Exchange Anisotropy in Epitaxial and Polycrystalline NiO/NiFe Bilayers

TL;DR

This study investigates how the crystalline orientation of NiO affects exchange anisotropy in NiO/NiFe bilayers, revealing that epitaxial films exhibit significant anisotropy despite theoretical expectations.

Contribution

It demonstrates that exchange anisotropy exists in (001)-oriented NiO/NiFe bilayers and attributes differences to surface anisotropy rather than magnetic compensation.

Findings

Exchange anisotropy is present in epitaxial NiO/NiFe bilayers.

Epitaxial films show about half the anisotropy of polycrystalline films.

Differences are due to surface anisotropy, not magnetic compensation.

Abstract

(001) oriented NiO/NiFe bilayers were grown on single crystal MgO (001) substrates by ion beam sputtering in order to determine the effect that the crystalline orientation of the NiO antiferromagnetic layer has on the magnetization curve of the NiFe ferromagnetic layer. Simple models predict no exchange anisotropy for the (001)-oriented surface, which in its bulk termination is magnetically compensated. Nonetheless exchange anisotropy is present in the epitaxial films, although it is approximately half as large as in polycrystalline films that were grown simultaneously. Experiments show that differences in exchange field and coercivity between polycrystalline and epitaxial NiFe/NiO bilayers couples arise due to variations in induced surface anisotropy and not from differences in the degree of compensation of the terminating NiO plane. Implications of these observations for models of induced exchange anisotropy in NiO/NiFe bilayer couples will be discussed.