Antiferromagnetic grain volume effects in metallic polycrystalline exchange bias systems

In this work we present a new interpretation of the role of the antiferromagnetic (AF) grain size in polycrystalline exchange bias thin films. It is found that at a finite temperature AF grains with sizes below a given critical volume VC do not contribute to the loop displacement because they are not thermally stable. There is a second critical volume VSET above which the AF grains cannot be set due to their anisotropy energy being too large. Therefore, only grains in the window between VC and VSET contribute to the loop displacement. Using this interpretation we can explain both the increase and the decrease in the exchange field with the AF grain size and the layer thickness.