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

Magnetochemistry recently emerged as a promising approach to control addressable spin arrays on surfaces. Here we report on the binding, spatial ordering, and magnetic properties of Fe on a highly regular Co-tetraphenylporphyrin (Co-TPP) template and highlight how the Fe controls the magnetism of the Co centers. As evidenced by scanning tunneling microscopy (STM) single Fe atoms attach to the saddle-shape conformers site-selectively in a unique coordination environment offered through a heptamer defined by the Co-N-C-C-C-N cyclic subunit. While the magnetic moment of Co is quenched for bare Co-TPP/Ag(111), the Fe presence revives it. Our X-ray magnetic circular dichroism (XMCD) experiments, complemented by density functional theory (DFT) calculations, evidence a ferromagnetic coupling between the Fe and the Co center concomitant with a complex charge redistribution involving the porphyrin ligand. Thus, we demonstrate an unusual metalloporphyrin coordination geometry that opens pathways to spatially order and engineer magnetic moments in surface-based nanostructures.

Construction d'un microscope à effet tunnel à basse température et études d'impuretés magnétiques en surfaces

Laurent Claude

This thesis reports on the design and construction of a novel experimental setup for the study of surface magnetic systems. This setup consists of a scanning tunneling microscope (STM) designed for ultra high vacuum (UHV) operating at T ≲ 1 K and strong magnetic fields (0 - 8.5 T). Such extreme experimental conditions have influenced the choice of the construction materials and components as well as the general design principles. This work comprises the design, realization and first tests of the STM, cryostat, damping system, sample preparation UHV chambers and transfer system. In parallel to this new setup realization, we have investigated the magnetic properties of dilute transition metal impurities deposited on non-magnetic metal surfaces. Measurements of those systems were performed with x-ray magnetic circular dichroism (XCMD). We have started by adressing the host electronic density effect on magnetic properties of the impurities. In order to make contact with the Friedel [1] and Anderson [2] models, 3d impurities (Fe and Ni) were deposited on alkali metal substrates (K, Na et Li) whose electronic structure is close to the Fermi electron gas. The gradual increase of the host electronic density with decreasing atomic weight going from K to Li allow us to study the effects of s - d electron hybridization on the magnitude magnetic moment of the impurity . In particular, we show that the magnetic moment of Fe is strongly reduced in going from K to Li, while the Ni moment disappears on Na and Li. The XMCD data also show that the orbital magnetic moment is atomic-like on K, but compared to the spin moment, significantly reduces as the substrate electron density increases. Finally, we report on a variable temperature STM study of the nucleation and growth processes of Pd/Pt(111). The experimental results are analyzed on the basis of mean field theory applied to surface diffusion processes and solved by numerical integration. This study allows us to highlight the different atomistic processes that contribute to the nucleation and growth of a prototype epitaxial system.

EPFL2005

Magnetic anisotropy of Fe and Co ultrathin films deposited on Rh(111) and Pt(111) substrates: An experimental and first-principles investigation

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We report on a combined experimental and theoretical investigation of the magnetic anisotropy of Fe and Co ultrathin layers on strongly polarizable metal substrates. Monolayer (ML) films of Co and Fe on Rh(111) have been investigated in situ by x-ray magnetic circular dichroism (XMCD), magneto-optic Kerr effect, and scanning tunneling microscopy. The experiments show that both magnetic adlayers exhibit ferromagnetic order and enhanced spin and orbital moments compared to the bulk metals. The easy magnetization axis of 1 ML Co was found to be in plane, in contrast to Co/Pt(111), and that of 1 ML Fe out of plane. The magnetic anisotropy energy (MAE) derived from the magnetization curves of the Fe and Co films is one order of magnitude larger than the respective bulk values. XMCD spectra measured at the Rh M2,3 edges evidence significant magnetic polarization of the Rh(111) surface with the induced magnetization closely following that of the overlayer during the reversal process. The easy axis of 1–3 ML Co/Rh(111) shows an oscillatory in-plane/out-of-plane behavior due to the competition between dipolar and crystalline MAE. We present a comprehensive theoretical treatment of the magnetic anisotropy of Fe and Co layers on Rh(111) and Pt(111) substrates. For free-standing hexagonally close-packed monolayers the MAE is in plane for Co and out of plane for Fe. The interaction with the substrate inverts the sign of the electronic contribution to the MAE, except for Fe/Rh(111), where the MAE is only strongly reduced. For Co/Rh(111), the dipolar contribution outweighs the band contribution, resulting in an in-plane MAE in agreement with experiment while for Co/Pt(111) the larger band contribution dominates, resulting in an out-of-plane MAE. For Fe films however, the calculations predict for both substrates an in-plane anisotropy in contradiction to the experiment. At least for Fe/Pt(111) comparison of theory and experiment suggests that the magnetic structure of the adlayer is more complex than the homogenous ferromagnetic order assumed in the calculations. The angular momentum and layer-resolved contributions of the overlayer and substrate to the MAE and orbital moment anisotropy are discussed with respect to the anisotropic hybridization of the 3d, 4d, and 5d electron states and vertical relaxation. The role of technically relevant parameters such as the thickness of the surface slab, density of k points in the Brillouin zone, and electron-density functionals is carefully analyzed.

2010

Interlayer exchange coupling in ordered Fe nanocluster arrays grown on Al2O3/Ni3Al(111)

Harald Brune, Alberto Cavallin, Fabio Donati, Jan Gui-Hyon Dreiser, Quentin Dubout, Luca Gragnaniello, Stefano Rusponi, Sergio Vlaic

We have combined magneto-optical Kerr effect, scanning tunneling microscopy, and x-ray magnetic circular dichroism to study the magnetic properties and the morphology of Fe nanoparticles grown on 2 ML thick Al2O3/Ni3Al(111)-(root 67 x root 67)R12.2 degrees with and without Pd seeding. The Ni3Al(111) substrate is ferromagnetic and shows two transition temperatures. The first, T-C1 = 81 +/- 3 K, is attributed to a 20-30 nm thick slightly Ni enriched region; the second, T-C2 = 240 +/- 12 K, is attributed to a much thinner and more strongly Ni enriched near interface region that contains Ni clusters embedded in the alloy matrix. The magnetic properties of the Fe cluster superlattice are strongly influenced by the superexchange coupling between Fe clusters and the underlying Ni clusters in that near interface region. Since the Ni clusters are at different distances from the oxide/metal interface, this coupling oscillates between ferro-and antiferromagnetic such that the overall magnetic moment is not increased by the Fe clusters. Pd seeding does not influence the magnetic properties of the system. The intrinsic Fe cluster properties, such as Curie temperature and easy magnetization axis, are accessed for T > T-C2. We find out-of-plane easy magnetization axes and T-C approximate to 300 K for cluster sizes above 440 atoms.