Co-doping modulated magnetocaloric effect and critical behavior of kagome MgMn6Sn6

Single crystals of MgMn_5.95Co_0.05Sn₆ were synthesized via the flux method, and their magnetic properties, magnetocaloric effects, and critical behavior were systematically investigated. Compared to the ferromagnetic (FM) MgMn₆Sn₆ parent compound, Co doping induces an antiferromagnetic-like low-magnetization state. The maximum magnetic entropy change $\left(|-\Delta S_M^{\max }|\right)$ under ΔH = 50 kOe was determined as 2.87 J/kg·K (H//ab) and 2.57 J/kg·K (H//c). Scaling analysis of the magnetic entropy change curves revealed a field-dependent magnetic phase transition: a first-order transition dominates at low magnetic fields (H < 40 kOe), while a crossover to second-order behavior occurs at higher fields (H > 40 kOe). Critical exponents β, γ, and δ were derived using modified Arrott plots, Kouvel-Fisher analysis, and the Widom scaling relation (δ = 1 + γ/β), with consistency validated through the universal scaling equation m = f_±(h). The critical exponents for H//ab align closely with the 3D-Ising universality class, whereas those for H//c exhibit hybrid characteristics-proximity to the 3D-Heisenberg model below T_C and mean-field behavior above T_C. These findings highlight the anisotropic critical behavior and universality crossover induced by Co doping in the MgMn₆Sn₆ system.