Phonon bottleneck effect leads to observation of quantum tunneling of the magnetization and butterfly hysteresis loops in (Et₄N)₃Fe₂F₉

A detailed investigation of the unusual dynamics of the magnetization of (Et4N)(3)Fe2F9 (Fe-2), containing isolated [Fe2F9](3-) dimers, is presented and discussed. Fe-2 possesses an S=5 ground state with an energy barrier of 2.40 K due to an axial anisotropy. Poor thermal contact between sample and bath leads to a phonon bottleneck situation, giving rise to butterfly-shaped hysteresis loops below 5 K concomitant with slow decay of the magnetization for magnetic fields H-z applied along the Fe-Fe axis. The butterfly curves are reproduced using a microscopic model based on the interaction of the spins with resonant phonons. The phonon bottleneck allows for the observation of resonant quantum tunneling of the magnetization at 1.8 K, far above the blocking temperature for spin-phonon relaxation. The latter relaxation is probed by ac magnetic susceptibility experiments at various temperatures and bias fields H-DC. At H-DC=0, no out-of-phase signal is detected, indicating that at T <= 1.8 K Fe-2 does not behave as a single-molecule magnet. At H-DC=1 kG, relaxation is observed, occurring over the barrier of the thermally accessible S=4 first excited state that forms a combined system with the S=5 state.