Pulsed field studies of the magnetization reversal in molecular   nanomagnets
  We report experimental studies of crystals of Mn12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4x10^3 T/s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets. 
J. Vanacken, S. Stroobants, M. Malfait, V.V. Moshchalkov, M. Jordi, J.   Tejada, R. Amigo, E.M. Chudnovsky and D.A. Garanin