CeRuPO: A rare example of a ferromagnetic Kondo lattice

Abstract

We have determined the physical ground state properties of the compounds CeRuPO and CeOsPO by means of magnetic susceptibility $\ensuremath{\chi}(T)$, specific heat $C(T)$, electrical resistivity $\ensuremath{\rho}(T)$, and thermopower $S(T)$ measurements. $\ensuremath{\chi}(T)$ reveals a trivalent $4{f}^{1}$ cerium state in both compounds. For CeRuPO a pronounced decrease of $\ensuremath{\rho}(T)$ below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ indicates the onset of coherent Kondo scattering, which is confirmed by enhanced $S(T)$. The temperature and magnetic field dependence of $\ensuremath{\chi}(T)$ and $C(T)$ evidence ferromagnetic (FM) order at ${T}_{C}=15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Thus, CeRuPO seems to be one of the rare examples of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at ${T}_{N}=4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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.