Phonon-softening in the Heusler alloy Ni2MnGa

Complete text of publication follows. Ni2MnGa is the only Heusler alloy which shows a martensitic transformation within the ferromagnetic phase. The transformation temperature TM strongly depends on the stoichiometry of the sample and can be adjusted between ∼175 and ∼450 K. The martensitic transformation is preceded by a strong softening of the TA2 (ζ - ζ 0)-phonons near ζ = 1/3. Therefore this alloy belongs to the very few systems in which the phonon softening is not centred at the Γ = point or at the zone boundary. The phonons of two single crystals were investigated by inelastic neutron scattering at the triple axis spectrometer FLEX at HMI. The two crystals mainly differ in the value of TM. The differences between TM and TC for sample 1 and 2 was 80 and 10 K, respectively. Both samples show a linear increase of the squared phonon energy with temperature near TM. However, the data of sample 1 show a distinct kink at TC. The cause of this behaviour are magneto-elastic interactions (1) which are one important driving force for the transformation (2). (author)