Progressive slowing down of spin fluctuations in underdoped LaFeAsO 1 − x F x

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ was studied over a broad doping, temperature, and magnetic field range ($x=$ 0\char21{}0.15, $T\ensuremath{\le}$ 480 K, ${\ensuremath{\mu}}_{0}H\ensuremath{\le}$ 30 T) by means of ${}^{75}$As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature ${({T}_{1}T)}^{\ensuremath{-}1}$ in underdoped superconducting samples ($x=$ 0.045, 0.05, and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally doped ($x=0.10$) and completely absent in overdoped ($x=0.15$) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in ${({T}_{1}T)}^{\ensuremath{-}1}$ clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.