Ralph Schenker, Michael N. Leuenberger, Grégory Chaboussant, Daniel Loss and Hans U. Güdel
Phonon bottleneck effect leads to observation of quantum tunneling of the magnetization and butterfly hysteresis loops in (Et₄N)₃Fe₂F₉
Phys. Rev. B 72, 184403/1-10 (2005)                 

Abstract: A detailed investigation of the unusual dynamics of the magnetization of (Et₄N)₃Fe₂F₉ (Fe₂), containing isolated [Fe₂F₉]^3– dimers, is presented and discussed. Fe₂ 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 T1.8 K Fe₂ 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.