  Diluted ferromagnetic semiconductors (DMSs) that combine the properties of
semiconductors with ferromagnetism have potential application in spin-sensitive
electronics (spintronics) devices. The search for DMS materials exploded after
the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a
series of DMS compounds isostructural to iron-based superconductors have been
reported. Among them, the highest Curie temperature $T_C$ of 230 K has been
achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As,
are p-type, i.e., the carriers that mediate ferromagnetism are holes. For
practical applications, DMS with n-type carriers are also advantageous. Here we
report the successful synthesis of a II-II-V diluted ferromagnetic
semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization
measurements show that the ferromagnetic transition occurs up to $T_{C} \sim$
45 K. Hall effect and Seebeck effect measurements jointly confirm that the
dominant carriers are electrons. Through muon spin relaxation ($\mu$SR), a
volume sensitive magnetic probe, we have also confirmed that the ferromagnetism
in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static.
