The unusual phenomena of unidirectional anisotropy was revealed over forty years ago in ferromagnetic (FM) Co fine particles with an oxidized antiferromagnetic (AF) shell.[1] This results in a shift of the hysteresis loop of the ferromagnet away from the zero-field axis. The same effect has since been observed in layered structures.[2-5] This effect was the first of several fascinating phenomena [6-8] caused by exchange coupling between neighboring layers with different magnetic order which have opened possibilities for exciting new practical applications.[9,10]
In the first model the authors[1] proposed that the interfacial exchange coupling was comparable to the atomic exchange coupling in bulk FM or AF materials. That model, however, fails to describe the small magnitude of the measured exchange anisotropy field, HE. Therefore, new models have been proposed which explain the measured HE value after taking into account domain wall formation in the AF layer and the presence of a random exchange field due to monatomic steps at the FM/AF interface.[11,12]
Other drastic discrepancies between theory and experimental data have also been reported.[2,13-15] For instance, it is well known that the coercivity, HC, of a FM film in contact with an AF layer is enhanced compared to the "free" FM layer. However, there is no model, which describes this enhancement. This phenomenon cannot be understood in terms of a spin coherent rotation model.[1] In this latter case the interfacial exchange coupling, leads only to a shift in the hysteresis loop. The coercivity of the AF/FM bilayer remains equal to the coercivity of the "free" FM layer: HCF = 2KF/MS, where KF and MS are the uniaxial anisotropy and the saturation magnetization of the ferromagnet, respectively. A model[11] incorporating a 1-dimensional planar domain wall in the AF layer, does predict the same value of HC for both small and large interfacial exchange couplings, and even to lowered HC for intermediate coupling magnitude.
It can be thought that in an AF/FM bilayer the magnetization process in an ultra-thin FM film proceeds most likely by either non-uniform spin rotation or by domain wall nucleation and motion. These magnetization processes should be accompanied by inhomogeneities in the AF spin distribution both across and along the interface. We will show that the increased coercive force in the AF/FM bilayer may be explained by taking into account the nucleation of domains in the AF layer with walls having components which are both parallel and perpendicular to the AF/FM interface.