Magnetism of individual atoms adsorbed on surfaces

Magnetic impurities in solids cause manifold changes in their macroscopic properties, such as anomalous low temperature resistance due to Kondo screening, reduction of the superconducting transition temperature due to local suppression of the order parameter, they create magnetic signatures in semiconductors, and lead to inelastic spin excitations in tunnel junctions. In the present paper we review what has been learnt about these effects from a surface science approach. Placing the magnetic impurities at well defined adsorption sites on single crystal surfaces makes their effect on the host, as well as their own magnetic properties better accessible to experiments, and also better understandable since the atomic environment of the impurity is exactly known lending comparison with theory more direct. After an introduction we discuss X-ray magnetic circular dichroism measurements which are spatially averaging and therefore report on ensemble properties. One of the recent progresses achieved in surface science is the preparation of well defined ensembles, such as surfaces with only single adatoms, each of them in an identical atomic environment and with sufficient mutual distance to exclude interactions. Due to this approach we can now determine the electronic configuration of individual adatoms, their hybridization with the host, and quantify their spin and orbital moments, as well as the spin–orbit induced magneto-crystalline anisotropy, which can be orders of magnitude larger than thin film and bulk values. In the second part we review recent progress in revealing the magnetic properties of individual atoms with the scanning tunneling microscope (STM). With this technique the spatial extent of the Kondo screening cloud and of subgap excitations in the superconductor quasiparticle density of states became apparent. We outline the first pioneering experiments measuring transport through reversible atomic point contacts containing magnetic atoms and measurements using the subgap features caused in superconducting STM tips to detect the magnetism of individual atoms. We then describe experiments using inelastic spin excitation spectroscopy to pin down the magnetic ground state and anisotropy energy of magnetic impurities. We continue with spin-polarized STM experiments reporting magnetization curves of individual magnetic adatoms and finish by a description of the most recent spin-excitation experiments revealing the necessary anisotropy environment for a high spin impurity to display the Kondo effect.

Propriétés magnétiques de nanostructures de cobalt adsorbées

Nicolas Weiss

This thesis reports results on magnetic properties of supported cobalt nanostructures. The nanostructures were grown on single crystal metal surface by Molecular Beam Epitaxy in an Ultra High Vacuum chamber. Our experimental setup is based on two in situ measurement techniques. The first is the Scanning Tunneling Microscope (STM) allowing to investigate the nanostructure morphology. The second, called Surface Magneto-Optical Kerr Effect (SMOKE), probes the magnetism of the nanostructures. By combining the data obtained with both techniques we were able to investigate the magnetism of 2D nanoparticles down to the atomic level. The first aim of the thesis consisted in disclosing the role played by the differently coordinated atoms in determining the Magnetic Energy Anisotropy in 2D nanostructures. To this purpose, we performed experiments on the Co/Pt(111) system, chosen as a model system since the Pt surface is well known to increase the out-of-plane uniaxial anisotropy as well as the magnetic moment of the Co islands. A preliminary work has been devoted to the growth of the Co islands as a function of the deposition and annealing temperatures. As a result, we learned to grow three different island's shapes : ramified, compact and double-layer islands. We then concentrated on the magnetism of these islands. Due to the nonlinear relationship between perimeter length and surface area, we were able to distinguish the different contributions to the anisotropy energy of surface and perimeter atoms. We found that the magnetic anisotropy in 2D Co nanostructures is predominantly due to the low-coordinated edge atoms, for which the anisotropy energy can be as large as 20 times the bulk value. This finding opens new possibilities to separately tune the anisotropy and moment of nanostructures. To exemplify this, and to illustrate once more the role of perimeter atoms, we produced Co island with a non-magnetic Pt core. As expected, these bimetallic islands have identical anisotropy and lower magnetic moment to their equally shaped pure Co counterpart. The second aim we focused on was the investigation of the magnetic properties of ultra-high density arrays of nanostructures created by self-assembly. We chose Co/Au(788) as a model system since the nanoscopic Co dots were found to have a very narrow size distribution and to be placed onto a lattice which is phase coherent on a macroscopic length scale. Moreover, the array has a density of 26 Tdot/in2 which allows the investigation of inter-particle interaction in ultra high density arrays. We showed that the narrow size distribution of the dots corresponds to an unprecedented narrow Magnetic Anisotropy Energy distribution with 35% width at half of the maximum. Moreover, we demonstrated the weak magnetic coupling between the Co dots despite their high density. In parallel to these fundamental researches, we developed a new experimental setup that will replace the current one. We designed a UHV chamber equipped with a homemade variable temperature STM combined with a SMOKE experiment. A quadrupole, focused on the sample, generates a 0.35 T magnetic field. This field is one order of magnitude larger than the present one and can be uniformly rotated in the plane of the quadrupole. Furthermore, the design of the new chamber allows STM imaging in this magnetic field. In this report, we present the adopted solutions as well as the first tests of the components (STM, SMOKE, cryostat).

EPFL2004

Magnetic anisotropy from single atoms to large monodomain islands of Co/]Pt(111)

Harald Brune, Pietro Gambardella, Stefano Rusponi, Nicolas Weiss

By studying the magnetic behavior of self-assembled Co islands on a single-crystal metal surface, Pt(111), we show how the magnetic anisotropy evolves from isolated atoms to monolayer islands and films. Single Co adatoms are found to have a giant magnetocrystalline anisotropy energy of E-a = 9.3 +/- 1.6 meV/atom arising from the combination of partly preserved orbital moments (m(L) = 1.1 mu(B)) and strong spin-orbit coupling induced by the Pt substrate. Combined scanning tunneling microscopy and X-ray magnetic circular dichroism experiments performed for differently sized small two-dimensional Co islands establish a clear connection between E-a and m(L), both quantities decrease sharply with the lateral coordination of the magnetic atoms. In accordance with this, Kerr magnetometry experiments, again performed in conjunction with scanning tunneling microscopy, reveal that the anisotropy energy of large two-dimensional Co islands is almost entirely determined by the relatively low number of perimeter atoms, having E-a = 0.9 +/- 0.1 meV/atom. These results confirm theoretical predictions and are of fundamental value the understanding of how the magnetic anisotropy develops in finite-size magnetic particles. Identification of the role of perimeter and surface atoms opens up new opportunities for engineering the anisotropy and the moment of a magnetic nanostructure. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.

2005

Growth, electronic structure and magnetism of supported metal nanowires

Pietro Gambardella

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.