Suppression of spin-density-wave transition and emergence of ferromagnetic ordering of Eu2+ moments in EuFe2-xNixAs2

We present a systematic study on the physical properties of EuFe2−xNixAs2 (0≤x≤0.2) by electrical resistivity, magnetic susceptibility, and thermopower measurements. The undoped compound EuFe2As2 undergoes a spin density wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu2+ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu2+ moments. For x≥0.06, the magnetic ordering of Eu2+ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for x≥0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in the EuFe2−xNixAs2 system are discussed.