Resonant tunneling in truly axial symmetry Mn12 single-molecule magnets:: Sharp crossover between thermally assisted and pure quantum tunneling

Magnetization measurements of a truly axial symmetry Mn-12-tBuAc molecular nanomagnet with a spin ground state of S = 10 show resonant tunneling. This compound has the same magnetic anisotropy as Mn-12-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.