NOVEL FEATURES IN THE RELAXATION-TIMES OF MN12AC

We report novel features of the relaxation time measured on single crystals of a Mn-12 acetate complex (Mn12Ac). Using a SQUID magnetometer working at 100 mg and up to 8 T, we were able to study the relaxation of the magnetization at low temperatures. For a given temperature and field, the relaxation of the bulk of the sample can be described by one relaxation time, tau. When the magnetic field is aligned along the easy axis of magnetization, the relaxation time is observed to deviate from thermal activation at approximately 1.9 to 1.5 K for H = 0 to 1 T. Below this crossover temperature, the relaxation time tends to saturate at values of around 10(7)-10(8) s. A critical account of this crossover temperature will be given, especially the possibility of magnetic quantum tunnelling.