Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co$_2$FeAl/MgO interface upon annealing

TL;DR

This study examines how annealing affects the interfacial perpendicular magnetic anisotropy and magnetic properties of Co$_2$FeAl/MgO films, revealing significant increases in anisotropy constants and insights into magnetic stability and damping.

Contribution

It provides new quantitative data on the evolution of interfacial anisotropy constants with annealing temperature in Co$_2$FeAl/MgO films, highlighting the impact of thermal treatment on magnetic properties.

Findings

Perpendicular interface anisotropy constant increases with annealing temperature.

Large anisotropy stabilizes perpendicular magnetization below 1.7 nm thickness.

Gilbert damping parameter shows a minimum at 40 nm thickness.

Abstract

We investigate thickness series of films of the Heusler alloy Co$_2$FeAl in order to study the effect of annealing on the interface with a MgO layer and on the bulk magnetic properties. Our results reveal that while the perpendicular interface anisotropy constant $K^{\perp}_{\rm S}$ is zero for the as-deposited samples, its value increases with annealing up to a value of $1.14\, \pm \,0.07$~mJ/m$^2$ for the series annealed at 320$^{\rm o}$C and of $2.07\, \pm \,0.7$~mJ/m$^2$ for the 450$^{\rm o}$C annealed series owing to a strong modification of the interface during the thermal treatment. This large value ensures a stabilization of a perpendicular magnetization orientation for a thickness below 1.7~nm. The data additionally shows that the in-plane biaxial anisotropy constant has a different evolution with thickness in as-deposited and annealed systems. The Gilbert damping parameter $\alpha$ shows minima for all series for a thickness of 40~nm and an absolute minimum value of $2.8\pm0.1\cdot10^{-3}$. The thickness dependence is explained in terms of an inhomogenous magnetization state generated by the interplay between the different anisotropies of the system and by crystalline disorder.