Field-dependence of the Magnetic Relaxation in Mn12-Acetate: A New Form of Spectroscopy

We report point by point measurements below the blocking temperature of the magnetic relaxation of Mn$_{12}$-acetate as a function of magnetic field applied along the easy axis of magnetization. Unexpectedly complex structure is observed which we attribute to the effect of higher-order terms of the spin Hamiltonianon on the tunneling process.