Structure and magnetic properties of stress-induced martensites in ferromagnetic shape memory alloy Mn₂NiGa

In-situ high pressure X-ray diffraction experiments on Heusler-type ferromagnetic shape memory alloy Mn₂NiGa under pressure up to 40 GPa have been carried out by using diamond anvil cell device with synchrotron radiation source, and magnetic measurements have also been performed on uninstalled samples by using vibration sample magnetometer. At ambient pressure Mn₂NiGa has body centered cubic structure. Two phase transitions in Mn₂NiGa were observed during high pressure experiments. The first at about 0.77 GPa is belong to martensitic transformation, and the second at about 20 GPa is isostructural transformation between two different martensites. Meanwhile, a lot of defects and distortion of lattice were generated in martensites due to pressurization. Consequently, coercive force of stress-induced martensites rises up to 204 kA/m and it is almost ten times larger than that of thermo-induced martensites. Pressure processes also make saturation magnetization of martensites decrease rapidly, which reveals the obvious defect effect.