Magnetic Interactions on Single-Crystal EuFe2As2 Studied by 57Fe Mossbauer Spectroscopy

Magnetic interactions on single-crystal AFe(2)As(2) (A: Eu and Sr) were studied by Fe-57 Mossbauer spectroscopy. An abrupt appearance of the effective magnetic hyperfine field (B-eff) is observed at the spin density wave transition temperature (similar to 190K for Eu and similar to 200K for Sr) in both samples. The value of B-eff in EuFe2As2 shows a small but significant increase around the Neel temperature T-N(Eu) (=19 K) of the Eu2+ sublattice, while B-eff in SrFe2As2 almost saturates at lower temperatures. Consistent with this, the isomer shift of EuFe2As2 indicates an anomalous temperature dependence below approximately T-N(Eu). These results provide direct evidence of the coupling between the localized Eu2+ moments and the conduction electrons from the FeAs layers and suggest that this coupling may affect the observed superconductivity in EuFe2As2 under pressure.