Controlling the Spin of Co Atoms on Pt(111) by Hydrogen Adsorption

We investigate the effect of H adsorption on the magnetic properties of individual Co atoms on Pt(111) with scanning tunneling microscopy. For pristine Co atoms, we detect no inelastic features in the tunnel spectra. Conversely, CoH and CoH2 show a number of low-energy vibrational features in their differential conductance identified by isotope substitution. Only the fcc-adsorbed species present conductance steps of magnetic origin, with a field splitting identifying their effective spin as S-eff = 2 for CoH and 3/2 for CoH2. The exposure to H-2 and desorption through tunnel electrons allow the reversible control of the spin in half-integer steps. Because of the presence of the surface, the hydrogen-induced spin increase is opposite to the spin sequence of CoHn molecules in the gas phase.

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

This thesis works reports on the magnetic properties of single transition metal atoms and hydrogenated compounds adsorbed on crystal surfaces. It is an experimental study, based on scanning tunneling microscopy (STM), initiated by the project of demonstrating the existence of magnetic remanence for single adsorbed atoms (adatoms). All the measurements have been performed with a scanning tunneling microscope operating at 0.4 K and equipped with 8.5 T superconducting coils. The investigated systems are cobalt atoms, pure and hydrogenated (cobalt hydrides), adsorbed on Pt(111) and graphene on Pt(111) surfaces. In the first case, we have immediately been confronted to the presence of cobalt hydrides on the surface. Three types of hydrides (CoH, CoH2, and CoH3) have been identified, quite different from the clean adatoms, and readily distinguished by their tunnel spectrum (STS). Twoofthesehydridesdisplaylow-energyvibrationalmodes(1−30meV),identifiedbytheir absence of response to a magnetic field, the effect of an isotopic substitution, and by comparison with known systems. The three hydrides can be deprotonated applying a sufficient voltage between tip and sample. CoH2 on Pt(111) presents a Kondo resonance, due to a half-integer spin (S = 3/2) and a peculiar magnetic anisotropy. This system corresponds to the first demonstration of the appearance of the Kondo effect upon hydrogen adsorption. The results of several attempts to observe remanence for single cobalt adatoms on Pt(111) with spin-polarized scanning tunneling microscopy (SP-STM) are also presented and discussed. The second system differs from the first one in the presence of a graphene layer on the Pt surface. The first experimental determination of the magnetic moment and magnetic anisotropy of adatoms on graphene is reported in this thesis. A magnetic moment of (2.2 ± 0.4) μB was measured by spin-excitation spectroscopy (SES), as well as a hard magnetization axis with an anisotropy energy of 8.1 ± 0.4 meV. This value, comparable to the record value of single cobalt atoms on Pt(111), has been mainly attributed to the strong hybridization between cobalt and graphene. Ab initio calculations confirmed our observations. Graphene thus represents an extremely promising substrate for future applications in nanoscale magnetism and spintronics. This system also exhibits three different cobalt hydrides in addition to the clean atoms. Their respective chemical composition, CoH, CoH2, and CoH3, was determined by direct comparison between high resolution STM images and simulated images obtained from ab initio calculated atomic structures. A controlled deprotonation method is presented. CoH3 is the only hydride to display spin excitations. Its magnetic moment is comparable to that of clean cobalt, it presents a hard magnetization axis as well, however, its magnetic anisotropy energy is sensibly lower, 1.7 ± 0.05 meV. The other two hydrides display vibrational excitations only. The effect of hydrogen on the magnetic properties of metal adatoms is therefore very strong in this system as well, and must be taken into account in future studies, hydrogen being a very common contaminant in ultra-high vacuum set-ups. Finally, a detailed study of the structure graphene mono- and bilayers on Ru(0001) was realized. Two different stackings, AB and AA, have been identified for the bilayers, the last one not observed to date for this system. The moiré structure of monolayer graphene and of this last type of bilayer has been determined: (11.57×11.57)R4.3◦ and (10.54×10.54)R4.7◦, respectively. Moreover, C-C bond distortions of more than 10%, observed in the STM images, have been studied in detail. A quantitative comparison with ab initio calculations allowed to identify their origin as the tilt of the graphene π-orbitals. These distortions are therefore purely artificial. This is an important result for future scanning tunneling microscopy studies of graphene.

EPFL2013

Magnetism of individual atoms adsorbed on surfaces

Harald Brune, Pietro Gambardella

Magnetic impurities in solids cause manifold changes in their macroscopic properties, such as anomalous low temperature resistance due to Kondo screening, reduction of the superconducting transition temperature due to local suppression of the order parameter, they create magnetic signatures in semiconductors, and lead to inelastic spin excitations in tunnel junctions. In the present paper we review what has been learnt about these effects from a surface science approach. Placing the magnetic impurities at well defined adsorption sites on single crystal surfaces makes their effect on the host, as well as their own magnetic properties better accessible to experiments, and also better understandable since the atomic environment of the impurity is exactly known lending comparison with theory more direct. After an introduction we discuss X-ray magnetic circular dichroism measurements which are spatially averaging and therefore report on ensemble properties. One of the recent progresses achieved in surface science is the preparation of well defined ensembles, such as surfaces with only single adatoms, each of them in an identical atomic environment and with sufficient mutual distance to exclude interactions. Due to this approach we can now determine the electronic configuration of individual adatoms, their hybridization with the host, and quantify their spin and orbital moments, as well as the spin–orbit induced magneto-crystalline anisotropy, which can be orders of magnitude larger than thin film and bulk values. In the second part we review recent progress in revealing the magnetic properties of individual atoms with the scanning tunneling microscope (STM). With this technique the spatial extent of the Kondo screening cloud and of subgap excitations in the superconductor quasiparticle density of states became apparent. We outline the first pioneering experiments measuring transport through reversible atomic point contacts containing magnetic atoms and measurements using the subgap features caused in superconducting STM tips to detect the magnetism of individual atoms. We then describe experiments using inelastic spin excitation spectroscopy to pin down the magnetic ground state and anisotropy energy of magnetic impurities. We continue with spin-polarized STM experiments reporting magnetization curves of individual magnetic adatoms and finish by a description of the most recent spin-excitation experiments revealing the necessary anisotropy environment for a high spin impurity to display the Kondo effect.

Elsevier2009

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

This thesis presents investigations of magnetic and structural properties of two dimensional metal-organic frameworks and of single atoms, both adsorbed on decoupling layers grown on metal surfaces, with focus on rare earth elements.We first report on a series of attempts at realizing self-assembled metal-organic networks on different decoupling layers, probed by scanning tunneling microscopy. The aim is to produce regular structures with high surface filling factors such that the magnetic properties of the rare earth atoms in the metal-organic networks can be investigated by X-ray ensemble measurements. The combinations of three molecular ligands (QDC, BDA, ZnTPyP) on three insulating layers (MgO, NaCl, graphene) with Tb as rare earth are reported. Of the three decoupling layers, only graphene is found to allow the synthesis of metal-organic structures. We successfully synthesized large islands of a rare earth-QDC coordination complex, with a majority of fivefold coordination of the rare earth atoms. The same structure is reproduced with Dy and Er.X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism measurements are then carried out on the Dy- and Er-QDC/gr/Ir(111) networks. Compared to single atoms on gr/Ir(111), both Dy and Er change their

4f

occupancy from 4

f^n

4f^{n-1}

and change their easy axis of magnetization. Both species are paramagnetic when incorporated into the metal-organic structure. The last part of the thesis is devoted to the study of Dy, Ho, and Gd on NaCl thin films. All three atoms display

4f

occupancy of the gas phase. The top-Cl adsorption site is determined with scanning tunneling microscopy. The magnetic properties are probed with X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism. Ho and Gd are paramagnetic. Dy has out-of-plane anisotropy with open hysteresis at non zero magnetic field. However, thermally assisted quantum tunneling of the magnetization prevents remanence at zero field.

EPFL2022