Diluted magnetic oxides have evolved into a popular branch of materials science during the last decade. In the first few years,people attributed the ferromagnetism to the magnetic dopants. However,the observation of ferromagnetism in undoped HfO_2 thin films made it more controversial and promoted extensive research on the ferromagnetism in various undoped oxides. Both of the experimental works and theoretical studies have shown that intrinsic defects in oxide nanomaterials play a crucial role in the origin of such an unexpected ferromagnetism,in spite of some contradicting views which kind of defects is predominant. In the past several years,we have conducted systematic and thorough research on the room temperature ferromagnetism in undoped ZrO_2 thin films,and clarify some physics behind it. We firstly prepared undoped ZrO_2 thin films by different ways,such as Pulsed electron beam deposition,magnetron sputtering,and electron beam evaporation,and successfully obtained ZrO_2 thin films with different crystalline structure,in particular a pure high-temperature stabilized one,by adjusting some preparation parameters during the deposition process or post-annealing treatment. A phase-dependent ferromagnetism was then confirmed to exist in such ZrO_2 thin films. Further,we conducted exhaustive defect analysis and characterization by X-ray photoelectron spectroscopy,photoluminescence spectra,and electron paramagnetic resonance,respectively,and found the oxygen vacancy,specifically the single ionized oxygen vacancy( V_O^+),has a remarkable influence on the enhancement of ferromagnetism. Herein,we will review the work in detail on the phase-dependent and oxygen vacancy-enhanced room temperature ferromagnetism in undoped ZrO_2 thin films.
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