  The Mn cluster complexes Mn12(RCOO)16(H2O)4O12 with R=CH3 and 2ClPh
(hereafter referred to as Mn12 acetate and Mn12 2-Cl benzoate, respectively)
exhibit macroscopic magnetic quantum tunneling. Dynamical magnetic measurements
indicate for both samples that the anisotropy energy barrier is nearly the same
and that the relaxation times have sharp minima at the same resonant fields.
However, we show that for both compounds a minority portion of the clusters
does not undergo this main relaxation process. From ac-susceptibility
experiments, we report evidence of a continuous distribution of faster
relaxation times, giving rise to a second relaxation regime. The zero-field
effective relaxation times of such a distribution follow an Arrhenius law with
energy barrier of approximately 23 and 30 K for the acetate and the benzoate,
respectively. In the case of Mn12 2-Cl benzoate, the field dependence of the
effective relaxation time points towards the existence of spin tunneling with
Hz=0 being a resonant field. The existence of a broad distribution of energy
barriers is corroborated in magnetization experiments by the observation of
logarithmic relaxation at short times.
