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We derive the low temperature properties of the molecular magnet Mo72Fe30,where 30 Fe(3+) paramagnetic ions occupy the sites of an icosidodecahedron andinteract via isotropic nearest-neighbour antiferromagnetic Heisenberg exchange.The key idea of our model (J.S. & M.L.) is that the low-lying excitations forma sequence of rotational bands, i.e., for each such band the excitationenergies depend quadratically on the total spin quantum number. Fortemperatures below 50 mK we predict that the magnetisation is described by astaircase with 75 equidistant steps as the magnetic field is increased up to acritical value and saturated for higher fields. For higher temperatures thermalbroadening effects wash out the staircase and yield a linear ramp below thecritical field, and this has been confirmed by our measurements (R.M.). Wedemonstrate that the lowest two rotational bands are separated by an energy gapof 0.7 meV, and this could be tested by EPR and inelastic neutron scatteringmeasurements. We also predict the occurrence of resonances at temperaturesbelow 0.1 K in the proton NMR spin-lattice relaxation rate associated withlevel crossings. As rotational bands characterize the spectra of many magneticmolecules our method opens a new road towards a description of theirlow-temperature behaviour which is not otherwise accessible.Comment: 7 pages, 6 figures, accepted for Europhysics Letter

Quantum rotational band model for the Heisenberg molecular magnet {Mo 72 Fe 30 }

Quantum rotational band model for the Heisenberg molecular magnet {Mo ₇₂ Fe ₃₀ }