摘要:
利用溶胶-凝胶技术制备了不同晶粒尺寸的Bi0.2Ca0.8MnO3,通过电子自旋共振研究了晶粒尺寸效应对Bi0.2Ca0.8MnO3电荷有序和自旋关联的影响. 电子自旋共振强度研究表明: 晶粒尺寸的减小削弱了长程电荷有序转变, 当晶粒尺寸减小到40 nm 时, 长程电荷有序转变完全消失; 在顺磁区域, 激活能随着晶粒尺寸的减小增加, 表明铁磁耦合增强. 所有样品线宽与温度曲线显示出典型的电荷有序特征, 这表明在40 nm样品中短程电荷有序态仍然存在,电荷有序态强度不受晶粒尺寸减小的影响. 在高温顺磁区域,居里-外斯温度随着晶粒尺寸的减小而降低, 表明晶粒尺寸减小削弱了铁磁相互作用. 因此, 电荷有序态的压制不能归因于Bi0.2Ca0.8MnO3纳米晶粒中铁磁双交换作用的增强. 在纳米尺度的电荷有序锰氧化物中,无序的表面自旋破坏了表面反铁磁排列构型, 从而引起了表面铁磁层. 晶粒尺寸减小对长程反铁磁电荷有序的削弱比对短程铁磁有序的削弱更加显著,铁磁有序将逐渐占据优势,这使得电子自旋共振强度曲线上电荷有序转变峰消失.
Abstract:
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.
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