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

Yusuke Nakai; Shunsaku Kitagawa; Tetsuya Iye; Kenji Ishida; Yoichi Kamihara; Masahiro Hirano; Hideo Hosono

doi:10.1103/PhysRevB.85.134408

paper

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

arXiv:1203.4937 · doi:10.1103/PhysRevB.85.134408

Abstract

$^{75}$ 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 $μ_{0} H_{int}$ 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 $μ_{0} H_{int}$ , 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 $T_{1}^{- 1}$ , $(T_{1})_{H ∥ ab}^{- 1} / (T_{1})_{H ∥ c}^{- 1}$ , 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 $(T_{1})_{H ∥ ab}^{- 1}$ 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.

5 pages, 5 figures, to appear in Phys. Rev. B

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

Abstract

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