Critical behavior of the spin density wave transition in underdoped Ba(Fe1-xCox)(2)As-2 (x <= 0.05): As-75 NMR investigation

We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an iron-based high-T-c superconductor Ba(Fe1-xCox)(2)As-2 by As-75 NMR, with primary focus on a composition with x = 0.02 (T-SDW = 99 K). We demonstrate that critical slowing down toward the three-dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition T-s = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between T-s and T-SDW, the dynamical structure factor of electron spins S(q,omega(n)) measured with the longitudinal NMR relaxation rate 1/T-1 exhibits a divergent behavior obeying a power law 1/T-1 proportional to Sigma S-q(q,omega(n)) similar to (T/T-SDW - 1)(-delta) with the critical exponent delta similar to 0.33.