Surface magnetism of ferromagnetic ultrafine particles

The electronic structures and atomic magnetic moments of Ni_n clusters (n= 2—6) have been studied. Compared with crystalline nickel, some clusters increase obviously in magnetism; some decrease obviously; and some show ferrimagnetism. The symmetry of clusters has great effect on magnetic moment. If they are similar in symmetry, the clusters are similar in magnetic moment. The magnetic moment for small clusters does not seem to increase or decrease monotonically with the change in their size, because adding or removing one atom may fully change the symmetry of small clusters. As the surface layer of ultrafine particles is made of many different polyhedrons with low symmetry and the alignment of the polyhedrons is complicated, the whole surface layer presents short-range order. The calculated results explain the abnormal phenomena about surface magnetism that have been in existence for a long time.     

铁磁超细微粒的表面磁性

用MS-Xα方法研究了Ni_n(n =2～6)团簇的电子结构和原子磁矩. 与晶体镍相比,有些团簇磁性明显增强,有些磁性明显减弱,有些则呈现出亚铁磁性. 分析发现,团簇的对称性对磁矩有着非常重要的影响. 对称性相似的团簇,磁矩大小也相仿. 仅有极少数原子的小团簇磁矩与团簇尺寸不存在递减或递增的规律性. 其原因是对于小团簇,增减一个原子可以引起对称性的根本变化. 超细微粒表面层由对称性较低的不同多面体构成,多面体排列复杂,整个表面层呈短程有序. 计算结果很好地解释了长期存在的表面磁性异常现象.