Phonon Bottleneck Effect Leads to Observation of Quantum Tunneling of the Magnetization and Butterfly Hysteresis Loops in (Et4N)3Fe2F9

TL;DR

This study reveals how phonon bottleneck effects in (Et4N)3Fe2F9 enable the observation of quantum tunneling of magnetization and butterfly hysteresis loops at very low temperatures, combining experimental and microscopic modeling insights.

Contribution

It demonstrates the role of phonon bottleneck in observing quantum tunneling and butterfly hysteresis in a specific Fe(III) dimer compound, with detailed experimental and theoretical analysis.

Findings

Butterfly hysteresis loops observed below 5 K due to phonon bottleneck.

Quantum tunneling of magnetization detected at 1.8 K, above the usual blocking temperature.

Magnetic relaxation occurs over the S=4 excited state, influenced by the phonon environment.

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 Hz 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. At H=0, no out-of-phase signal is detected, indicating that at T smaller than 1.8 K Fe2 does not behave as a single-molecule magnet. At 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.