Sustentation électromagnétique

La sustentation électromagnétique est une méthode permettant de faire léviter un objet en le faisant reposer sur un champ magnétique. Les forces magnétiques appliquées à cet objet s'opposent ainsi à l'action de son propre poids, empêchant sa chute. Il existe deux concepts fondamentaux concernant la physique et les propriétés de lévitation de la matière :

le concept électromagnétique (EML) : la lévitation est générée par des électroaimants régulés. Le Transrapid (allemand) et le Swissmetro sont des trains basés sur le concept EML ;
le concept électrodynamique (EDL) : la lévitation est basée sur les forces de répulsion générées par les courants induits (dits courants de Foucault) qui n'apparaissent que lors d'un déplacement relatif des corps en présence. Il est donc nécessaire de propulser initialement le train avant qu'il puisse léviter magnétiquement. Le seul projet actuellement très avancé utilisant ce concept de sustentation est le Maglev japonais.

Aujourd'hui, la plus grande

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

This thesis investigates the magnetic properties of single atoms and dimers adsorbed on graphene and oxide decoupling layers supported by single crystal metal substrates, using scanning tunneling microscopy (STM) and spin-polarized scanning tunneling microscopy (SP-STM). The goal is twofold: to use SP-STM to further advance the understanding of the interactions that determine the magnetic stability of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), and to use insights from these systems to motivate the study of new systems-- mainly, IrCo heterodimers on graphene/Ir(111).For Dy adatoms on graphene/Ir(111), the characteristic lifetime of the spin system is measured as a function of temperature and tunnel bias, probing the available magnetization reversal pathways in the energy level diagram. The necessity of including the intra-atomic exchange to correctly describe spin lifetimes is demonstrated. In addition, naturally abundant Dy isotopes possess two possible nuclear spin values. Models of both nuclear spin cases of Dy are compared, and shown to produce similar behavior in the temperature and bias ranges probed by SP-STM. Furthermore, accounting for both nuclear spin cases allows for modeling of X-ray magnetic circular dichroism (XMCD) magnetization sweeps. For Ho adatoms on MgO/Ag(100), novel measurement protocols are used to determine the zero-field stability, and the correct ground state model. These protocols are also used to induce magnetic state reversal via Landau-Zener tunneling at avoided level crossings due to the hyperfine interaction. Finally, the study of the IrCo heterodimer adsorbed on graphene is motivated with stability considerations derived from the studies of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), in addition to calculations based on density function theory (DFT). Strategies of engineering these heterodimers using statistical growth and atomic manipulation are successfully implemented. The heterodimer is highly mobile on graphene/Ir(111) and displays geometric instability, consistent with its predicted upstanding geometry. Despite this, measurement via STM and SP-STM is demonstrated. Observations are consistent with the robust magnetic stability expected from DFT studies, but additional investigation is needed to disentangle the measurements made thus far.

EPFL2022

Quantum dynamics in individual surface spins

Tobias Bilgeri

This thesis presents a combined experimental and theoretical study of the classical and quantum magnetization dynamics in single magnetic adatoms and molecules, and on the classical and quantum coherent control thereof. First, a detailed description of the methods and in particular electron spin resonance scanning tunneling spectroscopy is given. Next, we give a detailed account of how to experimentally update a standard scanning tunneling microscope for such experiments, which ultimately lead us to set up the world's second microscope with such capabilities. Subsequently, we show how we iteratively improved the setup, and how we were able to achieve a transmission of radiofrequency voltages to the tunneling junction with almost no additional losses beyond the intrinsic limitations of the materials used, a great improvement compared to most setups presently in use.Afterwards, we apply this technique to the metal-organic complex iron phthalocyanine. On MgO, we find it to form a spin-1/2 system, and we demonstrate quantum coherent control thereof, and investigate the magnetic interaction between surface-adsorbed molecules. Furthermore, we use this technique to experimentally demonstrate the magnetic stability of Dy adatoms on MgO. We find that this system, by several metrics, represents the worlds most stable single atom magnet discovered thus far. Using the capability of scanning tunneling microscopes to perform atomic manipulation, we ensemble atomically precise Dy nanostructures. By classically controlling the magnetic orientation of the Dy atoms, we are thus able to store information long-term in single adatoms and nanostructures even in absence of an external field, and create nanomagnets and local magnetic fields with unprecedented precision and stability.Finally, we investigate the magnetic properties of Ho/MgO. Depending on the precise adsorption site and charge configuration, we find either long lifetimes in the case of top-site 4f10 Ho, which was recently discovered as world's first single atom magnet, or fast sub-barrier relaxation due to strongly mixed quantum states. Using a novel theoretical approach, we are able to disentangle the different contributions of the individual species measured in ensemble-averaging X-ray absorption studies, and lift the discrepancies between previous X-ray and scanning tunneling microscopy based studies on Ho/MgO. By theoretically describing the perturbing effects of the experimental probes driving the spin degree of freedom out of thermal equilibrium, we are able to explain the measured dynamics in good agreement with the experimental data.

EPFL2021

Magnetic Stability of Single Lanthanide Atoms on Graphene

Romana Baltic

This thesis presents a study of the magnetism of surface supported atoms performed principally with XMCD spectroscopy and multiplet calculations. The objective of the research was twofold: first, to study the underlying interactions and conditions governing the magnetization stability of surface supported atoms, with the aim of achieving long magnetic lifetimes, and second, to assemble single atom magnets in an ordered pattern. Through our study of

4f

lanthanide atoms on supporting substrates, we established that their magnetic stability is governed by their quantum level structure, in particular their ground

J_z

state and the height of the energy barrier for thermally assisted magnetization reversal. These features are ruled through the crystal field interaction with their supporting surface. The adsorption site of adatoms governs the symmetry of the crystal field and, consequently, the coupling between the different

J_z

levels. This in turn enables magnetization reversal either through quantum tunneling between

J_z

states or via scattering with the electrons and phonons of the substrate. To reduce the scattering events, it is necessary to decouple the adatoms from the metal substrate by using decoupling layers. Here we show that a single layer of graphene is sufficient to decouple Dy atoms from the underlying Ir(111) substrate, resulting in a magnetic lifetime of about 1000~s at 2.5~K. In addition, we show that the moiré pattern of the graphene/Ir(111) surface can be used as a template for the self-assembly of these single atom magnets into well ordered superlattices. Further, by studying multiple graphene/metal substrates we show that the interaction of graphene with the supporting substrate greatly influences the magnetization stability of adsorbed atoms. Finally, we show that replacing graphene with an insulating layer does not result in a stable magnetization of adsorbed atoms if its superior decoupling is not accompanied with an adequate crystal field symmetry.

EPFL2018