Istituto dei materiali per l'elettronica ed il magnetismo     
Casoli F., Albertini F., Bolzoni F., Cabassi R. Magnetisation reversal in exchange-coupled FePt/Fe bilayers with different interface morphologies. In: Joint European Magnetic Symposia 2008 (Dublino, Irlanda, 14 - 19 settembre 2008).
Exchange-coupled bilayers, composed of a hard and a soft layer, have been proposed for next generation recording media [1]. They would in fact allow high anisotropy values, which warrant thermal stability, and could contemporarily exploit the hard-soft coupling to reduce the switching field. We have investigated perpendicular exchange-coupled bilayers made of a magnetically soft Fe layer (2 and 3.5 nm) over a hard FePt(001) layer (10 nm), and considered the effect of the extrinsic properties in the magnetic behaviour. We have taken into account the role of the hard layer morphology, which has been modified in a controlled way by exploiting growth temperature and in situ annealing. The bilayers have been grown based on L1_0-FePt epitaxial layers with high degree of chemical order (S≥0.76) and different morphologies. The differences concern both grains shape and size, with changes from interconnected maze-like grains to separated grains of reduced size. The resulting hard layer anisotropy is high (K>110^7 erg/cm^3), and the coercivity is increased by the grains separation (from 1.7 to 3 T). In the FePt/Fe bilayers the coercivity HC is strongly reduced compared to the hard layer value HChard (HC/HChard down to 0.37), indicating that high anisotropy perpendicular systems with moderate coercivity can be easily obtained [2]. Moreover, the hysteresis loops show that the magnetic regime passes from rigid magnet to exchange-spring when FePt growth evolves from quasi-continuous to island like, as expected for a reduction in the exchange-coupling between the hard and soft phases. In order to deepen the investigation of the magnetisation process as a function of the interface morphology, we have measured recoil curves of the samples by anomalous Hall voltage measurements. At fixed thickness of the soft layer, the reversible and irreversible magnetisation contributions during the reversal process strongly depend on the interface morphology, due to its influence on the magnetic phase diagram of the bilayer system. [1] R. H. Victora and Xiao Shen, IEEE Trans. Magn. 41 537 (2005) [2] F. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, and C. Bocchi, Appl. Phys. Lett. 92 142506 (2008)
Subject Exchange-spring magnets
Fe-Pt alloys
Exchange-coupled media

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