Tunable structural and magnetic properties of chemically synthesized dual-phase Co2NiGa nanoparticles

We report the structural and magnetic properties of chemically synthesized dual-phase Co2NiGa shape memory Heusler nanoparticles (NPs). Rietveld analysis of the X-ray diffraction (XRD) data reveals a dual-phase structure for all studied Co2NiGa NPs: the B2 ordered cubic austenite (beta) and the tetragonal (gamma or martensite beta') phases. We find that the fraction of the tetragonal gamma-phase and magnetic properties of NPs consisting of beta + gamma phases prepared using a heating rate of 0.5 K min(-1) can be tuned by varying the Ni-content. Higher saturation magnetization and Curie temperature are observed for Co2NiGa NPs with a higher fraction of the gamma-phase. Dual-phase beta + gamma Co2NiGa NPs exhibit the highest Curie temperature (1153 K) of all known Heusler NPs. Furthermore, the martensite beta'-phase is found to coexist with the beta-phase for NPs synthesized using a higher heating rate of 2 K min(-1). Co2NiGa NPs of mixed beta + beta' phase are paramagnetic at room temperature and exhibit low magnetization due to the presence of the martensite phase. High temperature XRD measurements of Co2NiGa NPs with beta + beta' phases confirm their irreversible phase transition of the beta-phase starting at a temperature of 837 K and the structural stability of the tetragonal beta'-phase. The developed new synthetic method makes it possible to fabricate Co2NiGa NPs with structure and properties tailored for high temperature magnetic shape memory devices at the nanoscale.