Tuning the phase transition temperature and realizing a giant magnetocaloric effect in Ni-Co-Mn-Ti alloys via spray-casting process

Recently, a giant magnetocaloric effect (GMCE) has been discovered in Ni-Co-Mn-Ti alloys, making them promising candidates for solid-state refrigeration. However, compared to other Heusler alloys, the absence of a sharp ferromagnetic phase transition in these compounds presents a limitation for practical applications. In this study, the effects of different microstructures on their magnetocaloric properties were compared by spray-casting rods of varying diameters. It was observed that the magnetic entropy changes under a 0–5 T magnetic field for a 10 mm spray-cast rod reached 49.4 J·kg⁻¹ ·K⁻¹, representing a 173% improvement compared to 18.1 J·kg⁻¹ ·K⁻¹ for an arc-melted sample, placing this value among the highest reported for similar alloy systems. Besides, the phase transformation temperature was reduced from 211 K in the arc-melted sample to 169.9 K in the 10 mm spray-cast sample, representing a decrease of 41.1 K. Through microstructural observations of rods with varying diameters, the increase in magnetic entropy was attributed to microstructural optimization and grain size enlargement. The increased grain size facilitates a sharper phase transition, while the rapid solidification process during spray-casting reduces the defect concentration. Our findings demonstrate that the spray-casting technique significantly enhances the homogenization in all-d-metal Ni-Co-Mn-Ti compounds, leading to improved magnetocaloric performance. This work establishes the relationship between microstructure and performance, providing a new insight into optimizing these materials for solid-state refrigeration applications through the spray-casting process.