Absence of ferromagnetism in bulk polycrystalline<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>Zn</mml:mi><mml:mn>0.9</mml:mn></mml:msub><mml:msub><mml:mi>Co</mml:mi><mml:mn>0.1</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>

The atomic structure and the magnetic properties of 5 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ Co-doped $\mathrm{ZnO}$ samples prepared by a high-pressure and high-temperature method have been investigated by various techniques, including x-ray diffraction with Rietveld refinements, scanning electron microscopy, high-resolution transmission electron microscopy, nanometer-sized element mapping, x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and extended x-ray absorption fine structure and magnetization measurements. It is found that Co ions with a valence of $2+$ substitute Zn ions in the wurtzite $\mathrm{ZnO}$ structure. No metallic or oxidic Co-rich clusters were detected in the samples. If ${\mathrm{Co}}^{2+}$ ions substitute ${\mathrm{Zn}}^{2+}$ ions, no intrinsic ferromagnetism is observed in $\mathrm{ZnO}$ at doping levels up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ Co even down to $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A paramagnetic behavior with partially antiferromagnetic interactions is observed. Thus, in bulk Co-doped $\mathrm{ZnO}$ system any observation of ferromagnetism is probably due to small clusters of second phase materials. However, in thin films the situation might be more complex, since other extrinsic influences, such as strain or proximity effects, may modify the electronic and magnetic properties.

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