  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.
