Effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles

► The H2-annealing induced a large enhancement of ferromagnetism in Cr-doped ZnO. ► Upon hydrogen annealing the coercivity of Cr-doped ZnO nanoparticles increases. ► PL data show a large enhancement of defect-related UV emission upon H2-annealing. ► The field-cooled M-T curves are well fitted by a combined theoretical model. ► The shallow donor defect or/and defect complexes may contribute to the magnetism.