Exchange coupling of NiFe/CrMnPtx bilayers prepared by a substrate bias sputtering method

We have investigated the unidirectional exchange anisotropy between a ferromagnetic Ni81Fe19 film and a disordered antiferromagnetic CrMnPtx (x=3, 6, or 9) film prepared by substrate bias sputtering. A hysteresis loop with the shape predicted by the planar domain-wall model was found in the sample without substrate bias, suggesting that the interfacial exchange coupling energy between the Ni81Fe19 and CrMnPtx layers is comparable to the antiferromagnetic domain-wall energy. The substrate bias on the Ni81Fe19 underlayers improved the texture of the CrMnPtx layers but also increased the interface roughness. The substrate bias effect on the exchange bias can be understood in terms of the planar domain-wall model. The interface roughness reduces the interfacial exchange coupling by introducing defects and disordering the antiferromagnetic moments at the interface. This leads to a triangular hysteresis loop. The texture improvement increases the exchange bias by increasing the domain-wall energy of the antiferromagnet. The differences between these substrate bias effects and those observed in the Ni81Fe19/Fe50Mn50 bilayer system are also discussed.