Characterisation of Novel Magnetocaloric Materials (MnCoGe) for magnetic refrigeration application

Conventional vapour compressor-based refrigeration systems are less energyefficient and harmful to the environment due to chlorofluorocarbon (CFC), which can deplete the ozone layer and contribute to global warming. Therefore, an alternative solution that is both energy-efficient and environmentally friendly is desirable. Studies on magnetocaloric materials (MCMs) with excellent properties are important to demonstrate their suitability and potential use in refrigeration. This work has focus on MnCoGe-based compound for magnetic cooling, which does not contain high-cost rare earth elements and can provide a favourable working temperature range near room temperature. MnCoGe has unique properties where it exhibits two stable crystallographic structures which can manipulate the magneto-structural transition by adjusting the composition. The magnetic behaviour was analysed through its structural properties, magnetic measurement, and magnetic entropy change (-ΔSM) for MnCoGe1-xAlx and MnCoGe1-xSix. The neutron diffraction is employed to study the magnetic structure and the moment of material. This measurement focus on MnCoGe0.97Al0.03 only due to the Echidna and Wombat neutron beam instruments offer very limited beam time. The room temperature x-ray diffraction shows that the MnCoGe1-xAlx (x=0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of TiNiSi-type structure for x ≤ 0.03 and the Ni2In-type structure for x > 0.03. For MnCoGe1-xSix (x=0, 0.05, 0.1, 0.15 and 0.2) alloys, the results indicate that the compounds have a major phase consisting of orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreased others (a and c) with the increase of Si concentration. The -ΔSM maximal value increased with the increase of Al content from 8.36 to 9.57 J·kg-1K-1 for x = 0.00 to 0.15. On the other hand, the MnCoGe1-xSix compounds shows decreasing pattern from -ΔSM~8.36 J kg-1 K-1 at x=0 to -ΔSM~5.49 J kg-1 K-1 at x=0.2 with 5 T applied field. The theoretical study mainly focuses on the MnCoGe0.97Al0.03 compound, as the compound shows that TiNiSi-type and Ni2In-type structures start to coexist together which given the best interest to further study the nature of FM - PM transition. The obtained critical parameters concluded the compound having long-range ferromagnetic order, which is second-order type magnetic transition, thus, verified the experimental studies and confirmed the reliability of the compounds for magnetic refrigeration application.