The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO1-xFx, were studied by means of susceptibility, specific heat, resistivity, and NMR measurements on polycrystals. All the results demonstrate that this compound is a magnetically nonordered heavy fermion metal with a Kondo temperature TK approximately 10 K, a Sommerfeld coefficient gamma=700 mJ/mol K2, and a mass enhancement factor of the order of 50. Analysis of the susceptibility data and of the spin relaxation time indicates that the strong electronic correlation effects originate from the Ce-4f electrons rather than from Fe-3d electrons. An enhanced Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2 approximately 0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the nonmagnetic side of a ferromagnetic instability.

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 4f 1 cerium state in both compounds. For CeRuPO a pronounced decrease of ρ(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 χ(T ) and C(T ) evidence ferromagnetic (FM) order at T C = 15 K. Thus, CeRuPO seems to be one of the rare example of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at T N = 4.4 K despite only minor changes in lattice parameters and electronic configuration. Additional 31 P 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 ionic like bonding between the layers.

We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr 3+ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La 3+and Pr 3+ , on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizeable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by ab-initio density-functional theory calculations.

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