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 such as permanent magnets [1], information storage [2] and magnetic micro actuators and systems (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 [4]. In the present work we discuss the reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers deposited by sputtering on a single crystal MgO (100) substrate. The magnetization process was investigated as a function of the soft layer thickness (tFe) combining magnetization loops at variable angle, MFM magnetic domain analysis and numerical micromagnetic simulations. The analytical micromagnetic model recently proposed by Asti and coworkers [5] can properly account for some of the observed features of magnetization hysteresis loops such as a positive nucleation field whose value increases with increasing tFe and a reduction of the perpendicular coercive field and remanence with tFe, but cannot satisfactorily describe the hysteretic behavior of real systems. We show that for thickness of the soft layer 1-2 times the actual interlayer exchange coupling length ?ex ( = 3.28 nm), numerical micromagnetic calculations is needed to reproduce experimental observations, showing that magnetization reversal process just below the coercive field involves the formation and evolution of magnetic domains whose magnetization is directed close to the surface normal for the hard layer and close to the film plane for the soft layer and rotation of the 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, JMMM 321, 545 (2009) [3] C.T. Pan and S.C. Shen, JMMM 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. Asti, M. Ghidini, R. Pellicelli, C. Pernechele, and M. Solzi, Phys. Rev. B 73, 1 (2006)