Multicaloric Effects in (MnNiSi)_1−x(Fe₂Ge)_x Alloys

MnNiSi-based alloys, substituted with isostructural Fe ₂ Ge as (MnNiSi) _1-x (Fe ₂ Ge) _x , were prepared by arc-melting to examine their structural, magnetocaloric, and barocaloric properties. A simultaneous (coupled) first-order magnetic and structural, that is, magnetostructural, transition from a low-temperature ferromagnetic (FM) (TiNiSi-type) orthorhombic phase to a high-temperature paramagnetic (PM) (Ni ₂ In-type) hexagonal phase was observed in all compositions, where 0.33 ≤ x ≤ 0.35, by magnetothermal and structural analyses. The magnetostructural transition temperature, T _t , decreased from 350 to 256 K (for μ ₀ H = 0.1 T, on heating) by providing chemical pressure used by increasing the compositional variable, x, to 0.35. However, application of hydrostatic pressure overall decreased both T _t at dT _t /dP ~7.3 K/kbar and thermal hysteresis, ΔT _t , significantly. Indeed, in the x = 0.33 composition, ΔT _t was drastically reduced by >80% from 30 K at ambient pressure to 5 K at P = 7.6 kbar. A maximum magnetic entropy change, ΔS _mag (μ ₀ H = 2 T), corresponding to 30 J/kg · K is noted at 270 K in (MnNiSi) _0.67 (Fe ₂ Ge) _0.33 at an applied pressure of 7.9 kbar. For the x = 0.34 and 0.35 compositions, an application of P ~ 8 kbar resulted in the partial, and complete, decoupling of the magnetic (FM → PM) and structural (orthogonal → hexagonal) transitions, respectively, and consequently, a large drop in ΔS _mag . Application of P = 7.9 kbar on the x = 0.35 composition converted the first-order T _t to the second-order Curie transition, T _C , and therefore loss of the structural transition indicating the stabilization of the high-temperature hexagonal structure. Overall, these features emphasize strong coupling between the magnetic spins and the lattice in MnNiSi-based alloys. Further fundamental and applied insight is obtained concerning pathways for optimizing the multicaloric response of MnNiSi-based alloys with isostructural substitution by Fe ₂ Ge for potential solid-state cooling applications.