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

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 ${\text{Ni}}_{50}{\text{Mn}}_{34}{\text{In}}_{16}$ magnetic superelastic composition. The paper reports on 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 ${\text{TA}}_{2}$ branch are relatively low and it exhibits a small dip anomaly at a wave number ${\ensuremath{\xi}}_{0}\ensuremath{\approx}1/3$, which softens with decreasing temperature. Associated with the softening of this phonon, we also observed the softening of the shear elastic constant ${C}^{\ensuremath{'}}=({C}_{11}\ensuremath{-}{C}_{12})/2$. 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 $⟨1\overline{1}0⟩$ directions. Additionally, we measured low-lying phonon-dispersion branches and elastic constants in applied magnetic fields aimed to characterize the magnetoelastic coupling.