Composites consisting of magnetic shape memory (MSM) particles embedded either in polyester [1] or polyurethane matrices were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni_50.9Mn_27.1Ga_22.0 (at.%) fibres. The crystal structure of the martensite is tetragonal (5M) with c<a=b. Magnetic characterizations of the deformed composites show indirect evidence for stress induced twin boundary motion within the MSM particles, as the compressed composite is easier to magnetize in the direction of compression compared with the directions perpendicular to it. The magnitude of this effect depends on the stiffness of the matrix material used and is therefore more pronounced for polyester than for polyurethane. The texture of all embedded MSM particles in the polyester composite is investigated before and after compression by means of synchrotron radiation [2]. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After 30% compression (height reduction), the MSM particles show a (004)-fibre texture in the direction of compression. This is unambiguous evidence for stress-induced twin boundary motion within the MSM particles. Finally, it is discussed whether it is more advantageous to use a polyurethane matrix in order to observe magnetic field-induced twin boundary motion.

[1] N. Scheerbaum, D. Hinz, O. Gutfleisch, K.-H. Müller, L. Schultz: Acta Mater. 55 (2007) 2707-2713.

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1. Dynamic observation of field induced twin boundary and magnetic domain wall motion in bulk NiMnGa
2. Alternative MSM alloy systems: Ni-Fe-Ga-Co and Ni-Co-Al
3. NiMnGa Fibres for use in polymer composites