Inelastic neutron scattering and magnetic susceptibilities of the single-molecule magnets [Mn4O3X(OAc)(3)(dbm)(3)] (X = Br, Cl, OAc, and F): Variation of the anisotropy along the series

The single-molecule magnets (SMMs) [Mn4O3X(OAc)(3)(dbm)(3)] (X = Br, Cl, OA c, and F) were investigated by a detailed inelastic neutron scattering (INS ) study. Up to four magnetic excitations between the zero-field split level s of the lowest S = 9/2 cluster ground-state have been resolved. From the d etermined energy-level diagrams and the relative INS intensities we can sho w that the inclusion of a rhombic term in the zero-field splitting (ZFS) Ha miltonian is essential in these compounds. On the basis of the Hamiltonian: (H) over cap (ZFS) = D[(S) over cap (2)(z) - 1/3S(S + 1)] + E((S) over cap (2)(x) - (S) over cap (2)(y)) + B-4(0)(O) over cap (0)(4), the following s ets of parameters are derived: For X = Cl: D = -0.529 cm(-1), \E\ = 0.022 c m(-1), and B-4(0) = -6.5 x 10(-5) cm(-1); for X = Br: D = -0.502 cm(-1), \E \ = 0.017 cm(-1), and B-4(0) = -5.1 x 10(-5) cm(-1); for X = OAc: D = -0.46 9 cm(-1), \E\ = 0.017 cm(-1), and B-4(0) = -7.9 x 10(-5) cm(-1); and for X = F: D = -0.379 cm(-1) and B-4(0) = -11.1 x 10-5 cm(-1). The wave functions derived from the energy analysis are in excellent agreement with the relat ive intensities of the observed INS transitions. The observed temperature m axima of the out-of-phase component of the variable frequency AC magnetic s usceptibility T-max[chi"] correlate very well with the energy splittings de termined by INS. Direct information about the rate of quantum tunneling is contained in the cluster wave functions derived in this study. The differen ce in the quantum tunneling between X = Cl and Br is shown to be directly r elated to differences in the rhombic anisotropy parameter \E\.