The physical principle of magnetic phase transition for the magnetocaloric materials: RCo2Mnx AND Tb1-xYxMn2Ge2 (R = Tb, Ho, Dy, 0 ≤ ???????? ≤ ????????. ????????) series compounds

Since Gd5(Si2Ge2) was found to have huge magnetic entropy near room temperature, various magnetocaloric materials have been produced and investigated. Meanwhile, different methods were used to optimize their magnetic properties, such as element doping, applying strong magnetic fields or mechanical pressure, etc. My work during this PhD study has been mainly focused on the Mn doped RCo2Mnx (R = Tb, Ho, Dy，0 ≤ 𝑥𝑥 ≤ 1.0) series compounds. It was found that all the Mn doped samples are isostructural to RCo2 series compounds with the cubic MgCu2 structure (Fd3􀴤 m space group). To further investigate the Mn doping effect on the magnetic properties such as the Curie temperature (TC), the coercivity (HC), the spontaneous magnetic moment (MS), and the magnetic entropy change (−ΔS), magnetic measurement were performed on a physical properties measurement system (PPMS). Meanwhile, critical exponent analysis was conducted for all the second order transitions belonging to the three-dimensional (3D) Heisenberg model, mean-field theory, or 3D Ising model. High resolution neutron and synchrotron measurements were then carried out to clarify the magnetic transitions and the crystal structure transitions during the magnetic-state transitions. By the way another series compounds Tb1-xYxMn2Ge2 (x = 0, 0.1, 0.2) was studied and included in the thesis.