Regulation of defect evolution and magneto-structural coupling in Ni-Co-Mn-Sn alloys by Co content and thermal history

In this work, the martensitic transformation (MT) behavior, defect evolution, and magnetic properties and magnetocaloric effect (MCE) of Ni_50-xCo_xMn₃₉Sn₁₁ (x = 6, 8, 10, 12 at.%) alloys were systematically investigated under different Co contents and thermal histories. Using XRD, DSC, M-T and M-H analyses, the combined effects of compositional tuning and thermal treatment on point defects, internal stress fields, atomic ordering, and magnetic structures were elucidated. The results show that increasing Co content lowers the MT temperature and enhances the ferromagnetism of the austenite, leading to a progressive evolution from a reversible thermoelastic transformation to the kinetic arrest state and finally to the strain-glass-like state. This behavior is attributed to the Co-induced modification of electronic structure and magnetic exchange interactions, which alters the free-energy balance between martensite and austenite. In addition, thermal history further modulates the transformation temperatures and magnetic performance within a given composition: water-quenched samples exhibit higher MT temperature and lower Curie temperature (T_C) of austenite, while aging reduces MT temperature and raises T_C, yielding the widest ferromagnetic austenite stability range and the strongest magneto-structural coupling. Especially, WQ-WQ sample of Co6 alloy exhibits outstanding MCE with a maximum magnetic entropy change (ΔS_Mmax) of 28.1 J/(kg·K), effective temperature window (ΔT_FWHM) of 16.4 K and refrigeration capacity (RC) of 405.4 J/kg. Further analysis reveals that WQ suppresses the ordering process and retains anti-site defects, whereas aging promotes vacancy diffusion and atomic rearrangement, resulting in higher chemical ordering degree and enhanced ferromagnetic exchange. The cooperative evolution of vacancies and anti-site defects governs the chemical ordering and magnetic exchange balance, enabling effective control of transformation behavior and magnetic properties. This study clarifies the intrinsic mechanism by which Co-doping and thermal history jointly modulate the magneto-structural coupling in Ni-Co-Mn-Sn alloys, providing theoretical guidance for the design and optimization of high-performance magnetocaloric materials.