Freezing and unfreezing of antiferromagnetic spins in CoO(111) epitaxial films on a ferromagnetic support

Epitaxial CoO(111)/Fe(110) bilayers grown on W(110) single crystal were studied using magneto-optic Kerr effect and X-ray magnetic linear dichroism techniques. A variety of possible configurations of frozen antiferromagnetic spins and in-plane easy axis of ferromagnet in CoO(111)/Fe(110) bilayers is presented. Our study shows that the blocking temperature, which marks the onset of exchange bias effect, strongly depends on the thickness of CoO layer. We confirmed that this dependence can be explained with the thermal stability of rotatable antiferromagnetic moments, that goes beyond classical finite-size effects. Partial unfreezing of antiferromagnetic spins at the vicinity of blocking temperature can be employed to reorient rotatable antiferromagnetic spins and refreeze them in a new orientation after subsequent cooling down the system.