Quantum tunneling and relaxation in ${\mathrm{Mn}}_{12}$-acetate studied by magnetic spectroscopy

The spin relaxation in the molecular magnet ${\mathrm{Mn}}_{12}\mathrm{ac}$ is investigated by high-frequency magnetic spectroscopy in the frequency domain. By reversing the external magnetic field the intensities of the $|\pm 10\overrightarrow{〉}|\pm 9〉$ transitions within the ground multiplet redistribute; below $T\approx 2\mathrm{K}$ resonant quantum tunneling is directly observed as a tunnel dip in the absorption spectra of polycrystals. We quantitatively describe our observations by taking into account inhomogeneous line broadening, the time dependence of the level population during the relaxation process, and the resonance behavior of the relaxation rate due to quantum tunneling near the avoided level crossing.