Observation of itinerant ferromagnetism in layered ${\mathrm{Sr}}_3$${\mathrm{Ru}}_2$${\mathrm{O}}_7$ single crystals

${\mathrm{Sr}}_3$ ${\mathrm{Ru}}_2$ ${\mathrm{O}}_7$ is a layered compound and a member of the Ruddlesden-Popper-type compound series of ${\mathrm{Sr}}_{\mathrm{n}+1\mathrm{}}$ ${\mathrm{Ru}}_{\mathrm{n}}$ ${\mathrm{O}}_{3\mathrm{n}+1}$ (n=1, 2, and n=∞) with n=2. Single crystals of ${\mathrm{Sr}}_3$ ${\mathrm{Ru}}_2$ ${\mathrm{O}}_7$ were grown and characterized, providing an unambiguous indication of a rich ferromagnetic state with ${\mathrm{T}}_{\mathrm{c}}$=104 K and a saturated moment of 1.30±0.05${\mathrm{\mu }}_{\mathrm{B}}$/Ru at T=5 K. Among many interesting features, a magnetic spin reorientation transition is observed, suggesting that moments in the ordered state order ferromagnetically with a slight canting on adjacent spins. Negative magnetoresistance, commonly occurring in ferromagnets, is relatively large (19% at 10 T and T=30 K), suggesting a significant suppression of spin-flip scattering by the applied magnetic field. The results are discussed with comparisons drawn with sister compounds ${\mathrm{Sr}}_2$ ${\mathrm{RuO}}_4$, ${\mathrm{SrRuO}}_3$ and other related systems.