Reversible room temperature ferromagnetism in undoped zinc oxide: Correlation between defects and physical properties

We report a systematic study of the structural, chemical, electrical, optical, and magnetic properties of undoped ZnO thin films grown under different conditions as well as the films that were annealed in various environments. Oxygen-annealed films displayed a sequential transition from ferromagnetism to diamagnetism as a function of the annealing temperature. An increase in the green band intensity has been observed in oxygen-annealed ZnO films. Reversible switching of room-temperature ferromagnetism and n-type conductivity have been demonstrated by oxygen and vacuum annealing. Electron paramagnetic resonance data were found to be in agreement with the results of magnetization and conductivity measurements. Possibility of external ferromagnetic impurity as the origin of the unconventional room temperature ferromagnetism in these films has been ruled out by secondary ion mass spectrometer and electron energy loss spectroscopy studies. Correlation between structural, electrical, optical, and magnetic properties has been established in terms of defects and defect complexes. Taken together, our data indicate that the ferromagnetic order in ZnO matrix might be defect-mediated. (C) 2010 American Institute of Physics. [doi:10.1063/1.3491037]