Lattice dynamics in magnetic superelastic Ni-Mn-In alloys. Neutron scattering and ultrasonic experiments

TL;DR

This study investigates the lattice dynamics of Ni-Mn-In alloys near the magnetic superelastic composition using neutron scattering and ultrasonic techniques, revealing phonon softening and magnetoelastic effects related to martensitic transformations.

Contribution

It provides the first detailed experimental phonon dispersion curves and elastic constants for Ni-Mn-In alloys, highlighting phonon softening and magnetoelastic coupling near the martensitic transition.

Findings

TA2 phonon branch exhibits a dip anomaly at ξ₀ ≈ 1/3 that softens with decreasing temperature.

Shear elastic constant C' softens as temperature decreases, indicating lattice instability.

Magnetic fields influence phonon dispersion, demonstrating magnetoelastic coupling.

Abstract

Neutron scattering and ultrasonic methods have been used to study the lattice dynamics of two single crystals of Ni-Mn-In Heusler alloys close to Ni$_{50}$Mn$_{34}$In$_{16}$ magnetic superelastic composition. The paper reports the experimental determination of the low-lying phonon dispersion curves and the elastic constants for this alloy system. We found that the frequencies of the TA$_{2}$ branch are relatively low and it exhibits a small dip anomaly at a wave number $\xi_{0} \approx 1/3$, which softens with decreasing temperature. Associated with the softening of this phonon, we also observed the softening of the shear elastic constant $C'=(C_{11}-C_{12})/2$. Both temperature softenings are typical for bcc based solids which undergo martensitic transformations and reflect the dynamical instability of the cubic lattice against shearing of $\{110\}$ planes along $<1\bar{1}0>$ directions. Additionally, we measured low-lying phonon dispersion branches and elastic constants in applied magnetic fields aimed to characterize the magnetoelastic coupling.