The softening of the TA2 phonon branch at [ξξ0] 2π/a, ξ ≅ 1/3, as seen through inelastic neutron scattering experiments, is well known to be associated to the Parent (P) → Intermediate (I) phase, preceding the martensitic transformation (MT) on cooling in Ni2MnGa alloys. There is still some controversy on the magnetic field dependence of both P → I and MT temperatures in these alloys, nevertheless the effect of magnetic field on the TA2 softening has never been analysed. In this work, preliminary results of the P → I softening under a magnetic field are presented.
Neutron scattering experiments were carried out at the Institute Laue-Langevin, at IN3 triple axis spectrometer. A stoichiometric Ni2MnGa single crystal (  axis perpendicluar to the scattering plane) has been studied in the 220 - 260 K temperature range. The measurements without applied magnetic field reproduce the softening as reported in "Phonon anomaly, central peak, and microstructures in Ni2MnGa", A. Zheludev et al., Phys. Rev B 51 (1995) 11310; the minimum in the energy being at T = 240 K for q ≅ [0.35, 0.35, 0], thus close to ξ = 1/3 as expected. At lower temperatures, i.e. 230 and 220 K, the value at the minimum increases and the dip in the branch is less pronounced. Under the application of a magnetic field of 2 T along , the branch at 230 K approaches that of 240 K, both being closer to the 230 K branch without applied magnetic field. Therefore a slight decrease in the P → I temperature under the application of the field could be inferred, although this is a preliminary qualitative estimation. Additional experimental data, extending both the field and temperature ranges are required in order to reach a quantitative estimation of the magnetic field effect.