Abstract
Giant exchange bias shifts of several Tesla have been reported in ferrimagnetic/ferromagnetic bilayer systems, which could be highly beneficial for contemporary high energy density permanent magnets and spintronic devices. However, the lack of microscopic studies of the reversal owing to the difficulty of measuring few nanometer-wide magnetic structures in high fields precludes the assessment of the lateral size of the inhomogeneity in relation to the intended application. In this study, the magnetic reversal process of nanoscale 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 measurements, probing solely on the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend critically 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.
Highlights
The concept of engineering exchange-coupled composites is the most promising approach to meet current challenges in fabricating high energy density permanent magnets.[1−3] Already in 1991, Kneller and Hawig[4] proposed to manufacture magnets with a magnetically hard and soft phase, exchange-coupled at a mutual interface
It is observed that the field required for reversing the FM layer becomes larger with decreasing number N, which is expected if the interfacial exchange coupling remains constant.[29,34]
The magnetic force microscopy (MFM) data recorded in fields from 0.8 to 1.05 T (Figure 3d−g) show steady states of the domain wall propagation that cannot be observed in the model used for simulation
Summary
The concept of engineering exchange-coupled composites is the most promising approach to meet current challenges in fabricating high energy density permanent magnets.[1−3] Already in 1991, Kneller and Hawig[4] proposed to manufacture magnets with a magnetically hard and soft phase, exchange-coupled at a mutual interface. It is observed that the field required for reversing the FM layer becomes larger with decreasing number N (thickness), which is expected if the interfacial exchange coupling remains constant.[29,34] The minor loops were captured to analyze the switching process of the softer FM layer and show two distinct switching mechanisms. The details of the underlying reversal mechanism of the fully reversible switching case were already reported for a similar TbFe/[Co/ Pt]5 heterostructure.[30] There, a nucleation-dominated three-stage magnetization reversal process was revealed, which is caused by grainto-grain variations in magnetic anisotropy of the Co/Pt ML and an inhomogeneous exchange coupling to the magnetically hard TbFe layer. Panel (k) schematically shows the additional local field Hz,FI arising from the FI layer that hinders the lateral domain wall propagation in the FM (Figure 3d−g), in contrast to the simulation in panels (e−h). The domain wall propagates through the film until all parts have switched parallel to the field (and parallel to the FI net magnetization), which is a fully irreversible process in agreement with the Experimental
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