Origin of the fast magnetization tunneling in the single-molecule magnet †Ni„hmp...„t-BuEtOH...Cl‡4

We present high-frequency angle-dependent EPR data for crystals of fNixZn1−xshmpdstBuEtOHdClg4 sx=1 and 0.02d. The x=1 complex behaves as a single-molecule magnet at low temperatures, displaying hysteresis and exceptionally fast magnetization tunneling. We show that this behavior is related to a fourth-order transverse crystal-field interaction, which produces a significant tunnel splitting s,10 MHzd of the ground state of this S=4 system. The magnitude of the fourth-order anisotropy, and the dominant axial term sDd, can be related to the single-ion interactions sDi and Eid at the individual NiII sites, as determined for the x=0.02 crystals. © 2005 American Institute of Physics. fDOI: 10.1063/1.1845871g