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

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 ${\xi }_0=1\mathrm{/}3\approx 0.33$, 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.