A 75 s NMR study of oriented CeFeAsO and CeFeAsO 0.84 F 0.16

We report the resistivity and $^{75}\text{A}\text{s}$ nuclear magnetic resonance (NMR) results of superconducting ${\text{CeFeAsO}}_{0.84}{\text{F}}_{0.16}$ and its parent compound CeFeAsO. The derivative of the resistivity with temperature and $^{75}\text{A}\text{s}$ NMR in CeFeAsO clearly show a signature of the onset of the long-range magnetic ordering. The resistivity in ${\text{CeFeAsO}}_{0.84}{\text{F}}_{0.16}$ drops sharply at 41 K and becomes zero at 38 K. Analyzing the $^{75}\text{A}\text{s}$ NMR spectra in ${\text{CeFeAsO}}_{0.84}{\text{F}}_{0.16}$ and CeFeAsO, we obtained the value of quadrupolar splitting frequency ${\ensuremath{\nu}}_{Q}=11.0$ and $9.9(\ifmmode\pm\else\textpm\fi{}0.2)\text{ }\text{MHz}$ and asymmetry parameter $\ensuremath{\eta}=0.09$ and 0.02, respectively. The shift $({K}_{ab})$ in the oriented $(H\ensuremath{\parallel}ab)$ ${\text{CeFeAsO}}_{0.84}{\text{F}}_{0.16}$ sample starts to decrease from 60 K (above ${T}_{c}$). Below 30 K, the NMR line broadens asymmetrically due to the formation of vortex lattice and the observed NMR spectra at 4\char21{}10 K clearly resemble the Redfield pattern.