Microscopic origin of magnetization reversal in exchange-coupled ferro-/ferrimagnetic bilayers

TL;DR

This paper investigates the microscopic mechanisms behind magnetization reversal in exchange-coupled ferrimagnetic and ferromagnetic bilayers, revealing thickness-dependent reversal modes driven by domain wall propagation or nucleation.

Contribution

It provides detailed microscopic insights into how layer thickness influences reversal mechanisms in exchange-coupled bilayers, combining experimental and micromagnetic simulation approaches.

Findings

Thick layers exhibit irreversible switching via domain wall propagation.

Thin layers show hysteresis-free, nucleation-dominated reversal.

Microscopic origin of reversal differs with ferromagnetic layer thickness.

Abstract

In this study, the magnetic reversal process of exchange-coupled bilayer systems, consisting of a ferrimagnetic TbFeCo alloy layer and a ferromagnetic [Co/Ni/Pt]N multilayer, was investigated. In particular, minor loop studies, probing solely the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend strongly on the thickness of the ferromagnetic layer. For thick layers, irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the ferromagnetic layer. In contrast, thin ferromagnetic layers show a hysteresis-free reversal, which is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt multilayer and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer, as confirmed by micromagnetic simulations.