Electronic states in magnetic fullerides studied by ESR under pressure

TDAE-C60 and AC60 are well known monoanion-C60 compounds, where TDAE is tetrakis-dimethylaminoethylene and A is Rb or Cs. The former compound is the soft ferromagnet (FM) below 16 K and the latter is a [2+2]-cycloadduct 1D polymer with the antiferromagnetic (AF) ground state below 50 K. Although several mechanisms for the FM in TDAE-C60 have been proposed, the present study under high pressures suggests the ordering of orbitals of Jahn–Teller distorted C60. So far, AF transition below 50 K in RbC60 has been interpreted as the metal–insulator transition driven by instability of half-filled 1D Fermi surface. However, the present study combined with the resistivity data under pressure revealed RbC60 to be a 3D Mott–Hubbard system with the AF-metallic phase arising from magnetic frustration in a fcc-like structure.