Thermally activated and field-tuned tunneling inMn12Ac studied by ac magnetic susceptibility

The magnetic ac susceptibility χ of oriented ${\mathrm{Mn}}_{12}$Ac crystallites has been measured as a function of temperature, field, and frequency. The field has been applied at different values of the angle θ with respect to the sample easy axis. For T=5tK, the isothermal and adiabatic χ limits have been determined as a function of field. For θ=0° and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field values ${\mathrm{H}}_{\mathrm{n}}$=${\mathrm{nH}}_1$ with n=0, 1, and 2 where ${\mathrm{H}}_1$=4.1tkOe. As θ increases these maxima shift to higher fields, that satisfy ${\mathrm{H}}_{\mathrm{n}}$cos θ=const, and decrease in amplitude. The relaxation time ${\mathrm{\tau }}_1$ follows Arrhenius' law with respect to temperature and decreases sharply at H=${\mathrm{H}}_{\mathrm{n}}$. 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.