Co/CoO 核/壳纳米结构中交换偏倚对颗粒间相互作用依赖性的研究
Dependence of Exchange Bias on Interparticle Interactions in Co/CoO Core/shell Nanostructures
AI总结：
本文研究了Co/CoO核/壳纳米结构中交换偏倚效应对颗粒间相互作用的依赖性，发现通过改变体积分数可调控颗粒间偶极与交换作用，进而显著影响矫顽力和EB场，在浓缩组件中偶极场叠加增强单向各向异性。
摘要：
本文报道了使用常规溶胶-凝胶技术合成的纳米晶 Co/CoO 核/壳结构中交换偏倚（EB）效应对颗粒间相互作用的依赖性。通过粉末X射线衍射（PXRD）研究和透射电子显微镜（TEM）图像分析，证实除了平均粒径约18 nm的核/壳 Co-CoO 之外，不存在其他晶相。通过引入化学计量的非磁性无定形二氧化硅基质（SiO$_2$），体积分数（$φ$）从20%变化到1%，导致颗粒间间距/相互作用发生改变。针对一系列 Co/CoO 核/壳纳米颗粒系统，研究了由颗粒间相互作用/间距变化引起的交换和偶极相互作用对 EB 效应的影响。对该系列样品的磁化强度热变化（$M-T$）和磁滞回线（$M-H$）的研究表明，在浓缩组件中磁性质强烈依赖于偶极相互作用，而在孤立纳米颗粒系统中则以单个纳米颗粒的响应为主导。对 EB 效应的分析揭示，矫顽力（$H_C$）和 EB 场（$H_E$）随体积分数的增加而单调增加。当纳米颗粒足够接近且颗粒间相互作用显著时，集体行为导致反铁磁（AFM）CoO 壳的有效厚度增加，从而产生高 $H_C$、$H_E$。此外，在浓缩组件中，偶极场叠加在局部交换场上，作为单向各向异性的额外来源，增强了 EB 效应。
英文摘要：
This article reports dependence of exchange bias (EB) effect on interparticle interactions in nanocrystalline Co/CoO core/shell structures, synthesized using conventional sol-gel technique. Analysis via powder X-Ray diffraction (PXRD) studies and transmission electron microscope (TEM) images confirm absence of crystalline phases other than core-shell Co-CoO with average particle size $\approx$18 nm. Volume fraction ($φ$) is varied (from 20\% to 1\%) by introduction of stoichiometric amount of non-magnetic amorphous silica matrix (SiO$_2$) which leads to a change in interparticle separation/interaction. The influence of exchange and dipolar interactions on the EB effect, caused by the variation in interparticle interaction/separation is studied for a series of Co/CoO core/shell nanoparticle systems. Studies of thermal variation of magnetization ($M- T$) and magnetic hysteresis loops ($M- H$) for the series point towards strong dependence of magnetic properties on dipolar interaction in concentrated assemblies whereas individual nanoparticle response is dominant in isolated nanoparticle systems. The analysis of the EB effect reveals a monotonic increase of coercivity ($H_C$) and EB field ($H_E$) with increasing volume fraction. When the nanoparticles are close enough and the interparticle interaction is significant, collective behavior leads to an increase in the effective antiferromagnetic (AFM) CoO shell thickness which results in high $H_C$, $H_E$. Moreover, in concentrated assemblies, the dipolar field superposes to the local exchange field and enhances the EB effect contributing as an additional source of unidirectional anisotropy.
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