Magnetization tunneling in single-molecule magnets

Abstract

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 [Mn 12O 12(O 2CC 6H 4- p-Me) 16(H 2O) 4] solvate. The two Mn 12 complexes are isomers that both differ in the positioning of the H 2O 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 12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn 4 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.