Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic transition in LaFeAs(O _1-xF _x) probed by 75As and 139La NMR

75As and 139La NMR results for LaFeAs(O _1-xF _x) (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 μ ₀H _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 μ ₀H _int, which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co) ₂As ₂ and BaFe ₂(As,P) ₂. The anisotropy of the spin-lattice relaxation rate T1-1, (T ₁)H∥ab-1/ (T ₁)H∥c-1, in the paramagnetic phase of x=0 and 0.025 is constant (∼1.5) but increases abruptly below T _S due to the enhancement of (T ₁)H∥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.