Magnetic field applied in the direction of the hard magnetization axis causes rotation of the lattice (twinning) of tetragonal NiMnGa martensite single crystal in such a way that easy axis of magnetization becomes aligned with the field and the crystal changes its dimension resulting in magnetic field induced strain (MFIS) reaching theoretically ~6%. Twinning interfaces move only if the magnetocrystalline anisotropy energy is larger than the twinning stress [1]. The reverse reorientation is achieved by mechanical action of elastic spring when the magnetic field is switched off. This is well known as the physical principle of MSM actuator. It is known to much lesser extent that, in order to optimize the actuation functionality, the NiMnGa crystal has to be first “trained”. It is not clear why this is needed and what the training actually does to the material.

In this work, the effect of training on magnetic actuation was investigated. Training of the NiMnGa martensite single crystal was performed by successive compressive deformations on two different faces of cuboid sample. When magnetic field was applied along the loading directions of the training, MFIS of ~5% was observed. No MFIS was observed by application of the magnetic field in the third perpendicular “untrained” direction, though it should be so, theoretically. The influence of the training effect and the number of compressive cycles needed to observe the MFIS is studied. Observed results are confronted with the measured characteristics of stress-strain curves.

[1] J. Enkovaara, A. Ayuela, A. T. Zayak, P. Entel, L. Nordstrom, M. Dube, J. Jalkanen, J. Impola, R. M. Nieminen, Mat. Sci. Eng. A 378 (2004) 52-60.

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