High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe$_8$

TL;DR

This study uses high-frequency EPR to precisely characterize the spin Hamiltonian parameters of the CsFe$_8$ molecular wheel, confirming its suitability as a model for antiferromagnetic Heisenberg rings and aligning with previous neutron and torque data.

Contribution

It provides accurate determination of exchange and anisotropy parameters of CsFe$_8$ using high-frequency EPR, validating the dimer model and its relevance as an AF Heisenberg ring model.

Findings

Exchange coupling J = -15(2) cm$^{-1}$

Uniaxial anisotropy D = -0.3940(8) cm$^{-1}$

Negligible rhombic anisotropy E

Abstract

High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T as well as Q-band ($f$ = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe$_8$. In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with $S \leq$ 4. Furthermore, the data can well be reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters $J$ and $D$. A fit to the dimer model yields $J$ = -15(2) cm$^{-1}$ and $D$ = -0.3940(8) cm$^{-1}$. A rhombic anisotropy term is found to be negligibly small, $E$ = 0.000(2) cm$^{-1}$. The results are in excellent agreement with previous inelastic neutron scattering (INS) and high-field torque measurements. They confirm that the CsFe$_8$ molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.