Thus our previous conclusion that magnetization reversal in the NiO/NiFe bilayer is primarily determined by the defect structure of AF layer is confirmed. Hence, to describe correctly the magnetization reversal process and, in particular, the enhanced coercivity and asymmetry in activity of domain formation centers of the AF/FM bilayers with unidirectional anisotropy, it is necessary to analyze inhomogeneities of spin configurations of the AF layer. In particular, in addition to the occurrence of a planar domain wall in AF layer,[2] variation of AF spins along the interface should be taken into account. The energy cost of such inhomogeneities results in an increase in the coercivity of the bilayer.
The asymmetry in the activity of domain nucleation centers reported here can be understood in terms of non-homogeneous magnetic anistropy in the AF layer (e.g., due to dislocations) and the existence of different AF states for opposite saturating magnetic fields. The AF exists in the ground state at one magnetic field orientation and develops a quasi-planar domain wall[2] upon field reversal. When the AF layer is in the ground state, domains with reversed magnetization occur in local regions having lowered anisotropy. However, when spins in the AF are twisted, the domains nucleate in local bilayer regions with enhanced anisotropy where the AF domain wall energy is increased. Calculations supporting the above suggested mechanism will be published elsewhere.
The Russian co-workers gratefully acknowledge a research grant from the Russian Foundation for Basic Research No. 97-02-16879.