Anomalies related to theTA2-phonon-mode condensation in the HeuslerNi2MnGa alloy

We present specific-heat, elastic constants, and magnetic susceptibility measurements in a ferromagnetic ${\mathrm{Ni}}_2$MnGa Heusler alloy, in a temperature range where partial condensation of the [110]${\mathrm{TA}}_2$ phonon at q=0.33, accompanied by the development of a micromodulated structure has been reported to occur. All these quantities have been found to exhibit anomalous behavior at the temperature of condensation. From the results presented, it can be concluded that the appearance of the micromodulated structure takes place via a phase transition which is very weakly first order. It is found that at the transition point there is a significant reduction of the dynamical stability of the lattice not just for the transverse phonon at q=0.33, but also for any shear long wavelength distortion. It is particularly remarkable the large softening of C′ (around 60%) at the transition point. Finally, it is also shown that the intermediate transition is related to a magnetoelastic interaction.