Microscopic Origin of Magnetization Reversal in Exchange-coupled Ferro-/Ferrimagnetic Bilayers

Abstract

Exchange-coupled systems employing ferrimagnets (FI) provide high tunability, interfacial exchange interaction, and zero magnetic moment at the compensation temperature [1], which is highly beneficial for many applications such as spin valves [2] and magnetic tunnel junction devices [3]. Micromagnetic simulations showed that a partial domain wall is formed at the FI/ferromagnet(FM) interface layer during the reversal of the FM. The minor loop becomes fully reversible if this domain wall generates a hard-axis field that overcomes the anisotropy field of the FM [4]. In this study, the magnetic reversal process of exchange-coupled bilayer systems, consisting of a FI TbFeCo alloy layer and a FM [Co/Ni/Pt]N multilayer (ML), was investigated [5]. In particular, minor loop studies, probing solely the reversal characteristics of the softer FM layer, reveal two distinct reversal mechanisms, which depend strongly on the thickness of the FM layer. For thick layers (N=9, 10.8 nm), 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 FM layer. In contrast, thin FM layers (N=5, 6.0 nm) show a hysteresis-free reversal, which was already reported in previous work [6] and is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt ML and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer. The results were confirmed by micromagnetic simulations.