Magnetic Properties of Iron Catalyst Particles in HiPco Single Wall Carbon Nanotubes

We have investigated the magnetic response of residual metal catalyst in the raw and super purified HiPco single wall carbon nanotubes (HiPco_raw and HiPco_SP SWCNTs). It has been shown that the residual metal catalyst is in the form of nanoparticles, even in the HiPco_SP SWCNTs that should contain a minimal amount of the metal. Mossbauer spectroscopy of the HiPco_raw SWCNTs proved the catalyst nanoparticles are in the form of Fe3C. Analysis of the synchrotron X-ray diffraction data provided an average diameter of nanoparticles about 1.9 am. Magnetic studies by means of temperature dependence of magnetization, magnetization isotherms and susceptibility suggested that the nanoparticles obey the behavior of weakly interacting superparamagnetic systems in both samples. Further analysis of the data revealed a core-shell structure of the nanoparticles in the HiPco_raw nanotubes, with a magnetically oriented core and a paramagnetic shell, which is almost removed in the case of the HiPco_SP catalyst nanoparticles.