Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic transition in LaFeAs(O_{1-x}F_x) probed by ^{75}As and ^{139}La NMR

${}^{75}$As and ${}^{139}$La NMR results of LaFeAs(O${}_{1-x}$F${}_x$) ($x$=0, 0.025, and 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic structural phase transition temperature $T_S$, antiferromagnetic transition temperature $T_N$ and internal magnetic field ${\mu }_0{H}_{\mathrm{i}\mathrm{n}\mathrm{t}}$ 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 ${\mu }_0{H}_{\mathrm{i}\mathrm{n}\mathrm{t}}$, which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co)${}_2$As${}_2$ and BaFe${}_2$(As,P)${}_2$. The anisotropy of the spin-lattice relaxation rate $T_1^{-1}$, $(T_1)^{-1}{H\parallel ab}/(T_1)^{-1}{H\parallel c}$,intheparamagneticphaseof$x = 0$and0.025isconstant($\sim 1.5$), but increases abruptly below $T_S$ due to the enhancement of $(T_1{)}_{H\parallel ab}^{-1}$ 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.