  In the investigations of antiferromagnetic (AF)/ ferromagnetic (FM) bilayer
samples, often distinct experimental techniques yield different values for the
measured exchange anisotropy field (HE). We propose that the observed
discrepancy may be accounted in part by the dependence of the unidirectional
anisotropy with the value of the external applied field (h). Using a simple
microscopic model for representing the AF/FM interface, which incorporates the
effect of interface roughness, we show that the interface energy between the AF
and FM layer indeed varies with h, as recently observed in anisotropic
magnetoresistance measurements, lending support to our proposal.
