Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8

TL;DR

This study reveals a significant isotope effect on the specific heat of the molecular nanomagnet Fe8, showing how nuclear spins influence quantum tunneling and spin-lattice relaxation at very low temperatures.

Contribution

It demonstrates the impact of 57Fe nuclear spins on the specific heat and tunneling dynamics in Fe8, introducing a model for tunneling-lattice coupling and observing a crossover in tunneling mechanisms.

Findings

Enhanced specific heat due to nuclear spins below 1 K

Tunneling becomes inelastic when nuclear-spin-mediated

Crossover from nuclear-spin to phonon-induced tunneling under transverse field

Abstract

Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe nuclear spins leads to a huge enhancement of the specific heat below 1 K, ascribed to a strong increase in the spin-lattice relaxation rate Gamma arising from incoherent, nuclear-spin-mediated magnetic quantum tunneling in the ground-doublet. Since Gamma is found comparable to the expected tunneling rate, the latter process has to be inelastic. A model for the coupling of the tunneling levels to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.