Tuning magnetostructural and magnetocaloric properties of MnNiGe alloys through strategic use of self- and dual doping approaches

The present work aims to address the effect of self-doping and a combined perturbation of self- and vacancy doping on the structural, magnetic, and magnetofunctional properties of MnNiGe-based magnetic equiatomic alloys of nominal compositions Mn_1-xNi₁₊ₓGe (for x = 0.2 and 0.15), Mn1+yNi_1-yGe (for y = 0.2 and 0.22), and Mn1+zNi_0.9-zGe (z = 0.03 and 0.06). Experimental findings reveal that all six alloys undergo martensitic type first-order phase transition and the transition is found to be shifted towards lower temperature with increasing doping concentration. Structural investigation reveals the incomplete nature of the martensitic phase transitions. A substantial magnetic entropy change near the martensitic phase transition temperature suggests the materials applicability as magnetic refrigerant. Interestingly, perturbation at the magnetically ordered transition metal site (Mn site) through self-doping shows larger impact on the structural and magnetic properties compared to the perturbation at the nonmagnetic transition metal site (Ni site). The introduction of vacancy at the Ni-site results in a larger impact on the incompleteness in the martensitic phase transition present in dual-doped alloys. Field-induced metamagnetic transition, nonmonotonicity in the inverse magnetocaloric effect behavior, and the presence of a possible spin-reorientation transition have also been noticed in the alloys with coexisting self- and vacancy doping.