Magnetization reversal mechanism in perpendicular exchange-coupled Fe/L10-FePt bilayers

The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (t(Fe) = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (similar to 2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction.