Experimental evidence of multiple magnetic relaxation processes in Mn-12 acetate and Mn(12)2-Cl benzoate

The Mn cluster complexes Mn-12(RCOO)(16)(H2O)(4)O-12 With R = CH3 and 2ClPh (hereafter referred to as Mn-12 acetate and Mn-12 2-Cl benzoate, respectiv ely) exhibit macroscopic magnetic quantum tunneling. Dynamical magnetic mea surements indicate for both samples that the anisotropy energy barrier is n early the same and that the relaxation times have sharp minima at the same resonant fields. However, we show that for both compounds a minority portio n of the clusters does not undergo this main relaxation process. From a.c.- 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 Arr henius law with energy barrier of approximately 23 and 30 K for the acetate and the benzoate, respectively. In the case of Mn-12 2-Cl benzoate, the fi eld dependence of the effective relaxation time points towards the existenc e of spin tunneling with H-z = 0 being a resonant field. The existence of a broad distribution of energy barriers is corroborated in magnetization exp eriments by the observation of logarithmic relaxation at short times. (C) 1 999 Elsevier Science Ltd. All rights reserved.