Abstract
A detailed investigation of the unusual dynamics of the magnetization of , containing isolated dimers, is presented and discussed. possesses an ground state with an energy barrier of 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 concomitant with slow decay of the magnetization for magnetic fields applied along the 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 , 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 . At , no out-of-phase signal is detected, indicating that at does not behave as a single-molecule magnet. At , relaxation is observed, occurring over the barrier of the thermally accessible first excited state that forms a combined system with the state.
4 More- Received 23 February 2005
DOI:https://doi.org/10.1103/PhysRevB.72.184403
©2005 American Physical Society