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

We have investigated the unidirectional exchange anisotropy between a ferro magnetic Ni81Fe19 film and a disordered antiferromagnetic CrMnPtx (x=3, 6, or 9) film prepared by substrate bias sputtering. A hysteresis loop with th e shape predicted by the planar domain-wall model was found in the sample w ithout substrate bias, suggesting that the interfacial exchange coupling en ergy between the Ni81Fe19 and CrMnPtx layers is comparable to the antiferro magnetic domain-wall energy. The substrate bias on the Ni81Fe19 underlayers improved the texture of the CrMnPtx layers but also increased the interfac e roughness. The substrate bias effect on the exchange bias can be understo od in terms of the planar domain-wall model. The interface roughness reduce s 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 inc reasing the domain-wall energy of the antiferromagnet. The differences betw een these substrate bias effects and those observed in the Ni81Fe19/Fe50Mn5 0 bilayer system are also discussed. (C) 2000 American Institute of Physics . [S0021-8979(00)04601-6].