Characterisation of interface spin clusters in polycrystalline exchange bias thin films

A study of exchange bias systems has been made focusing on the characteristics of the interfacial spin clusters found in-between the bilayers. The materials studied were primarily IrMn (10nm) / CoFe (2nm) thin films of varying grain sizes. They were selected as the bulk properties have already been well characterised and they exhibit the effects being looked for.

A combination of vibrating sample magnetometry and electron microscopy has been used to investigate the nature of the interfacial spin clusters. Measurements were selected which specifically highlight the properties of the clusters. The first set of measurements consisted of increasing the setting field. The second, reducing the measurement temperature of median blocking curve measurements. The third involved taking hysteresis loops at liquid Helium temperatures.

The first set of measurements clearly showed an increase in the exchange bias that was due to the interfacial spin clusters. This increase was shown to correlate with the spin cluster size of the samples and showed quasi-superparamagnetic behaviour. The second set of measurements showed a temperature dependent variation in coercivity that implied the coercivity was due to the interfacial spin clusters. The final set of measurements showed a change in behaviour of the spin clusters whereby they started to act as a spin glass, which again was cluster size dependent. All of these results could be explained by virtue of an energy barrier distribution of the spin clusters whereby they could take on one of three different states, thermally unstable, partially stable, or fully stable. Taken as a whole, the results and conclusions give a good insight into the nature of the interfacial spin clusters and their effects on the exchange bias and coercivity of IrMn / CoFe polycrystalline thin films.