Structural And Phase Transition In Transition Metal Compounds

Since the discovery of superconductors,the interplay between magnetism and superconductivity has been concerned,particularly in unconventional superconductors,such as heavy fermions,copper oxides and iron pnictides or chalcogenides.In the case of these compounds,superconductivity always appears near static antiferromagnetic order.Magnetic structure and associated phase transition is very important for understanding the origin of unconventional superconductivity microscopically.Neutron itself is a charge 0 fermion with spin magnetic moment,ideally suited to study magnetism of materials.Thus,we can study static magnetic order,magnetic phase transition and spin excitation behavior by elastic diffraction and inelastic scattering experiments,respectively.We firstly introduced the related study of magnetism in transition metal compounds,such as heavy fermions,copper oxides and iron pnictides or chalcogenides.The basic knowledge of determining magnetic structure by neutron diffraction experiments and the classification of magnetic structure in solids are also included.Finally,we mainly introduced the basic principle and process of determining magnetic structure by Rietveld whole pattern fitting.In Chapter 2,we reported neutron diffraction and transport results on the newly discovered superconducting nitride ThFeAsN with Tc=30 K.No magnetic transition,but a weak structural distortion around 160 K,is observed cooling from 300 K to 6 K.Analysis on the resistivity,Hall transport and lattice parameters suggests this material behaves as an electron optimally doped pnictide superconductors due to extra electrons from nitrogen deficiency or oxygen occupancy at the nitrogen site,which together with the low arsenic height may enhance the electron itinerancy and reduce the electron correlations,thus suppress the static magnetic order.In Chapter 3,we provided detailed information on the growth of heavy-fermion Yb_1-xTb_xAgGe single crystals.The measurement of resistivity,specific heat capacity,magnetic susceptibility et al and neutron diffraction experiments are also included.Transport measurements show the temperature and number of magnetic transition will increase with increasing Tb doping,thus enhancing the complexity of magnetism in Yb_1-xTb_xAgGe.Resistivity,specific heat capacity and neutron diffraction patterns indicate that two different kinds of YbAgGe single crystals with kagome structure could be synthesized.One of them has magnetic transition,the others not,however.In our opinion,the difference of magnetism may be related to the change of microscopic crystal structure between two kinds of YbAgGe single crystals.In Chapter 4,we reported the results of crystal structure and magnetic phase transition in 3d transition-metal Cr₂GaN.Neutron diffraction patterns illustrate the existence of magnetic phase transition around 170K and magnetic-elastic coupling in Cr₂GaN.Meantime,we find that the little change of crystal structure at 170K may happen.The high-resolution synchrotron and neutron diffraction data are needed on further analysis in determining the crystal structure and magnetic structureIn Chapter 5,Here we systematically studied the uniaxial pressure dependence of the resistivity in Sr₁-BaFe_1.97Ni_0.03As₂,where nonlinear behaviors are observed near the nematic transition temperature.We show that it can be well explained by the Landau theory for the second-order phase transitions considering that the external field is not zero.The effect of the coupling between the isotropic and nematic channels is shown to be negligible.Moreover,our results suggest that the nature of the magnetic and nematic transitions in Sr₁-BaFe₂As₂ is determined by the strength of the magnetic-elastic coupling.In Chapter 6,a summery was presented in the end.