Ligand Influence on Local Magnetic Moments in Fe-Based Metal-Organic Networks

Planar metal-organic networks are highly promising materials due to their modular nature and wide-ranging possible applications from spintronics up to biosensing. Spin state transitions connect local magnetic properties with structural modifications. In this paper, we report on ab initio calculations for two metal organic planar networks, the Fe-phthalocyanine (Pc) polymer and its precursor material Fe-tetracyanobenzene (TCNB). The spin-polarized generalized gradient approximation to density functional theory with an explicit treatment of the Hubbard-U correction (SGGA+U) indicates a spin state transition between the well confirmed S = 1 state for Fe-Pc and a local, high-spin S = 2 state at the Fe site for Fe-TCNB. The high-spin state at the Fe site is confirmed by X-ray absorption spectroscopy (XAS) measurements of the Fe-TCNB network on the Au(111) substrate in connection with a multiplet analysis. We propose a possible spin state transition between Fe-TCNB and Fe-Pc by the on-surface synthesis of the latter compound. The ab initio results prove also a high chemical stability of the Fe-TCNB network, metallic and ferromagnetic behavior, as well as a partial screening of the Fe spin S = 2 by two antiparallel electrons on the ligand sites to a state with total spin of S = I. All of this makes the Fe-TCNB network an interesting material for spintronics applications.