Precision ESR Measurements of Transverse Anisotropy in the Single-molecule Magnet Ni$_4$

TL;DR

This paper introduces a precise ESR-based method to measure tunnel splitting and transverse anisotropy in single-molecule magnets, demonstrated on Ni$_4$, enabling independent determination of anisotropy parameters.

Contribution

The study develops a novel ESR measurement technique to accurately determine transverse anisotropy in SMMs, independent of other Hamiltonian parameters.

Findings

Narrower resonance peaks in diluted Ni$_4$ samples

Precise measurement of tunnel splitting at multiple frequencies

Independent determination of transverse anisotropy magnitude

Abstract

We present a method for precisely measuring the tunnel splitting in single-molecule magnets using electron-spin resonance, and use these measurements to precisely and independently determine the underlying transverse anisotropy parameter, given a certain class of transitions. By diluting samples of the SMM Ni$_4$ via co-crystallization in a diamagnetic isostructural analogue we obtain markedly narrower resonance peaks than are observed in undiluted samples. Using custom loop-gap resonators we measure the transitions at several frequencies, allowing a precise determination of the tunnel splitting. Because the transition under investigation occurs at zero field, and arises due to a first-order perturbation from the transverse anisotropy, we can determine the magnitude of this anisotropy independent of any other Hamiltonian parameters. This method can be applied to other SMMs with tunnel splittings arising from first-order transverse anisotropy perturbations.