A wheel-shaped single-molecule magnet of [(Mn3Mn4III)-Mn-II]: Quantum tunneling of magnetization under static and pulse magnetic fields

The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (=H-3(5-NO2-hbide)) with Mn(OAc)(2)center dot 4H(2)O in methanol, followed by recrystallization from 1,2-dichloroethane, yielded a wheel-shaped single-molecule magnet (SMM) of [(Mn3Mn4III)-Mn-II(5-NO2-hbide)(6)]center dot 5C(2)H(4)Cl(2) (1). In 1, seven manganese ions are linked by six tri-anionic ligands and form the wheel in which the two manganese ions on the rim and the one in the center are Mn-II and the other four manganese ions are Mn-III ions. Powder magnetic susceptibility measurements showed a gradual increase with chi T-m values as the temperature was lowered, reaching a maximum value of 53.9 emu mol(-1)K. Analyses of magnetic susceptibility data suggested a spin ground state of S=19/2. The zero-field splitting parameters of D and B-4(0) were estimated to be -0.283(1)K and -1.64(1) x 10(-5) K, respectively, by high-field EPR measurements (HF-EPR). The anisotropic parameters agreed with those estimated from magnetization and inelastic neutron scattering experiments. AC magnetic susceptibility measurements showed frequency-dependent in- and out-of-phase signals, characteristic data for an SMM, and an Arrhenius plot of the relaxation time gave a re-orientation energy barrier (Delta E) of 18.1 K and a pre-exponential factor of 1.63 x 10(-7) s. Magnetization experiments on aligned single crystals below 0.7 K showed a stepped hysteresis loop, confirming the occurrence of quantum tunneling of the on magnetization (QTM). QTM was, on the other hand, suppressed by rapid sweeps of the magnetic field even at 0.5 K. The sweep-rate dependence of the spin flips can be understood by considering the Landau-Zener-Stuckelberg (LZS) model.