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 186K (for x 0.09). A large magnetic entropy change of Delta S-M = - 17.6 J/kg K for DH 5 T was observed in the vicinity of TM 186K 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 (DST 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of Delta S-M of - 5.8 and - 4.8 J/kg K for DH 5 T in the vicinity of TC were observed for x 0.09 and 0.085, respectively. A concentration- dependent phase diagram of transition temperatures (magnetic, structural, and magnetostructural) has been generated using magnetic, XRD, and DSC data. The role of magnetic and structural changes on transition temperatures is discussed. (C) 2013 AIP Publishing LLC.