Progressive slowing down of spin fluctuations in underdoped LaFeAsO1−xF

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO1−xFx was studied over a broad doping, temperature, and magnetic field range (x= 0–0.15, T≤ 480 K, μ0H≤ 30 T) by means of 75As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature (T1T)^−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 (T1T)^−1 clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.