Growth, electronic structure and magnetism of supported metal nanowires

This thesis describes experiments with one-dimensional atomic chains grown by step decoration of vicinal Pt surfaces. The objective of the research was twofold: first, to characterize metal epitaxial growth on stepped substrates; second, to make use of this knowledge to fabricate arrays of monatomic metal wires in order to investigate their electronic and magnetic properties. The first part focuses on molecular beam epitaxy (MBE) of Ag, Cu, Co, and Ni on densely stepped Pt surfaces (one monatomic step every 20 Å). Growth was investigated on the atomic scale as a function of the substrate temperature and coverage by means of thermal energy atom scattering (TEAS) and scanning tunneling microscopy (STM). A wide variety of growth scenarios has been characterized depending on the choice of the overlayer material and on the interplay between surface diffusion, strain and alloying. Uniform arrays of one-dimensional atomic chains of Ag, Cu, and Co can be fabricated in a well defined temperature range. The growth of alternate wires of different metals was also investigated. In the second part we have studied the chemisorption of O2, CO, and H2, at Pt and at Ag-decorated Pt steps. We have identified the Pt edge atoms at the top of the step as the most active sites for O2 dissociation. Controlled decoration of the Pt steps by monatomic Ag wires was used to locally vary the reactivity of the Pt edge atoms and to further elucidate the dissociation process. Ag decoration results in a selective modification of the adsorption rate of O2, CO, and H2 on Pt stepped surfaces. The third part deals with the electronic and magnetic properties of Co monatomic wires grown on Pt(997). We have investigated the wire-induced valence band states by means of angle resolved photoemission (ARPES). Co 3d states attributed to monatomic chains display a double-peaked structure that suggests the presence of a one-dimensional exchange-split band. The magnetic behavior of the Co chains was studied by x-ray magnetic circular dichroism (XMCD). Co monatomic chains are superparamagnetic with blocking temperature between 5 and 10 K. The one-dimensional character of the wires shows up in a pronounced uniaxial magnetic anisotropy and in large values of the orbital magnetic moment compared to bulk Co and thin films.

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

Magnetic properties of surface-adsorbed single rare earth atoms, molecules, and atomic scale clusters

Aparajita Singha

This thesis presents combined experimental and theoretical investigations of nanoscale, surface-supported magnets based on rare earths (RE) to understand and control the magnetic properties down to the scale of single atoms. We present the effects of adatom-substrate interaction on the magnetic properties of isolated single RE atoms using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and multiplet analysis. Our systematic investigations of Dy, Ho, Er, and Tm adatoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111), reveal that the REs can possess two types of 4f occupations, namely divalent 4f^n and trivalent 4f^(n-1). The trivalent state is realized in presence of low promotion energy of the RE and strong hybridization of their external spd shell with the surrounding environment. Notably, none of the REs exhibit magnetic hysteresis, suggesting that magnetic relaxation is faster than about 10 seconds. We also report on the effect of adatom-adatom interaction by studying the size-dependent magnetic properties of Er clusters adsorbed on Cu(111). Combining XMCD, scanning tunneling microscopy (STM), and mean-field nucleation theory we reveal that the adatom-adatom interaction dominates over the adatom-substrate interaction in Er clusters starting from the size of three atoms. Consequently the easy axis of Er changes from in-plane for the single atoms and dimers to out-of-plane for trimers and bigger clusters. In addition, the out-of-plane magnetic anisotropy of 2.9 meV/atom results in magnetic hysteresis at 2.5 K in all clusters starting from trimers. With a magnetic lifetime of approximately 2 minutes at 0.1 T, the Er trimers are one of the smallest metal-supported ferromagnetic clusters observed so far. The investigation of adatom-adatom interaction is further extended by studying 4f-3d heterodimers namely, Ho-Co adsorbed on thin insulating layers of MgO. Their magnetic easy axis is oriented out-of-plane. Using spin-polarized STM we have detected spin-excitations in these heterodimers at ±20 and ±8 meV. We have identified the origin behind these spin-excitations using an effective spin Hamiltonian model, which indicates that, given a ferromagnetic exchange interaction between Ho and Co, the most intense feature at ±20 meV corresponds to a transition in which the spin moment of Co is strongly diminished. This is further accompanied by an overall change of the total magnetic moment of the heterodimer, i.e., ¿J=-1. In contrast, the weaker transition at ±8 meV occurs following a change in the out-of-plane moment of Ho while the total moment of the heterodimer remains intact i.e., ¿J=0. Notably, we observe an effective g factor of 3.1 for the ±20 meV transition, which significantly exceeds the free electron value of 2. In addition, the ferromagnetic coupling between Ho and Co is very unusual compared to the ferrimagnetic exchange interaction known for the bulk 4f-3d compounds, especially those derived from the late lanthanides. Nonetheless, this marks the first evidence of spin-excitations in the smallest 4f containing cluster. The knowledge gained on the fundamental aspects of magnetism in surface-supported REs combined with the continued parallel search for magnetic stability down to single atoms, enabled us to achieve magnetic remanence in single adatoms with lifetimes of the order of 1000 s at 2.5 K. In addition we have achieved significantly enhanced hysteresis and magnetic lifetime in TbPc2.

EPFL2017

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