Spin configurations in hard-soft coupled bilayer systems: from rigid magnet to exchange spring transitions

TL;DR

This paper analyzes the equilibrium magnetization profiles in hard-soft magnetic bilayer systems, deriving analytical solutions for rigid interfaces and numerical results for relaxed interfaces, revealing transition behaviors from rigid to exchange-spring states.

Contribution

It provides the first analytical solution for the magnetization profile in a rigid interface bilayer and numerical analysis for relaxed interfaces, enhancing understanding of spin configurations.

Findings

Analytical sine-Gordon solution for rigid interface profiles.

Critical soft layer thickness for spin deviations.

Numerical profiles for multilayer relaxed interfaces.

Abstract

We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M(z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M(z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.