Multicaloric Effects in (MnNiSi)_1-x(FeGe)_xAlloys

MnNiSi-based alloys, substituted with isostructural Fe2Ge as (MnNiSi)1-x(Fe2Ge)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) (Ni2In-type) hexagonal phase was observed in all compositions, where 0.33le x le0.35 , by magnetothermal and structural analyses. The magnetostructural transition temperature, T-{mathrm {t}} , decreased from 350 to 256 K (for mu -{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-{mathrm {t}} at dT t/ dP 7.3 K/kbar and thermal hysteresis, Delta T-{mathrm {t}} , significantly. Indeed, in the x= 0.33 composition, Delta T-{mathrm {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, Delta S-{mathrm {mag}} (mu -{0}H= 2 T), corresponding to 30~J/kg cdot mathrm {K} is noted at 270 K in (MnNiSi)0.67(Fe2Ge)0.33 at an applied pressure of 7.9 kbar. For the x= 0.34 and 0.35 compositions, an application of P sim ~8 kbar resulted in the partial, and complete, decoupling of the magnetic (FM to PM) and structural (orthogonal to hexagonal) transitions, respectively, and consequently, a large drop in Delta S-{mathrm {mag}}. Application of P= 7.9 kbar on the x= 0.35 composition converted the first-order T-{mathrm {t}} to the second-order Curie transition, T-{mathrm {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 Fe2Ge for potential solid-state cooling applications.