Inelastic neutron scattering and transmission electron microscopy have been used to study a single crystal of the ${\mathrm{Ni}}_2$ MnGa shape-memory ferromagnetic Heusler alloy in a wide temperature range above the martensitic phase transformation at ${\mathit{T}}_{\mathit{M}}$=220 K. Significant, though incomplete, softening in the [ζζ0] ${\mathrm{TA}}_2$ phonon branch has been observed at a wave vector ${\mathrm{\zeta }}_0$≊0.33. The anomaly in the dispersion curve is shown to persist at high temperature, even above the Curie point. A temperature-dependent peak in the elastic diffuse scattering is also present at the same wave vector ${\mathrm{\zeta }}_0$, which develops into a Bragg peak representative of an intermediate phase between the high-temperature Heusler and the low-temperature martensitic structures.

• Received 12 December 1994

DOI:https://doi.org/10.1103/PhysRevB.51.11310