Spin Excitations of High Spin Iron(II) in Metal-Organic Chains on Metal and Superconductor

Many-body interactions in metal-organic frameworks (MOFs) are fundamental for emergent quantum physics. Unlike their solution counterpart, magnetization at surfaces in low-dimensional analogues is strongly influenced by magnetic anisotropy (MA) induced by the substrate and still not well understood. Here, on-surface coordination chemistry is used to synthesize on Ag(111) and superconducting Pb(111) an iron-based spin chain by using pyrene-4,5,9,10-tetraone (PTO) precursors as ligands. Using low-temperature scanning probe microscopy, their structures and low-energy spin excitations of coordinated Fe atoms are compared with high S = 2 spin-state. Although the chain and coordination centers are identical on both substrates, the long-range spin-spin coupling due to a superexchange through the ligand on Ag is not experimentally observed on Pb(111). This reduction of spin-spin interactions on Pb in tunneling spectra is ascribed to the depletion of electronic states around the Fermi level of the Pb(111) superconductor as compared to silver.