Basic research of the influence of real structure on magnetic field induced strain (MFIS) in NiMnGa alloys - Magnetic field induced strain (MFIS) in textured polycrystalline ferromagnetic martensitic NiMnGa alloys

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Single crystals of NiMnGa are reported to show magnetic field induced strain (MFIS) due to twin boundary motion (TBM). This behavior is found in a low temperature martensitic phase, which forms from a cubic L21 phase ordering around 750 °C upon cooling. Prerequisites for MFIS are a uniaxial magnetic anisotropy and highly mobile twin boundaries. Both features depend not only on composition but also on structure and microstructure. The goal of the present project is to establish the conditions under which MFIS by TBM can be achieved in polycrystalline NiMnGa. The results will be used to prepare improved magnetically driven actuators by efficient preparation routes. Later, the results of this study will be applied to novel alloy systems to further improve ferromagnetic shape memory elements. At the beginning, the preparation of highly textured large grained polycrystalline samples of such compositions will be developed, which are reported to show MFIS by TBM in single crystals. Two routes of preparation will be studied, directional solidification and recrystallization after deformation. Then the rules to treat such polycrystals to achieve MFIS will be established. From preliminary studies it is expected that multi-step thermo-mechanical and/or thermo-magnetic treatments (training) will be necessary. Finally, the elements of the microstructure shall be identified, which promote resp. prevent MFIS by TBM. This step requires extended cooperation within the priority program SPP1239, which means that well defined samples of this project will be provided for elaborated structural characterization.