Mössbauer Spectroscopic Study of Cobalt-doped Superconductor CaFe<sub>1-x</sub>Co<sub>x</sub>AsF (x=0.0, 0.1)

We have studied magnetic properties of cobalt-doped and undoped CaFe_1-xCo_xAsF (x=0.0, 0.1) by using XRD, EDX, SQUID, and ⁵⁷Fe Mössbauer spectroscopy through wide temperature range. Polycrystalline samples were synthesized by 2-step solid-state reaction method using high purity Ca, Fe, Co, As and CaF₂ powders. The structural and the magnetic aspects of the observations have been examined by comparing results of the cobalt doped and the parent compounds together. The strategy of Mössbauer spectroscopic analysis has been set in terms of the structural and the spin density wave characteristics, according to which the temperature dependence of hyperfine parameters have been explained. Subsequently, various constituent phases in the materials have been separated out by comparing the results obtained from the SQUID with those from the Mössbauer spectroscopy. It has been found out that undoped CaFeAsF has two different kinds of phase transition happening around ~120 K, about 10 K apart from each other. On the other hand, M-T curve of cobalt-doped CaFe_0.9Co_0.1AsF showed that the spin density wave phase and the superconducting phase coexisted below ~120 K, and finally displayed the onset of the superconductivity at 22 K. Moreover, superconductive CaFe_0.9Co_0.1AsF at 22 K still showed paramagnetic doublet, indicating there was no abrupt changes in hyperfine parameters observed across either the spin density wave transition or the superconductive transition.