RESONANT MAGNETIZATION TUNNELING IN THE TRIGONAL PYRAMIDAL MN(IV)MN-3(III) COMPLEX [MN4O3CL(O2CCH3)(3)(DBM)(3)]

The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)(3)(dbm)(3)], where dbm (-) is the monoanion of dibenzoylmethane, functions as a single-molecu le magnet. High-field EPR data are presented for an oriented microcrys talline sample to characterize the electronic structure of the (MnMn3I II)-Mn-IV complex. These data show that the complex has a S = 9/2 grou nd state, experiencing axial zero-field splitting (D (S) over cap(z)(2 )) With D = -0.53 cm(-1) and a quartic zero-field splitting (B-4(0) (O ) over cap(4)(0))With B-4(0) = -7.3 x 10(-5) cm(-1). Magnetization ver sus 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 hyste resis is seen. Steps are seen on each hysteresis loop. This is clear e vidence that each (MnMn3III)-Mn-IV complex functions as a single-molec ule magnet that is magnetizable. Furthermore, the steps on the hystere sis loops are due to resonant magnetization quantum mechanical tunneli ng. In response to an external field each molecule reverses its direct ion of magnetization not only by being thermally activated over a pote ntial-energy barrier, but by the magnetization tunneling through the b arrier. Additional evidence for resonant magnetization tunneling was f ound in the change in the temperature at which the out-of-phase ac mag netic susceptibility is observed as a function of Bn external de field . The results of magnetization relaxation experiments carried out in t he 0.394-0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhen ius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this p lot 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 x 10(-2) s(-1). This S = 9/2 single-molecule magnet exhibits a tunneling of its direct ion of magnetization at a rate of 3.2 x 10(-2) s(-1) in the 0.394-0.60 0 K range. Thus, resonant magnetization tunneling is seen for a half-i nteger-spin (S = 9/2) ground-state magnet in the absence of an externa l magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.