Unusual properties of the molecular nanomagnet Mn(12)ac

The magnetic properties of the Mn(12)ac molecular cluster were studied by c omplex susceptibility and specific heat measurements, Extensive ac suscepti bility measurements done under a magnetic field on good quality single crys tals and zero field powder sample specific heat results indicate that the u sual uniaxial Hamiltonian, as well as the recently proposed fourth order co rrections, are not enough to account for all the observations. Very low tem perature specific heat data indicate the presence of zero field splitting o f the ground state which is much larger than expected. Finally, specific he at measurements done under a magnetic field permitted us to observe and mea sure the lattice spin relaxation by calorimetric methods with time constant s that are in agreement with the magnetic relaxation measurements, (C) 1998 American Institute of Physics. [S0021-8979(98)49111-4].