³¹P NMR studies of an iron-based superconductor Ba_0.5Sr_0.5Fe₂(As_1−xP_x)₂ with T_c = 29 K

We report ³¹P NMR studies of an oriented polycrystalline superconductor of Ba_0.5Sr_0.5Fe₂(As_1−xP_x)₂ with x ∼ 0.4 (T_c = 29 K) at T = 14−325 K and B₀ = 1 and 5 T. The ³¹P Knight shift K_ab at B₀ ⊥ c shows a nearly T-independent uniform spin susceptibility above T_c and a spin singlet decrease below T_c. The ³¹P nuclear spin-lattice relaxation rate 1/T₁ shows an asymptotic behavior of a + bT (a and b are constants) at T > 100 K and the minimum at 40 K with an upturn toward T_c. The a term in 1/T₁ indicates the presence of two-dimensional antiferromagnetic spin fluctuations. The negative θ = −15 K of the Curie-Weiss-type antiferromagnetic spin susceptibility $\chi (Q)\propto 1/(T+\theta )$ in the analysis of 1/T₁T suggests antiferromagnetic instability in the superconducting state. Discussions are made from the self-consistent renormalization (SCR) theory for the spin fluctuations with interlayer correlation.