Effect of Pt doping Ni on the structural and magnetic properties of hexagonal MnNiGa alloy

The structure and magnetic properties Mn1.2PtxNi1-xGa (x = 0, 0.2, 0.5, 1) alloys prepared by melting-spinning method were systematically studied. X-ray diffraction experiments confirmed that all the alloys exhibited hexagonal structure. Both a and c axes expanded with the increase of N content, which resulted in the increase of distance between Mn-Mn atoms in neighboring layers, thus a decrease of Curie temperature due to the weakening of ferromagnetic exchange interaction between Mn-Mn was observed. The coercivity of Mn1.2NiGa is about 300 Oe at 5 K, and it is enhanced to 8000 Oe in Mn1.2PtGa alloy. Simultaneously, the saturation magnetic field was increased by N doping, suggesting the enhancement of magnetocrystalline anisotropy. It is suggested that the large coercivity is attributed to the pinning of glass phase on the ferromagnetic phase and the increase of spin-orbit coupling. The glass phase transition temperature decreases with increasing N content, which makes the temperature stability of coercivity in Mn1.2PtGa is not so good as in Mn1.2Pt0.5Ni0.5 Ga alloy.