Molecular Nanomagnets

Abstract

Quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets (SMM's). Magnetization tunneling is described for two crystallographically different forms of [Mn 12 O 12 (O 2 CC 6 H 4 - p -Me) 16 (H 2 O) 4 ] solvate. The two Mn 12 complexes are isomers. The magnetization versus magnetic field hysteresis loops are 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 unusual orientation of Jahn-Teller elongation axes in one isomer leads to greater rhombic zero-field splitting that is the origin of the faster magnetization tunneling. In addition to data for Mn 12 complexes, magnetization relaxation rate versus temperature responses of three mixed-valence Mn 4 complexes are also reported. In all three cases the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-independent region as well as a temperature-dependent region. The temperature-independent relaxation rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.