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

${}^{75}$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 ${\mu }_0{H}_{\mathrm{int}}$ 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 ${\mu }_0{H}_{\mathrm{int}}$, 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 $T_1^{-1}$, $(T_1{)}_{H\parallel ab}^{-1}/(T_1{)}_{H\parallel c}^{-1}$, 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 $(T_1{)}_{H\parallel ab}^{-1}$ 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.

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

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• Nematicity and magnetism in FeSe and other families of Fe-based superconductors
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• NMR Search for the Spin Nematic State in LaFeAsO Single Crystal
• Crystallographic, Electronic, Thermal and Magnetic Properties of Single-Crystal SrCo2As2
• Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• Progressive slowing down of spin fluctuations in underdoped LaFeAsO${}_{1-x}$F${}_x$
• Crystallography and Physical Properties of BaCo2As2, Ba{0.94}K{0.06}Co2As2 and Ba{0.78}K{0.22}Co2As2
• Relationship between Superconductivity and Antiferromagnetism in LaFe(As${}_{1-x}$P${}_x$)O Revealed by ${}^{31}$P-NMR
• Antiferromagnetic spin correlations and pseudogap-like behavior in Ca(Fe1-xCox)2As2 studied by 75As nuclear magnetic resonance and anisotropic resistivity
• A unified phase diagram of F-doped LaFeAsO by means of NMR and NQR parameters
• Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• Mapping out the spin fluctuations in Co-doped LaFeAsO single crystals by NMR
• Suppression of superconductivity and structural phase transition against pressure in Ca${}_{10}$(Ir${}_4$As${}_8$)(Fe${}_{2-x}$Ir${}_x$As${}_2$)${}_5$