Polycrystalline ferromagnet-antiferromagnet bilayers exhibit a wide range of interesting magnetic effects. This paper describes a model for these bilayers which explains not only the unidirectional anisotropy that gives rise to the well known shifted hysteresis loops, but also the hysteretic effects observed in rotational torque and ferromagnetic resonance experiments. The model is based on independent uniaxial antiferromagnetic grains with unconstrained (Heisenberg) or planar (XY) spins, coupled to a ferromagnetic film both by direct coupling to the net moments at the interfaces of the grains and by spin flop coupling. Because of coupling at the interface, rotating the ferromagnetic magnetization applies a torque to the antiferromagnetic spins at the interface of each grain. The torque winds up partial domain walls parallel to the interface in the antiferromagnetic grains. If the domain walls get wound up past a postulated critical angle, the order in the antiferromagnetic grain becomes unstable and changes. These instabilities in the antiferromagnetic spin configuration are found to contribute to experimentally observed rotational hysteresis and rotatable anisotropy. The types of spins, the type of coupling, and the energies of the coupling compared to the antiferromagnetic domain wall energies determine the unidirectional anisotropy. If the interfacial coupling is weaker than the domain wall energy, the unidirectional anisotropy is set by the direct coupling, with no contribution from the spin-flop coupling. If the interfacial coupling is stronger, then the unidirectional anisotropy is set by the domain wall energy, but depends on the type of spins. For Heisenberg spins, domain walls are never wound up past 180°, which maintains the antiferromagnetic order of the biased state. For this type of spins, spin-flop coupling only reduced the unidirectional anisotropy from its value in the absence of spin-flop coupling. For XY spins, strong interfacial coupling winds domain walls up past 180° for both direct and spin-flop coupling. These domain walls can detach from the interface and erase the biased state.
Citation: Physical Review B (Condensed Matter and Materials Physics)
Pub Type: Journals
antiferromagnet, exchange anisotropy, exchange bias, ferromagnetic resonance, hysteresis, rotational torque, spin valve