Magnetic and Structural Phase Transitions of EuFe2As2 Studied via Neutron and Resonant X-ray Scattering

Employing neutron and resonant x-ray scatterings, we have investigated the magnetic and structural phase transitions of a high quality single crystal of undoped pnictide EuFe2As2 to reveal the nature of these transitions. It has been found that structural order parameter delta = (a - b)/(a + b) and the square of Fe magnetic order parameter have the same temperature dependence, which is attributed to the linear-quadratic coupling between structural and Fe magnetic order parameters in the Landau free-energy expansion. This result also suggests that Fe magnetic order drives the structural phase transition which is the weak first-order phase transition.