Thickness-Dependent Magnetoelasticity and its Effects on Perpendicular Magnetic Anisotropy in Ta|CoFeB|MgO Thin Films

TL;DR

This study investigates how the magnetoelastic coupling in ultra-thin Ta|CoFeB|MgO films varies with thickness and strain, revealing a significant impact on magnetic anisotropy and explaining nonlinear behaviors in ultrathin films.

Contribution

It introduces a thickness-dependent model of magnetoelasticity affecting magnetic anisotropy, extending the Nel model for ultrathin films.

Findings

Magnetoelastic coupling depends strongly on CoFeB thickness.

Thickness dependence can be modeled as surface and volume contributions.

The model explains nonlinear magnetic anisotropy in ultrathin films.

Abstract

We report measurements of the in-plane magnetoelastic coupling in ultra-thin Ta|CoFeB|MgO layers as a function of uniaxial strain, conducted using a four-point bending apparatus. For annealed samples, we observe a strong dependence on the thickness of the CoFeB layer in the range 1.3-2.0 nm, which can be modeled as arising from a combination of effective surface and volume contributions to the magnetoelastic coupling. We point out that if similar thickness dependence exists for magnetoelastic coupling in response to biaxial strain, then the standard N\'eel model for the magnetic anisotropy energy acquires a term inversely proportional to the magnetic layer thickness. This contribution can significantly change the overall magnetic anisotropy, and provides a natural explanation for the strongly nonlinear dependence of magnetic anisotropy energy on magnetic layer thickness that is commonly observed for ultrathin annealed CoFeB|MgO films with perpendicular magnetic anisotropy.