Magnetic Coupling of Porphyrin Molecules Through Graphene

Abstract

Graphene, a material with unique properties, [ 1 ] is expected to complement todays Si-based information technology with new and more effi cient functions. [2][3][4] It exhibits desirable properties for spin electronic applications such as high charge carrier mobility, low intrinsic spin-orbit interaction, as well as low hyperfi ne interaction. [5][6] In particular, magnetic molecules in contact with graphene constitute a tantalizing approach towards organic spin electronics because of the reduced conductivity mismatch at the interface. In such a system a bit is represented by a single molecular magnetic moment, which must be stabilized against thermal fl uctuations. [ 7 ] Here, we show in a combined experimental and theoretical study that the moments of paramagnetic Co-octaethylporphyrin (CoOEP) molecules on graphene can be aligned by a remarkable anti-ferromagnetic coupling to a Ni substrate underneath the graphene. This coupling is mediated via the π electronic system of graphene, while no covalent bonds between the molecule and the substrate are established, and the molecules sit at a distance of ≈ 3.3 Å above the graphene plane. The laterally extended π electron system of graphene exhibits metal-like electronic properties along the plane, and molecule-like properties perpendicular to the plane, which makes graphene highly relevant for the design of hybrid metal-organic spintronic materials. Several studies have focused on the spin-split electronic states of graphene or the injection of spin-polarized currents from metallic ferromagnetic electrodes. [8][9][10] Electronic spin transport and spin precession have been observed in graphene over micrometer distances even at room temperature. [ 11 , 12 ] Molecules adsorb on graphene mainly by Van der Waals interaction, thereby the molecular properties of the adsorbate, like its reactivity, are preserved. This opens the door to an optimization of the undisturbed molecular design of an adsorbate independently of its interaction with the substrate.We use X-ray magnetic circular dichroism (XMCD) in the absorption of soft X rays as an element-selective probe of the magnetization of CoOEP molecules in the submonolayer regime on a graphene-passivated Ni fi lm, [ 13 ] supported by a W(110) single crystal surface. In the upper and lower panel of Figure 1 a, we show Co L 2,3 X-ray absorption (XA) spectra and XMCD spectra of 0.7 molecular monolayers (ML) of CoOEP on top of graphene/Ni, respectively, measured in zero fi eld at remanence of the ferromagnetic Ni fi lm. The astonishing presence of a fi nite Co XMCD signal proves on the one hand a net 3 d magnetic moment localized on the Co ions and on the other hand an unexpected magnetic coupling between the magnetization of the Ni layer and these Co moments, stabilizing them against thermal fl uctuations. The XA spectrum as well as the XMCD spectra exhibit a particular fi nestructure at the Co L 3 edge, which, by comparison to CoPc bulk measurements, [ 14 ] is consistent with a d 7 low-spin state of Co. The XMCD difference cu...

Surface chemistry of porphyrins and phthalocyanines

“…For example, CoOEP was found to engage in antiferromagnetic coupling to Ni/W(1 1 0), from which it was separated by a graphene layer. Besides the spin magnetic moment, a large in-plane orbital magnetic moment was observed, which was also antiferromagnetically coupled to the Ni spin [681,682]. Similar observations were made for CoOEP on graphene/Ni(1 1 1) [683].…”

Antiferromagnetic coupling of Cr-porphyrin to a bare Co 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].…”

“…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].…”

“…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].…”

Anisotropy and Damping of Molecules/Cobalt Hybrid Thin Films

“…Stronger coupling [15] and thus tuning of the properties is expected compared to the physisorption case obtained by evaporation. Ab-initio calculations [10,16,17] demonstrate that what is important is the delocalized electrons as the conduction band of graphene [18]. In molecules, this corresponds to the π-bands which tend to strongly hybridize at the surface with a metal.…”