Low-moment antiferromagnetic ordering in triply charged cubic fullerides close to the metal-insulator transition

We report on the ambient pressure C 13 and C 133 s NMR study of disorder-free A15-structured Cs3 C60, which shows non-BCS superconductivity at 38 K under pressure. Temperature-independent C 13 and C 133 s spin-lattice relaxation rates prove an insulating ground state with a small effective magnetic moment 1.3-1.6 μB for T>100?K. Below 46 K, A15 Cs3 C60 orders into an antiferromagnetic structure best described by a magnetic wave vector, q= (1 2, 1 2, 1 2) with a staggered magnetic moment ∼0.9 μB per C60. The large (U/W) ratio (∼2.2), the opening of a gap in the electronic excitation spectrum and the small C 60 3- magnetic moment all underline the importance of electronic correlations in A15 Cs3 C60.