Correlation Between Magnetism and Electronic Structure of Zn1−xCoxO Nanoparticles

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Zn1−xCoxO nanoparticles (~9 nm) were produced with x ranging from 0 to 0.2 using a forced hydrolysis method. X-ray diffraction measurements confirm the samples to be single phase, and reveal a systematic change in the lattice parameters upon cobalt doping. The unit cell volume V decreases up to x = 0.025 after which it stays roughly constant. The band gap energy (Eg), determined from the photoluminescence spectra gradually increases from x  = 0 to 0.025 and then remains nearly constant for x > 0.025. Room temperature hysteresis loops, obtained using vibrating sample magnetometry, show a similar trend in the saturation magnetization (Ms). Undoped ZnO nanoparticles show a weak magnetic hysteresis; doping causes an increase in Ms up to x = 0.025 and then decreases to lower values for x > 0.025. The magnetic moment per Co ion μ decreases rapidly with x nearly following μ(x) ∝ 1/x, indicating that the moments from the Co ions have little impact on the observed magnetic properties. Electron paramagnetic resonance (EPR) data confirmed that the pure samples are free of any magnetic impurities, while all the doped samples show spectra corresponding to Co2+. The variation of the integrated EPR signal intensity with x also shows a maximum at x = 0.025. X-ray photoelectron spectroscopy confirm that the dopant is incorporated as high spin Co2+ ions for low x, but increasing fractions of the dopant ions change to Co3+ as x increases to 0.2. These results along with the strong correlation observed between the structural (V), electronic (Eg), and magnetic (Ms) properties of Zn1−xCoxO nanoparticles, and the rapid decrease in magnetic moment with increasing x, indicate that the observed changes in the magnetic properties are related to changes in the electronic structure of ZnO nanoparticles caused by dopant incorporation.