Following the discovery of fullerenes, nanoparticles and nanotubes, numerous attempts to fill the nanoscale cavities of these materials were made. A successful filling of nanotubes and/ or nanoparticles is of great interest, since it allows confinement of amounts of material small enough to promise novel physical properties, as well as a protection of the encapsulated metals from oxydation by resistant carbon cages. The production of encapsulated nanometer-sized ferromagnetic particles has thus recently allowed to obtain the first experimental evidence of the NČel-Brown model of magnetization reversal. We produce carbon-encapsulated spherical cobalt particles by a modified arc discharge technique, with an average diameter that can be varied between 5 and 45 nm by changing the deposition parameters. The magnetic properties were analysed on macroscopic samples with a magnetometer, as well as on individual particles by electron holography measurements in the transmission electron microscope. The oxide layer on the surface of unprotected particles influenced strongly their magnetic behavior, justifying the encapsulation approach. The encapsulated particles showed ferromagnetic hysteresis loops with marked size-dependent properties: the coercive field and remanent magnetization were found to vary with size and temperature, as measured on both macroscopic samples and individual particles. The results are interpreted as reflecting the transition from single- to multidomain character as the particle size increases, which occurs at diameters around 25 nm at 300 K. It appears thus that our particles fall within the size range of "optimum magnetic hardness", i.e. in the range where particles are single-domain but do not exhibit superparamagnetism.
Departement de Physique, Ecole Polytechnique Federale de Lausanne
CH-1015 Lausanne, Switzerland
Phone: +41 21 6934410; Fax: +41 21 6933604