Direct measurement of the tunneling rate of the magnetization in Fe8 via 57Fe nuclear spin-lattice relaxation by strong collision

TL;DR

This study measures the magnetization tunneling rate in Fe8 molecular magnets using $^{57}$Fe nuclear spin-lattice relaxation, demonstrating a direct link between relaxation rates and incoherent tunneling probabilities at very low temperatures.

Contribution

It provides the first direct experimental measurement of the tunneling rate in Fe8 via nuclear relaxation, confirming the strong collision mechanism as the dominant relaxation process.

Findings

Nuclear relaxation is driven by a strong collision mechanism.

The tunneling rate derived from relaxation matches theoretical estimates.

Relaxation measurements are consistent across different sample orientations.

Abstract

$^{57}$Fe and $^1$H relaxation measurements have been performed in single crystal and oriented powder of enriched $^{57}$Fe8 molecular cluster in the temperature range 0.05--1.7 K in zero external field and with small perturbing longitudinal field ($< 1$ T). On the basis of the experimental results it is argued that in zero external field the nuclear spin-lattice relaxation ($1/T_1$) mechanism is driven by a strong collision mechanism whereby $1/T_1$ is a direct measure of the incoherent tunneling probability in the low lying magnetic energy states of the molecular nanomagnet. The approximate value of the effective tunneling rate vs $T$ and $H$ derived directly from $1/T_1$ is shown to be consistent with theoretical estimates based on known parameters of the Hamiltonian.