Relaxation of the magnetization from interface defects

We study the nucleation of a domain wall induced by an interface step defect in a bilayer consisting of a four-fold crystalline anisotropy ferromagnetic film exchange coupled with a two-sublattice uniaxial antiferromagnetic (AF) substrate. For thin ferromagnetic (F) films we find a Nèel wall, and the surface magnetic profile is a replica of the interface pattern. For thick films the magnetic pattern at the free surface may deviate significantly from the interface pattern. The relaxation is due to the local character of the interface exchange coupling. We find that above a threshold value of the F film thickness the surface is uniformly magnetized in the direction perpendicular to the easy axis of the AF substrate. We discuss how the relaxation affects the hysteresis curves.