Introduction

Since the discovery of unidirectional anisotropy[1] numerous investigations of this phenomenon have been carried out in ferromagnetic (FM) thin films deposited onto antiferromagnetic (AF) material. The simplest model, proposed by the authors who discovered this phenomenon, incorporates an exchange coupling across the flat interface between FM and AF spins. Because this model fails to describe the magnitude of the measured exchange anisotropy field, H_E, new models have been proposed, involving either planar[2] or vertical[3,4] domain walls in the AF layer. At present, it is established that the exchange shift of the hysteresis loop of a thin exchange-biased FM film depends upon the behavior of ordered spins in the AF. However, the picture is still not entirely clear.

In particular, it is known that the magnitude of the coercive force, H_C, of AF/FM bilayers increases with decreasing temperature below the Neel temperature of AF layer.[1,5-7] However, there is no model describing mechanisms responsible for this phenomenon. It is clear that the enhanced coercivity cannot be understood in terms of a spin-coherent rotation model.[1] In this case the exchange anisotropy field, H_E, leads only to a shift in a hysteresis loop.[1] The coercivity of the bilayer remains equal to the coercivity of "free" FM layer. A model[2] which takes into account the planar domain-wall formation in the AF leads to the same value of H_C for both small and large interfacial exchange couplings, and even to lowered H_C for intermediate interfacial coupling magnitude. The magnetization process in ultra-thin FM films proceeds by either domain wall nucleation and motion or by non-uniform spin rotation. These processes could be accompanied by two-dimensional inhomogeneities in the AF spin distribution resulting in an increase in the coercive force. In this paper, we study the magnetization reversal of epitaxial NiO/NiFe bilayers grown on both single crystal (001) MgO and polycrystalline Si substrates. Moreover, the influence of dislocations on domain wall nucleation and motion was investigated. In addition, the behavior of exchange-biased NiO/NiFe bilayers is compared with the reversal properties of unbiased NiFe films grown without an AF buffer.