Resonant magnetization tunneling in Mn12 acetate:: The absence of inhomogeneous hyperfine broadening

We present the results of a detailed study of the thermally assisted-resonant-tunneling relaxation rate of Mn-12 acetate as a function of an external, longitudinal magnetic field and find that the data can be fit extremely well to a Lorentzian function. No hint of inhomogeneous broadening is found, even though some is expected from the Mn nuclear hyperfine interaction. This inconsistency implies that the tunneling mechanism cannot be described simply in terms of a random hyperfine field.