氧化镁纳米多晶的微结构和磁性

Magnetism of MgO nanoparticles

实验发现,宏观晶体是非磁性的氧化镁时,其多晶样品有弱铁磁性.本文用第一性原理电子结构方法研究了氧化镁表面、纳米颗粒和晶界的磁性.计算结果表明:绝缘的氧化镁表面可以是导电的,并且有与之相关的铁磁性;磁性表面的共同特征是在表面上有氧原子富集,包括(111)表面的纯氧原子层,(114)表面的氧原子链;其他高晶面指数表面也会有氧原子富集区域;氧化镁纳米颗粒的磁性出现在高晶面指数表面以及不同晶面交界的棱及其顶角等有氧原子富集的区域,这种由氧原子富集而形成的磁性有巡游特征.氧化镁Σ7[111]和Σ5[001]晶界的计算结果表明:在没有氧原子富集的情况下,多晶样品中晶界的磁性很弱,而在有氧原子富集的情况下,晶界磁性比较强.因此可以推断多晶样品的磁性主要出现在多晶表面、晶粒包围孔隙、微裂纹界面、晶界和其他晶体缺陷等有氧原子富集的区域.这种残余磁性可以通过热处理等结构优化过程而削弱甚至消除.

MgO polycrystal is found to be weakly magnetic experimentally, although its single crystal is non magnetic. In this work, the magnetic properties of surfaces of crystal and nano-particles of MgO are studied by the first-principles density functional theory. The obtained results show that there are the oxygen-rich regions in all the magnetic surfaces discussed in this work, especially in the （111） surface with pure oxygen layer and the （114） surface with pure oxygen chains. Other surfaces with high Miller indices generally have the oxygen-rich regions. For MgO nano-particles, the facets with high Miller indices and the edges and vertexes formed by different orientation surfaces are oxygen-rich possibly and have strong magnetism. The itinerant magnetism is indentified for the magnetism on the surfaces of MgO crystal and the surfaces of MgO nano-particles. That the special MgO ∑7[111] grain boundary is not magnetic means that the magnetism of MgO grain boundary is weak if the chemical composition in grain-boundary region is slightly different from that in the crystal. It can be inferred that the magnetism of MgO polycrystal is mainly contributed by the polycrystal surface, the micro-pores, micro-voids and micro-cracks.