Magnetization tunneling in single-molecule magnets

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM's). SMM's are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H5-p-Me)(16)(H2O)(4)] solvale. The two Mn,, complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the Mn-III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn-12 complexes. One Mn-12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn,, isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and de magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn, complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state. (C) 2001 Elsevier Science Ltd. All rights reserved.