  Technical advancement of oxide molecular beam epitaxy (MBE) has opened new
avenues for studying various quantum transport phenomena in correlated
transition-metal oxides, as exemplified by the exotic superconductivity of
Sr$_2$RuO$_4$ and quantum oscillations of SrRuO$_3$. On the other hand, film
research of another Ruddlesden-Popper strontium ruthenate Sr$_3$Ru$_2$O$_7$
which exhibits a unique quantum phase related to metamagnetism in bulk systems
did not progress well. Here we report the fabrication of high-quality
Sr$_3$Ru$_2$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 Sr$_{n+1}$Ru$_n$O$_{3n+1}$, which sensitively depends on the ratio
of in-plane to out-of-plane Ru-Ru distances.
