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.
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