Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys

TL;DR

This study uses neutron scattering to map phonon behavior in Ni-Mn-Al Heusler alloys, revealing low-lying TA2 phonon modes, anomalies at specific wave numbers, and magnetic satellite features, advancing understanding of lattice dynamics.

Contribution

First experimental determination of low-lying phonon dispersion curves in Ni-Mn-Al Heusler alloys, identifying phonon softening and anomalies related to magnetic ordering.

Findings

TA2 phonon modes are relatively low in frequency.

An anomaly (dip) at wave number 1/3 softens with decreasing temperature.

Presence of magnetic ordering satellites.

Abstract

Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni$_{54}$Mn$_{23}$Al$_{23}$ Heusler alloy over a broad temperature range. The paper reports the first experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA$_2$ modes are relatively low. This branch exhibits an anomaly (dip) at a wave number $\xi_{0} ={1/3}\approx 0.33$, which softens with decreasing temperature. Associated with this anomalous dip at $\xi_{0}$, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.