Mössbauer study of the interfacial environment of ferromagnetic atoms exchange coupled to CoO (abstract)

Abstract

Although unidirectional anisotropy was first observed over 40 years ago in surface oxidized Co particles, the basic physics of the mechanism remains elusive. Unidirectional and exchange anisotropies are observed when ferromagnetic (FM) and antiferromagnetic (AFM) materials are coupled to each other. Because of the interfacial nature of the coupling, the exchange strength is sensitive to the interfacial environment. The simple model of an epitaxial atomically smooth interface predicts exchange field strengths two orders of magnitude larger than the observed values. Cross-sectional pictures of NixCo(1−x)O/permalloy exchange couples by transmission electron microscopy (TEM) reveal interfacial complexities such as roughness and strain gradients that could account for the reduction of the interfacial exchange strength. In this work, we studied the interfacial environment of the first few atomic layers on the FM side. We deposited the Mössbauer isotope Fe57 as an interlayer of variable thickness (4⩽t⩽16 Å) between a 300 Å CoO base layer and a 150 Å Ni capping layer. Using room temperature conversion electron Mössbauer spectroscopy, we measured the changes in the chemical and magnetic environment of the Fe atoms as a function of their distances from the interface. These measurements revealed that all the Fe atoms were magnetically ordered. The resulting complex spectra indicated a mix of Fe2+, Fe3+, and multiple metallic Fe subspectra within the first few monolayers of the interface. A particular metal fraction appears for tFe>8 Å and dominates the thicker Fe films. The existence of several metal subspectra indicates that Fe and Ni alloy at the interface. The Mössbauer measurements show that the Fe moments, metal and ionic, lie in the plane of the film. Measurements of two different 8 Å Fe57 interlayer films verified the reproducibility of the complex spectra. The origin of oxidation of the interfacial atoms is being studied by comparing the spectra of films prepared under various deposition conditions.