Charge-induced maximal spin states of a polynuclear transition-metal complex

TL;DR

This paper theoretically explores how adding electrons to a polynuclear transition-metal complex can induce maximal spin states, revealing potential for charge-controlled molecular magnetism and spin-blockade effects.

Contribution

It introduces a phenomenological model that predicts charge-induced maximal spin states in transition-metal complexes, highlighting the role of electron correlations and the Nagaoka mechanism.

Findings

Maximal total spin states (S=3/2 or 7/2) can be achieved at certain charge states.

Unpaired electron spins couple to a maximal spin due to the Nagaoka mechanism.

Maximal spin states may be observed as spin-blockade effects in electron tunneling experiments.

Abstract

We theoretically investigate the ground state spin of a polynuclear transition-metal complex as a function of the number of added electrons taking into account strong electron correlations. Our phenomenological model of the so-called [$2\times2$]-grid molecule incorporates the relevant electronic degrees of freedom on the four transition-metal centers (either Fe$^{2+}$ or Co$^{2+}$) and the four organic bridging ligands. Extra electrons preferably occupy redox orbitals on the ligands. Magnetic interactions between these ligands are mediated by transition-metal ions {\em and vice versa}. Using both perturbation theory and exact diagonalization we find that for certain charge states the maximally attainable total spin (either $S_{\mathsf{tot}}=3/2$ or $S_{\mathsf{tot}}=7/2$) may actually be achieved. Due to the Nagaoka mechanism, all unpaired electron spins couple to a total maximal spin, including unpaired electron spins on the metal-ions in the case of Co$^{2+}$. The parameters are chosen to be consistent with cyclovoltammetry experiments in which up to twelve redox states have been observed. The above effect may also be realized in other complexes with an appropriate connectivity between the redox sites. The maximal spin states of such a charge-switchable molecular magnet may be experimentally observed as spin-blockade effects on the electron tunneling in a three-terminal transport setup.