Coexistence of superconductivity and ferromagnetism in BaFe$_{1.8}$Co$_{0.2}$As$_2$

Thermal expansion and heat capacity measurements were performed on three single crystals of BaFe$_{1.8}$Co$_{0.2}$As$_2$ with superconducting transition temperatures $T_c$ = 16.5, 19, and 22 K . The thermal expansion coefficients $\mu_i$ (i = a and c axis) are highly anisotropic. Magnetization measurements reveal the presence of ferromagnetism at the same transition temperature as superconductivity in some of the samples. The ferromagnetism has a small moment on the order of 0.5x10$^{-3}$ $\mu_B$/Fe ($\mu_B$ is the Bohr magneton). Thus, two phases: superconductivity and magnetism coexist in some BaFe$_{1.8}$Co$_{0.2}$As$_2$ samples. This ferromagnetism could be associated with a canted (non- collinear) antiferromagnetic order. This material is based upon work supported by the U. S. Department of Energy (DE-FG02-07ER46269) and the National Science Foundation (DMR-050476 and DMR-0552458).