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 Ni54Mn23Al23 Heusler alloy over a broad temperature range. The paper reports the 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 approximate to 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.