Lattice dynamics and external magnetic-field effects in Ni-Fe-Ga alloys

Abstract

Precursor phenomena were investigated in a Ni-Fe-Ga alloy close to the stoichiometric Heusler composition ${\text{Ni}}_{2}\text{FeGa}$. In particular, the phonon-dispersion curves, the diffuse scattering and the magnetic properties of a single crystalline ${\text{Ni}}_{51.5}{\text{Fe}}_{21.5}{\text{Ga}}_{27}$ alloy were measured as a function of temperature. The ${\text{TA}}_{2}$ branch along the [110] direction of the $\text{L}{2}_{1}$ phase shows a significant phonon softening around $\ensuremath{\xi}=0.35$ resulting in a marked dip which becomes more pronounced as the temperature decreases. Diffuse neutron-scattering measurements performed along $[\overline{\ensuremath{\xi}}\ensuremath{\xi}0]$ direction around Bragg reflections also reveal the presence of small satellite peaks at $\ensuremath{\xi}=0.33$ whose intensity increases on approaching the martensitic transformation temperature. Both elastic and inelastic-scattering anomalies confirm the occurrence of premartensitic phenomena in Ni-Fe-Ga alloys. The influence of an external magnetic field (6 T) on the anomalous phonon is shown to be negligible and just a small shift of the transformation temperature takes place because of the magnetic field.