  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.
