Coordination dependent magnetic properties of 3d and 4d metal nano-structures

In this thesis, the magnetic properties of self-assembled 3d and 4d metal nano-structures supported on surfaces have been investigated. The atomic coordination within the nano-structures was found to profoundly affect important quantities such as the magnetic moment and the magnetic anisotropy. The use of thin, atomically flat insulating Xe spacers of 1-15 monolayers (ML) thickness allowed for a study of coordination effects in the two limits of strong and weak coupling with an underlying metal substrate. The systems were characterized by surface-sensitive methods, based on synchrotron radiation (X-ray magnetic circular dichroism, and X-ray scattering/diffraction) and variable temperature scanning tunneling microscopy (VT-STM). The VT-STM was developed and implemented during this PhD work. First, the magnetism of Rh nano-structures on a Xe buffer layer has been investigated. Rh is non-magnetic in bulk but shows a finite magnetic moment upon reducing cluster sizes to below 100 atoms. Within this work a small, non-zero magnetic moment was found for Rh nano-structures situated on Xe. The effect of intra-cluster Rh-Rh coordination was observed to affect both the spin and orbital part of the magnetic moment, leading to strongly oscillating values at smallest cluster sizes. Further, the analysis of the spectroscopic data suggests an interpretation for the absence of magnetism in directly deposited Rh on Ag(100) that is based on the formation of a kinetically promoted Ag-Rh alloy. Second, the buffer layer assisted growth (BLAG) was studied for sub-monolayer Co nano-clusters on Ag(111) and Pt(111) surfaces. The observation of the cluster formation process in the very early stages of BLAG revealed the paramount importance of the substrate in determining both magnetism and structural properties of the nano-clusters. On Ag(111), a weakly interacting substrate, the clusters form on the buffer layer independently from the metal substrate and show no magnetic anisotropy at this stage. As soon as the Xe is desorbed by sample annealing an in-plane anisotropy forms, as a consequence of the contact with the substrate. X-ray scattering and diffraction data support this interpretation and also show that in the limit of a single monolayer of Xe on Ag(111) the BLAG is a 'simple' atomic diffusion process, with a very high mobility of Co atoms on Xe. On a thick Xe buffer layer instead, due to a lower Xe-Xe binding energy and to the higher surface energy of Co compared to Xe, the deposition of Co provokes a re-arrangement of the Xe atoms. On the other hand, on Pt(111) the BLAG process fails to ensure a cluster formation process ontop the buffer layer and independent of the metal substrate. Here in fact, electric dipolar interactions occurring between Co atoms and the substrate through the Xe layer, are strong enough to destroy the Xe ML order and bring the Co atoms in direct contact with the Pt(111) before Xe atoms are thermally desorbed. This complex process becomes evident from VT-STM investigations and by the occurrence of perpendicular magnetic anisotropy right after Co deposition on the Xe ML/Pt(111). In a detailed discussion it is shown that magnetic properties like magnetic anisotropy and orbital/spin moments are strongly entangled with their morphology. Both morphology and magnetism are determined by the interaction with the environment. This opens the way to more complex systems, where the interaction with the medium is tuned such as to gain nano-structures with a pre-defined structure and function. Third, the knowledge about the cluster-substrate interactions during BLAG was exploited to build highly ordered arrays of Co nano-structures on a patterned template substrate. In this case the hexagonal boron nitride (h-BN) nanomesh on Rh(111) was used. These systems have been employed to study the effect of hybridization of the Co d band with capping layers such as Pt, Au, Al2O3 and MnPt on the magnetic moment of Co. It was found that in all these cases Co clusters have no remanence, due to the small size and weak coupling with the h-BN atoms. However, it could be shown that capping the clusters strongly influence the clusters magnetization, in a non-trivial way.

Growth and magnetism of 2D bimetallic nanostructures

Géraud Moulas

This thesis addresses the growth and magnetic characterization of 2D bimetallic nanostructures deposited by atomic beam epitaxy (ABE) on Pt(111). These structures possess both tunable high perpendicular magnetic anisotropy (PMA) and magnetic moment. These properties make them appealing as model systems in order to learn how to control the properties of the futures media used in magnetic data storage. Our study combined two in situ measurements techniques : variable-temperature scanning tunneling microscopy (VT-STM) as a local probe allows insight on the morphology of nanostructures, while magneto-optic Kerr effect (MOKE) is a spatially integrating technique giving access to the variation of the overall magnetization of a sample. The first part of this work focuses on the growth of iron on the Pt(111) surface. Growth was investigated on the atomic scale as a function of the substrate temperature in the case of low coverages. We have fitted the mean cluster size as a function of the annealing temperature with mean field rate equations for diffusion-controlled growth. The activation parameters for monomer, dimer and trimer diffusion could be inferred from this procedure. The formation of monatomic Fe wires has also been evidenced on the temperature range 160 K–260 K. The origin of their formation was discussed. In a second part, we have made use of our knowledge on the growth of cobalt and iron on Pt(111) in order to fabricate "core-shell" Co nanostructures of which density, size and shape were controlled. Therefore, we could realized both compact and ramified structures within the size range 800–1800 atoms. The study of the mechanism of magnetization reversal of these model structures has revealed a strong size and shape dependence. This is due to the shape-induced non-uniformity of the local magnetization and the number of pinning centers. The conclusions are that ramified structures with arms longer than 150 Å reverse their magnetization by nucleation and domain-wall motion while compact structures reverse coherently their magnetization. The third part deals with the magnetic properties of one monolayer thick bimetallic "Co core-shell" nanostructures on Pt(111). The blocking temperature TB marks the transition between superparamagnetic and blocked states and is inferred from the magnetic anisotropy. Here, we performed magnetic zero-field susceptibility measurements so as to determine TB in our samples. From our experiments, we show the possibility to make up a fine tuning of the nanostructure magnetic anisotropy and overall magnetization. In the case of the FexCo1-x alloy, TB adopt a bell-shape with x and exhibit a maximum at x = 0:5. The various lateral and vertical interfaces between Co from one side and Fe, Pt or Pd from the other side are at the origin of substantial TB variation. Those variations are inferred from the symmetry breaking and the strong hybridization between the d orbitals of these elements.

EPFL2008

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

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