Twin interaction and large magnetoelasticity in Ni-Mn-Ga single crystals

We demonstrate experimentally the existence of triple twins in Ni-Mn-Ga magnetic shape memory single crystals with a modulated five-layered martensite structure using optical observations andx-ray diffraction. Subsequently, we investigate the response of the crystals with triple-twin segments to compressive loading up to several MPa. Such loading typically resulted in an abrupt rearrangement of the twin microstructure to a configuration with many fine twins (1–10 µm in size) ending at a twin boundary. This type of twin microstructure exhibited recoverable deformation with up to 0.3% macroscopic strain and an estimated 2.5% local strain, while the recoverable strain was much smaller for other studied microstructure configurations. The results indicate that by the creation of a suitable twin microstructure, the originally pseudoplastic or magnetoplastic material can be made rubberlike elastic or magnetoelastic with the macroscopic recoverable strain comparable to 2.5%.