Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets

TL;DR

This paper investigates how higher-lying spin multiplets influence spectroscopic transition intensities in molecular nanomagnets, revealing effects observable in INS and EPR techniques, and provides analytical tools for their analysis.

Contribution

It introduces a systematic analytical approach to quantify many-spin effects on spectroscopic intensities, including spin mixing and anisotropy contributions, in molecular nanomagnets.

Findings

INS intensities depend oscillatory on momentum transfer Q due to many-spin wave functions

Spin mixing affects transition intensities within the spin multiplet differently

Analytical methods differentiate contributions from spin mixing and ligand-field anisotropy

Abstract

Many molecular magnetic clusters, such as single-molecule magnets, are characterized by spin ground states with defined total spin S exhibiting zero-field-splittings. In this work, the spectroscopic intensities of the transitions within the ground-state multiplet are analyzed. In particular, the effects of a mixing with higher-lying spin multiplets, which is produced by anisotropic interactions and is neglected in the standard single-spin description, are investigated systematically for the two experimental techniques of inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR), with emphasis on the former technique. The spectroscopic transition intensities are calculated analytically by constructing corresponding effective spin operators perturbationally up to second order and consequently using irreducible tensor operator techniques. Three main effects of spin mixing are observed. Firstly, a pronounced dependence of the INS intensities on the momentum transfer Q, with a typical oscillatory behavior, emerges in first order, signaling the many-spin nature of the wave functions in exchange-coupled clusters. Secondly, as compared to the results of a first-order calculation, the intensities of the transitions within the spin multiplet are affected differently by spin mixing. This allows one, thirdly, to differentiate the higher-order contributions to the cluster magnetic anisotropy which come from the single-ion ligand-field terms and spin mixing, respectively. The analytical results are illustrated by means of the three examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid molecule, and the single-molecule magnet Mn12.