High-field Torque Magnetometry for Investigating Magnetic Anisotropy in Mn-12-acetate Nanomagnets

The single-molecule superparamagnet [Mn12O12(OAc)16(H2O)4].2AcOH.4H2O (Mn-12-acetate) has attracted considerable attention because it shows exceedingly slow paramagnetic relaxation at low temperature. The cluster has S-4 symmetry in the solid state and comprises four Mn(IV) ions (S = 3/2) and eight Mn(III) ions (S = 2) which are magnetically coupled to give an S=10 ground state. The ground manifold is largely split in zero magnetic field and many efforts have been spent to determine the zero-field splitting (zfs) parameters alpha, beta and gamma appearing in the fourth-order spin-Hamiltonian H = alpha Sz^2 + beta Sz^4 + gamma (S+^4 + S-^4) + muB.B.g.S. These are of paramount importance for defining the magnetic anisotropy of the cluster, which in turn determines the slow relaxation of the magnetization and quantum tunneling effects at low temperatures. We want to show that cantilever torque magnetometry in high fields is a suitable technique for determining second- and fourth-order anisotropic contributions in high-spin molecules, such as Mn12-acetate. The main advantage of the method lies in its high sensitivity which allows to use very small single crystals. Torque curves have been recorded at 4.2 K by applying the magnetic field (0-28 T) very close to the ab-plane of the tetragonal unit cell. The zfs parameters obtained by this procedure [alpha = -0.389(5) cm-1 and beta = -8.4(5) x 10^-4 cm-1] are in excellent agreement with those determined by spectroscopic techniques, such as high frequency EPR and inelastic neutron scattering.