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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2012 Apr 4|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics