Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate

Abstract

We report the discovery of an antiferromagnetic coupling of the magnetic moment of chromium(II) tetraphenylporphyrin (CrTPP) molecules to the magnetization of the clean ferromagnetic Co(001) substrate. We assign this unusual molecule-substrate exchange coupling to the less than half-filled chromium 3d orbitals interacting with Co valence band electrons via porphyrin-ligand molecular orbitals. X-ray magnetic circular dichroism, x-ray photoelectron spectroscopy, and scanning tunneling microscopy are combined with DFT + U calculations and provide evidence for a surprising type of antiferromagnetic 90• indirect magnetic exchange coupling. Spin-bearing metal-organic molecules have attracted considerable attention in recent years [1][2][3][4]. In particular, squareplanar coordination complexes offer a wide range of tunable chemical functionalities [5][6][7][8], which derive from the sensitive interaction of the open shell of the metal center with the planar ligands as well as with the surrounding molecular architecture. In "on-surface" configurations novel properties can arise when the magnetic and electronic characteristics of the metal-organic species are subtly modified by the surface-molecule interaction. Accordingly, it was discovered that metallophthalocyanines and metalloporphyrins with 3d and 4f block metal centers exhibit remarkable electronic, optoelectronic, and magnetic properties when adsorbed on surfaces [1,2,[5][6][7][8][9][10][11][12][13][14][15][16]. Strikingly, the magnetic exchange coupling of the spin of metalloporphyrins or metallophthalocyanines with that of ferromagnetic (FM) substrates (Co and Ni) was found to induce stable magnetic order in the paramagnetic molecules at and above room temperature [1,2,11]. It was reported that metallophthalocyanines and metalloporphyrins align their magnetic moments parallel to those of the bare FM substrate. The origin of this alignment has been much discussed [1,2,7,10,11] and assigned either to direct or indirect magnetic exchange coupling via the chemical bonds across the interface. Understanding and controlling molecule-substrate interactions are key ingredients for designing novel functional surfaces [15].To date, an antiferromagnetic (AFM) indirect exchange interaction has been reported for planar molecules assembled on oxygen reconstructed Ni and Co substrates [5,6] or when a graphene interlayer [12] was introduced between the molecule and substrate. An indirect AFM coupling was also observed for the rare-earth double-decker Tb-bis-phthalocyanine adsorbed on a metallic FM substrate, where one macrocycle acts as an interlayer [13]. However, for transition-metal porphyrins and phthalocyanines on bare FM substrates an AFM coupling has not yet been observed. * jan.girovsky@psi.ch † peter.oppeneer@physics.uu.seHere we report an AFM coupling of Cr(II)-tetraphenylporphyrin molecules adsorbed on bare FM Co. We describe this discovery by a 90• indirect exchange coupling between the less than half-filled 3d shell of the Cr ion and Co substrate mediated by nitrog...

Ferrimagnetism in 2D networks of porphyrin-X and -XO (X=Sc,...,Zn) with acetylene bridges

“…The results presented in this subsection align with the experimental and theoretical conclusions of the work on porphyrin-Cr adsorbed at the Co surface. 9 Namely, the local spin-moments induced at nitrogens are AF-coupled to Cr, and the spin of less than halfly-occupied 3d-shell of Cr is AF-coupled to the more than halfly-occupied 3dshell of Co substrate -as in our case, it holds for the Cr and O localized magnetic moments.…”

“…This is because different valence states of a metal, metal-oxygen moiety, or metal-Li complex lay closely in the energy. [8][9][10][11] Not only the single layers of the metal-organic networks exist, also the double-and triple-decker layers of metal-phthalocyanines have been grown, on both the ferro-and antiferromagnetic substrates. 12 For catalytic reactions with the intramolecular charge transfer, such as the water splitting, the intermolecular connections must be very weakly conducting (noncovalent); for example the hydrogen bridges (O-H...O).…”

Surface chemistry of porphyrins and phthalocyanines

“…The validity of the Nmediated indirect superexchange mechanism was later verified by a study of (Cl)MnTPP on Co/Cu(1 0 0) [543]. The antiferromagnetic coupling of CrTPP to a Co(0 0 1) substrate was also explained with an indirect exchange coupling via the porphyrin N atoms [673]. For (O)VPc on Fe, Co and Ni films on Cu(0 0 1), antiferromagnetic coupling was observed on Fe and (less pronounced) on Co, while no coupling was observed with the Ni substrate.…”

Controlling the magnetism of adsorbed metal–organic molecules

“…Controlling the magnetism of adsorbed metal-organic molecules on a Co substrate, where the antiparallel exchange coupling is attributed to the less than half-filled 3d shell of the Cr 2+ ion [27], and for Mn phthalocyanine molecules on ferromagnetic EuO, where the interaction between the half-filled 3d shell of Mn 2+ and the Eu 5d electrons is held responsible for the antiparallel coupling [28]. The coupling of a Cu-tetraazaporphyrin to a magnetite(100) surface changes even sign as a function of the magnetization direction, which is interpreted as evidence for a strongly anisotropic exchange coupling between the Cu moment and the magnetite surface resulting from the simultaneous presence of competing superexchange coupling paths [29].…”