  The magnetic anisotropy and exchange coupling between spins localized at the
positions of 3d transition metal atoms forming two-dimensional metal-organic
coordination networks (MOCNs) grown on the Au(111) metal surface are studied.
In particular, we consider MOCNs made of Ni or Mn metal centers linked by TCNQ
(7,7,8,8-tetracyanoquinodimethane) organic ligands, which form rectangular
networks with 1:1 stoichiometry. Based on the analysis of X-ray magnetic
circular dichroism (XMCD) data taken at T= 2.5 K, we find that Ni atoms in the
Ni-TCNQ MOCNs are coupled ferromagnetically and do not show any significant
magnetic anisotropy, while Mn atoms in the Mn-TCNQ MOCNs are coupled
antiferromagnetically and do show a weak magnetic anisotropy with
in-planemagnetization. We explain these observations using both
amodelHamiltonian based on mean-fieldWeiss theory and density functional theory
calculations that include spin-orbit coupling. Our main conclusion is that the
antiferromagnetic coupling between Mn spins and the in-plane magnetization of
the Mn spins can be explained neglecting effects due to the presence of the
Au(111) surface, while for Ni-TCNQ the metal surface plays a role in
determining the absence of magnetic anisotropy in the system.
