Microstructure and magnetism of Co-doped PbPdO2 films with different grain sizes

Abstract

Single-phase Co-doped PbPdO₂ films with grain sizes ranging from 30 nm to 120 nm were synthesized using a sol–gel spin-coating method. The doped Co ions exhibit a mixed state of 2+ and 3+, which is caused by the existence of Pb vacancies and the low electronegativity of Co. Ferromagnetism, paramagnetism, antiferromagnetism and a charge ordering state were observed in all of the Co-doped PbPdO₂ films. Additionally, the charge ordering signal determined by the surface state of the nanograins is gradually weakened with decreasing grain size. The carrier-mediated mechanism bridged to the bound magnetic polaron model was used to explain the coexistence of the ferromagnetism, paramagnetism and antiferromagnetism. For the ferromagnetism, the tendency of the saturation magnetization to increase with temperature was found in all the Co-doped PbPdO₂ films with a nanograin structure. This abnormal magnetic behavior is believed to be one of the peculiar features of spin gapless semiconductors.