Technical advancements in oxide molecular beam epitaxy (MBE) have opened new avenues for studying various quantum transport phenomena in correlated transition-metal oxides, as exemplified by the exotic superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and quantum oscillations of ${\mathrm{SrRuO}}_3$. On the other hand, film research of another Ruddlesden-Popper strontium ruthenate ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, which exhibits a unique quantum phase related to metamagnetism in bulk systems, has not progressed well. Here, we report the fabrication of high-quality ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ thin films by oxide MBE and the observation of a strain-induced ferromagnetic ground state. The change in magnetic exchange coupling evaluated by first-principles calculations indicates a systematic relation between the compression of the $c$-axis length and induced ferromagnetism. Giant epitaxial strain in high-quality films will be a key to a comprehensive understanding of the magnetism in Ruddlesden-Popper strontium ruthenates ${\mathrm{Sr}}_{n+1}{\mathrm{Ru}}_n{\mathrm{O}}_{3n+1}$, which sensitively depends on the ratio of in-plane to out-of-plane Ru-Ru distances.

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• Epitaxial strain
• Ruthenates
• First-principles calculations
• Molecular beam epitaxy