Magnetic Coupling of Fe₃C Nanoparticles Within Double-Wall Carbon Nanotubes

Studying the novelty of magnetism in carbon nanotubes is still intriguing and it has not been completely explored, despite a number of promising technological applications. In this study, we focus on the magnetic properties of bundles of double-walled, metallic carbon nanotubes with encapsulated cementite (Fe₃C) nanoparticles of ~3 nm in length inside their hollow core. These nanoparticles are not unintentional impurities, but have been deliberately introduced to fill the nanotubes with ferromagnetic material. Results of VSM magnetometry of the filled nanotubes give an anisotropy constant of ~3x10⁶ erg/cm³, much larger than the anisotropy of free cementite nanoparticles, indicating a strong magnetic coupling of the nanoparticles with the nanotubes. On the other hand, the calculated activation volume for magnetization reversal, of ~400 nm³, shows that around 300 nanoparticles of cementite are mutually coupled through the conduction electrons of the nanotubes. Considering a spin diffusion length of 130 nm along the direction of the nanotubes, the activation volume suggests that magnetic coupling also occurs between neighboring nanotubes in the direction perpendicular to their axis, with a perpendicular magnetic correlation length of ~5-6 nm, i.e. involving four parallel nanotubes.