Magnetic properties of carbon-coated Sm-Co-C and Mn-Al-C alloy nanoparticles

Abstract

The Hut%nan-Kratschmer method of preparing fullerenes has been modified for the preparation of carboncoated nanccrystals of a variety of metals and metal carbides [l-S]. Here the graphite anode is hollowed out and packed with a mixture of metal or metal oxide powder and graphite cement. In the arc, metal-containing clusters form. The cluster stoichiometry depends on the chemistry between the metal atoms, carbon, and oxygen. The clusters ditfuse until they ate deposited on a surface, either the high temperature cathode or the room temperature walls of the reactor. The nanocrystalline phases produced depend on the surface temperature and the cooling rate of the clusters determined by their diffusion path and the amount of helium buffer gas [6]. This method is therefore uset%l in preparing metastable phases. The carbon coating common to all nanocrystals prepared by this method arises when the particles cool. Since graphite melts at a higher temperature than the metals or metal carbides we have made, phase segregation occurs in the cooling nanoparticle, forming a graphitic shell. The coating in some cases prevents degradation of air or water-sensitive compounds, a problem encountered with important magnetic materials such as NhFe,.,B. Ifthe particles are formed tirn a femrmagnetic material, their size is small enough to support only a single magnetic domain. These particles are said to be superparamagnetic, and all the atomic spins align to yield a large particle moment. The particle moment rotates not through domain wall motion, but by rotating all the atomic spins together. Theories of superparamagnetism [7,8] show that the temperature dependence of the coercivity, H, for a uniaxial spherical particle with a single magnetic domain is given by the equation,