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

Nanocomposite multilayer films of hard and soft magnetic phases with interface exchange coupling have been the subject of a great interest both for fundamental studies and for their potential applications in many technological fields (permanent magnets [1], information storage [2] and MagMAS [3]). Such nanocomposite systems show a complex magnetic behaviour which is still under debate being influenced by many different factors such as microstructural features, presence of defects and disorder, nature of the interface between the two magnetic phases, etc. [4]. In the present work we discuss the magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers as a function of the soft layer thickness (tFe) combining magnetization loops at variable angle, MFM magnetic domain analysis and numerical micromagnetic simulations (figure 1) [5]. The analytical model proposed by Asti et al. for an ideal Fe/FePt bilayer [6] can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and reduced remanence with increasing tFe. However, the model cannot satisfactorily describe the magnetization process of real systems where the formation and evolution of magnetic domains can take place, being also affected by microstructural features. For thicknesses of soft layer exceeding the FePt exchange length (~ 2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. It has been shown that, just above the coercive field, the magnetization reversal does not proceed in a single step as predicted by the analytical model, but in 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. [1] R. Skomski and J.M.D. Coey, Phys. Rev. B 48 15812 (1993) [2] D. Suess, J. Lee, J. Fidler, T. Schrefl, J. Magn. Magn. Mat. 321, 545 (2009) [3] C.T. Pan and S.C. Shen, J. Magn. Magn. Mat. 285, 422 (2005) [4] F. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, and P. Luches, Acta Materialia 58, 3594 (2010) [5] G. Varvaro, F. Albertini, E. Agostinelli, F. Casoli, D. Fiorani, S. Laureti, P. Lupo, P. Ranzieri, B. Astinchap and A.M. Testa, New J. Phys. 14, 073008 (2012) [6] G. Asti, M. Ghidini, R. Pellicelli, C. Pernechele, and M. Solzi, Phys. Rev. B 73, 1 (2006)