Magnetic neutron diffraction study of Ba(Fe_1-xCo _x)₂As₂ critical exponents through the tricritical doping

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe_1-xCo_x)₂As₂ for x=0.039, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the (x,T) plane, x_tr≈0. 022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long-range antiferromagnetic order that may be described near the transition by simple power laws, with β=0.306±0.060 for x=0.039, β=0.208±0. 005 for x=0.022, and β=0.198±0.009 for x=0.021. For the x=0.039 sample, the data are reasonably well described by the order parameter exponent β=0.326 expected for a three-dimensional Ising model while the x=0.022 and x=0.021 samples are near the β=0.25 value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.