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(T), specific heat C(T), electrical resistivity rho(T), and thermopower S(T) measurements. chi(T) reveals a trivalent 4f(1) cerium state in both compounds. For CeRuPO a pronounced decrease of rho(T) below 50 K indicates the onset of coherent Kondo scattering, which is confirmed by enhanced S(T). The temperature and magnetic field dependence of chi(T) and C(T) evidence ferromagnetic (FM) order at T-C=15 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 K despite only minor changes in lattice parameters and electronic configuration. Additional P-31 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.