Abstract
Polycrystalline $\mathrm{Ba}\mathrm{Co}{\mathrm{O}}_{3}$ was synthesized by a citrate technique using thermal treatments at high oxygen pressure. Magnetic susceptibility measurements were carried out at very low ac and dc fields on the compound. The magnetic properties of the material between 14 and $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were identified with the appearance of nanoscale ferromagnetic regions. The observed magnetic irreversibilities can be understood in the framework of a superparamagnetic model. The real and imaginary parts of the complex susceptibility are consistent with an Arrhenius-like thermal relaxation law with an energy barrier of about $0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Through this model, the mean size of the ferromagnetic clusters could be determined; it is about $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter, if spherical geometry is assumed for them.
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