CoO-NiO superlattices: Interlayer interactions and exchange anisotropy with Ni81Fe19 (invited)

Abstract

A strong interlayer exchange interaction is observed in polycrystalline antiferromagnetic CoO-NiO thin-film superlattices. This was determined by measuring the exchange field He resulting from coupling these superlattices with Ni81Fe19. The temperature above which He is zero (the blocking temperature) is taken as a measure of the ordering temperature of the superlattice. In CoO-NiO superlattices with short repeat distances the CoO and NiO layers order at a single temperature that is between the ordering temperatures of bulk CoO and NiO. These ordering temperatures are the same as for CoxNi1−xO alloy films with the same overall composition. The temperature dependence of He in some of the superlattice exchange couples deviates from the linear behavior expected for cubic antiferromagnets. In addition, the exchange anisotropy induced by some CoO-NiO superlattices is greater than that induced by CoxNi1−xO. The higher He and nonlinear temperature dependence suggest that the interlayer coupling has a strong effect on the magnetocrystalline anisotropy of the layers within the superlattice. Thick-NiO/thin-CoO/Ni81Fe19 trilayers were produced to investigate the thickness dependence of the oxide-oxide interaction. When the CoO layers are sufficiently thin (≤20 Å), the CoO ordering temperature approximates the NiO value, indicating quite strong coupling throughout the CoO layer. The effect of the coupling is much weaker for thicker CoO layers (≥30 Å).