PREPARATION AND PROPERTIES OF FERROMAGNETIC CARBON-COATED FE, CO, ANDNI NANOPARTICLES

Carbon-coated iron, cobalt, and nickel particles were produced by an a re discharge process modified in the geometry of the anode and the flo w pattern of helium gas. Field emission scanning electron microscopy s hows that the resulting material consists of only carbon-coated metal particles without any nanotubes or other unwanted carbon formations pr esent. The diameters of iron, cobalt, and nickel particles range predo minantly from 32 to 81 nm, 22 to 64 nm, and 16 to 51 nm, respectively. X-ray diffraction analysis confirmed that the as-made particles are c arbon-coated elements rather than metal carbides. High resolution tran smission electron microscopy reveals that the as-made cobalt and nicke l particles are covered by 1-2 graphitic layers, while iron particles are surrounded by amorphous carbon. When the samples were treated by a nnealing or immersion into nitric acid, particles completely coated by carbon resisted both postdeposition treatments. However, further grap hitization of the carbon coating by either of the two treatments was o bserved. Particles only partially coated by carbon were not protected, but sintered by annealing or dissolved in the acid. The magnetic prop erties of the as-made particles were measured by a vibrating sample ma gnetometer. The values of the saturation magnetic moment per gram of e ach type of metal particle are 56.21, 114.13, and 34.9 emu/g represent ing 26%, 71%, and 64% of the saturation moments of the bulk ferromagne tic elements iron, cobalt, and nickel, respectively. All the metal par ticles were shown to be ferromagnetic with a ratio of remnant to satur ation magnetization M(R)/M(S) similar to 0.3 at room temperature (25 d egrees C). In this article the detailed preparation and the properties of these carbon-coated metal particles will be discussed. (C) 1996 Am erican Institute of Physics.