Domain state model for exchange bias:: Influence of structural defects on exchange bias in Co/CoO

The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer show exchange bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the exchange bias field on the number of defects. The exchange bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced non-magnetic defects, also structural defects can enhance the exchange bias coupling. Twin boundaries in undiluted CoO increase the exchange bias coupling in Co/CoO by more than a factor of 2 compared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or grain boundaries, might also stabilize a domain state, suggesting that the domain state model for exchange bias is more generally applicable to understand the origin of the exchange bias phenomenon.