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Theory of high-symmetry tetramer single molecule magnets

We present a microscopic theory of single molecule magnets. From our exact single-ion spin matrix elements for four arbitrary spins, we study the single-ion anisotropy of equal spins exhibiting $T_d$, $D_{2d}$, or $C_{4v}$ molecular group symmetry. Each group generates site-dependent single-ion anisotropy. For weak anisotropy, accurate Hartree expressions for the magnetization, specific heat, electron paramagnetic resonance (EPR) absorption, and inelastic neutron scattering cross-section are given. For $D_{2d}$, azimuthal single-ion anisotropy leads to the observed Ni$_4$ EPR splittings.