Coexistence of Ferromagnetic and Antiferromagnetic Fluctuations in Iron-germanide Superconductor

The recently discovered iron-germanide superconductor YFe₂Ge₂ is of great interest because it is a possible new unconventional iron-based superconductor other than iron pnictides/chalcgenides. In contrast to the parent compounds of most iron-based superconductors, YFe₂Ge₂ does not exhibit long range magnetic order, but instead shows superconductivity below 1.8K. The nature of its magnetism has not been determined. Here, we report inelastic neutron scattering measurements of single crystalline YFe₂Ge₂. Both stripe and ferromagnetic spin fluctuations are observed over a wide range of energies. These results suggest that the magnetic ground state is highly frustrated and that both stripe spin fluctuations and ferromagnetic fluctuations are candidates for mediating the electron pairing in YFe₂Ge₂.