Low-temperature specific heat of Fe-6 and Fe-10 molecular magnets

The energy splitting of the low-lying levels has been investigated on two m agnetic molecular clusters Fe-6 and Fe-10 by means of low-temperature zero- field specific-heat measurements. Significant deviations from the usual C s imilar to T-2 law were observed above the maximum of the main Schottky anom alies as a result of nonnegligible contributions from the excited spin stat es with S>1 and the estimated lattice contributions follow a phenomenologic al power law C/R similar to T-alpha with alpha-2.7 for both these compounds . The singlet-triplet energy gaps evaluated by the Schottky anomaly, T-0=19 .2 K for Fe6 and 4.56 K for Fe-10, are smaller than what we can estimate by a simplified spin-Hamiltonian approach in the strong exchange approximatio n and using the energy levels obtained by the high-field magnetization and susceptibility measurements. This discrepancy asks for a more complex descr iption of the low-lying states of these molecular clusters, beyond the stro ng exchange approximation. At very low temperatures T<<1 K, two low-energy Schottky anomalies were also observed in Fe-10, probably due to a small fra ction of defected rings or to hyperfine contributions.