Effect of heat treatment on the martensite phase and magnetic properties of chemically synthesized Co2NiGa nanoparticles

To achieve the martensite (M) phase at room temperature, chemically synthesized polycrystalline Co₂NiGa nanoparticles (NPs) with a preponderance of the austenite (A) phase were reheated at different temperatures. Heat treatment at 1373 K led to a single M phase structure in stoichiometric Co₂NiGa nanoparticles with excellent magnetic and shape memory properties. Room-temperature single M phase NPs transformed completely into the A phase when heated to 723 K, confirming the martensitic transition in Co₂NiGa NPs. The synthesized NPs displayed a soft ferromagnetic nature with magnetic moment at 40 kOe under an applied field (M_sat) ranging from 35.1 emu g⁻¹ to 40.4 emu g⁻¹ at 5 K and from 13.0 to 26.4 emu g⁻¹ at 300 K. The Curie temperatures of the M and A phases were 350 ± 2 K and 896 ± 2 K, respectively. The analysis of the magnetic properties revealed the presence of a dead layer with a thickness of ≈1.67 nm and the single-domain nature of the M phase Co₂NiGa NPs. By carefully selecting post-synthesis heat treatment conditions, the M ↔ A phase transition temperature and magnetic properties of Co₂NiGa NPs can be tailored to suit potential nanoactuator applications.