Hysteresis from antiferromagnet domain-wall processes in exchange-biased systems: Magnetic defects and thermal effects

The partial domain-wall theory of exchange bias predicts bias field magnitudes and film thickness dependencies consistent with certain experimental systems. However, the theory does not account for the coercivity enhancement that accompanies the hysteresis loop shift in single domain materials. We show theoretically that the presence of an attractive domain-wall potential in the antiferromagnet, arising from magnetic impurities, for example, can provide an energy barrier for domain-wall processes that control the coercivity. Asymmetric hysteresis loops are observed and modifications to the angular dependence of exchange bias suggest a mechanism of rotational hysteresis in terms of wall pinning. Similar domain-wall processes are also seen at finite temperatures, where an analogous pinning arises from a displacement of the domain wall from the interface due to thermal fluctuations.