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