Observation of two-dimensional spin fluctuations in the bilayer ruthenate ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ by inelastic neutron scattering

We report an observation of two-dimensional incommensurate magnetic fluctuations in the layered metallic perovskite ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7.$ The wave vectors where the magnetic fluctuations are strongest are different from those observed in the superconducting single layer ruthenate ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and appear to be determined by Fermi surface nesting. No antiferromagnetic ordering is observed for temperatures down to 1.5 K. For temperatures $T\gtrsim 20\mathrm{K},$ the fluctuations become predominately ferromagnetic. Our inelastic neutron scattering measurements provide concrete evidence of the coexistence of competing interactions in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ and of the low-energy scale of the fluctuations.