Istituto dei materiali per l'elettronica ed il magnetismo     
Casoli F., Albertini F., Nasi L., Fabbrici S., Pareti L., Cabassi R., Bolzoni F., Bocchi C., Asti G., Ghidini M., Pellicelli R., Pernechele C., Solzi M. Perpendicular exchange-spring bilayers based on L10-FePt: magnetic behaviour as a function of the soft layer thickness and hard layer morphology. In: 14th Workshop on Magnetism and Intermetallics (Coimbra, Portogallo, 14 - 15 febbraio 2008).
Exchange-coupled hard/soft bilayers are nowadays considered a very promising system for perpendicular magnetic recording, allowing in principle high anisotropy values, which warrant thermal stability, while maintaining moderate switching fields [1,2]. More generally, the exchange-spring architecture constitutes an interesting approach for the realization of nanostructured permanent magnets with high-performances. As a first approach to perpendicular exchange-spring systems, we have realized bilayers made of a magnetically soft Fe layer over a hard FePt(001) layer and studied their magnetic behaviour as a function of Fe thickness (t_Fe). The characteristics of different magnetic regimes have been identified, in agreement with the predictions of an analytical micromagnetic model, which has been developed to trace the magnetic phase diagram as a function of hard and soft layer thickness. With FePt thickness fixed to 10 nm, two different coupling regimes are found on increasing t_Fe from 1.5 to 10 nm: the Rigid Magnet (RM) and Exchange-Spring (ES). The RM regime is characterized by the irreversible and simultaneous inversion of the two phases, while the ES regime shows a reversible portion of the demagnetizing curve due to the spring-like rotation of magnetic moments in the soft layer. In order to study in depth the effect of the nanoscale structure on the hard/soft coupling, we have prepared three bilayers series with different morphologies of the FePt layer. Each series is constituted of an epitaxial L1_0-FePt layer of 10 nm, and two bilayers obtained by depositing Fe layers of 2 and 3.5 nm on the FePt layer. The three series differ in the growth and annealing characteristics of the L1_0-FePt layer, which lead to different morphologies. The differences concern both grains shape and size, with changes from interconnected maze-like grains to regular, separated grains, and reduction in the average grain size from 55 to 45 nm. The thickness of the Fe layer has been chosen with the aim of exploring both the RM and ES regimes. In all the bilayers series the FePt layer has a high degree of chemical order (S≥0.8) and thus a large value of uniaxial anisotropy (Keff>110^7 erg/cm^3), while the coercivity is increased by the grains separation. Independently of the hard layer morphology, the deposition of the Fe layer strongly reduces the coercivity of the FePt layer. Moreover, the control of the hard phase morphology allows to modify the magnetic regime at fixed t_Fe (from RM to ES), due to the nanoscale structure effect on the hard/soft coupling, and to tailor the hysteresis loop characteristics. The investigated samples have been grown by RF sputtering and characterized by X-Ray Reflectivity and Diffraction, Atomic Force and Transmission Electron Microscopies, Alternating Gradient Force Magnetometry and Anomalous Hall Effect Voltage measurements. [1] R. H. Victora and Xiao Shen, IEEE Trans. Magn. 41, 537 (2005) [2] D. Suess, T. Schrefl, R. Dittrich, M. Kirschner, F. Dorfbauer, G. Hrkac, J. Fidler, J. Magn. Magn. Mater. 290-291, 551 (2005)
Subject Fe-Pt alloys
Exchange-spring magnets

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