The effect of pressure and temperature on the magnetic and magnetocaloric properties of an MnNi_0.9Ge_1.1 alloy

The magnetic and magnetocaloric properties of a self-doped MnNiGe alloy of nominal composition MnNi_0.9Ge_1.1 have been investigated in ambient as well as in high pressure conditions. It orders ferromagnetically below around 225 K and undergoes first order martensitic phase transition (MPT) to an antiferromagnetic (AFM) martensite phase below 147 K. This self-doping results in a significant decrease in the lattice volume and hence the Mn–Mn intra-layer distance which induces ferromagnetism (FM) in otherwise AFM alloys. MPT affects this FM ordering and the alloy becomes predominantly AFM in nature below the structural transition temperature. The observed values of the magnetocaloric effects (MCE) are reasonably large at the magnetic (−5.5 J kg⁻¹ K⁻¹ for magnetic field changing from 0 to 50 kOe around 210 K) and structural (8.3 J kg⁻¹ K⁻¹ for magnetic field changing from 0 to 50 kOe around 136 K) transition temperatures in ambient condition. MCE is found to decrease with increasing external hydrostatic pressure (P) at MPT region, whilst this external P has vanishingly small effect on MCE around the magnetic transition temperature. (paper)