Spin fluctuation observed by ⁷⁵As NMR relaxation rate in iron-pnictide superconductor Ba_0.66K_0.34Fe₂As₂ : Comparison with nematic fluctuation

Spin-echo decay rate 1/T₂ was measured to investigate the low-energy spin fluctuations in the hole-doped iron superconductor Ba_0.66K_0.34Fe₂As₂. In a wide temperature range, the relaxation curves are well-fitted by considering the Lorentzian and the Gaussian decay components, and the relaxation rates 1/T_2L and 1/T_2G were obtained. In general, the 1/T_2L is described by contributions of longitudinal magnetic fluctuations along the applied field and transverse one as the nuclear spin-lattice relaxation rate 1/T₁[1-3]. We elucidated that the 1/T_2L is well-scaled with the 1/T₁ , indicating that the contribution of 1/T₁ is dominant. Both 1/T_2L and 1/T₁ increase on cooling toward the superconducting temperature T_c, implying the development of magnetic fluctuations. The magnetic anisotropy observed both in 1/T_2L and 1/T₁ suggests that the striped magnetic fluctuations develop toward T_c. Below 100 K, the magnetic anisotropy in 1/T_2L is slightly smaller than that in 1/T₁, possibly indicating slight longitudinal magnetic fluctuations. On the contrary, the Gaussian component 1/T_2G is temperature-independent, resulting from the direct dipole interactions between ⁷⁵As nuclear spins. The comparison between the 1/T_2LT monitoring spin fluctuations and the nematic susceptibility suggests a possible coupling between spin and orbital fluctuations, but the coupling is weaker than that in electron-doped systems (e.g. LaFe_1-xCo_xAsO and Ba(Fe_1-xCo_x)₂As₂))[4-6]. The origin of the weaker coupling in a hole-doped system Ba_0.66K_0.34Fe₂As₂ will be discussed.