Unusual properties of the molecular nanomagnet Mn12ac

The magnetic properties of the Mn12ac molecular cluster were studied by complex susceptibility and specific heat measurements. Extensive ac susceptibility measurements done under a magnetic field on good quality single crystals and zero field powder sample specific heat results indicate that the usual uniaxial Hamiltonian, as well as the recently proposed fourth order corrections, are not enough to account for all the observations. Very low temperature specific heat data indicate the presence of zero field splitting of the ground state which is much larger than expected. Finally, specific heat measurements done under a magnetic field permitted us to observe and measure the lattice spin relaxation by calorimetric methods with time constants that are in agreement with the magnetic relaxation measurements.