The ferromagnetic properties of (Zn, Co)O were observed by a magnetometer at low temperatures. This is, however, inconsistent with the results from the magneto-optical measurements. They reflect the presence of Co metal precipitates in (Zn, Co)O. On the other hand, the formation of Co metal precipitates was strongly suppressed by the additional Cu doping. It appears that the formation of Co metal precipitates is related to the carrier concentration in the system.
