Interplay between Fe 3d and Ce 4f magnetism and Kondo interaction in CeFeAs_{1-x}P_{x}O probed by ^{75}As and ^{31}P NMR

Abstract

A detailed ${}^{31}$P (I=1/2) and ${}^{75}$As (I=3/2) NMR study on polycrystalline CeFeAs${}_{1-x}$P${}_x$O alloys is presented. The magnetism of CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a heavy fermion system without long range order whereas CeFeAsO exhibits a Fe-3$d$ SDW type of ordering accompanied by a structural transition from tetragonal ($TT$) to orthorhombic ($OT$) structure. Furthermore Ce-4$f^1$ orders antiferromagnetically (AFM) at low temperature. At the critical concentration where the Fe-magnetism is diminished the Ce-Ce interaction changes to a ferromagnetic (FM) type of ordering. Three representative samples of the CeFeAs${}_{1-x}$P${}_x$O ($x$=0.05, 0.3 and 0.9) series are systematically investigated. 1) For the $x$=0.05 alloy a drastic change of the line width at 130 K indicates the AFM-SDW type of ordering of Fe and the structural change from $TT$ to $OT$ phase. The line width roughly measures the internal field in the ordered state and the transition is most likely first order. The small and nearly constant shift for ${}^{31}$P and ${}^{75}$As suggests the presence of competing hyperfine interactions between the nuclear spins and the 4$f$ and 3$d$ ions of Ce and Fe. 2) For the $x$=0.3 alloy, evolution of the Fe-SDW type order takes place at around 70 K corroborating the results of bulk measurement and $\hat{I}¼SR$. Here we found strong evidence for phase separation of paramagnetic and magnetic SDW phases. 3) In contrast to heavy fermion CeFePO for the $x$=0.9 alloy a phase transition is found at 2 K. The field-dependent NMR shift gives strong evidence of FM ordering. Above the ordering the spin lattice relaxation rate ${}^{31}$(1/T${}_1$) shows unconventional, non-Korringa like behaviour which indicates a complex interplay of Kondo and FM fluctuations.

10 (+) pages, 11 figures, Accepted in Journal of Physics condensed Matter