The impact of high-temperature annealing on magnetic properties, structure and martensitic transformation of Ni₂MnGa-based glass-coated microwires

In this article we present our experimental results on the effect of high temperature annealing on magnetic and structure performance of NiMnGa-based glass coated microwires. The samples were annealed at 1173 K and 1273 K for 1h. The as-prepared sample exhibits weak ferromagnetic behaviour with magnetic remanence near to zero and average coercivity about 6 Oe. Annealing of NiMnGa microwires leads to increase in coercivity up to 230 Oe for the sample annealed at 1273 K. Additionally, high-temperature annealing induces martensitic transformation (MT). Annealing significantly influences the Curie temperature (Tc) of the samples, bringing it closer to room temperature, thereby making them more suitable for magnetic solid-state refrigeration applications. The observed changes can be attributed to several factors, such as internal stress relaxation, nanocrystalline structure, recrystallization processes, and variations in the magnetic ordering of phases present in the as-prepared and annealed states. While the insulating and flexible glass coating enhances the mechanical properties of the microwires, it is important to acknowledge that it can also significantly affects their magnetic properties. The current results confirm the stability of ferromagnetic and martensitic transformation of nanocrystalline NiMnGa-based glass-coated wires after heat treatment up to 1273 K.