We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature $K_1(\mathrm{rt}{\text{)=1.65×10}}^5\hspace{0.17em}{\mathrm{J m}}^{\text{−3}}$ and $K_2$ is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. $K_1(\mathrm{rt}{\text{)=1.7×10}}^5\hspace{0.17em}{\mathrm{J m}}^{\text{−3}}$ and $K_2(\mathrm{rt}{\text{)=0.9×10}}^5\hspace{0.17em}{\mathrm{J m}}^{\text{−3}}$ referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with $K_1(\mathrm{rt}{\text{)=−2.3×10}}^5\hspace{0.17em}{\mathrm{J m}}^{\text{−3}}$ and $K_2(\mathrm{rt}{\text{)=0.55×10}}^5\hspace{0.17em}{\mathrm{J m}}^{\text{−3}}.$ The temperature dependence of $K_1\mathrm{(T)}$ of five-layered martensite follows magnetization power law with exponent $\text{n=3}$ suggesting a single ion origin of the magnetic anisotropy in Ni–Mn–Ga martensite.

We study the temperature dependence and low and high temperature limits of the magnetic shape memory effect (MSME) in five-layered tetragonal Ni–Mn–Ga martensite. Using a simple model we show that, additionally to the limits posed by transformation to austenite or an intermartensitic transformation, the temperature dependence of the magnetic anisotropy, tetragonality of the lattice, and twinning stress play important role when considering the temperature limits of the MSME. With decreasing temperature, the lattice distortion and magnetic anisotropy increase, but saturate in a low temperature region. The twinning stress does not saturate and its temperature dependence has exponential-like character that increases rapidly in the low temperature region. The model predicts that the low-temperature limit of the MSME is 165 K for ${\mathrm{Ni}}_{\text{49.7}}{\mathrm{Mn}}_{\text{29.1}}{\mathrm{Ga}}_{\text{21.2}}$ composition. This agrees very well with the value of 173 K determined from direct measurements. The high temperature limit is transformation to austenite at 315 K. The interval of the MSME existence is therefore 173–315 K. Quasistatic measurements of the MSME in the range 203–313 K up to the 1.15 T magnetic field show that the onset of the MSME shifts to a higher field and that a maximum field-induced strain increases with decreasing temperature.

Ni–Mn–Ga single crystals with a twinning stress of about 0.1 MPa were studied. They showed a tendency to stay in a single variant state and to retain only one or very few twin boundaries during martensite reorientation induced by an external stress or magnetic field. This makes the crystals problematic for application in a magnetic actuator. To solve the issue, we introduced many parallel twin boundaries into the crystals by bending. However, this twin microstructure was not stable under cycling load. Additionally, it exhibited a twinning stress of 0.8 MPa—about ten times higher than a crystal with a single boundary.

Twin relationships and stress-induced reorientation were studied in Ni2Mn1.14Ga0.86 single crystal with five-layered modulated martensite crystal structure. Very low twinning stress of about 0.1 MPa was found for twin boundaries which deviated a few degrees from the (011¯) crystallographic plane. However, twin boundaries oriented exactly parallel to the (011¯) plane exhibited considerably higher level of twinning stress, above 1 MPa. X-ray diffraction experiments and calculations based on approximation of the martensite crystal lattice as a tetragonal lattice with a slight monoclinic distortion identified the two different kinds of twin interfaces as type II and type I twin boundaries.