Low-temperature specific heat of Li : Fe6 molecular magnets

We have studied the low-temperature (T) specific heat (C) of Li : Fe6 molec ular magnets. The Schottky anomaly related to the singlet-triplet energy ga p is shifted towards lower temperature as compared with that observed on Na :Fe6. This evidences the effects of the internal-guest substitution in thes e antiferromagnetically coupled iron(III) rings. The singlet-triplet energy gap (E(1)/k(B) = 14.2 +/- 0.4 K) and its zero-field splitting (D(1)/k(B) = 3.9 +/- 1.0 K) evaluated by the C-versus-T data fitting are compared to th e values estimated by the high-field torque experiments. (C) 2000 Elsevier Science B.V, All rights reserved.