Magnetic neutron diffraction study of Ba(Fe1-xCox)2As2 critical exponents through the tricritical doping

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe1-xCox)(2)As-2 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) approximate to 0.022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples showlong-range antiferromagnetic order that may be described near the transition by simple power laws, with beta = 0.306 +/- 0.060 for x = 0.039, beta = 0.208 +/- 0.005 for x = 0.022, and beta = 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 beta = 0.326 expected for a three-dimensional Ising model while the x = 0.022 and x = 0.021 samples are near the beta = 0.25 value for a tricritical system in themean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions. DOI: 10.1103/PhysRevB.87.134507