Resonant magnetization tunneling in the molecular magnet Mn12 acetate

I review the evidence for thermally assisted resonant tunneling between spin states in the molecular magnet known as Mn-12 acetate. The hysteresis loops for a sample show steps at regular intervals of magnetic field, every 4.5 kOe. At these same fields, the magnetic relaxation rate increases markedly. This system can be modeled as a double-well potential with energy levels corresponding to projections of the spin along the anisotropy axis. A magnetic field tilts this potential and regularly brings levels in opposite wells into resonance, allowing tunneling between the wells. The field interval between steps quantitatively agrees with the independently measured value of the anisotropy parameter. The data imply that the tunneling is driven in part by an internal transverse magnetic field, perhaps of hyperfine origin. However, the resonances have a Lorentzian line shape with no hint of the inhomogeneous broadening expected from a random internal field.