Growth and magnetism of 2D bimetallic nanostructures

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

The consolidated European synthesis of CO2 emissions and removals for the European Union and United Kingdom: 1990–2020

Quantification of land surface–atmosphere fluxes of carbon dioxide (CO2) and their trends and uncertainties is essential for monitoring progress of the EU27+UK bloc as it strives to meet ambitious targets determined by both international agreements and internal regulation. This study provides a consolidated synthesis of fossil sources (CO2 fossil) and natural (including formally managed ecosystems) sources and sinks over land (CO2 land) using bottom-up (BU) and top-down (TD) approaches for the European Union and United Kingdom (EU27+UK), updating earlier syntheses (Petrescu et al., 2020, 2021). Given the wide scope of the work and the variety of approaches involved, this study aims to answer essential questions identified in the previous syntheses and understand the differences between datasets, particularly for poorly characterized fluxes from managed and unmanaged ecosystems. The work integrates updated emission inventory data, process-based model results, data-driven categorical model results, and inverse modeling estimates, extending the previous period 1990–2018 to the year 2020 to the extent possible. BU and TD products are compared with the European national greenhouse gas inventory (NGHGI) reported by parties including the year 2019 under the United Nations Framework Convention on Climate Change (UNFCCC). The uncertainties of the EU27+UK NGHGI were evaluated using the standard deviation reported by the EU member states following the guidelines of the Intergovernmental Panel on Climate Change (IPCC) and harmonized by gap-filling procedures. Variation in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), originate from within-model uncertainty related to parameterization as well as structural differences between models. By comparing the NGHGI with other approaches, key sources of differences between estimates arise primarily in activities. System boundaries and emission categories create differences in CO2 fossil datasets, while different land use definitions for reporting emissions from land use, land use change, and forestry (LULUCF) activities result in differences for CO2 land. The latter has important consequences for atmospheric inversions, leading to inversions reporting stronger sinks in vegetation and soils than are reported by the NGHGI. For CO2 fossil emissions, after harmonizing estimates based on common activities and selecting the most recent year available for all datasets, the UNFCCC NGHGI for the EU27+UK accounts for 926 ± 13 Tg C yr−1, while eight other BU sources report a mean value of 948 [937,961] Tg C yr−1 (25th, 75th percentiles). The sole top-down inversion of fossil emissions currently available accounts for 875 Tg C in this same year, a value outside the uncertainty of both the NGHGI and bottom-up ensemble estimates and for which uncertainty estimates are not currently available. For the net CO2 land fluxes, during the most recent 5-year period including the NGHGI estimates, the NGHGI accounted for −91 ± 32 Tg C yr−1, while six other BU approaches reported a mean sink of −62 [] Tg C yr−1, and a 15-member ensemble of dynamic global vegetation models (DGVMs) reported −69 [] Tg C yr−1. The 5-year mean of three TD regional ensembles combined with one non-ensemble inversion of −73 Tg C yr−1 has a slightly smaller spread (0th–100th percentiles of [] Tg C yr−1), and it was calculated after removing net land–atmosphere CO2 fluxes caused by lateral transport of carbon (crop trade, wood trade, river transport, and net uptake from inland water bodies), resulting in increased agreement with the NGHGI and bottom-up approaches. Results at the category level (Forest Land, Cropland, Grassland) generally show good agreement between the NGHGI and category-specific models, but results for DGVMs are mixed. Overall, for both CO2 fossil and net CO2 land fluxes, we find that current independent approaches are consistent with the NGHGI at the scale of the EU27+UK. We conclude that CO2 emissions from fossil sources have decreased over the past 30 years in the EU27+UK, while land fluxes are relatively stable: positive or negative trends larger (smaller) than 0.07 (−0.61) Tg C yr−2 can be ruled out for the NGHGI. In addition, a gap on the order of 1000 Tg C yr−1 between CO2 fossil emissions and net CO2 uptake by the land exists regardless of the type of approach (NGHGI, TD, BU), falling well outside all available estimates of uncertainties. However, uncertainties in top-down approaches to estimate CO2 fossil emissions remain uncharacterized and are likely substantial, in addition to known uncertainties in top-down estimates of the land fluxes. The data used to plot the figures are available at https://doi.org/10.5281/zenodo.8148461 (McGrath et al., 2023)

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

18 páginas, 7 figuras, 6 tablas.-- PACS number(s): 75.30.Gw, 75.75.-c, 71.15.Mb, 78.70.DmWe performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about −0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.P.B. and J.H. acknowledge support of the Austrian Science Funds under Project. No. P19712-N16. Financial support from the Swiss National Science Foundation (Grants No. 200020-109800 and No. 200020-112322) and from the European Science Foundation (EUROCORES 05-SONS-FP-009 SANMAG) are gratefully acknowledged. P.G. acknowledges support from the Spanish Ministerio de Ciencia e Innovación (MAT2007-62341) and Agència de Gestió d’Ajuts Universitaris i de Recerca (2009 SGR 695).Peer reviewe

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

23 páginas,15 figuras, 6 tablas.-- PACS number(s): 75.30.Gw, 75.75.-c, 71.15.Mb, 78.70.DmWe 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.The calculations described in this paper were performed using HPC resources from GENCI-CINES (Grant No. 2009- x2009095045) at the CINES (Montpellier, France) and from the Vienna Scientific Cluster (VSC). Financial support from the Swiss National Science Foundation under Grants No. 200020-109800 and No. 200020-112322, the Spanish Ministerio de Ciencia e Innovación (Grant No. MAT2007-62341), the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (2009 SGR 695), and from the European Science Foundation EUROCORES 05-SONS-FP-009 SANMAG are gratefully acknowledged.Peer reviewe

Magnetic properties of ultrathin FexCo(1-x) films on Pt(111)

14 pages, 14 figures.-- PACS nrs.: 75.70.Ak; 75.30.Gw; 73.20.At.et al.The magnetism of 1-ML-thick films of Fe(x)Co(1−x) on Pt(111) was investigated both experimentally, by x-ray magnetic circular dichroism and magneto-optical Kerr effect measurements, and theoretically, by first-principles electronic structure calculations, as a function of the film chemical composition. The calculated Fe and Co spin moments are only weakly dependent on the composition and close to 3µ(B)/atom and 2µ(B)/atom, respectively. This trend is also seen in the experimental data, except for pure Fe, where an effective spin moment of only Seff=(1.2 ± 0.2) µ(B)/atom was measured. On the other hand, both the orbital moment and the magnetic anisotropy energy show a strong composition dependence with maxima close to the Fe(0.5)Co(0.5) stoichiometry. The experiment, in agreement with theory, gives a maximum magnetic anisotropy energy of 0.5 meV/atom, which is more than 2 orders of magnitude larger than the value observed in bulk bcc FeCo and close to that observed for the L10 phase of FePt. The calculations clearly demonstrate that this composition dependence is the result of a fine tuning in the occupation number of the d(x^2−y^2) and d(xy) orbitals due to the Fe-Co electronic hybridization.Financial support from the Swiss National Science Foundation (Grants No. 200020- 109800 and No. 200020-112322) and from the Austrian Science Fund (FWF) within the Joint Research Programme S90 are gratefully acknowledged. Financial support of this work was also provided by the Austrian Science Foundation (Grant No. WK W004) and Oak Ridge National Laboratory (Subcontracts No. 4000043271 and No. 4000063148). We acknowledge the ESRF for provision of beam time and financial support from the EUROCORES 05-SONS-FP-009 SANMAG project of the European Science Foundation.Peer reviewe