Canted ferromagnetic order in nonsuperconducting Eu(Fe1-xNix)2As2

The magnetic order in a series of nonsuperconducting Eu(Fe1-xNix)2As2 single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for x 0.02, in which the spins lie in the ab plane, to a pure canted ferromagnetic order for 0.04 x 0.20, in which the spins are canted with a small angle off the c axis. By comparison with the c-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in Eu(Fe1-xNix)2As2 might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu2+ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the Eu2+ moments in an intermediate temperature range is observed for x = 0.20, in which some in-plane spin fluctuations persists above the long-range ordering temperature TEu.