Fig. 3

Fe spin fluctuations. Temperature dependence of the ⁷⁵As spin–lattice relaxation rate divided by temperature (T ₁ T)⁻¹ measured at 15 T. The error bars reflect the uncertainty in the fitting procedure. At high temperatures, (T ₁ T)⁻¹ is well described by a Curie–Weiss (CW) law (solid line). Below ~240 K, it deviates from the diverging behavior and drops at lower temperatures, forming a large peak centered at ~190 K. At low temperatures below 120 K, (T ₁ T)⁻¹ reaches a constant value comparable to that observed at the high temperature limit, implying that the spin fluctuations are completely gapped out. (T ₁ T)⁻¹ sharply turns up at ~50 K, indicating critical slowing down of spin fluctuations toward a magnetic order. In the inset, T _N ~ 45 K (green arrow) was identified from the sharp peak observed for H || c. (T ₁ T)⁻¹ drops at T _c, determined by the resistivity measurements under H || ab (red arrow) and H || c (blue arrow), microscopically probing bulk superconductivity in the magnetically ordered state