Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic transition in LaFeAs(O1-xFx) probed by 75As and 139La NMR

As-75 and La-139 NMR results for LaFeAs(O1-xFx) (x = 0, 0.025, and 0.04) are reported. Upon F doping, the tetragonal-to-orthorhombic structural phase transition temperature T-S, antiferromagnetic transition temperature T-N, and internal magnetic field mu H-0(int) are gradually reduced for x < 0.04. However, at x = 0.04, T-N is abruptly suppressed to be 30 K along with a tiny mu H-0(int), which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co)(2)As-2 and BaFe2(As,P)(2). The anisotropy of the spin-lattice relaxation rate T-1(-1) , (T-1)H-parallel to ab(-1)/(T-1)H parallel to c(-1), in the paramagnetic phase of x = 0 and 0.025 is constant (similar to 1.5) but increases abruptly below T-S due to the enhancement of (T-1)H-parallel to ab(-1) by the slowing-down of magnetic fluctuations. This indicates that the tetragonal-to-orthorhombic structural distortion enhances the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit interaction.