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The one-band Hubbard model at half-filling on a truncated tetrahedron (C 12 ), and on the C 60 molecule is studied. Within the Hartree-Fock approximation, we find a magneticlike instability of the "Fermi sea" towards a spin-ordered phase for an intermediate value of the coupling (U/t) c ≈ 2.6. The ordered phase presents a spin arrangement similar to that of the classical Heisenberg model defined on the same clusters. We study the linear excitations around the Hartree-Fock ground states using the random-phase approximation.On a finite cluster, we expect that these results signal the presence of a rapid crossover between a paramagnetic region and a regime where spin correlations are important. The relationship with the Heisenberg limit (large U/t) is discussed. Finally, we comment on implications of our results for purely repulsive models of superconductivity in alkali-metaldoped fullerenes.

We report results on the temperature dependence of the susceptibilities of a set of MBE-grown short-period EuTe/PbTe antiferromagnetic superlattices having different EuTe layer thicknesses. In-plane and orthogonal susceptibilities have been measured and display a strong anisotropy at low temperature, confirming the occurrence of a magnetic phase transition in the thicker samples, as seen also in neutron diffraction studies. We suggest that dipolar interactions stabilize antiferromagnetic long-range order in an otherwise isotropic system and we present numerical and analytical results for the low- §

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