CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations

The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO_1-xF_x, 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 T_K∼10  K, a Sommerfeld coefficient γ=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≃0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the nonmagnetic side of a ferromagnetic instability.