Quantized antiferromagnetic spin waves in the molecular Heisenberg ring CsFe$_8$

TL;DR

This study uses inelastic neutron scattering to observe quantized spin waves and a ground state gap in the molecular Heisenberg ring CsFe$_8$, modeled effectively with a simple Hamiltonian.

Contribution

It provides the first experimental observation of quantized antiferromagnetic spin waves in a finite molecular ring and demonstrates accurate modeling with minimal parameters.

Findings

Observation of quantized spin wave excitations

Detection of a ground state energy gap

Excellent data fit with a simple Heisenberg model

Abstract

We report on inelastic neutron scattering (INS) measurements on the molecular spin ring CsFe$_8$, in which eight spin-5/2 Fe(III) ions are coupled by nearest-neighbor antiferromagnetic Heisenberg interaction. We have recorded INS data on a non-deuterated powder sample up to high energies at the time-of-flight spectrometers FOCUS at PSI and MARI at ISIS, which clearly show the excitation of spin waves in the ring. Due to the small number of spin sites, the spin-wave dispersion relation is not continuous but quantized. Furthermore, the system exhibits a gap between the ground state and the first excited state. We have modeled our data using exact diagonalization of a Heisenberg-exchange Hamiltonian together with a small single-ion anisotropy term. Due to the molecule's symmetry, only two parameters $J$ and $D$ are needed to obtain excellent agreement with the data. The results can be well described within the framework of the rotational-band model as well as antiferromagnetic spin-wave theories.