Lattice dynamics and migration enthalpies inCoPt3and FePd

Abstract

The frequencies of the normal modes of vibration of CoPt 3 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 K͒ for CoPt 3 and in the L1 0 ordered phase ͑at 300 and 860 K͒ for FePd. Dispersion curves were also measured in the fcc disordered states, at 1060 and 1020 K for CoPt 3 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 et al., J. Phys.: Condens. Matter 4, 9321 ͑1992͔͒ and its extension to the L1 2 ordered structure. The phonon properties of FePd 3 reported in the literature have been analyzed similarly and are compared with the results for CoPt 3 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.

Phonons in disordered alloys: Comparison between augmented-space-based approximations for configuration averaging to integration from first principles

“…[ζζζ] The filled circles are the experimental data [41]. The force constants used are given in Table I.…”

“…It was therefore interesting to test the suitability of both the approximations in case of such a system where one of the constitutents forming the alloy has a different structure than the alloy itself in it's elemental phase. second, inelastic neutron scattering data was available for Fe 50 Pd 50 [41]. It would therefore have been possible to compare the ICPA and the ASR results with the experimental data directly enabling the understanding of the nature of interactions between various pairs of species in the random phase.…”

Saddle-point energies and Monte Carlo simulation of the long-range order relaxation in CoPt

“…Nevertheless, qualitative comparison to CoPt 3 , in which the migration energy was calculated using both Schober's and Flynn's models, can be made. Schober's model initially developed for bcc and fcc pure metals [10] has been extended to the A 3 B compounds with L1 2 structure [11] and applied to CoPt 3 [53]. Averaging the values of the migration energy over the different kinds of atomic jumps, we get a migration energy of 1.3 eV at 300 K. Flynn's model [9] was applied to CoPt 3 using the elastic constants deduced from the slope of the phonon dispersion curves at the center of the Brillouin zone [53].…”

“…Schober's model initially developed for bcc and fcc pure metals [10] has been extended to the A 3 B compounds with L1 2 structure [11] and applied to CoPt 3 [53]. Averaging the values of the migration energy over the different kinds of atomic jumps, we get a migration energy of 1.3 eV at 300 K. Flynn's model [9] was applied to CoPt 3 using the elastic constants deduced from the slope of the phonon dispersion curves at the center of the Brillouin zone [53]. A migration energy of 0.95 eV was found in the fcc-disordered state at 1060 K. The difference in the amplitude between the calculated value of the migration energy in CoPt and the values deduced from Schober's and Flynn's models in CoPt 3 might be explained by the atomic mass and size effects.…”

Monte Carlo simulations of the L10long-range order relaxation in dimensionally reduced systems

“…Atomic migration has been investigated using experimental resistivity measurements and lattice dynamics to deduce the atomic migration energies at different temperatures and states of atomic order. The study is almost completed in FePd [6,7,8,10] whereas, the other two systems are still under investigation. In parallel to these experimental investigations, numerical simulations of the thermodynamical properties and structural kinetics have been performed using Monte Carlo methods and molecular dynamics [5,9].…”