Thermal activation in exchange biased bilayers

Magnetisation reversal in an exchange biased ferromagnetic/antiferromagnetic bilayer is extremely complex due to the coupling between the spins at the interface. Particular importance should be paid to the issue of thermally activated and hence time dependent reversal of the antiferromagnet during the reversal of the adjacent ferromagnet. This reversal of the antiferromagnet is one mechanism that can lead to an increased coercivity of the ferromagnetic layer. Waiting time experiments on two different systems, IrMn( X Å)/NiFeCo(100 Å) and a NiFe(100 Å)/NiMn(250 Å) bilayer, have shown that the degree of reversal in the antiferromagnet is highly dependent upon the particular antiferromagnet used. Measurements of bilayers with different thicknesses of the antiferromagnet show that the reversal rate is dependent upon the thickness. It is difficult to deduce the particular mechanism for reversal as the data is consistent both with a reversal by coherent rotation of grains and also by the formation of helical domains. The reversal is best viewed as a thermally activated process over a distribution of energy barriers, the particulars of which are determined by the morphology of the interface and the mechanism(s) for reversal. Further waiting time experiments show that this thermally activated reversal of the AFM is not the only mechanism determining the coercivity and that other mechanisms such as spin-flop coupling at a partially compensated interface may also be important.