Critical behavior of the spin density wave transition in underdoped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Ba(Fe</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Co</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>)</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>x</mml:mi><mml:mo>≤</mml:mo><mml:mn>0.05</mml:mn></mml:mrow></mml:math>):<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">As</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>75</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>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 75As 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, Ts = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and T_SDW, the dynamical structure factor of electron spins S(q,Wn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33.

Critical behavior of the spin density wave transition in underdopedBa(Fe1−xCox)2As2(x≤0.05):As75NMR investigation Journal Articles