Phonon-Assisted Tunneling in High-Spin Molecules: Experimental Evidence

We report on the observation of quantum tunneling in ${\mathrm{Mn}}_{12}-\mathrm{acetate}$ single crystals at a high frequency (680 MHz) and low temperature (down to 20 mK). A peak in the imaginary part of the ac susceptibility as a function of magnetic field is observed. It arises from transitions between the two states of the fundamental doublet of the high-spin molecule. The tunneling rate is increased by setting the magnetic field perpendicular to the easy axis of magnetization of the crystals. Below 0.1 K the peak decreases strongly down to 0.02 K. This effect is explained with a phonon-induced tunneling process.