Resonant magnetization tunneling in Mn-12 acetate: The absence of inhomogeneous hyperfine broadening

We present the results of a detailed study of the thermally assisted-resona nt-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 we ll to a Lorentzian function. No hint of inhomogeneous broadening is found, even though some is expected from the Mn nuclear hyperfine interaction. Thi s inconsistency implies that the tunneling mechanism cannot be described si mply in terms of a random hyperfine field.