Abstract
The findings of a Fe-57 and Eu-151 Mossbauer spectroscopy investigation between 1.8 and 299.5 K of EuFeAs2 and the 14 K superconductor EuFe0.97Ni0.03As2 are reported. In both compounds, the Fe sublattice orders in the antiferromagnetic spin-density-wave fashion with the Neel temperatures and Fe saturation magnetic moments of 106.2(1.9) K, 0.78(1)mu(B )and 56.6(2.2) K, 0.47(1)mu(B), respectively. The Neel temperatures and the saturation hyperfine magnetic fields of the antiferromagnetically ordered Eu sublattice in both compounds are 44.4(5) K, 294.2(7) kOe and 43.5(1) K, 290.5(1) kOe, respectively. The 3% substitution of Fe by Ni in EuFeAs2 has a dramatic effect on the magnetism of the Fe sublattice and virtually no effect on the magnetism of the Eu sublattice. The presence of Fe and Eu magnetic order in EuFe0.97Ni0.03As2 is direct proof of the coexistence of superconductivity and magnetism. The increase of the magnitude of the main component of the electric field gradient tensor, at both Fe and Eu sites, with decreasing temperature is explained by a T-3/2 power-law relation. The Debye temperatures of EuFeAs2, EuFe0.97Ni0.03As2, and the FeAs2 impurity phase are 355(18), 428(14), and 594(25) K, respectively.