  A millisecond magnetar engine has been widely suggested to exist in gamma-ray
burst (GRB) phenomena, in view of its substantial influences on the GRB
afterglow emission. In this paper, we investigate the effects of the magnetar
engine on the supernova (SN) emission which is associated with long GRBs and,
specifically, confront the model with the observational data of SN 2006aj/GRB
060218. SN 2006aj is featured by its remarkable double-peaked
ultraviolet-optical (UV-opt) light curves. By fitting these light curves, we
demonstrate that the first peak can be well accounted for by the breakout
emission of the shock driven by the magnetar wind, while the primary supernova
emission is also partly powered by the energy injection from the magnetar. The
magnetic field strength of the magnetar is constrained to be $\sim 10^{15}$ G,
which is in good agreement with the common results inferred from the afterglow
emission of long GRBs. In more detail, it is further suggested that the UV
excess in the late emission of the supernova could also be due to the leakage
of the non-thermal emission of the pulsar wind nebula (PWN), if some special
conditions can be satisfied. The consistency between the model and the SN
2006aj observation indicates that the magnetar engine is likely to be
ubiquitous in the GRB phenomena and even further intensify their connection
with the phenomena of superluminous supernovae.
