Antiferromagnetic Layer Structure Effects on Exchange Coupling Strength in Ion-Beam Deposited Ni80Fe20/Cobalt Oxide Bilayers

Abstract

A series of Ni80Fe20/Cobalt oxide bilayers were prepared by a dual ion-beam deposition technique that produced a layer composition that varied from rock-salt CoO (a=4.27 Å) to spinel Co3O4 (a=8.21 Å) with increasing O2 content in the assist beam. Each polycrystalline layer (∼20 nm) exhibited a columnar structure perpendicular to the film surface. A strong temperature dependence of coercivity Hc and exchange bias field Hex was found in these Ni80Fe20/cobalt oxide bilayers. At T=10 K, films prepared with 34% O2 in the assist beam exhibited an exchange shift Hex∼-200 Oe that persisted at temperatures higher than 30 K. The variations in Hc and Hex as a function of %O2 are related to the interfacial roughness and magnetic state of CoO or Co3O4. The transition temperature of Co3O4 has increased above the bulk value via exchange coupling with the permalloy.