Study of magnetization reversal mechanism in exchange-coupled systems: perpendicular Fe/L10-FePt bilayers with variable Fe layer thickness

Bulk and multilayer systems where hard and soft magnetic phases are exchange coupled at the nanometer scale have received a great deal of attention for their potential applications in different technological fields such as permanent magnet, information storage media (exchange coupled media) and magnetic microactuators [1-4]. Composite bilayers constitute a suitable model system to study fundamental properties of nanocomposite magnetic systems being the properties of individual layers relatively easier to control. Different hard and soft materials have been proposed in the literature and the influence on the magnetic behavior of intrinsic and extrinsic properties (such as interface structure, thickness of the soft and hard layer, and layers microstructure) has been extensively studied and debated[2]. The fundamental role played by the soft layer thickness and the film morphology has been experimentally and theoretically proved. An analytical micromagnetic model has been suggested [3] to describe the magnetization reversal of an ideal and infinite Fe/FePt bilayer as a function of thickness and intrinsic properties of both hard and soft phases, and two different regimes (rigid-magnet and exchange spring) were predicted. In this work, we discuss the reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers deposited by sputtering on MgO. The reversal mechanism has been investigated as a function of the soft layer thickness (tFe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, measured by Vector VSM, MFM magnetic domain analysis and numerical micromagnetic simulations. The analytical micromagnetic model proposed in the literature can properly account for some of the observed features of magnetization hysteresis cycles such as a positive nucleation field whose value increases with increasing the soft layer thickness tFe and a reduction of the perpendicular coercive field and remanence with tFe, but it was demonstrated that it cannot satisfactorily describe the hysteretic behavior of real systems. We showed that for thickness of the soft layer larger than 1-2 times the actual interlayer exchange coupling length ?ex (<= (?ex,Fe)Theor = 3.28 nm), numerical micromagnetic calculations are more suitable to reproduce experimental observations, and we were able to give a more complete picture of the reversal mechanism. Actually, just below coercive field, the magnetization reversal does not proceed with a single step switching, as predicted by the model, but with a more complex mechanism: 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] E.F. Kneller and R. Hawig, The Exchange-Spring Magnet: A new material principle for permanent magnets, IEEE Transactions on Magnetics, 27 3588 1991. [2] R. Skomski and J.M.D. Coey, Giant energy product in nanostructured two-phase magnets, Physical Review B, 48 15812 1993 . [3] D. Suess D. Suess, J. Lee, J. Fidler, T. SchreflExchange-coupled perpendicular media, Journal of Magnetism and Magnetic Materials, 321 545 2009. [4] C.T. Pan and S.C. Shen, Magnetically actuated bi-directional microactuators with permalloy and Fe/Pt hard magnet, Journal of Magnetism and Magnetic Materials, 285, 422 2005. [5] F. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, and P. Luches, Role of interface morphology in the exchange-spring behavior of FePt/Fe perpendicular bilayers, Acta Materialia, 58 3594 2010. [6] G. Asti, M. Ghidini, R. Pellicelli, C. Pernechele, and M. Solzi, Magnetic phase diagram and demagnetization processes in perpendicular exchange-spring multilayers, Physical Review B, 73 1 2006.