Mixed-valence behavior and strong correlation effects of metal phthalocyanines adsorbed on metals

Abstract

We investigate the hierarchy of local correlation and hybridization effects in metal-organic molecules adsorbed on metals. Using x-ray magnetic circular dichroism and ligand field multiplet calculations, we demonstrate that the 3d electronic ground state of monolayer metal-phthalocyanine (CoPc, FePc) on Au (111) is given by the coherent superposition of two charge states, d n E + d n+1 , where E represents a substrate electron antiferromagnetically coupled to the central metal ion and d n the many-body ionic orbital configuration of the unperturbed molecule. These results differ from previous models of hybrid metal-organic systems and provide a consistent description of their magnetic moments and Kondo physics in terms of spin and orbital multiplicity. The magnetic properties of transition-metal (TM) compounds depend critically on the competition between d-d electron correlation and covalency, that is, the transfer of charge between d-orbitals and delocalized ligand states. 1 Charge transfer (CT) affects the magnetization of TM ions, their coupling through indirect exchange paths, as well as the conductivity of important classes of materials, including TM oxides, high-T c superconductors, and molecular complexes. Recently, CT processes that take place at the interface between magnetic molecules and metal surfaces have come under intensive attention. 2-4 Transport measurements of single molecules trapped between metallic electrodes 5 and layered metal/organic heterostructures 6 have revealed the role played by interfacial hybridization in determining efficient charge and spin injection across TM complexes, a prerequisite to developing molecular spintronic devices. 6,7 A comprehensive description of the electronic and magnetic properties of metal-organic hybrids, however, is complicated by the fact that CT can occur between TM and ligand orbitals, but also between any of these states and the electron reservoir of a supporting metal surface or electrode. Clearly, understanding such processes would be of great importance in modeling and controlling the properties of magnetic molecules interfaced with metals.Density functional theory (DFT) provides the basis of our current understanding of hybrid metal-organic systems. [2][3][4]8,9 Yet, due to the difficulty of modeling exchange and electron correlation phenomena using effective one-electron potentials, DFT affords only a partial description of the magnetic properties of TM complexes. 1 Methods beyond standard DFT that include Coulomb repulsion through semiempirical Hubbard U terms have been successfully employed to describe the ground state of isolated molecules. 10 However, DFT + U remains a mean-field scheme, which precludes a proper modeling of valence and spin fluctuations induced by CT.A notable case where CT has a dramatic influence on the magnetism of a TM complex is that of Co-phthalocyanine (CoPc), a well-known molecule with spin S = 1/2 (Ref. 11).DFT predicts that CoPc adsorbed on Au(111) (Refs. 12 and 13), Cu (111) (Ref. 14), and even ferromagnetic Fe(110...

Spin coupling and relaxation inside molecule–metal contacts

“…Another relevant point is that, within the error, T K does not present significant spatial variations, as expected for spin flip events involving a single-electron orbital. This is remarkably different from the adsorption-induced delocalization of the metal spin observed previously in Co porphyrin complexes on Cu(111), for which T K has been found to vary strongly over the porphyrin radius 13,17 . Moreover the total magnetic moment of the Co porphyrins does not change on deposition on Cu, whereas it increases by about 1 µ B for CuPc and NiPc on Ag.…”

“…Negligible hybridization of the d x2-y2 orbital with the substrate hinders charge transfer towards either Cu or Ni atoms, preserving the spin of the central metal ions 12 . This is in contrast to molecules that include Mn, Fe, and Co ions, where empty d-orbitals projecting outside the molecular plane directly hybridize with the substrate, leading to either quenching [13][14][15][16][17] or Kondo screening 18,19 of the metal spin.…”

Restoring the Co Magnetic Moments at Interfacial Co-Porphyrin Arrays by Site-Selective Uptake of Iron

“…For the latter we used the same code as described in ref 56. In these calculations we varied the charge transfer and crystal field parameters acting on the Co-center to fit the XAS data.…”

“…The quenching of the spin moment follows the same arguments presented for CoPc on Au(111). 56 Here, the Co ion assumes a d 7 In (c) and (d), the Ag background and a step function to compensate nonresonant absorption have been subtracted from the data. All X-ray absorption spectra have been normalized to the absorption intensity of the L 3 -edge maximum at θ = 0°.…”

Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets

“…5 There is a substantial amount of literature on the characteristic multiplet structure of the metal 2p-to-3d transitions in such molecules. [6][7][8][9][10][11][12][13][14][15] For Mn see Refs. 16 Here we focus on the crystal field generated by the negatively charged N atoms surrounding the central metal ion of porphyrins and phthalocyanines.…”