Magnetic properties of multi-walled carbon nanotubes.

: The magnetic properties of multiwall carbon nanotubes (MWCNTs) grown by microwave plasma enhanced chemical vapor deposition (MPECVD) were studied by electron paramagnetic resonance and magnetization measurements. The ferromagnetic characters of virgin grown MWCNTs are mainly attributed to the catalyst nanoparticles embedded on the tips. After acid etching for purification, the trace of magnetic resonance prominently distributed from the remnant of catalyst nanoparticles within the lower sectional parts of the tubes and partially might come from the unpaired spins of CNTs. The free radicals from the unbounded carbons and the itinerant conduction electrons of metallic CNTs also contribute to the electron spin resonance. Magnetization measured by a superconducting quantum interference device (SQUID) reveals fairly a small hysteretic loop with small coercive field for both virgin and purified MWCNTs.