Long Spin Lifetime in Rare-Earth Single-Atom Magnets at Surfaces

Boris Sorokin
EPFL, 2022
EPFL thesis

Abstract

Further miniaturisation of magnetic storage devices requires an advent of new types of magnets, since classical ferromagnetic materials show lack of remanence at nano- and subnanoscale. A single atom can represent the smallest possible bit of information. Even though the isolated single atoms are paramagnetic, the interaction of surface adsorbed atoms with a substrate can result in high magnetic anisotropy energy and long magnetisation lifetime. In this thesis the magnetic properties of surface supported rare-earth single atoms are investigated with X-ray absorption spectroscopy, X-ray magnetic circular dichroism and multiplet simulations. The first goal of the project was to find new adatom/substrate combinations that exhibit a long adatom magnetic lifetime, possibly extending the current limits of the atomic magnetic moment stability to higher temperatures and longer spin relaxation times. The second objective was to implement the external electric field control of the adatom magnetic properties.Firstly, we present the experimental and theoretical research on Dy and Ho single atoms deposited on BaO. A comparison of our results with similar studies sheds light on the impact of the substrate crystal field, in particular Dy-O bond covalency, on the magnetic stability of the rare-earth adatoms. Next, the detailed experimental investigation of Dy single atoms on ZnO is presented. This adatom/substrate pair is used as first attempt to implement the electric field control of the adatom magnetic properties. Lastly, we study the Dy adatoms on graphene layers grown on various metals. The vibrational modes of the metallic substrates are shown to strongly affect the Dy spin lifetime, while addition of a second graphene layer does not allow to significantly improve the adatom magnetic stability.

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Boris Sorokin
EPFL, 2022
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/296917?ln=en

About this result

Related concepts (26)

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Related publications (51)

Related publications

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

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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

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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

Synthesis and Properties of Molecular Nanostructures on Surfaces

Nasiba Abdurakhmanova

One central aim of nanotechnology is the creation and exploitation of nanostructured materials with pre-programmed architecture and properties. The fabrication of functional nanostructures relies on versatile protocols of synthetic chemistry enabling the precise control of reactions at the molecular level. The surface-supported nanostructures investigated in this thesis have been synthesized using extended concepts of synthetic chemistry. To this end, mainly two chemical bonding types were employed: metal-organic coordination and covalent bonding. The resulting structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Furthermore, the electronic and magnetic properties of the coordination structures were investigated by x-ray absorption spectroscopy (XAS) at the synchrotron radiation facility in Grenoble, France. The thesis is organized into the following three topics: - Organizational chirality of metal-organic coordination networks on a metal surface. The metal coordination networks formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and Mn as well as Cs adatoms on Ag(100) are presented in chapter 3. The metal atoms form very similar chiral complexes with TCNQ. The complexes organize into densely packed and highly ordered domains whose organizational chirality depends on the metal center. This issue is addressed by theoretical modeling. It is shown that the exibility of the metal-organic coordination bond plays a key role in the supramolecular chirality. Mn forms rigid and directional bonds resulting in a heterochiral packing of the complexes. This organization avoids electrostatic repulsion between adjacent cyano groups of TCNQ. In the Cs-TCNQ4 structure this is achieved by an umbrella-like folding of four TCNQs around the central Cs atom, which yields a denser homochiral packing. This is enabled by the flexible nature of the ionic bonds. Moreover, the alkali atoms allow a mean to modify the electrostatic properties of the organic adlayer, which is important for the design of metal-organic interfaces in electronic devices. - Electronic and magnetic properties of the Me-TCNQ structures on metal surfaces. Previous investigations based on x-ray photoemission spectroscopy and density functional theory calculations for the Me-TCNQx systems on Cu(100) and Ag(100) surfaces show that two charge transfer (CT) channels are present, i.e. metal adatom-TCNQ and substrate-TCNQ. The choice of the substrate significantly influences CT, thereby controlling the electronic properties. In chapter 4 the magnetic properties of Ni-TCNQ and Mn-TCNQ structures on Ag(100) and Au(111) is presented. It is shown that the magnetic properties of Mn-TCNQ structures are essentially the same on Ag(100) and Au(111) surfaces, whereas for Ni-TCNQ the influence of the substrate is very pronounced. Both Ni-TCNQ/Au(111) and Ni-TCNQ/Ag(100) show ferromagnetic (FM) behavior, with stronger coupling observed on Au(111). This fact is especially interesting since the Ni impurities are found to be non-magnetic on both substrates. The observations are linked to the different CT channels and possible coupling mechanisms are identified. - The covalent bond formation reactions. Chapter 5 discusses the on-surface controlled synthesis of carbon-based nanostructures. The investigation of the properties of carbon-based nanostructures requires a better control over the synthesis process. To this end, one of the strategies proposed is the use of well-defined precursor molecules, which are deposited and activated on the surface to obtain the final structure. The surface catalyzed cyclodehydrogenation (SCDDH) reaction was used for the synthesis of fullerenes and nanotube caps. The selectivity of the C-C bond formation process is shown. Based on this result the isomeric pure C84 higher fullerene was synthesized. The same method was applied to the synthesis of nanotube caps. The precursor-based approach was also applied to the synthesis of N=9 and N=15 graphene nanoribbons (GNRs). Polyphenyl precursor molecules have been employed, which allow a synthetically simple mean to adjust the width of the ribbons. However, the resulting GNRs comprise many defects and the coupling mechanism is not very effective. The aspects of surface supported reactions are discussed and possible improvement strategies are proposed.

EPFL2012

Tailoring the Magnetism of Co Atoms on Graphene through Substrate Hybridization

Harald Brune, Alberto Cavallin, Fabio Donati, Jan Gui-Hyon Dreiser, Quentin Dubout, Luca Gragnaniello, Donat Fabian Natterer, François Patthey, Marina Pivetta, Stefano Rusponi

We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001) feature exceptionally large orbital and spin moments, as well as an out-of-plane easy axis with large magnetic anisotropy. Conversely, the magnetic moments are strongly reduced for Co/G/Ir(111), and the magnetization is of the easy-plane type. We demonstrate how the Co magnetic properties, which ultimately depend on the degree of hybridization between the Co 3d orbitals and graphene p bands, can be tailored through the strength of the graphene-substrate coupling.

Amer Physical Soc2014

Synthesis and Properties of Molecular Nanostructures on Surfaces

Nasiba Abdurakhmanova

EPFL2012

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EPFL2022