Nature of the magnetic ground state in the mixed valence compound CeRuSn: a single-crystal study

We report on detailed low temperature measurements of the magnetization, the specific heat and the electrical resistivity on high quality CeRuSn single crystals. The compound orders antiferromagnetically at \(T_{\rm N} = 2.8\) K with the Ce\(^{3+}\) ions locked within the \(a-c\) plane of the monoclinic structure. Magnetization shows that below \(T_{\rm N}\) CeRuSn undergoes a metamagnetic transition when applying a magnetic field of 1.5 and 0.8 T along the \(a\) and \(c\)--axis, respectively. This transition manifests in a tremendous negative jump of \(\sim 25\)% in the magnetoresistance. The value of the saturated magnetization along the easy magnetization direction (\(c\)--axis) and the magnetic entropy above \(T_{\rm N}\) derived from specific heat data correspond to the scenario where only one third of the Ce ions in the compound being trivalent and carrying a stable Ce\(^{3+}\) magnetic moment, whereas the other two thirds of the Ce ions are in a nonmagnetic tetravalent and/or mixed valence state. This is consistent with the low temperature CeRuSn crystal structure i.\,e.\,, a superstructure consisting of three unit cells of the CeCoAl-type piled up along the \(c\)--axis, and in which the Ce\(^{3+}\) ions are characterized by large distances from the Ru ligands while the Ce-Ru distances of the other Ce ions are much shorter causing a strong 4{\it f}-ligand hybridization and hence leading to tetravalent and/or mixed valence Ce ions.