Lattice dynamics and migration enthalpies in CoPt3 and FePd

The frequencies of the normal modes of vibration of CoPt3 and FePd single crystals have been measured using inelastic neutron scattering. The measurements were performed in the L1(2) ordered phase (at 300 and 930 K) for CoPt3 and in the L1(0) ordered phase (at 300 and 860 K) for FePd. Dispersion curves were also measured in the fcc disordered states, at 1060 and 1020 K for CoPt3 and FePd, respectively. The activation enthalpy of atomic migration has been evaluated from the phonon density of states by applying Schober's model [H. R. Schober , J. Phys.: Condens. Matter 4, 9321 (1992)] and its extension to the L1(2) ordered structure. The phonon properties of FePd3 reported in the literature have been analyzed similarly and are compared with the results for CoPt3 and FePd. The contribution of the long-range order to the migration enthalpy estimated in the present analyses agrees well in magnitude with the previous evaluation by Monte Carlo simulation for alloys of the fcc, L1(2), and L1(0) structures

Structural origin of magnetic anisotropy in Co-Pt alloy films probed by polarized XAFS

Polarized X-ray absorption fine structure (XAFS) measurements at the Co K and Pt L3 edges show that the perpendicular magnetic anisotropy found in epitaxial fcc CoPt3 (111) films stems from the existence of anisotropic local ordering. Such ordering, induced during the codeposition process and dependent on the growth temperature, is characterized by preferential CoCo pairs in the film plane, balanced by preferential CoPt pairs out of the plane, resulting from some Co 2D-segregation. Polarized XAFS at the Pt edge reveals similar anisotropic local ordering in epitaxial hcp Co3Pt (0001) films exhibiting a larger magnetocrystalline anisotropy compared to that of bulk hcp Co. Besides, a polarization dependence of the Co XANES profile is observed only for the Co3Pt films exhibiting hcp symmetry