Critical behavior of the spin density wave transition in underdoped Ba(Fe1−xCox)2As2 (x≤0.05): As75 NMR investigation

We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an iron-based high-Tc superconductor Ba(Fe1−xCox)2As2 by As75 NMR, with primary focus on a composition with x=0.02 (TSDW=99 K). We demonstrate that critical slowing down toward the three-dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition Ts=105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and TSDW, the dynamical structure factor of electron spins S(q,ωn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law 1/T1∝ΣqS(q,ωn)∼(T/TSDW−1)−δ with the critical exponent δ∼0.33.