Magnetocrystalline anisotropy in L10 FePt and exchange coupling in FePt/Fe3Pt nanocomposites

The magnetic and structural properties of Fe-Pt nanocomposites and related idealized structures have been investigated by a combination of experimental and theoretical techniques. The dependence of magnetocrystalline anisotropy (MCA) of L1(0) FePt on the ratio of the tetragonal lattice parameters, c/a, has been calculated with a relativistic version of the full potential local orbital method, assuming complete chemical order and fixed unit-cell volume. It has been found that the well known tetragonal lattice distortion in this phase has a relatively small influence on the MCA (compared to the influence of chemical ordering) and even reduces the MCA. The calculated in-plane anisotropy is negligible. The structure, magnetic properties and magnetization reversal processes of Fe100-xPtx (x = 40, 45, and 50) powders produced by mechanical milling and subsequent annealing have been investigated. Structural studies reveal that upon annealing of the as-milled powders consisting of fine Fe/FePt (A1)/Pt lamellae, chemically highly ordered L1(0) FePt and, in the case of the Fe-rich compositions, L1(2) Fe3Pt are formed. The nanometre scale multilayer structure preserved after annealing gives rise to large effects of exchange interactions between the crystallites of the phases. With decreasing Pt concentration x, the remanence enhancement increases, due to the increase of the Fe3Pt fraction, whereas the coercivity and the switching fields for irreversible magnetization reversal are reduced.