Perpendicular magnetic anisotropy in Au/FeCo/Au ultrathin films: Combined experimental and first-principles study

TL;DR

This study combines experimental measurements and first-principles calculations to investigate perpendicular magnetic anisotropy in ultrathin FeCo films on Au, revealing thickness-dependent anisotropy behavior and surface effects.

Contribution

It provides new insights into the thickness-dependent magnetic anisotropy of FeCo/Au ultrathin films through combined experimental and computational approaches.

Findings

Perpendicular anisotropy observed below 0.74 nm thickness.

Surface contribution significantly affects magnetic anisotropy.

An irregular dependence of anisotropy on layer thickness was found.

Abstract

Magnetic tunnel junctions with a magnetic layer with perpendicular anisotropy are currently used in computer memories that do not require voltage sustaining. An example of layers with perpendicular magnetic anisotropy are ultrathin FeCo films on Au substrate. Here, we present an experimental and computational study of Fe$_{0.75}$Co$_{0.25}$ layers with a thickness up to two nanometers. The investigated FeCo layers are surrounded on both sides by Au layers. In experiment, we found perpendicular magnetic anisotropy in FeCo polycrystalline films with a thickness below 0.74 nm (five atomic monolayers). We also measured a strong surface contribution to effective magnetic anisotropy. From the density functional theory we determined structural, electronic, and magnetic properties of the FeCo films. The calculations showed an irregular dependence of magnetic anisotropy on FeCo layer thickness. We observed perpendicular anisotropy for one- and three-atom FeCo monolayers and in-plane anisotropy for two- and four-atom monolayers. However, the determination of running averages of magnetic anisotropy (of three successive thicknesses of atomic monolayers) leads to a close linear thickness dependence of magnetic anisotropy, similar to the experimental result. The reason why the properties of the several-atom-thick FeCo layers differ so significantly from those of the bulk parent material is the disappearance of the central region of the layer whose properties approximate those of the bulk, and instead the existence of near-interface regions with properties altered by the presence of discontinuities.