Contrasting Effects of Bi and Si Substitution at the Ni Site on Magnetostructural Transitions and Magnetocaloric Properties in Ni–Mn–In Heusler Alloys

We investigated the structural, magnetic, magnetocaloric, and magnetotransport properties of Ni₅₀Mn₃₅In₁₅ Heusler alloys by partial substitution of Ni with 3 at.% Bi (Ni₄₇Bi₃Mn₃₅In₁₅) and Si (Ni₄₇Si₃Mn₃₅In₁₅) synthesized by arc-melting. X-ray diffraction confirms a predominant L₂₁ cubic structure (space group Fm-3m), while SEM/EDX verifies compositional homogeneity. Temperature-dependent magnetization measurements reveal that the Bi-substituted alloy exhibits a first-order magnetostructural transition associated with the martensitic transformation, followed by a second-order ferromagnetic–paramagnetic transition near the Curie temperature. In contrast, the Si-substituted alloy shows a single second-order transition with negligible thermal hysteresis, indicating suppression of the martensitic phase. The Curie temperature decreases from 324 K for the parent alloy to 313 K and 286 K for the Bi- and Si-substituted alloys, respectively. A maximum magnetic entropy change of 6.0 Jkg^-1K^-1 and 4.5 Jkg^-1K^-1 is obtained under an applied magnetic field change of 50 kOe for the Bi- and Si-substituted alloys, respectively, with corresponding relative cooling power values of 303 Jkg^-1 and 345 Jkg^-1. These results demonstrate that lattice expansion (Bi) and contraction (Si) distinctly modify Mn–Mn exchange interactions, enabling tunable magnetocaloric performance in Ni–Mn–In Heusler alloys.