Crystallography and magnetic field-induced strain by Co doping NiCoMnGa Heusler alloy

Crystallographic, thermal strain, and magnetic properties of Ni _50-xCo_xMn_38.5Ga_18.5 (0≤x≤9), were investigated across the martensite transition temperature T_M and the reverse martensite transition temperature T_R at atmospheric pressure. These transition temperarures increased gradually with increasing Co component x. Moreover, temperature hysteresis in the thermal cycles of the magnetization across the T_R and T_M, became larger with increasing x. Wide temperature hysteresis of T_M -T_R = 65 K was observed in the thermal strain measurement. The metamagnetic transition from the paramagnetic martensite phase to the ferromagnetic austenite phase appeared between 330 K and 390 K. The magnetostriction of this alloy was 0.11% at 350 K under atomospheric pressure. This value of the magnetostriction is larger than that of TbDyFe single crystal under atomospheric pressure. The samples of Ni_50-xCo_xMn_38.5Ga_18.5 in this study were polycrystal, and it is easy for processing and handling. Moreover, the magnetostriction effect occurred at a temperature between room temperature and 390 K, therefore it is useful in the high temperature region, e.g. the engine room of the motorcar.