Precursor effects and premartensitic transformation in Ni2MnGa.

Abstract

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 ($Tg{T}_{1}=260$ K), a premartensitic phase (${T}_{1}gTg{T}_{M}$), and a martensitic ($Tl{T}_{M}=220$ 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 (${\ensuremath{\zeta}}_{M}, {\ensuremath{\zeta}}_{M}, 0$), ${\ensuremath{\zeta}}_{M}\ensuremath{\approx}0.43$. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.