New Features of the Coercivity in Exchange-Coupled Ni_(81)Fe_(19)/CoO Bilayers

The coercivity behaviour of the Nis1Fe19 film exchange-coupled with an antiferromagnetic CoO underlayer has been investigated systematically. It has been found that the coercivity is greatly enhanced not only in the easy axis direction but also in the hard axis direction when the temperature is below the Néel temperature of CoO. Also, the thickness dependence of coercivity at low temperature follows the scaling relation as 1/tNiFe with α=2.5 at the hard axis, which is quite in contrast with the case of the easy axis, i.e. α=1.5, predicted theoretically and verified experimentally previously. The increase of the temperature leads to the decrease of the coercivity at both the easy and hard axes, but the scaling relations are held except the narrow region just below the Néel temperature of CoO, at which the coercivity varies as 1/t NiFe with α=1.0 for both the easy and hard axes. Based upon Hoffmann's ripple theory and Malozemoff's random field model, a simple interpretation of the experiment findings is presented.