hysteresis loop timescale.