We report zero-field muon spin relaxation $\left(\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}\text{SR}\right)$ measurements on $R\text{FeAsO}$ with $R=\text{La}$, Ce, Pr, and Sm. We study the interaction of the FeAs and $R$ (rare-earth) electronic systems in the nonsuperconducting magnetically ordered parent compounds of $R{\text{FeAsO}}_{1-x}{\text{F}}_x$ superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Mössbauer spectroscopy and neutron-scattering data. These studies provide microscopic evidence of long-range commensurate magnetic Fe order with the Fe moments not varying by more than 15% within the series $R\text{FeAsO}$ with $R=\text{La}$, Ce, Pr, and Sm. At low temperatures, long-range $R$ magnetic order is also observed. Different combined Fe and $R$ magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of $R$ order on the iron subsystem in the case of different symmetry of Fe and $R$ order parameters resulting in a Fe spin reorientation in the $R$-ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear $\text{Fe-}R$ Heisenberg interactions in $R\text{FeAsO}$. A strong Fe-Ce coupling due to non-Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above $T_N^{\text{Ce}}$. Finally, we argue that the magnetic $R\text{-Fe}$ interaction is probably not crucial for the observed enhanced superconductivity in $R{\text{FeAsO}}_{1-x}{\text{F}}_x$ with a magnetic $R$ ion.

• Received 17 April 2009

DOI:https://doi.org/10.1103/PhysRevB.80.094524