Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12

TL;DR

This study analyzes how dipolar and exchange interactions influence the temperature-dependent linewidths and shifts in EPR spectra of Fe8 and Mn12 single-molecule magnets, highlighting the dominant role of spin-spin interactions.

Contribution

It provides a quantitative analysis of the temperature dependence of EPR linewidths and shifts considering inter-molecular interactions and distributions in anisotropy and g-factor, aligning well with experimental data.

Findings

Spin-spin interactions mainly cause temperature dependence of linewidths and shifts.

Fe8 shows a stronger temperature dependence than Mn12 due to narrower D distribution.

Fe8's line-shift suggests weak ferromagnetic exchange and dipolar interactions.

Abstract

We examine quantitatively the temperature dependence of the linewidths and line shifts in electron paramagnetic resonance experiments on single crystals of the single-molecule magnets Fe$_8$ and Mn$_{12}$, at fixed frequency, with an applied magnetic field along the easy axis. We include inter-molecular spin-spin interactions (dipolar and exchange) and distributions in both the uniaxial anisotropy parameter $D$ and the Land\'{e} $g$-factor. The temperature dependence of the linewidths and the line shifts are mainly caused by the spin-spin interactions. For Fe$_8$ and Mn$_{12}$, the temperature dependence of the calculated line shifts and linewidths agrees well with the trends of the experimental data. The linewidths for Fe$_8$ reveal a stronger temperature dependence than those for Mn$_{12}$, because for Mn$_{12}$ a much wider distribution in $D$ overshadows the temperature dependence of the spin-spin interactions. For Fe$_8$, the line-shift analysis suggests two competing interactions: a weak ferromagnetic exchange coupling between neighboring molecules and a longer-ranged dipolar interaction. This result could have implications for ordering in Fe$_8$ at low temperatures.