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 nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio are chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fee nickel or cobalt and major peaks of graphite 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 function of temperature in the range 20-900 degrees C give a Curie temperature 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 hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20-650 degrees C for Ni, and 20-900 degrees C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing. (C) 1998 American Institute of Physics. [S0021-8979(98)04302-3].