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A method for the measurement of the anisotropy constant of the antiferromagnet (AF) KAF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature ⟨TB⟩ of a CoFe∕IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5±0.5)×106erg∕cc has been obtained at room temperature for a 4nm thick IrMn layer.

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.

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