Magnetic Properties and Structural Phase Transformations of NiMnGa Alloys

The magnetic properties and structural phase transformations of Heusler alloys Ni2+ Mn1 Ga and Ni2 Mn1+ 2Ga1+ 2 were investigated. It was found that the martensitic transformation temperature increases and the Curie temperature decreases with decreasing Mn concentration in the Ni2+ Mn1 Ga system, but the magnetic properties decrease faster than those do in the Ni2 Mn1+ 2Ga1+ 2 system. It suggests that the influence of the conduction electron concentration and the distance between Mn atoms on the Curie temperature is stronger than the magnetic dilution effect. The martensitic transformation temperature shows a peak value at x = 0 06 in the Ni2 Mn1+ 2Ga1+ 2 system. A large stress-free magnetic-field-induced strain (MFIS) of 1.8% has been obtained at room temperature in a single crystal sample with the composition in the first system.