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$^{75}$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-orthorhombicstructural phase transition temperature $T_S$, antiferromagnetic transitiontemperature $T_N$ and internal magnetic field $\mu_0H_{\rm int}$ are graduallyreduced for $x<0.04$. However, at $x=0.04$, $T_N$ is abruptly suppressed to be30 K along with a tiny $\mu_0H_{\rm int}$, which is distinct from thecontinuous disappearance of the ordered phases in the Ba122 systems ofBa(Fe,Co)$_2$As$_2$ and BaFe$_2$(As,P)$_2$. The anisotropy of the spin-latticerelaxation rate $T_1^{-1}$, $(T_1)^{-1}_{H\parallel ab}/(T_1)^{-1}_{H\parallelc}$, in the paramagnetic phase of $x = 0$ and 0.025 is constant ($\sim 1.5$),but increases abruptly below $T_S$ due to the enhancement of$(T_1)^{-1}_{H\parallel ab}$ by the slowing down of magnetic fluctuations. Thisindicates that the tetragonal-to-orthorhombic structural distortion enhancesthe anisotropy in the spin space via magnetoelastic coupling and/or spin-orbitinteraction.

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