Magnetic properties of FeCo nanoparticles encapsulated in carbon

The as-prepared and heat treated iron/cobalt nanoparticles with r = Co/Fe = 0.08 and 1 were obtained by laser decomposition. At low temperatures all the materials are in a ferromagnetic blocked state. At higher temperatures (T > 200 K) the as-prepared materials exhibit superparamagnetic properties. In this case the nanoparticle size distribution deduced from the m(H) curves and from the difference between the FC and ZFC susceptibilities was compared with the TEM data. The heat treated materials have larger magnetizations and a maximum (161 emu/g) was achieved for the r = 1 material heat treated at 900°. For this material an anomalous form of the hysteresis loop was explained in terms of a superposition of single and multi-domain magnetizations.