  The merger of a double neutron star (NS-NS) binary may result in a rapidly
rotating massive NS with an extremely strong magnetic field (i.e., a
millisecond magnetar). In this case, the magnetic spin-down of the NS remnant
provides an additional source of sustained energy injection, which would
continuously power the merger ejecta. The thermal emission from the merger
ejecta would give rise to a bright optical "magnetar-powered merger-nova". In
this work, we carry out a complete search for magnetar-powered merger-nova from
Swift short gamma-ray burst (SGRB) sample. We focus on short GRBs with extended
emission or internal plateau, which may signify the presence of magnetars as
the central engine. We eventually find three candidates of magnetar-powered
merger-nova from the late observations of GRB 050724, GRB 070714B and GRB
061006. With standard parameter values, the magnetar remnant scenario could
well interpret the multi-band data of all three bursts, including the extended
emission and their late chromatic features in the optical and X-ray data. The
peak luminosities of these merger-novae reach several times $10^{42}~{\rm
erg~s^{-1}}$, more than one order of magnitude brighter than the traditional
"kilo-novae" with peak luminosity of $\sim 10^{41}~{\rm erg~s^{-1}}$. Intense,
multi-color late time observations of short GRBs are encouraged to identify
more merger-novae in the future.
