Development and structural characterization of exchange-spring-like nanomagnets in (Fe,Co)-Pt bulk nanocrystalline alloys

FePt-based alloys are currently under scrutiny for their possible use as materials for perpendicular magnetic recording. Another possible application is in the field of permanent magnets without rare-earths, magnets that may operate at higher temperatures than the classic Nd-Fe-B magnets. Within this study, FeCoPt alloys prepared by rapid solidification from the melt are structurally and magnetically characterized. In the as-cast FeCoPt ribbons, a three-phase structure comprising well-ordered CoFePt and CoPt L1(0) phases embedded in a disordered fcc FePt matrix was evidenced by XRD, HREM and SAED. Extended transmission electron microscopy analysis demonstrates the incipient formation of ordered L1(0) phases. X-ray diffraction was used to characterize the phase structure and to obtain the structural parameters of interest for L1(0) ordering. In the as-cast state, the co-existence of hard magnetic CoFePt and CoPt L1(0) tetragonal phases with the soft fcc FePt phase is obtained within a refined microstructure made of alternatively disposed grains (grain sizes from 1 to 7 nm). Following a thermal treatment of 1 h at 670 degrees C, the soft magnetic fcc matrix phase transforms to tetragonal L1(0) phases (disorder-order transition). The resulting CoPt and CoFePt L1(0) phases have grains of around 5-20 nm in size. In the as-cast state, magnetic measurements show a quite large remanence (0.75 T), close to the value of the parent L1(0) FePt phase. Coercive fields of about 200 kA/m at 5 K were obtained, comparable with those reported for some FePt-based bulk alloys. Upon annealing both remanence and coercivity are increased and values of up to 254 kA/m at 300 K are obtained. The polycrystalline structure of the annealed FeCoPt samples, as well as the formation of multiple c-axis domains in different CoPt and CoFePt regions (which leads to a reduction of the magneto-crystalline anisotropy) may account for the observed coercive fields that are lower than in the case of very thin FeCoPt films. A Curie temperature of about 820 K (close to 550 degrees C) is reported for the Fe35Co15Pt50 alloy which opens wide possibilities for the use of such magnets in high operating temperature industrial applications. The present results indicate that ternary FeCoPt alloys hold a great potential as a novel class of rare earth free exchange-spring coupled nanocomposite magnets. (C) 2015 Elsevier B.V. All rights reserved.