Spin and exchange coupling for Ti embedded in a surface dipolar network

We have studied the spin and exchange coupling of Ti atoms deposited on a Cu2N/Cu(100) surface using density functional theory with generalized gradient approximation +U. In agreement with experiments, we find that Ti has the highest binding on top of Cu atoms. We also find that the spin of individual Ti atoms deposited on the Cu2N/Cu(100) surface increases as Ti coverage on the surface is decreased. For U = 0, the spin of a Ti atom starts at S = 0 at high coverages and increases to S = 1/2 as the coverage is decreased, which agrees very well with results obtained from STM experiments. At higher values of U, the spin of Ti is found to be close to one regardless of coverage. We also calculate the exchange coupling for Ti dimers on the Cu2N/Cu(100) surface, and we find that the exchange coupling across a "void" of 3.6 angstrom is antiferromagnetic, whereas indirect (superexchange) coupling through a N atom is ferromagnetic. For a square lattice of Ti on Cu2N/Cu(100), we find a novel spin striped phase.