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 LaFeAsO1-xFx was studied over a broad doping, temperature, and magnetic field range (x = 0-0.15, T <= 480 K, mu H-0 <= 30 T) by means of As-75 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.