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 χ(T), specific heat $C(T)$, electrical resistivity ρ(T), and thermopower $S(T)$ measurements. χ(T) reveals a trivalent 4f¹ cerium state in both compounds. For CeRuPO a pronounced decrease of ρ(T) below 500.3em0exK indicates the onset of coherent Kondo scattering, which is confirmed by enhanced $S(T)$. The temperature and magnetic field dependence of χ(T) and $C(T)$ evidence ferromagnetic (FM) order at TC=150.3em0exK. Thus, CeRuPO seems to be one of the rare examples of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at TN=4.50.3em0exK despite only minor changes in lattice parameters and electronic configuration. Additional ³¹P NMR results support these scenarios. 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.