First-order phase transitions in CaFe2As2 single crystal: a local probe study

57Fe Mossbauer spectroscopy has been used to investigate the structural and magnetic phase transitions of CaFe2As2 (TN = 173 K) single crystals. For this compound we found that VZZ is positive and parallel to the c-axis of the tetragonal structure. For CaFe2As2 a magnetic hyperfine field Bhf was observed at the 57Fe nucleus below TN ~ 173 K. Analysis of the temperature dependence of Bhf data using the Bean–Rodbell model shows that the Fe spins undergo a first-order magnetic transition at ~ 173 K. A collinear antiferromagnetic structure is established below this temperature with the Fe spin lying in the (a, b) plane. Below TN the paramagnetic fraction of Fe decreases down to 150 K and for lower temperatures all the Fe spins are magnetically ordered.