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Raman spectroscopic studies of oxygen defects in Co-doped ZnO films exhibiting room-temperature ferromagnetism

The optical properties, Raman spectra, and magnetization of Zn1−xCoxO (0≤x<0.1) thin films are reported. Optical transmission spectra confirm the substitution of Co2+ cations for Zn2+ ions at the tetrahedral sites of ZnO. Raman spectra of films annealed in air show an additional broad vibrational mode, attributed to disordered –Zn–O–Co– local vibrations. These modes disappear on vacuum annealing, suggesting the formation of oxygen vacancies close to the dopant sites (–Zn––Co–). Magnetization measurements show that air-annealed films lack significant moment, whereas vacuum-annealed samples with intermediate Co concentrations (0.012<x<0.05) exhibit room-temperature ferromagnetism. Using the framework of the bound magnetic polaron model, the polaron size is estimated to be roughly 7.8 Å for this system.