Effetto del campo magnetico nella dispersione fononica di la lega magnetica con memoria di forma Ni2MnGa

The subject of this thesis is the study of the effect of magnetic field in the phonon dispersion of the ferromagnetic shape memory alloy Ni2MnGa. As well as other shape memory metals, this material presents a martensitic phase transformation that gives origin to the shape memory effect. At temperatures above 370 K the material is in a paramagnetic and paraelastic state. It shows a high temperature austenitic phase with an L21 FCC structure. Below this temperature the material becomes ferromagnetic with <100> as the easy magnetization axes. If the temperature is lowered below 260 K the material undergoes a pre-martensitic transformation that consists in a modulation characterized by a shuffling of the {110} planes in the {110} direction with a periodicity of six atomic planes. According to the previous publications the phase can be detected by neutron diffraction where super-structure peaks are found at 1/6 of the distance between the austenitic Bragg reflections. Interestingly, this material shows an anomaly in one of its acoustic phonon branches, the TA_2 (transversal acoustic 2) in the {&#958;&#958;0} direction. This branch is closely related with the pre-martensitic transformation. At q=(1/3 1/3 0) the phonon branch shows a softening. This means that the energy of the phonon is lower than what is expected from a harmonic, and stable structure. Several studies analyze the dependance of this dip with temperature and external stress* No previous work is found in the literature that studies the effect of the magnetic field in the phonon dispersion. At room temperature, this material (in stoichiometric composition) is ferromagnetic and in its austenitic high symmetry phase. Strictly speaking, however, the magnetization breaks the cubic symmetry. This study determines if this magnetic anisotropy is reflected in the phonon dispersion. It is expected that the relative orientation between the magnetization and the phonon propagation would directly affect the energy of the phonon in mean value and spread. The experiments performed consist in doing energy scans at constant Q, corresponding to q=(1/3 1/3 0) around different Bragg peaks with and without magnetic field. Also for different relative orientations between the phonon propagation and the magnetization denominated ¿favorable¿ and ¿non favorable¿ depending on whether they are correspondent with the pre- martensitic transformation or not. The results show no shift in the energy of the phonon, nor in its spread. Instead an increase in the spectral weight was observed. The intensity of the measurements with magnetic field is found to be higher than without magnetic field but only for the ¿favorable¿ case. The experiment is repeated with a simplified experimental set up to be able to recreate the results. While the phonon again does not change in energy nor in spread, the intensity does not shows a clear increase in intensity for the favorable case. Surprisingly for the non favorable condition the phonon disappears under the magnetic field. An hypothesis of a complete elimination of the anomaly, that would result in a splitting of the acoustic branch is proposed but is not consistent with the previous experiments.