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

在氦气氛中，采用电弧蒸发法制备了包覆钴颗粒的碳纳米胶囊。研究了不同Co/C比和不同氦气压下制备的Co纳米胶囊的晶体结构、形貌和磁性。1.纳米胶囊中Co粒子的晶体结构和颗粒尺寸主要以面心立方相为主。对fcc-Co颗粒进行热处理，并未引起hcp的结构转变。Co粒子的直径随着气压的增加而增大(50Torr时约为18 nm，500Torr时约为25 nm)。2.Coercive Force Hc随粒子尺寸的变化从640 Oe急剧下降到300 Oe，但从d=21 nm到25 nm变化不大，这表明直径小于20 nm的Co粒子具有单一的磁域。3.饱和磁化当Co填充孔洞的面积占复合棒总横截面的16%以上时，制备的样品的饱和磁化强度ms大于块体Co的饱和磁化强度ms的88%，这是由于非磁性石墨涂层的存在，保护了核心Co粒子的氧化和聚结。碳包覆钴颗粒的磁化率与块体钴的磁化率具有相似的温度依赖关系。

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.