Alternating current (ac) magnetic susceptibility data are presented for six distorted cubane complexes of the composition [MnIVMnIII 3O3X]. Each of these complexes has a well isolated S = 9/2 ground state. There is zero-field splitting (ZFS) in the ground states where D, the axial ZFS parameter, is found to be in the range of −0.27 to −0.38 cm-1. As a result of the big spin ground state and appreciable magnetic anisotropy, an out-of-phase ac magnetic susceptibility signal is seen for each of the six Mn4 complexes. The out-of-phase ac susceptibility signal reflects slow magnetization relaxation which is taken to indicate that individual molecules are acting as magnets. Alternating current susceptibility data are presented for a frozen glass of one of the Mn4 complexes to confirm that the out-of-phase ac signal is associated with isolated molecules. The factors that influence whether a given complex can function as a single-molecule magnet are described. The above Mn4 complexes represent only the second type of molecules that exhibit enough magnetic anisotropy to function as single-molecule magnets.

The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII 3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜ z 2) with D = −0.53 cm-1 and a quartic zero-field splitting (B 4 0Ô4 0)with B 4 0 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII 3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by being thermally activated over a potential-energy barrier, but by the magnetization tunneling through the barrier. Additional evidence for resonant magnetization tunneling was found in the change in the temperature at which the out-of-phase ac magnetic susceptibility is observed as a function of an external dc field. The results of magnetization relaxation experiments carried out in the 0.394−0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhenius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this plot gives an activation barrier of 11.8 K. Below 0.6 K the relaxation rate is independent of temperature with a rate of 3.2 × 10-2 s-1. This S = 9/2 single-molecule magnet exhibits a tunneling of its direction of magnetization at a rate of 3.2 × 10-2 s-1 in the 0.394−0.600 K range. Thus, resonant magnetization tunneling is seen for a half-integer-spin (S = 9/2) ground-state magnet in the absence of an external magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.