Magnetic Excitations in Ferromagnetically Coupled Spin-1 Nanographenes

TL;DR

This study explores the magnetic excitations of ferromagnetically coupled spin-1 nanographenes, demonstrating synthesis, characterization, and modeling of high-spin molecular assemblies with potential for advanced magnetic carbon materials.

Contribution

It introduces a method to synthesize and analyze covalently bonded high-spin nanographene dimers and trimers, advancing understanding of their magnetic excitations and interactions.

Findings

Successful synthesis of S=2 dimers and S=3 trimers on Au(111)

Identification of magnetic excitation spectra via IETS

Validation of Heisenberg model for describing ferromagnetic exchange

Abstract

In the quest for high-spin building blocks to form covalently bonded 1D or 2D materials with controlled magnetic interactions, $\pi$-electron magnetism provides an ideal framework to engineer large ferromagnetic interactions between nanographenes. As a first step in this direction, we investigate the spin properties of ferromagnetically coupled triangulenes, triangular nanographenes with spin $S = 1$. Combining in-solution synthesis of rationally designed molecular precursors and on-surface synthesis, we achieve covalently bonded $S = 2$ triangulene dimers and $S = 3$ trimers on Au(111). Starting from the triangulene dimer, we thoroughly characterize its low-energy magnetic excitations using inelastic electron tunneling spectroscopy (IETS). IETS reveals conductance steps identified as a quintet to triplet excitation, and a zero-bias peak stemming from higher-order spin-spin scattering of the 5-fold degenerate ferromagnetic ground state. The Heisenberg picture captures the relevant parameters of inter-triangulene ferromagnetic exchange, and its successful extension to the larger $S = 3$ system confirms the model's accuracy. We expect that the addition of ferromagnetically coupled building blocks to the toolbox of magnetic nanographenes opens new opportunities to design carbon materials with complex magnetic ground states.