magnetic excitations and spin correlations in the spherical keplerate molecular magnet {Mo72Fe30}

As one of the biggest molecular magnets that have been synthesized, the polyoxomolybdate Mo72Fe30 allows us to investigate a highly symmetric and frustrated spin structure and provides us an ideal test-bed for the theory on frustrated quantum spin systems. In our work, the low-field magnetic susceptibility measurement yields a Curie temperature about -20 K, indicating weak antiferromagnetic exchange interactions between the nearest neighbors. The low lying magnetic excitations of Mo72Fe30 are characterized in terms of field-dependent heat capacity and inelastic neutron scattering measurements. The energy gaps resolved from the experimental data are located at about 0.1 and 0.7 meV, consistent with the quantum rotational band model for Mo72Fe30 system. The diffuse neutron scattering study with polarization analysis on Mo72Fe30 polycrystals reveals characteristic spin correlations close to a coplanar three-sublattice ground state. Our observations agree with the previous theoretical model for this system, but also suggest that refinements should be done to the existing theoretical models.