This chapter deals with magnetic composites consisting of carbon nanostructures and impurity species. The host systems included here are the prototypes fullerene, graphene and carbon nanotubes. In each case discussed, emphasis will be placed on the mechanisms by which the electronic ground state of the aggregate adopts magnetic properties. In many cases, the considered carbon allotrope provides a framework that preserves the magnetism of the guest species, as paradigmatically realized by some metallofullerenes with enclosed lanthanide atoms or clusters. In some systems, however, magnetism is not imported by the guest species but evolves as the carbon allotrope interacts with the externally added moieties. This situation is well exemplified by the much-studied compound tetrakis-dimethylamino-ethylene-C60 (TDAE-C60).

In sections 12.1 and 12.2, we refer first to magnetic metallofullerenes, arguably the most traditional among the structures included here. After all, research interest in fullerenes with enclosed metal components emerged soon after the discovery of C60 [139]. From metal atoms or metal atom clusters as guest species inside fullerene cages, we turn to single group V atoms as encapsulated components. Systems of the form A@C60, with A = N, P, have proven to be efficient in preserving the spin of the enclosed atom, which makes them interesting as physical realizations of qubits in quantum computing. The following Section (12.3) discusses a variety of magnetic phases that originate from electron transfer to fullerenes. This mechanism is operative in very diverse composites, ranging from fullerides to hybrids of fullerenes and organic molecules.

The remainder of this chapter deals with compounds involving graphene and nanotubes. Specifically, graphene is considered as substrate of two adatom types, hydrogen and fluorine, and computational as well as experimental findings on the magnetic phases of hydrogenated and fluorinated graphene are surveyed. The final section of this chapter summarizes various results on carbon nanotubes in combination with magnetic metal components, ranging from atoms to nanoparticles.

Magnetic Metallofullerenes

The magnetism of endohedral metallofullerenes with enclosed paramagnetic guest species arises from the interplay of various effects. Thus, the intrinsic magnetic moment of the enclosed metal component may affect the magnetic moment of the unit as a whole. Further, this component may be in a cationic state, as a consequence of electron transfer from the encapsulated atom or cluster to the fullerene enclosure.