  The magnetic analogue of the Josephson effect can be exploited to develop a
new class of nano-spin oscillators that we denote as spin superfluid Josephson
oscillators. Such a device, consisting of two exchange coupled easy-plane
metallic ferromagnets separated by a thin normal metal spacer, is proposed and
analyzed. A spin chemical potential difference drives a $2\pi$ precession of
the in-plane magnetization of each ferromagnet. The $2 \pi$ precession angle
gives maximum values of the giant magnetoresistance, resulting in large output
power compared to conventional spin Hall oscillators. An applied ac current
results in a time-averaged magnetoresistance with Shapiro-like steps. The
multistate mode-locking behavior exhibited by the spin Shapiro steps may be
explored for applications in neuromorphic computing. As an experimental
characterization method, electrical measurements of spin superfluid Josephson
junctions can provide additional signatures of spin superfluidity.
