Quantum molecular nonomagnets

We describe the magnetic and thermal properties of a molecular crystal of nanomagnets. Each nanomagnet is formed by a magnetic cluster of 12 interacting Mn ions and behaves at low temperature as macrospins. High field magnetization, complex susceptibility and specific heat results show that the clusters have a S=10 ground state with a high magnetocrystalline anisotropy and are magnetically well isolated from each other. The relaxation time is approximated by an Arrhenius law tau=tau(0)e(Delta/kT) where Delta is the anisotropy energy barrier, with values being in fair agreement with high field magnetization and EPR measurements. Deviations from this ideal superparamagnetic behavior in a wide range of relaxation times and an unusual dependence with magnetic field are explained to be due to Quantum Tunneling of the Magnetization through the anisotropy energy barrier. Zero field specific heat results indicate that the usual uniaxial Hamiltonian is not enough to explain the results, being necessary to use the recently proposed fourth order corrections. Very low temperatures specific heat data indicate a ground state splitting much larger than expected. Finally, specific heat measurements done under magnetic field permitted to observe and measure the lattice spin relaxation by calorimetric methods with time constants that are in agreement with the magnetic relaxation measurements.