Exact ground state properties of the classical Heisenberg model for giant magnetic molecules - art. no. 100407

We find the exact ground state energy and magnetic moment for an arbitrary magnetic field H of the classical Heisenberg model of spins on the vertices of an icosidodecahedron. This model provides an accurate description of th e magnetic properties of the giant paramagnetic molecule {Mo72Fe30} in whic h 30 Fe3+ ions are coupled via antiferromagnetic exchange. The strong frust ration of the magnetic interaction in the molecule is relaxed when the angl e between nearest-neighbor spins is 120 degrees. We predict that the magnet ic moment is linear with H until saturating at a critical field H-c, and th is is consistent with the results of a recent experiment at 0.46 K. We deri ve our results using a graph-theoretical construction and a special propert y, three-colorability, of the icosidodecahedron. We also consider spins on the vertices of an octahedron, icosahedron, and dodecahedron.