Model for exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

This paper describes a model for polycrystalline ferromagnet-antiferromagne t bilayers. Independent antiferromagnetic grains are coupled to a ferromagn etic film both by direct coupling to the net moments at the interfaces of t he grains and by spin-flop coupling. Rotation of the ferromagnetic magnetiz ation applies a torque to the antiferromagnetic spins at the interface of e ach grain which winds up partial domain walls in the antiferromagnet. The m odel explains both the unidirectional anisotropy that gives rise to the wel l-known shifted hysteresis loops, and the hysteretic effects observed in ro tational torque and ferromagnetic resonance experiments. The unidirectional anisotropy comes from grains in which the antiferromagnetic order is stabl e as the magnetization is rotated. The hysteretic effects come from grains in which the antiferromagnetic order irreversibly switches as the domain wa ll is wound up past a postulated critical angle. For all of the models cons idered here, spin-flop coupling does not contribute to the unidirectional a nisotropy. [S0163-1829(99)01305-3].