We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe1−xCox)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, xtr≈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 3D 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.
@article{arxiv.1303.4775,
title = {Magnetic neutron diffraction study of Ba(Fe1-xCox)2As2 critical exponents through the tricritical doping},
author = {D. M. Pajerowski and C. R. Rotundu and J. W. Lynn and R. J. Birgeneau},
journal= {arXiv preprint arXiv:1303.4775},
year = {2015}
}