EFFECT OF ANNEALING ON THE STRUCTURE AND MAGNETIC-PROPERTIES OF GRAPHITE ENCAPSULATED NICKEL AND COBALT NANOCRYSTALS

We report the structure and magnetic studies of carbon coated nanocrys tals of nickel and cobalt synthesized in a special low carbon to metal ratio are chamber. Powder x-ray diffraction profiles show peaks assoc iated with single phase of fee nickel or cobalt and major peaks of gra phite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a fu nction of temperature in the range 20-900 degrees C give a Curie tempe rature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M-H hysteresi s loop of the particles have been measured at room temperature after a nnealing in the temperature range 20-650 degrees C for Ni, and 20-900 degrees C for Co. The dependence of room temperature saturation magnet ization, remanent magnetization, and coercive field of the particles o n annealing temperature is reported. These data are described by trans ition of particles form single domain to multidomain as a result of pa rticle growth due to annealing. We also present the particle size dist ribution measurements that show log-normal behavior, and indicate subs tantial particle size growth due to annealing. (C) 1998 American Insti tute of Physics. [S0021-8979(98)04302-3].