Low temperature specific heat of molecular nanomagnets and the contribution of the internal fields

We present the specific heat measurements at very low temperatures of the single molecule magnets Mn(12)ac and Fe-8. Below 3 K, both systems present superparamagnetic blocking effects in zero external magnetic field and an excess specific heat contribution around 1 K. The experimental results are explained considering a distribution of internal fields of dipolar and hyperfine origin as well as quantum tunneling of the magnetization. (C) 2001 Elsevier Science B.V. All rights reserved.