Berlin 2008 – wissenschaftliches Programm
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MM: Fachverband Metall- und Materialphysik
MM 23: Poster session
MM 23.43: Poster
Dienstag, 26. Februar 2008, 14:45–18:00, Poster B
Lattice dynamics of ferromagnetic shape memory alloys from inelastic neutron scattering — •Tarik Mehaddene1, Juergen Neuhaus2, Winfried Petry1,2, Klaudia Hradil2,3, and Philippe Bourges4 — 1Physik-Department E13, Technische Universitaet Muenchen, D-85747 Garching, Germany — 2Forschungsneutronenequelle Heinz Maier-Leibnitz, D-85747 Garching, Germany — 3Institut fuer Physikalische Chemie, Georg-August-Universitaet, D-37077 Goettingen, Germany — 4Laboratoire Leon Brillouin (LLB), CEA Saclay, F-91191 Gif sur Yvette Cedex, France
The tendency of shape memory alloys to undergo a martensitic transtion shows up in the anomalous phonon softening of particular phonon modes in their austenitic phase. We report on phonon measurements in both austenitic and martensitic phases of Ni-based alloys. We show that, contrary to NiMnSn, the anomalous softening of the TA2[110] phonons in NiMnGa is strongly enhanced below the Curie temperature. The measurements revealed that low restoring forces against the shearing of the (111) atomic planes along [11-2] develop upon cooling. The vibrational spectra measured in two different martensitic phases of NiMnGa, namely the tetragonal 5M and the orthorhombic 7M structures, revealed fundamental differences. A dispersive low-energy excitation develops from the elastic modulation peaks in the 5M structure and come into interaction with TA2[110] phonon branch in the q-range 0.25-0.3 r.l.u where the latter shows a wiggle in the dispersion. No wiggle is seen in the TA2[110] branch of the 7M structure. It shows a normal sinus-like dispersion curve.