Near room temperature giant magnetocaloric effect in (MnNiSi)_1-x(Fe₂Ge)_x alloys

The magnetocaloric effect (MCE) of the pseudo-binary alloy (MnNiSi)_1-x(Fe₂Ge)_x with the one composition variable (x) was studied. The ferromagnetic (FM) to paramagnetic (PM) transition temperature of MnNiSi was reduced by adding Fe and Ge in the ratio 2:1 (or Fe₂Ge), stabilizing the hexagonal phase of MnNiSi. The transition temperature can be tuned from 356 K to 218 K by varying x in the range of 0.32–0.36, respectively. Interestingly, the alloy with x = 0.34 exhibited magnetostructural transition near room temperature with values of ΔS_max of 57 J kg⁻¹ K⁻¹ and RCP of 342 J kg⁻¹. The material was processed by arc melting, without any subsequent heat treatment. TEM analyses were used to identify the structural transitions. The first order transition was modeled by Arrott plots, Landau theory and the Bean-Rodbell model. Maximum entropy change (ΔS_max) values of up to 57 J kg⁻¹ K⁻¹ were obtained, higher than previously reported values for these alloys.