57Fe and 151Eu Moessbauer spectroscopy and magnetization studies of Eu(Fe0.89Co0.11)2As2 and Eu(Fe0.9Ni0.1)2As2

The superconducting compound Eu(Fe0.89Co0.11)2As2 and the normal conducting compound Eu(Fe0.9Ni0.1)2As2 were studied by 151Eu and 57Fe Moessbauer 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 TM=18 and 20 K, respectively) with a tilting angle of about 360 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.