CeRuPO: A rare example of a ferromagnetic Kondo lattice

We have determined the physical ground state properties of the compounds CeRuPO and CeOsPO by means of magnetic susceptibility $\chi \left(T\right)$, specific heat $C\left(T\right)$, electrical resistivity $\rho \left(T\right)$, and thermopower $S\left(T\right)$ measurements. $\chi \left(T\right)$ reveals a trivalent $4f^1$ cerium state in both compounds. For CeRuPO a pronounced decrease of $\rho \left(T\right)$ below $50\mathrm{K}$ indicates the onset of coherent Kondo scattering, which is confirmed by enhanced $S\left(T\right)$. The temperature and magnetic field dependence of $\chi \left(T\right)$ and $C\left(T\right)$ evidence ferromagnetic (FM) order at $T_C=15\mathrm{K}$. Thus, CeRuPO seems to be one of the rare examples of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at $T_N=4.5\mathrm{K}$ despite only minor changes in lattice parameters and electronic configuration. Additional ${}^{31}\mathrm{P}$ NMR results support these scenarios. $\mathrm{LSDA}+U$ calculations evidence a quasi-two-dimensional electronic band structure, reflecting a strong covalent bonding within the CeO and RuP layers and a weak ioniclike bonding between the layers.