This article takes a broad view of the understanding of magnetic bistabilityand magnetic quantum tunneling in single-molecule magnets (SMMs), focusing onthree families of relatively simple, low-nuclearity transition metal clusters:spin S = 4 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III)complexes are related by the fact that they contain triangular Mn3 units inwhich the exchange may be switched from antiferromagnetic to ferromagneticwithout significantly altering the coordination around the Mn(III) centers,thereby leaving the single-ion physics more-or-less unaltered. This allows fora detailed and systematic study of the way in which the individual-ionanisotropies project onto the molecular spin ground state in otherwiseidentical low- and high-spin molecules, thus providing unique insights into thekey factors that control the quantum dynamics of SMMs, namely: (i) the heightof the kinetic barrier to magnetization relaxation; and (ii) the transverseinteractions that cause tunneling through this barrier. Numerical calculationsare supported by an unprecedented experimental data set (17 differentcompounds), including very detailed spectroscopic information obtained fromhigh-frequency electron paramagnetic resonance and low-temperature hysteresismeasurements. Diagonalization of the multi-spin Hamiltonian matrix is necessaryin order to fully capture the interplay between exchange and local anisotropy,and the resultant spin-state mixing which ultimately gives rise to thetunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 andNi4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) ofthe molecules highlighted in this study proves to be of crucial importance.
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