Lattice dynamics across the magnetic transition in(Mn,Fe)1.95(P,Si)

Abstract

The lattice dynamics in MnFe 0.95 Si 0.50 P 0.50 were investigated experimentally using 57 Fe nuclear inelastic scattering and inelastic x-ray scattering across the first-order magnetic transition which occurs close to room temperature. The lattice dynamics characterization was supported by a macroscopic magnetic characterization, an x-ray diffraction study, and a hyperfine interactions characterization using Mössbauer spectroscopy. The Fe specific and the x-ray generalized density of phonon states were obtained both in the ferromagnetic and in the paramagnetic state. A prominent shift, 2 meV at 20 meV, in the x-ray generalized density of phonon states across the first-order magnetic transition, that involves vibrations with essentially Fe character, is revealed corroborated by a change in the local environment quantified in the isomer shift and the quadrupole splitting. Above 35 meV the vibrational modes are practically insensitive to the magnetic transition. The entropy change induced by a 1 T magnetic field across the magnetic transition, ∼10 J/K/kg, is only a fraction of the Fe vibrational entropy change, 62(21) J/K/kg.

Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6H1.6

“…6 (c)) are somewhat smaller than those reported in Ref. [49] for magnetocaloric (MnFe) 1.95 (P,Si) (FM: 3661 m s ; PM: 3267 m s ).…”

“…Our results reveal that hydrogen does not only shift the temperature of the first-order transition, but also significantly affects the magnetoelastic response of the Fe subsystem, as observed experimentally and theoretically. We would like to mention that NRIXS investigations on other magnetocaloric materials [49,75], although they also demonstrated a clear redshift in the Fe-partial VDOS across the firstorder phase transition, did not reveal any changes in the shape of the Fe-partial VDOS across the transition. Thus, La(Fe,Si) 13 and La(Fe,Si) 13 H appear to be exceptional materials with respect to strong spin-phonon coupling and strong adiabatic EPI for particular Fe-specific phonon modes.…”

“…The vibrational density of states (VDOS) obtained experimentally from the NRIXS data allows us to determine on the atomic level the average Debye velocity of sound, v D , in LaFe 11.4 Si 1.6 H 1.6 across the magnetostructural phase transition. v D has been determined by Herlitschke et al [75] employing NRIXS on the magnetocaloric material MnFe 4 Si 3 , and by Bessas et al [49] on magnetocaloric (Mn,Fe) 1.95 (P,Si). The velocity of sound is an important physical quantity, which, to the best of our knowledge, surprisingly has not been considered yet in the literature for the important magnetocaloric La(Fe,Si) 13 and La(Fe,Si) 13 -H compounds.…”

“…g(E) E 2 is the reduced phonon DOS. g(E), at low phonon energies E, is quadratic in E. The ratio g(E) E 2 can be described at low energies by a constant called the Debye level [49], which can be determined straight-forwardly from the experimental VDOS g(E) [49,75] in the limit E → 0. ρ in Eq. ( 5) is the mass density of LaFe 11.4 Si 1.6 H 1.6 , which has not yet been reported across the transition in the literature, to the best of our knowledge.…”

“…(iii) Is the magnetostructural first-order FM-to-PM phase transition in the hydrogenated material reflected in the T -dependence of the average velocity of sound, v D , as determined from the NRIXS data? In the literature, this method has been applied to the magnetocaloric (Mn,Fe) 1.95 (P,Si) compound, and v D was shown to be larger in the FM state than in the PM state [49].…”