Abrupt Transition Between Thermally-Assisted and Pure Quantum Tunneling in Mn12

Single-molecule magnets are organic materials which contain a large (Avogadro’s) number of identical magnetic molecules; Mn12 acetate is a particularly simple and much-studied example of this class. The Mn12 clusters are each composed of twelve Mn atoms (see Fig. 1) coupled by superexchange through oxygen bridges to give a sizable spin magnetic moment, S=10. These magnetic molecules are regularly arranged on a tetragonal crystal lattice with spacings between them sufficiently large that inter-cluster magnetic interactions are weak. As illustrated by the double well potential of Fig. 2, strong uniaxial anisotropy (of the order of 60 K) yields doubly degenerate ground states in zero field and a set of excited levels corresponding to different projections m s = ±10, ±9, ....., 0 of the total spin along the easy c-axis of the crystal. Measurements[1, 2] below the blocking temperature of 3 K reveal a series of steps in the curves of M versus H at roughly equal intervals of magnetic field, as shown in Fig. 3, due to enhanced relaxation of the magnetization whenever. levels on opposite sides of the anisotropy barrier coincide in energy. Strong temperature dependence was found which indicates that thermal processes play a central role. The steps in the magnetization curves have thus been attributed to thermally-assisted quantum tunneling of the spin magnetization.