Interplay of Rare Earth and Iron magnetism in RFeAsO with R = La, Ce, Pr, and Sm: A muon spin relaxation study and symmetry analysis

TL;DR

This study uses muon spin relaxation to investigate the magnetic interactions between iron and rare earth elements in RFeAsO compounds, revealing complex magnetic structures and interactions that influence magnetic ordering but are likely not key to superconductivity enhancement.

Contribution

It provides detailed microscopic insights into the magnetic structures and interactions in RFeAsO compounds, especially highlighting the non-Heisenberg nature of R-Fe coupling and the impact of R magnetic order.

Findings

Fe moments are consistent across R series with less than 15% variation.

Long-range R magnetic order is observed at low temperatures.

Strong Fe-Ce coupling results in induced Ce magnetization above T_N{Ce}.

Abstract

We report zero field muon spin relaxation (muSR) measurements on RFeAsO with R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R (rare earth) electronic systems in the non superconducting magnetically ordered parent compounds of RFeAsO{1-x}Fx superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Moessbauer 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 RFeAsO with R = 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 Fe--R Heisenberg interactions in RFeAsO. 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{Ce}. Finally, we argue that the magnetic R--Fe interaction is probably not crucial for the observed enhanced superconductivity in RFeAsO{1-x}Fx with a magnetic R ion.