Ralph Schenker, Michael N. Leuenberger, Grégory Chaboussant, Hans U. Güdel and Daniel Loss
Butterfly hysteresis and slow relaxation of the magnetization in (Et₄N)₃Fe₂F₉: manifestations of a single-molecule magnet
Chem. Phys. Lett. 358, 413-418 (2002)                 

Abstract: (Et₄N)₃Fe₂F₉ exhibits a butterfly-shaped hysteresis below 5 K when the magnetic field is parallel to the threefold axis, in accordance with a very slow magnetization relaxation in the timescale of minutes. This is attributed to an energy barrier D = 2.40 K resulting from the S = 5 dimer ground state of [Fe₂F₉]^3- and a negative axial anisotropy. The relaxation partly occurs via thermally assisted quantum tunneling. These features of a single-molecule magnet are observable at temperatures comparable to the barrier height, due to an extremely inefficient energy exchange between the spin system and the phonons. The butterfly shape of the hysteresis arises from a phonon avalanche effect.