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

Abstract

To whom correspondence should be addressed 1 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.

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…According to a recent study by Faraggi et al [64], the electronic and magnetic structure of the 3d transition metal atoms based MOCNs is very sensitive to the specific 3d manyfold occupation that may give rise to delocalized or localized spin densities depending on the existence of hybridizations between 3d states and molecular orbitals. Therefore, in the present work the 3d states of Co or Cr were treated within the GGA+U approach [82] and Dudarev's scheme [83].…”

“…Indeed, the conformational freedom of the organic acceptor increases upon the uptake of electrons and the cyano groups are then able to bend [63,84]. According to the results of a recent theoretical study [64], there is a significant charge transfer (between 1 and 2 e − ) from the Ni and Mn atoms to the TCNQ lowest unoccupied molecular orbital (LUMO). This could enable the bending of the cyano groups thus facilitating a complete or partial accommodation of the mismatch.…”

“…Namely, to achieve the long-range FM order in the system of adatoms at the TI surface one has to organize their ordered arrays [14,34,35] and prevent them from diffusion [44,47] or intercalation inside the bulk or the van der Waals gaps [34,[51][52][53][54]. Lately, a type of system which may provide these conditions were successfully grown on metallic substrates-selfassembled metal-organic coordination networks (MOCNs) [55][56][57][58][59][60][61][62][63][64]. In such experiments, strong electron acceptors like tetracyanoethylene (TCNE, C 6 H 4 ), tetracyanobenzene (TCNB, C 10 H 2 N 4 ), and tetracyanoquinodimethane (TCNQ, C 12 H 4 N 4 ) are the most frequently used molecules.…”

“…The latter is provided by the strong chemical bonding to the cyano groups of the organic ligands which prevents adatoms from diffusion on the surface or below it. Importantly, the local spins of the metallic centers don't quench upon contact with the substrate and the FM exchange interactions mediated by the organic ligands have been shown to exist in Fe-and Ni-based MOCNs deposited on the noble metal surfaces [58,60,64].…”

The DFT+U: Approaches, Accuracy, and Applications

“…The itinerant spin density, however, forms at an energy penalty determined by the dispersion of the conduction band; the larger the density of states at the Fermi level, the easier is for the itinerant spin density to form. The addition of an extra interaction term that accounts for the strong on-site coulomb U correction has proved to lead to good results [54]. One more advantage of the DFT+U is that it can be used to model systems containing up to few hundred atoms [55].…”