Magnetic anisotropy of Fe and Co ultrathin films deposited on Rh(111) and Pt(111) substrates: An experimental and first-principles investigation

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi
2010
Article

Résumé

We report on a combined experimental and theoretical investigation of the magnetic anisotropy of Fe and Co ultrathin layers on strongly polarizable metal substrates. Monolayer (ML) films of Co and Fe on Rh(111) have been investigated in situ by x-ray magnetic circular dichroism (XMCD), magneto-optic Kerr effect, and scanning tunneling microscopy. The experiments show that both magnetic adlayers exhibit ferromagnetic order and enhanced spin and orbital moments compared to the bulk metals. The easy magnetization axis of 1 ML Co was found to be in plane, in contrast to Co/Pt(111), and that of 1 ML Fe out of plane. The magnetic anisotropy energy (MAE) derived from the magnetization curves of the Fe and Co films is one order of magnitude larger than the respective bulk values. XMCD spectra measured at the Rh M2,3 edges evidence significant magnetic polarization of the Rh(111) surface with the induced magnetization closely following that of the overlayer during the reversal process. The easy axis of 1–3 ML Co/Rh(111) shows an oscillatory in-plane/out-of-plane behavior due to the competition between dipolar and crystalline MAE. We present a comprehensive theoretical treatment of the magnetic anisotropy of Fe and Co layers on Rh(111) and Pt(111) substrates. For free-standing hexagonally close-packed monolayers the MAE is in plane for Co and out of plane for Fe. The interaction with the substrate inverts the sign of the electronic contribution to the MAE, except for Fe/Rh(111), where the MAE is only strongly reduced. For Co/Rh(111), the dipolar contribution outweighs the band contribution, resulting in an in-plane MAE in agreement with experiment while for Co/Pt(111) the larger band contribution dominates, resulting in an out-of-plane MAE. For Fe films however, the calculations predict for both substrates an in-plane anisotropy in contradiction to the experiment. At least for Fe/Pt(111) comparison of theory and experiment suggests that the magnetic structure of the adlayer is more complex than the homogenous ferromagnetic order assumed in the calculations. The angular momentum and layer-resolved contributions of the overlayer and substrate to the MAE and orbital moment anisotropy are discussed with respect to the anisotropic hybridization of the 3d, 4d, and 5d electron states and vertical relaxation. The role of technically relevant parameters such as the thickness of the surface slab, density of k points in the Brillouin zone, and electron-density functionals is carefully analyzed.

Official source

https://infoscience.epfl.ch/record/151966?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi
2010
Article

Résumé

Official source

https://infoscience.epfl.ch/record/151966?ln=fr

À propos de ce résultat

Concepts associés (35)

Concepts associés

Chargement

Publications associées (85)

Chargement

Afficher plus

Publications associées

Chargement

Publications associées (85)

Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

Sergio Vlaic

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.

EPFL2013

Chargement

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

Chargement

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

Chargement

Afficher plus

Propriétés magnétiques de nanostructures de cobalt adsorbées

Nicolas Weiss

EPFL2004

EPFL2013

Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

Sergio Vlaic

EPFL2013