Perpendicular exchange bias independent of NiO layer thickness in NiO/CoPt structures with orthogonal spin configuration

We report on an investigation of the orthogonal coupling system of a NiO/CoPt bilayer on a Pt(111)/glass substrate, with CoPt showing strong perpendicular magnetic anisotropy. This system displays a favorable perpendicular exchange bias with an exchange bias field as high as-900 Oe, exceeding most of the results reported so far from various NiO/FM exchange coupling systems. In contrast to the general bottom-pinned stacking sequence, in this research we placed an antiferromagnetic layer on top of a ferromagnetic layer; an interesting new phenomenon was observed-the loop shift amplitude does not change with the thickness of the antiferromagnet, which is explained in detail by modeling and computing. We demonstrate a modified random field model by applying Malozemoff's assumption to a 'spin flop' coupling system, in which interface roughness plays a crucial role. A simulation on the basis of the random field model agrees well with the experimental observations.