Abstract
The magnetic ac susceptibility x of oriented Mn12Ac crystallites has been measured as a function of temperature,
field, and frequency. The field has been applied at different values of the angle u with respect to the
sample easy axis. For T55 K, the isothermal and adiabatic x limits have been determined as a function of
field. For u50° and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field
values Hn5nH1 with n50, 1, and 2 where H154.1 kOe. As u increases these maxima shift to higher fields,
that satisfy Hncosu5const, and decrease in amplitude. The relaxation time t 1 follows Arrhenius’ law with
respect to temperature and decreases sharply at H5Hn . The observed phenomenology unambiguously proves
the existence of field-tuned tunneling between excited magnetic states which are thermally populated. At 5 K,
the effective activation energy and the spin states involved in the tunneling process have been obtained.