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(H ₂O) ₄O ₁₂ with R=CH ₃ and 2ClPh (hereafter referred to as Mn ₁₂ acetate and Mn ₁₂ 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 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 Arrhenius 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 field dependence of the effective relaxation time points towards the existence of spin tunneling with H_z=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.