Electron spin resonance study on charge ordering and spin ordering in nanocrystalline Bi0.2Ca0.8MnO3

Bi-based manganites Bi0.2Ca0.8MnO3 samples with different paiticle sizes were prepared by the sol-gel technique. The effect of paiticle size on the charge ordering (CO) and spin correlations of Bi0.2Ca0.8MnO3 was investigated by electron spin resonance (ESR). The variation in ESR intensity with temperature shows that the long-range CO transition is suppressed by the size reduction, and completely disappears as the paiticle size is reduced to 40 nm. In the paramagnetic (PM) region, the ESR intensity is fitted by the Arrhnius formula. The result shows that the activation energy is significantly enhanced with decreasing of paiticle size, especially in the 40 nm sample, indicating the enhancement of ferromagnetic (FM) correlations. However, the temperature dependence of ESR line width displays a typical CO characteristics for all samples. Thus, it is suggested that there is the short-range CO state in the 40 nm sample, while the long-range CO transition is completely suppressed. It is found that the onset temperatures of CO states are almost the same in all samples, indicating that the strength of CO correlations is not influenced by size reduction in this compound. A positive Curie-Weiss (CW) temperature is obtained from the line width in high-temperature PM regime, which confirms the existence of FM correlations in this system. Moreover, the value of CW temperature shows a significant decrease with particle size reduction, which indicates that the FM interactions can be weakened by size reduction in this system. Based on the research of ESR intensity and line width, it is concluded that the suppressed CO cannot be attributed to the enhancement of double-exchange FM interactions in Bi0.2Ca0.8MnO3 nanoparticles. To explain these behaviors, a core-shell model based on surface effect is proposed. In nanosized CO manganites, the disordered surface spins destroy the collinear antiferromagneitc (AFM) configuration, and favor FM surface layer coupled with the inner AFM core. With the reduction of paiticle size, the weakening of long-range AFM CO is more significant than that of short-range FM ordering due to the increase of surface spin disorders. With the reduction of paiticle size, the FM ordering will gradually dominate in the competition between FM ordering and AFM ordering, which results in the disappearance of CO transition peak in the ESR intensity curve.