Precursor effects and premartensitic transformation inNi2MnGa

Inelastic neutron scattering and neutron diffraction were used to study a single crystal of the ${\mathrm{Ni}}_2$MnGa shape memory Heusler alloy in a wide temperature range covering the parent phase ($T>\mathrm{}{T}_1=260\mathrm{}$ K), a premartensitic phase ($T_1>T>\mathrm{}{T}_M$), and a martensitic ($T<\mathrm{}{T}_M=220\mathrm{}$ K) phase region. Several anomalies in the phonon dispersion curves in the parent phase were observed. The premartensitic phase involves a transverse modulation of the parent cubic structure with a simple periodicity of ($\frac{1}{3}, \frac{1}{3}, 0$). The approximately tetragonal lattice of the low-temperature martensite is distorted by transverse modulations with incommensurate wave vectors (${\zeta }_M, {\zeta }_M, 0$), ${\zeta }_M\approx 0.43$. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.