Crystalline Electric Field and Cluster Effects in Borocarbide Superconductors

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For a detailed investigation of the competition between superconductivity and magnetic order in the borocarbides, the knowledge of the electronic ground state of the magnetic ions is indispensable. The magnetic ground state is determined by the crystalline electric field (CEF) at the rare earth site which lifts the J(J+l)-fold degeneracy of the lowest lying J-multiplet. Many of the magnetic properties of the borocarbides can be understood if the single ion magnetism — given by the CEF — is known and the results can be used to make predictions with the help of mean-field calculations. The determination of the CEF-Hamiltonian is an indispensable basis for the understanding of the richness of the magnetic structures that are observed in the borocarbides: it is the competition between the RKKY interaction and the CEF that give rise to the observed variety. The single ion anisotropy, which is determined by the CEF, favors commensurate structures with short wavelength and the magnetic moments parallel to an easy axis. In contrast to this, the RKKY interaction is determined by the polarizability of the conduction electrons and, therefore, prefers incommensurate magnetic structures with a long wavelength. Quantitative information about the magnetic exchange is particularly valuable for models that explain the coexistence of magnetic order and superconductivity on a microscopic level. In the absence of single crystal neutron data, the only way to get an idea about the nature of the coupling is the investigation of the rare earth pair interaction in diluted systems.