Defect-mediated ferromagnetism and controlled switching characteristics in ZnO
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John T. Prater Materials Science Division, Army Research Office, Research Triangle Park, North Carolina 27709 (Received 21 December 2010; accepted 15 February 2011)
We report a detailed study of the structural, chemical, electrical, and magnetic properties of undoped ZnO thin films grown under different conditions and the films that were annealed in various environments and irradiated with an ultraviolet laser. Samples prepared in low oxygen pressure or subsequently annealed in vacuum have always been strongly magnetic. Oxygen-annealed films displayed a sequential transition from the ferromagnetic to the diamagnetic state as a function of the annealing temperature. Reversible switching of room temperature ferromagnetism and n-type conductivity have been demonstrated either by annealing in different environments or by a novel laser irradiation treatment. Enhancements in both the electrical conductivity and magnetic moment have been controlled precisely with laser pulses, without altering the crystal structure. Electron paramagnetic resonance data were found to be in good agreement with the magnetization and conductivity measurements. Our secondary ion mass spectrometer and electron energy loss spectrometer studies conclusively rule out the presence of any external ferromagnetic impurities.
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.74
such as TiO2, In2O3,9 and nonstoichiometric CaB6.10 In case of TiO2 and HfO2 systems, the ferromagnetism has been attributed to the presence of oxygen vacancies (VO).11 RTFM has also been reported in undoped ZnO powders, nanoparticles, nanorods, nanowires, and thin films.12–15 An important but often overlooked aspect of the controversy regarding the observation of ferromagnetism in TM-doped ZnO is related to the role of intrinsic defects, metal precipitates, and TM–defect complexes.16 However, the ferromagnetism in undoped ZnO is linked to the poor crystalline quality, strain, grain boundaries, and intrinsic defects, namely, VO, zinc vacancies (VZn), and zinc interstitials (IZn). Recent ab initio calculations have suggested that the magnetic moment arises from the unpaired 2p electrons at the O sites surrounding the VZn with each nearest neighbor O atom carrying a magnetic moment ranging from 0.49 to 0.74 lB.17 Recently, RTFM was induced in undoped ZnO powder by applying a mechanical force18 and then destroyed upon oxygen annealing. It was suggested in this case that the onset of FM had its origin in the unpaired spin states generated by mechanical strain and/or dislocations at the crystalline domain boundaries. In our experience, point defects such as VO, VZn, and IZn are always present in the ZnO films grown by PLD and that these intrinsic defects play an important role in tailoring the physical properties of the films. We find that undoped ZnO films can be tailored to be ferromagnetic by invoking nonstoichiometric processing conditions,
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Ó Materials Research Society 2011
I. INTRODUCTION
Recent research efforts on the growth of diluted ma
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