Size effects on the magnetic and optical properties of CuO nanoparticles |
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Authors: | Shama Rehman A Mumtaz S K Hasanain |
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Affiliation: | (1) Department of Physics, Quaid-I-Azam University, Islamabad, Pakistan |
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Abstract: | Optical and magnetic studies on CuO nanoparticles prepared by a chemical route are reported and the effect of size variation
on these properties is discussed. SEM images show that the nanoparticles are interlinked into microspheres with the cages
containing visible nanoscale holes. Diffuse reflectance spectroscopy indicates a consistent red shift in the fundamental band
gap (indirect band gap) from 1.23 to 1 eV as the size decreases from 29 to 11 nm. This observed red shift is attributed to
the presence of defect states within the band gap. A clear blue shift is observed in the direct band gap of these nanoparticles
presumably due to the quantum confinement effects. Air-annealed samples show a paramagnetic response whereas particles annealed
in a reducing atmosphere show additionally a weak ferromagnetic component at room temperature. For both types of particles,
the paramagnetic and ferromagnetic moments, respectively, increase with decreasing size. The role of oxygen vacancies is understood
to relate to the generation of free carriers mediating ferromagnetism between Cu spins. AC susceptibility measurements show
both the antiferromagnetic transitions of CuO including the one at 231 K which is associated with the onset of the spiral
antiferromagnetic phase transition. |
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Keywords: | |
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