Synthesis and magnetic characterization of Ni nanoparticles and Ni nanoparticles in multiwalled carbon nanotubes

Nickel nanoparticles were synthesized by reduction of nickel acetylacetonate in a monosurfactant system. These nanoparticles, mostly amorphous, were used as catalyst for the growth of multiwalled carbon nanotubes by the catalytic decomposition of methane at 500 degrees C. TEM analysis reveals a wide size distribution of the diameter of the particles centred around two main values. A detailed characterization of the magnetic properties of the Ni nanoparticles and Ni nanoparticles carbon nanotubes embedded is hereby presented. Both the systems show superparamagnetic behaviour above the blocking temperature T-B. Magnetization data are well fitted by an equation formed by two weighted Langevin functions and display the correct scaling of M/M-S versus H/T for superparamagnetic nanoparticles. The hysteresis loops obtained below T-B agree with the ferromagnetism of single-domain particles, confirmed also by the expected temperature dependence of the coercive field H-C. Differences in the coercive fields at increasing and decreasing applied magnetic fields are to be ascribed to a NiO layer originating an exchange bias with inner Ni. The smallness of the coercive fields difference (around 5 Oe) confirms that the used preparation method produces a very thin NiO layer around Ni nanoparticles. (c) 2005 Elsevier B.V. All rights reserved.