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The single-molecule magnets (SMMs) [Mn4O3X(OAc)3(dbm)3] (X = Br, Cl, OAc, and F) were investigated by a detailed inelastic neutron scattering (INS) study. Up to four magnetic excitations between the zero-field split levels of the lowest S = 9/2 cluster ground-state have been resolved. From the determined energy-level diagrams and the relative INS intensities we can show that the inclusion of a rhombic term in the zero-field splitting (ZFS) Hamiltonian is essential in these compounds. On the basis of the Hamiltonian:  Ĥ ZFS = D[ − 1/3 S(S + 1)] + E( − ) + , the following sets of parameters are derived:  For X = Cl:  D = −0.529 cm-1, |E| = 0.022 cm-1, and = −6.5 × 10-5 cm-1; for X = Br:  D = −0.502 cm-1, |E| = 0.017 cm-1, and = −5.1 × 10-5 cm-1; for X = OAc:  D = −0.469 cm-1, |E| = 0.017 cm-1, and = −7.9 × 10-5 cm-1; and for X = F:  D = −0.379 cm-1 and = −11.1 × 10-5 cm-1. The wave functions derived from the energy analysis are in excellent agreement with the relative intensities of the observed INS transitions. The observed temperature maxima of the out-of-phase component of the variable frequency AC magnetic susceptibility T max[χ‘ ‘] correlate very well with the energy splittings determined by INS. Direct information about the rate of quantum tunneling is contained in the cluster wave functions derived in this study. The difference in the quantum tunneling between X = Cl and Br is shown to be directly related to differences in the rhombic anisotropy parameter |E|.

The synthesis and structural characterisation of three small nickel(II) cages are reported, all stabilised by pivalate ligands. The magnetic properties of the cages have been studied by a combination of susceptibility measurements and inelastic neutron scattering. For a dinuclear cage, [Ni2(mu-OH2)(O2CCMe3)4(HO2CCMe3)4] 1 the ground state is S=2, with a ferromagnetic exchange interaction between the Ni(II) centres of J=0.32 meV and D(S=2)=-0.09 meV in the ground state. For a tetranuclear heterocubane cage, [Ni4(mu(3)-OMe)4(O2CCMe3)4(MeOH)4] 2, two ferromagnetic exchange interactions are found and an S=4 ground state observed. While the zero-field splitting of this state cannot be determined unambigiously the most likely value is DS=4=-0.035 meV. For a tetranuclear nickel butterfly, [Ni4(mu3-OH)2(O2CCMe3)6(EtOH)6] 3, three exchange interactions are required, two anti-ferromagnetic and one weakly ferromagnetic; the resulting ground state is S=0. The data enable us to estimate the zero-field splitting of single Ni(II) ions involved in the cage as Di=+1.0 meV. Both and are therefore expected to be new single molecule magnets.

PACS. 75.30.Et -Exchange and superexchange interactions. PACS. 75.50.Xx -Molecular magnets. PACS. 78.70.Nx -Neutron inelastic scattering. Abstract. -We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V IV15 As6O42]·9D2O (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V 4+ (S=1/2) is determined by three weakly coupled spin-1/2 on a triangle. The INS spectra at low-energy allow us to directly determine the effective exchange coupling J0 = 0.211(2) meV within the triangle and the gap 2∆ = 0.035(2) meV between the two spin-1/2 doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions.