High mobility of twin boundary is crucial for magnetic shape memory effect. The twin boundary can be moved by applied magnetic field or mechanical stress. In Ni-Mn-Ga 10M martensite there are two different, field movable, a-c twin boundaries type I and II due to monoclinic lattice. For single twin boundary of both types we experimentally evaluated the equivalence of magnetic and mechanical force and the validity of generally used energy model using direct stress-strain and magnetization measurements. For type II, highly mobile twin boundary, the equivalence seems to be valid and model broadly agrees with measurement. However, for type I the calculated magnetic stress is much larger than mechanical stress needed for twin boundary motion.
We investigated the effect of large compressive stress on magnetic shape memory effect in modulated 10M martensite of Ni-Mn-Ga Heusler alloy. The single crystalline sample deformed approximately elastically up to highest load 540 MPa. Structural and microstructural changes of a single crystal were analysed by X-ray diffraction 2D scan mapping in 2θ and ω. Although the crystal structure, lattice cell parameters, and twinned microstructure (a/b and modulation twinning) exhibited only small changes after the loading, the volume of the sample undergoing the magnetically induced reorientation decreased sharply while magnetic field necessary for the reorientation gradually increased. The extrapolation suggested that no magnetically induced reorientation might occur after compression of about 1 GPa.
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