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 °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 TB. Magnetization data are well fitted by an equation formed by two weighted Langevin functions and display the correct scaling of M/MS versus H/T for superparamagnetic nanoparticles. The hysteresis loops obtained below TB agree with the ferromagnetism of single-domain particles, confirmed also by the expected temperature dependence of the coercive field HC. 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.