More evidence for a distribution of tunnel splittings in Mn12–acetate

In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn12 has been measured in response to a field that is swept back and forth across the resonances corresponding to steps N=4,5,…9. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter XE=−ln(|E|/2D). We show that dipolar shuffling is a small effect and is not responsible for the nonexponential relaxation.In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn12 has been measured in response to a field that is swept back and forth across the resonances corresponding to steps N=4,5,…9. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter XE=−ln(|E|/2D). We show that dipolar shuffling is a small effect and is not responsible for the nonexponential relaxation.