Thermally activated and field-tuned tunneling in Mn12Ac studied by ac magnetic susceptibility

The magnetic ac susceptibility chi 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 theta with respect to the sample easy axis. For T = 5 K, the isothermal and adiabatic chi limits have been determined as a function of field. For theta = 0 degrees and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field values H-n = nH(1) with n = 0, 1, and 2 where H-1 = 4.1 kOe. As theta increases these maxima shift to higher fields, that satisfy H-n cos theta = const, and decrease in amplitude. The relaxation time tau(1) follows Arrhenius' law with respect to temperature and decreases sharply at H = H-n. 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.