Magnetic domains and twin microstructure of single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

Magnetic Shape Memory Effect or more precisely magnetic-field-induced-structural reorientation (MIR) generate large strain (up to 12 %) and fast response (around 1 kHz) in a moderate magnetic field below 1 T. The strain is caused by twin microstructure reorientation in which the maximum generated strain is determined by difference of lattice constants of the pseudotetragonal structure a = b > c, i.e. ε_MAX = ε₀ = 1-c/a. The reorientation is mediated by twin boundary motion. Very high mobility is, therefore, necessary condition for the existence of the effect. Recently considering more precise structural description of 10M martensite of Ni-Mn-Ga alloys as monoclinic, i.e. a > b > c and γ π 90, we showed that there are two different kinds of mobile a-c twin boundary, Type I and II with different microstructure [1]. The boundaries differ not only in magnitude of twinning stress needed to move twin boundary, i.e. in mobility, but also in temperature dependence of the mobility or twinning stress [2]. Despite of the simplicity of the moving interface the twinned structure is complex [3]. The model of the movable Type II twin boundary is shown in the figure. The twinned structure consists apart of the moving a-c twin boundary the modulation twinning bands and a-b twinning.