Abstract
Zero-field–cooled magnetization, magnetic relaxation, and a.c. susceptibility measurements have been performed on a Mn12-Ac oriented powdered sample as a function of temperature, field, and orientation. Magnetization jumps and susceptibility peaks have been observed about magnetic-field values Bn = nB1, where B1 ≈ 5 kG. These anomalies are due to the existence of relaxation rate maxima near Bn. From these experimental results we infer the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation.