Modeling Ferro- and Antiferromagnetic Interactions in Metal-Organic Coordination Networks

TL;DR

This study investigates the magnetic interactions in metal-organic networks formed with TCNQ and transition metals Mn and Ni, revealing how electronic structure influences ferromagnetic or antiferromagnetic coupling.

Contribution

It demonstrates how first principles calculations and models can explain the differing magnetic behaviors in Ni- and Mn-TCNQ networks based on their electronic structures.

Findings

Ni-TCNQ exhibits ferromagnetic coupling due to a hybrid band crossing the Fermi level.

Mn-TCNQ shows very weak antiferromagnetic correlations.

Spin polarization is delocalized along the system in Ni-TCNQ.

Abstract

Magnetization curves of two rectangular metal-organic coordination networks formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences that are explained using first principles density functional theory and model calculations. We find that the existence of a weakly dispersive hybrid band with M(3d) and TCNQ character crossing the Fermi level is determinant for the appearance of ferromagnetic coupling between metal centers, as it is the case of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The spin magnetic moment localized at the Ni atoms induces a significant spin polarization in the organic molecule; the corresponding spin density being delocalized along the whole system. The exchange interaction between localized spins at Ni centers and the itinerant spin density is ferromagnetic. Based on two different model Hamiltonians, we estimate the strength of exchange couplings between magnetic atoms for both Ni- and Mn-TCNQ networks that results in weak ferromagnetic and very weak antiferromagnetic correlations for Ni- and Mn-TCNQ networks, respectively.