Magnetization measurements of a truly axial symmetry Mn₁₂-tBuAc molecular nanomagnet with a spin ground state of $S=10$ show resonant tunneling. This compound has the same magnetic anisotropy as Mn₁₂-Ac but the molecules are better isolated and the crystals have less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a well-resolved step fine structure which is due to level crossings of excited states. All step positions can be modeled by a simple spin Hamiltonian. The results establish a sharp crossover between thermally assisted and pure quantum tunneling, as had been previously predicted.
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Wernsdorfer et al. (2006) studied this question.