Chemical Switching of the Magnetic Coupling in a MnPc Dimer by Means of Chemisorption and Axial Ligands

We present an ab initio density functional theory study of the magnetic properties of manganese phthalocyanine dimers, where we focus on the magnetic coupling between the Mn centers and on how it is affected by external factors like chemisorption or atomic axial ligands. We have studied several different configurations for the gas phase dimers, which resulted in ferromagnetic couplings of different magnitudes. For the bare dimer we find a significant ferromagnetic coupling between the Mn centers, which decreases by about 20% when a H atom is adsorbed on one of the Mn atoms and is reduced to about 7% when a Cl atom is adsorbed. The magnetic coupling is almost fully quenched when the dimer, bare or with the H ligand, is deposited on the ferromagnetic substrate Co(001). Our calculations indicate that the coupling between the two Mn atoms principally occurs via a superexchange interaction along two possible paths within a Mn-N-Mn-N four-atom loop. When these electrons get involved in chemical bonding outside the dimer itself, an appreciable alteration of the overlap between Mn and N molecular orbitals along the loop occurs, and consequently, the magnetic interaction between the Mn centers varies. We show that this is reflected by the electronic structure of the dimer in various configurations and is also visible in the structure of the atomic loop. The chemical tuning of the magnetic coupling is highly relevant for the design of nanodevices like molecular spin valves, where the molecules need to be anchored to a support