Regensburg 2007 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 17: Magnetic Shape Memory Alloys
MA 17.2: Vortrag
Mittwoch, 28. März 2007, 15:30–15:45, H10
Ab initio determination of symmetry reduced structures by a soft-phonon analysis in Ni2MnGa — •Tilmann Hickel, Blazej Grabowski, Matthé Uijttewaal, and Jörg Neugebauer — Max-Planck-Institut für Eisenforschung GmbH, Postfach 140444, 40074 Düsseldorf, Germany
The ferromagnetic shape memory compound Ni2MnGa undergoes a martensitic phase transition from a high to a low symmetry structure at 200 K. In the low symmetry phase several shuffling structures have been observed experimentally, but their appearance and importance for the shape memory effect are not yet completely understood. In order to identify the stable structures and the phase transition paths, we have calculated free energy surfaces as function of key reaction coordinates (lattice constants, c/a-ratio) in DFT. Due to the large phase space of possible atomic positions and their small formation energies of only a few meV per unit cell, the determination of the shuffling structures is a major challenge in these studies. To overcome this issue, we have developed a scheme which is based on the computation of the phonon spectra using the quasiharmonic approximation. Starting with the symmetric structure, the TA2 phonon dispersion shows a softening along the [110] direction. From the eigenvectors of the unstable phonon modes and by setting up the corresponding modulated harmonics in supercell calculations, we were able to systematically and efficiently identify stable shuffling structures. Using these structures, the effect of symmetry breaking on magnetic properties has been computed and the results have been compared with recent experiments.