Studies of magnetism and charge density wave order in rare earth Nickel dicarbides

Abstract

Rare earth nickel dicarbides, RNiC2, display various aspects of fundamental interest: a noncentrosymmetric, orthorhombic parent crystal structure and a quasi-one-dimensional electronic structure, which gives rise to commensurate as well as incommensurate charge density wave (CDW) transitions. A CDW transition is an instability of the lattice and the electronic band structure, which is characterized by a periodic lattice distortion accompanied by the opening of additional energy band gaps. RNiC2 compounds show a variety of rare earth magnetic ground states reaching from simple ferromagnetism to complex antiferromagnetic states, where the specific state is strongly determined by crystalline electric field effects. Most interestingly, these magnetic, electronic and lattice degrees of freedom appear to be interconnected as e.g. revealed for SmNiC2, where the suppression of the CDW within the ferromagnetically ordered ground state motivated a large number of studies in literature.The main focus of the present thesis is to reveal details of the magnetic ground state of TmNiC2 due to crystalline electric field effects as well as anisotropic transport properties and a characterization of its orthorhombic to commensurate CDW transition. For this purpose, two TmNiC2 single crystals were grown and prepared for x-ray diffraction, heat capacity, magnetic and transport studies and polycrystalline material was provided for inelastic neutron studies. Crystalline electric field parameters and the magnetic energy level splitting were evaluated via simultaneous fitting of single crystal inverse susceptibility, heat capacity, and inelastic neutron data and revealed a quasi-dublet ground state of TmNiC2 which is well separated by more than 20 meV from excited states. The CDW state of TmNiC2 was characterized through the combination of thermodynamic and transport studies, which revealed the transition temperature to be 375 (3) K. Close similarities to transport features of LuNiC2 are discussed. In addition to TmNiC2, three further members of the RNiC2 family have been studied. Measurements of single crystalline GdNiC2 revealed distinct changes of the paramagnetic Curie temperature in each charge density wave state and the anisotropic behavior of GdNiC2 was discussed via simulations of dipole-dipole interaction as well as RKKY interaction. Finally, PrNiC2 and HoNiC2 were investigated, where for PrNiC2 a highly anisotropic electrical resistivity with, however, nearly temperature independent anisotropy is observed. For HoNiC2, an additional magnetic spin-reorientation transition at 2.2 K is detected and analyzed by magnetic susceptibility studies.