⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy and magnetization studies of Eu(Fe_0.89Co_0.11) ₂As₂ and Eu(Fe_0.9Ni_0.1) ₂As₂

The superconducting compound Eu(Fe_0.89Co_0.11) ₂As₂ and the normal conducting compound Eu(Fe _0.9Ni_0.1)₂As₂ were studied by ¹⁵¹Eu and ⁵⁷Fe Mössbauer spectroscopy (MS) and magnetometry at temperatures ranging from 5 to 297 K. The ¹⁵¹Eu MS studies showed that in both compounds, the europium magnetic moments order helically (at T_M = 18 and 20 K, respectively) with a tilting angle of about 36° to the c-axis, whereas in pure EuFe₂As₂ where the Fe is magnetically ordered, they order perpendicular to the c-axis. We concluded that this change is due to the disappearance of iron magnetism in the mixed compounds. Although the iron ions are nonmagnetic in both the superconducting and the normal conducting compound, ⁵⁷Fe MS studies display, at 5 K, magnetic hyperfine structure due to transferred magnetic hyperfine fields (∼1 T) from the magnetically ordered Eu sublattice.