Donor defects enhanced ferromagnetism in Co:ZnO films

We experimentally report the role of donor defects on the origin of room temperature ferromagnetism in Zn0.985Co0.015O films. Undoped ZnO, Co:ZnO and (Co, N):ZnO thin films are fabricated by radio-frequency magnetron sputtering. The local structural measurements indicate that Co2+ substitutes for Zn2+ in the ZnO wurtzite lattice with the upper limit of ∼ 10% Co metal of the total cobalt dopants. The Raman spectra reveal vibrational modes at 273, 470, 639, 691 and 854 cm-1 in addition to the host phonons of ZnO, indicating the substitutional behavior of N3- for O2- and the increase in donor defects. Acceptor doping with nitrogen compensates the electron carriers and hence decreases the electron carrier concentration. Combined with the enhancement of ferromagnetism, it is found that the donor defects other than electron carriers are responsible for the room temperature ferromagnetism in Co:ZnO films