Two-Way Martensitic Transitions in Co–Ni–Ga Ferromagnetic Shape Memory Alloy Nanoparticles

Three sets of Co–Ni–Ga alloy nanoparticles have been synthesized by a template-free chemical route. Structural, morphological, shape memory, and magnetic properties of room-temperature martensite (M) phase, dual (M + secondary γ) phase, and austenite (A) phase Co–Ni–Ga nanoparticles are reported. Temperature-dependent XRD analysis revealed that Co36Ni36Ga28 nanoparticles exhibiting a single M phase at room temperature completely transform to A phase at ∼1000 K. Upon cooling to room temperature, the A phase transforms back to single M phase, confirming the two-way martensitic transition in Co–Ni–Ga nanoparticles. Structural analysis shows that the γ-phase does not influence the martensitic transition of biphasic (M + γ) Co41Ni34Ga25 nanoparticles. These nanoparticles display saturation magnetization ranging from 2.9 to 15.3 emu/g at room temperature. The γ phase could be introduced in A phase Co44Ni26Ga30 nanoparticles when heated to 1073 K. Curie temperatures of A and M phases are higher than the martensitic transition temperatures in all of the samples, qualifying them as ferromagnetic shape memory alloy nanoparticles. Observation of M ↔ A phase transition and Co–Ni–Ga nanoparticles with tunable magnetic properties make them excellent candidates for low and high-temperature nanoactuators and other ferromagnetic shape memory applications.