Rare Earth - Transition Metal Intermetallic Compounds Gdmn <sub> 2 </ Sub> Ge <sub> 2 </ Sub> And Rmn <sub> 6 </ Sub> Sn <sub> 6 </ Sub> (r = Tb, Dy, Ho) And Magnetic Study

Due to their special crystalline structure and magnetic interaction, the rare earth-transition intermetallic compounds GdMn2Ge2 and RMn6Sn6 (R=Tb, Dy, Ho) display abundant magnetic properties and magnetic transitions. In this thesis, we have established the theoretical model of GdMn2Ge2 and RMn6Sn6 (R=Tb, Dy, Ho) compounds in the approximation of molecular field theory of exchange interaction. Based on this model, we have studied the influence of different kind of exchange interaction, crystalline parameters, magnetocrytalline anisotropic energy and their competition on the magnetic properties of GdMn2Ge2 and RMn6Sn6 (R=Tb, Dy, Ho) compounds. The intrinsic magnetic parameters, such as exchange coupling parameters and magnetocrytalline anisotropic constants, have been estimated by fitting the experimental data. Through the detailed theoretical analysis and large number of calculation, we have gotten the following conclusion:(1) There are five and four possible magnetic phases in GdMn2Ge2 and in RMn6Sn6 (R=Tb, Dy, Ho) compounds, respectively. When the intrinsic magnetic parameters or external conditions (such as temperature or applied field) vary, these compounds would display first or second order magnetic transitions from one magnetic phase to another. Our study shows that the dependence of the interlayer Mn-Mn exchange interaction on the intralayer Mn-Mn distance plays a very important role on the magnetic properties of GdMn2Ge2 compound.(2) The temperature dependence of the saturation magnetization and the magnetic curves along different crystalline axis at different temperature of GdMn2Ge2 and RMn6Sn6 (R=Tb, Dy, Ho) compounds have been calculated. Based on these calculations, the corresponding H-T magnetic phase diagrams have been achieved for applied field along different crystalline axis.(3) In terms of energy, the physical mechanism of the spin-reorientation transition of RMn6Sn6 (R=Tb, Dy, Ho) compounds has been analyzed. For TbMn6Sn6, this transition results from the competitionbetween the rare earth and manganese sublattices' magnetocrystalline anisotropic energy; while for DyMn6Sn6 and HoMn6Sn6, besides the reason mentioned, the competition of the different order magnetocrytalline anisotropic constants of rare earth sublattice also contributes.(4) The magnetic parameters of GdMn2Ge2 and RMn6Sn6 (R=Tb, Dy, Ho) compounds have been achieved through the optimal fitting of various experimental data.