Magnetic Behaviour of Non-Contacting Ni Nanoparticles Encapsulated in Vertically Aligned Carbon Nanotubes

Magnetic properties of carbon nanotubes (CNT) obtained by plasma-enhanced chemical vapour deposition (PECVD) have been studied. The growth of these nanotubes has been activated from Ni catalyst nanoparticles. Samples consist of Ni nanoparticles encapsulated at the tip of vertically aligned multiwalled carbon nanotubes (VACNTs) forming an homogeneous and dense large area monolayer of isolated (non-contacting) nanoparticles. The magnetic characterisation has been performed in the temperature range of 5-300 K with magnetic fields up to 9 T. The results show that the wide size range (30-180 nm) of the particles originates the coexistence of blocked and superparamagnetic particles and leads to the strong ferromagnetic behaviour of the whole assembly. The coercivity decreases monotonically with increasing temperature and the value for the intrinsic coercivity is 225 Oe. The encapsulation of Ni nanoparticles by VACNTs preserves them from aggregation. This makes possible to tune the coercivity by controlling size distribution of particle monolayers. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This work was supported in part by the University of Oviedo under Grant No. UNOV-08-MA-100 and by the Spanish MEC through grant Ref. MAT2008-06784. Projects CSD2006-12 and DPI2006-03070 of the Spainsh MEDU and the Generalitat de Catalunya (project No. SGR2005-00666). The authors thank Serveis Cientifico-Tècnics of the Universitat de Barcelona (SCT-UB) for measurement facilities