First-principles study of lattice instabilities in ferromagnetic L1₂ Fe₃Ni: direct force constants method versus linear response

Abstract

The phonon-dispersion relations for ferromagnetic Fe 3 Ni in the L1 2 structure are computed along the high symmetry directions using two powerful first-principles techniques: direct force constants method, which is used in conjunction with a standard ab-initio method, and the linear response approach that is based entirely on the density-functional-perturbation theory. The two methods have been applied with success to reveal the experimental observations that characterize the Invar problem in Fe 3 Ni most especially in the [110] direction. They also help to shed more light on the additional softening of the TA mode being speculated in [111] direction.

Lattice dynamics and structural stability of orderedFe3Ni,Fe3PdandGruner1, Adeagbo2, Zayak3 et al. 2010Phys. Rev. BSelf Cite

“…Although the technical parameters are less tight, the obtained dispersions agree well on a qualitative level with the ones obtained within the direct approach (except for discrepancies which can be partially related to the use of LDA in the latter case as the typical underestimation of the lattice constant as well as an extension of the instability of the TA 1 branch to the Γ-point, see also Ref. 65) and are therefore not explicitly presented in this manuscript.…”

“…5 agrees well with previous ab initio results for the same material obtained by the same method (but less restrictive accuracy). 65 Differences are mainly present at the M-point, where the complete softening of the TA 1 mode is less pronounced in the present study and slightly shifted away from the Brillouin zone boundary. In addition, the softening in [111] direction is more pronounced in our study.…”

Phonon density of states of self-assembled isolated Fe-rich Fe-Pt alloy nanoclusters

“…[12][13][14] Thus, detailed knowledge of the phonon spectrum is essential for the understanding of Invar-related properties. 15 Recently, Fe 3 Pt nanowire arrays ͑with Invar composition͒ have been fabricated. 16,17 The fundamental question arises whether the Invar effect and the related martensitic transition are suppressed in Fe 3 Pt ͑or other Invar-type͒ nanostructures due to finite-size effects.…”

“…21 An essential prerequisite for the occurrence of the martensitic transformation in Invar alloy NCs is the existence of the ͓110͔ TA 1 phonon mode, since softening of this mode is the origin of this instability in the bulk. 3,[5][6][7][8]15 Therefore, a fundamental question is whether the ͓110͔ TA 1 mode still exists in Invar-alloy NCs. This is not clear, since in isolated metallic NCs, due to their large surface-to-volume ratio, distinct modifications of the phonon spectrum as compared to the bulk PDOS have been predicted in Refs.…”

Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys

“…For the systems under consideration, experimentally determined phonon dispersion relations obtained by inelastic neutron scattering are available for disordered Fe Fe 3 Pt. 21,[23][24][25][26][27][28] Theoretical calculations of the full phonon dispersion, on the other hand, have been reported so far mainly for ordered Fe 3 Ni, 65,81 , disordered Fe 72 Pd 28 , 85,86 and Fe-Pd alloys in the Pd-rich composition range. [87][88][89] Several of the above mentioned investigations rely on empirical or semi-empirical descriptions of the interatomic forces.…”