Intrinsic defects and grain boundaries formulated magnetism and dielectric response in ZnO:(Mn + Tb) nanocrystals

•Zn0.98−xMn0.02TbxO (x = 3%, 5%) nanocrystals exhibit room temperature ferromagnetism.•The specific grain boundary area is higher than the threshold value.•Mn and Tb/Mn co-doping induced various intrinsic defects in these nanocrystals.•Dielectric parameters were significantly enhanced by (Tb, Mn) co-doping.•Intrinsic defects and grain boundaries formulate magnetism and dielectric response.