SYNTHESIS AND MAGNETIC PROPERTIES OF COBALT SINGLE CRYSTALLINE PARTICLES ENCAPSULATED IN CARBON CAGES

利用石墨/钴复合材料在氦气中电弧蒸发制备了包裹Co颗粒的碳纳米胶囊。研究了不同Co/C比、不同He压力下不同复合棒制备的Co纳米胶囊的晶体结构、形貌和磁性能。Co颗粒的晶体结构和粒度纳米胶囊中的Co颗粒主要处于fcc相。热处理后的fcc-Co颗粒未引起hcp的结构转变。Co颗粒直径随着He气体压力的增加而增大（50 Torr时约为18 nm， 500Torr时约为25nm）。当粒径d从18 nm增加到21 nm时，矫顽力Hc从640 Oe急剧下降到300 Oe，但从d=21 nm到25 nm，矫顽力Hc变化不大，说明粒径小于0.20 nm的Co颗粒存在单磁畴。饱和磁化当Co填充孔的面积占复合棒总横截面的16%以上时，制备样品的饱和磁化Ms大于块状Co的Ms的88%。Ms的降低是由于非磁性石墨涂层的存在，该涂层保护核心Co颗粒不被氧化和聚结。碳包覆Co颗粒的Ms表现出与大块Co相似的温度依赖性。

Carbon nanocapsules encaging Co particles were produced by arc evaporation of graphite/cobalt composite in helium gas. Crystal structures, morphology and magnetic properties of Co nanocapsules, which were prepared from various composite rods with varying Co/C ratio as well as under different He pressures, were studied.1.Crystal Structure and Paticle Size of Co ParticlesCo particles in nanocapsules were predominantly in a fcc phase. Heat treatment of the fcc-Co particles did not bring about structural transformation to hcp. Diameter of Co particles increased with the increase of He gas pressure (ca.18 nm at 50 Torr to ca.25nm at 500Torr).2.Coercive Force as a Function of Particle SizeCoercive Force Hc decreased steeply from 640 Oe to 300 Oe when the particle size d increased from 18 nm to 21 nm, but it did not show any large changes from d=21 nm to 25 nm, suggesting that the Co particles with diameters less than ca.20 nm had single-magnetic domains.3.Saturation magnetizationWhen the area of a Co-packed hole occupied more than 16% of the total cross-section of the composite rod, the saturation magnetization Ms of the prepared sample was more than 88% of the Ms for bulk Co. The reduction in Ms is due to the presence of non-magnetic graphite coating, which protect the core Co particles from oxidation and coalescence. The Ms of carbon-coated Co particles showed a temperature dependence similar to that of bulk Co.