Field tuning of thermally activated magnetic quantum tunnelling in Mn-12-Ac molecules

Zero-field-cooled magnetization, magnetic relaxation, and a.c. susceptibility measurements have been performed on a Mn-12-Ac oriented powdered sample as a function of temperature, field, and orientation. Magnetization jumps and susceptibility peaks have been observed about magnetic-field values B-n = nB(1), where B-1 approximate to 5 kG. These anomalies are due to the existence of relaxation rate maxima near B-n. From these experimental results we infer the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation.