Exploring the magnetic properties of the largest single molecule magnets

TL;DR

This study uses first-principles calculations and effective spin models to analyze the complex magnetic properties of the largest known single-molecule magnets, revealing hierarchical energy spectra and the significance of weak couplings.

Contribution

It introduces a detailed theoretical framework for understanding the magnetic excitations and ground states of giant Mn wheels, highlighting the role of geometry and weak interactions.

Findings

Weakly coupled Mn7 subunits carry effective S=2 spins

Energy spectrum shows hierarchy and degeneracies

Low-energy excitations are Heisenberg-ring-like

Abstract

The giant $\{ \mathrm{Mn}_{70} \}$ and $\{ \mathrm{Mn}_{84} \}$ wheels are the largest nuclearity single-molecule magnets synthesized to date and understanding their magnetic properties poses a challenge to theory. Starting from first principles calculations, we explore the magnetic properties and excitations in these wheels using effective spin Hamiltonians. We find that the unusual geometry of the superexchange pathways leads to weakly coupled $\{ \mathrm{Mn}_{7} \}$ subunits carrying an effective $S=2$ spin. The spectrum exhibits a hierarchy of energy scales and massive degeneracies, with the lowest energy excitations arising from Heisenberg-ring-like excitations of the $\{ \mathrm{Mn}_{7} \}$ subunits around the wheel, at energies consistent with the observed temperature dependence of the magnetic susceptibility. We further suggest an important role for weak longer-range couplings in selecting the precise spin ground-state of the $\mathrm{Mn}$ wheels out of the nearly degenerate ground-state band.