Phase diagram and magnetocaloric effects in aluminum doped MnNiGe alloys

The magnetocaloric and thermomagnetic properties of the MnNiGe1−xAlx system have been studied by temperature-dependent x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. The partial substitution of Al for Ge in MnNiGe1−xAlx results in a first order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ΔSM = −17.6 J/kg K for ΔH = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. The value is comparable to those of giant magnetocaloric materials such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (ΔST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ΔSM of −5.8 and −4.8 J/kg K for ΔH = 5 T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. A concentration-dependent phase diagram of transition...