Correlation between antiferromagnetic interface coupling and positive exchange bias

The induced moment in antiferromagnetic (AFM)–ferromagnetic (FM) $({\mathrm{FeF}}_2-\mathrm{Fe}$ and ${\mathrm{MnF}}_2-\mathrm{Fe})$ bilayers has been studied from the shift along the magnetization axis of the exchange-biased hysteresis loops. The magnetization shift depends strongly on the cooling field and microstructure of the AFM layer. The shift for small cooling fields can be opposite to the cooling field, indicating that, in some cases, the presence of the FM layer induces an antiferromagnetic coupling at the interface. Samples with negative magnetization shifts (antiferromagnetic coupling) exhibit large changes in exchange bias $H_E$ as a function of cooling field and positive exchange bias. Samples with positive magnetization shifts (ferromagnetic coupling) show almost no change in $H_E$ with cooling field and the exchange bias field remains always negative. These results confirm the theoretical assumption that an antiferromagnetic interface coupling is necessary to observe positive exchange bias.