Lattice dynamics in ferromagnetic shape memory alloys

Lattice dynamics in ferromagnetic
shape memory alloys

Would you be able to deform a piece of metal up to 11% of its length just by pressing it with fingers or applying a magnetic field? Yes ! if this is a magnetic shape memory alloy. These are smart materials able to recover large strain under moderate applied magnetic fields. Nibased alloys such as Ni2MnGa are typical candidates for such a use. Prerequisite for the occurrence of a shape memory effect is a structural transformation from a hightemperature phase (austenite) to a lowtemperature phase (martensite). This transformation is due to lowrestoring forces against specific atomic vibrations (phonons) easily identified from their anomalous softening with decreasing temperature.

Fig.: Isointensity map on a logarithmic scale measured at room temperature in a single variant martensitic Ni2MnGa sample along [110] in a transverse polarisation. We clearly see the acoustic phonon branch (W∝q) and the spinwave (magnon, W∝q2 ). The high elastic signal (red) arises from the incoherent scattering of Ni. It develops to a well defined peak around 0.28 r.l.u which represents the elastic peak of the modulated structure. An extra inelastic scattered intensity merging from the elastic modulation peak is seen (dotted lines). Its origin is still an open issue and has to be addressed.

We propose, in the frame of this Diplomarbeit, the investigation of the lattice dynamics in the austenitic and martensitic phases of different Nibased shape memory alloys by means of inelastic neutron scattering on single crystals. This work aims to address open questions regarding the interplay of the magnetic properties and the vibration degrees of freedom. The measurements will be performed using the threeaxis spectrometers PUMA and PANDA at the Forschungsneutronenquelle Heinz MaierLeibnitzin Garching. A part of the experiments will eventually be performed at the Laboratoire Léon Brillouin in Saclay (France). This work is a part of a priority program (SPP1239, http://www.magneticshape.de/) of the Deutsche Forschungsgemeinschaft entitled: change of the microstructure and form of solid materials by external magnetic fields. The work is topic to a close collaboration with the group of Dr. Schneider (HMI, Berlin) for the sample preparation together with Prof. Entel's and Prof. Acet's groups from Duisburg University for the firstprinciple calculations and the magnetic properties.

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