Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

his thesis reports on the magnetic properties of bi- and tri-metallic nanostructures at surfaces. The main goal is to build the smallest nanostructure ferromagnetic at room temperature and the assembly of those structures in high density arrays. To this purpose we have developed an approach consisting in a fine engineering of the nanostructure chemical structure at the atomic level. We grow core-shell nanostructures with atomically sharp interfaces between the different constituents and we investigate the effects produced by these interfaces as a function of their chemistry and structure. This approach is then used to grow organized arrays of bimetallic nanostructures that represent model systems for next generation high density magnetic storage media. The magnetic properties of the different experimental systems were investigated by magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the understanding of the effects of atomically sharp interfaces among several elements. Two dimensional cobalt nanostructures have been grown on Pt(111) single crystal surface and subsequently their edges have been decorated by Fe, Pt or Pd. This one-dimensional decoration produces element dependent variations of the magnetic anisotropy energy with strong enhancement for Fe decoration and reduction for Pt and Pd. Furthermore we observe a crystallographic direction dependence on the effect of Pt and Pd decoration since Co/Pt or Co/Pd staking with {111}-orientation strongly enhances the magnetic hardness of the nanostructures. The effect of atomically sharp Co/Fe one-dimensional interface has been compared with direct alloying of the two elements finding the first one to give the highest effect for shell thicknesses smaller than 5 atoms. With the help of fully relativistic ab-initio we were able to reproduce the experimental results and unravel their pure electronic origin. The electronic hybridizations at the interface between the elements gives rise to "hot spots" in the electronic band structure responsible of a strong variation of the magnetocrystalline anisotropy resulting in the enhanced magnetic hardness. By growing Co-core Fe-shell Pd-capped nanostructures we were able to enhance the magnetization thermal stability of nanostructures by almost a factor of 3 with respect single-metal nanostructures of the same size. In the second part of this thesis, the first attempt of growing organized array of bimetallic nanostructures is presented. Taking advantage of the well-known self organized growth of Co nanodots on Au(111)-vicinal surfaces, we were able to grow Co-core Fe-shell nanodots on Au(11,12,12) single crystal. The thermal stability enhancement produced by the atomically sharp Co/Fe interface is higher than the direct alloying of the two elements as in the previous case. We argue that with this method, combined with three-dimensional growth of nanostructures arrays, it will be possible to produce multimetallic nanodots composed by ≈ 3000 atoms with magnetization thermal stability suitable for magnetic storage media. In the last part another model system for magnetic nanostructure superlattices is presented. Fe nanostructures have been grown on the Pd-seeded and unseeded alumina thin film formed by oxidation of a Ni3Al(111) single crystal surface at high temperature. We found that the sample stoichiometry near the surface evolves with the preparation cycles. This leads to the formation of a Ni-rich region, close to the alumina surface, ferromagnetic up to 250 K. By means of X-ray magnetic circular dichroism measurements we were able to estimate the amount of Ni in excess. Fe clusters deposited on the alumina surface are magnetically coupled to the Ni-rich region independently on the presence of Pd. Nevertheless, the system behavior cannot be explained by a simple ferromagnetic or antiferromagnetic coupling. We argue that the Fe nanostructures are coupled to the substrate by an interlayer exchange coupling across the alumina thin film. The coupling constant is expected to change sign depending on the distance between the Fe clusters and the Ni-rich region resulting in a peculiar magnetic behavior of the system.

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008

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

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

This thesis presents our studies on nanostructures growth and magnetism, mainly based on scanning tunneling microscopy (STM), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD) measurements. In most of these experiments, nanostructures were grown by molecular beam epitaxy on single crystal substrates under ultra high vacuum conditions. Our combined morphological and magnetic investigations aim at an atomic-level description of the nanostructures magnetic properties. In particular, we are searching for strategies to design the smallest ferromagnet with stable magnetization at room temperature to be used in devices such as magnetic random access memories, storage media, and sensors. A critical factor in pursuing this aim is the determination and control of the magnetization reversal mechanism. Different reversal mechanisms may occur in the nanostructures we considered: either all the spins quasi-coherently rotate or a domain wall is nucleated and propagated along the particle. We prepared monolayer-high Co islands on Pt(111) with compact and ramified shape by adjusting the deposition temperatures and coverages for a sequence of deposition and annealing steps. Combining field-dependent magnetization curves and temperature dependent zero-field susceptibility measurements we identify a size- and shape-dependent transition from quasi-coherent to domain wall propagation reversal regimes. In a second study we investigated a system which according to a recent publication presents many of the characteristics desired in bit patterned magnetic media for ultra-high density storage devices. In particular, it was claimed that Co nanoclusters, growing in registry with the GdAu2/Au(111) moiré periodic substrate, and featuring a density of 52 Tera/in2, present out-of-plane remanent magnetization at T = 300 K. At odds with this claim, no perpendicular magnetic anisotropy was found in our combined MOKE and STM study, which shows an in-plane remanent signal appearing only for coalesced islands. A third study is devoted to FeRh, an alloy material which, thanks to its peculiar magnetic phase diagram, has been proposed coupled to FePt for advanced magnetic recording schemes. We present the first observation of low temperature stabilization of the ferromagnetic (FM) phase in FeRh chemically ordered crystals. Nanocrystals measuring 3.3 nm in diameter have B2 structure with alternating atomic Fe and Rh layers. XMCD demonstrates FM alignment of the Fe and Rh magnetic moments of 3 and 1 μB, respectively. In sharp contrast to FeRh bulk and thin films, which show antiferromagnetic iii Abstract (AFM) order below 300 K, the low-temperature FM 3.3 nm nanocrystals are the hallmark of a size-dependent AFM-FM transition temperature. Finally we present our results towards creation by chemical vapor deposition of a “mille-feuille” structure composed of graphene and magnetic nanoclusters. To realize this objective we optimized the cluster size, composition, and annealing temperature to obtain a compromise between graphene syntesis and cluster coalescence. We demonstrate by STM that annealing to T = 720 K in ethylene stabilizes Ir-seeded clusters against diffusion and coalescence, and by Auger Electron Spectroscopy (AES) that Ni-coated clusters can be covered with up to half of the Carbon contained in a graphene layer.

EPFL2013

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

EPFL2008

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

Quentin Dubout

EPFL2013

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

EPFL2013