Effect of magnetic field on the TA2 phonon branch softening in Ni2MnGa

Vicente Recarte 1José Ignacio Pérez-Landazábal 1Eduard Cesari 2Volodymyr A. Chernenko 3Mónica Jiménez-Ruiz 4

1. Depto. de Física, Universidad Pública de Navarra (UPN), Campus de Arrosadía, Pamplona E-31006, Spain
2. Dept. de Física, Universitat de les Illes Balears (UIB), Palma de Mallorca E-07122, Spain
3. Istituto per l'Energetica e le Interfasi Consiglio Nazionale delle Ricerche Unità di Lecco (IENI Lecco), Corso Promessi Sposi, 29, Lecco 23900, Italy
4. Institut Laue Langevin (ILL), Avenue des martyrs, Grenoble 38042, France

Abstract
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 ( [100] 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 [100], 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.

 

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Presentation: Poster at E-MRS Fall Meeting 2007, Symposium E, by Eduard Cesari
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-04-23 16:06
Revised:   2009-06-07 00:44