Electron paramagnetic resonance in Zn1−xCoxO

► Zn1−xCoxO with 0.01≤x≤0.05 were synthesized by sol-gel method (citrate rute). ► For Co concentration higher than 3% the mean size particle are higher than 25 nm. ► There are shallow free radicals when the mean size particle is higher than 25 nm. ► Ferromagnetism is the behavior in Co-doped ZnO.