Bulk electronic structure studied by hard X-ray photoelectron spectroscopy of the valence band: The case of intermetallic compounds

In this work, results of hard X-ray photoelectron spectroscopy (HAXPES) of Heusler compounds and new materials for spintronics are presented. The class of Heusler materials includes some interesting half-metallic and ferromagnetic properties that were predicted by theory. HAXPES allows a direct comparison of the measured and the calculated electronic structure. Valence band spectroscopy of bulk materials by HAXPES is illustrated for the case of the half-metallic ferromagnet Co2MnGe. The feasibility of HAXPES to explore the valence band electronic structure in deeply buried metallic layers is demonstrated for buried Co2MnSi films. The films exhibit the same valence density of states as bulk samples and confirm the promise of an epitaxial, single-crystalline Co2-based Heusler compound film as a ferromagnetic electrode for spintronics devices. The study of complete CoFe(B)/MgO/CoFe(B) tunneling junctions demonstrates the capability of HAXPES to explore the electronic structure in deeply buried layers in a non-destructive way. The improvement of the TMR by annealing of the junction is explained by an improvement of the structure together with a change of the composition in the CoFeB layers.

Core-level Photoemission Spectroscopy (PES) has played a very important role in our understanding of the electronic structure of correlated transition metal and rare-earth compounds. The appearance of strong satellite structures accompanying the main PES spectra in correlated systems is well known, and systematic variations in the position and intensities of these satellites provide us with important clues to their electronic structures. In spite of these successes, the surface sensitivity of PES has often led to controversies regarding surface versus bulk electronic structure, and hence, hard X-ray photoelectron spectroscopy (HAXPES) is very important and promising. HAXPES is a bulk sensitive probe of the electronic structure due to its ability to overcome surface sensitivity of conventional PES. Unlike soft X-ray PES, 2p core-level HAXPES have shown additional well-screened features with significant intensity at the low binding energy side of the main peak. These features were explained well by the configuration-interaction model including a screening channel derived from coherent states near Fermi energy. Here, we review these advances and examine the application of HAXPES to studies of the strongly correlated electron systems, especially for 3d transition metal compounds. The details of the well-screened features are also discussed.

Recent Advances in Topological Quantum Materials by Angle-Resolved Photoemission Spectroscopy

The substitutional series of Heusler compounds ${\text{NiTi}}_{1\ensuremath{-}x}{M}_{x}\text{Sn}$ (where $M=\text{Sc},\text{V}$ and $0lx\ensuremath{\le}0.2$) were synthesized and investigated with respect to their electronic structure and transport properties. The results show the possibility to create $n$-type and $p$-type thermoelectrics within one Heusler compound. The electronic structure and transport properties were calculated by all-electron ab initio methods and compared to the measurements. Hard x-ray photoelectron spectroscopy was carried out and the results are compared to the calculated electronic structure. Pure NiTiSn exhibits massive ``in gap'' states containing about 0.1 electrons per cell. The comparison of calculations, x-ray diffraction, and photoemission reveals that Ti atoms swapped into the vacant site are responsible for these states. The carrier concentration and temperature dependence of electrical conductivity, Seebeck coefficient, and thermal conductivity were investigated in the range from 10 to 300 K. The experimentally determined electronic structure and transport measurements agree well with the calculations. The sign of the Seebeck coefficient changes from negative for V to positive for Sc substitution. The high $n$-type and low $p$-type power factors are explained by differences in the chemical-disorder scattering-induced electric resistivity. Major differences appear because $p$-type doping (Sc) creates holes in the triply degenerate valence band at $\ensuremath{\Gamma}$ whereas $n$-type doping (V) fills electrons in the single conduction band above the indirect gap at $X$ what is typical for all semiconducting transition-metal-based Heusler compounds with $C{1}_{b}$ structure.

Addressing the oxidation state of functional materials such as transition metal oxides is a current critical challenge and requires new methodologies to characterize their electronic structure within surface to bulk resolution at the nanometer scale. One approach to this issue is the combination of co‐localized soft and hard X‐ray photoelectron spectroscopies for a non‐destructive depth profiling. In this work, we demonstrate the capability to characterize the oxidation state of cobalt in LiCoO 2 thin films, a model electrode material in the Li‐ion battery field, within the first 15 nm. The capability of the methodology to address the surface evolution of the cobalt oxidation state is tested through a proof of concept surface modification introduced by Ar‐ion sputtering. To address the cobalt local electronic structure at different depths from the extreme surface, we exploited Auger‐free Hard X‐ray photoelectron spectroscopy (HAXPES) spectra to fit the Co 2p core‐level features in the X‐ray photoelectron spectroscopy (XPS) spectra. This approach may pave the way for a better understanding of the surface electronic structure changes in transition metal oxides driven by their applications in a broad range of technologies.

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{Fe}(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5})$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at ${T}_{\mathrm{an}}=\phantom{\rule{0.16em}{0ex}}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different ${T}_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_{1}$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_{1}$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_{1}$-ordering in the present CFGG film.

Electron spectroscopic methods are powerful and efficient tools for characterization of chemical and electronic structures of surface and interface layers of solids. The electron spectroscopic methods most widely applied for surface chemical analysis, the X‐ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) are providing information on the elemental composition of the surface and interface layers, as well as on the chemical state of the components. In addition, these techniques can offer possibilities for depth‐resolved and/or laterally resolved analysis in a nondestructive (up to several nanometers depth) or destructive (in combination with ion sputtering, up to several hundred nanometers depth) way. Quantitative surface chemical analytical applications of these methods are greatly helped by physical quantities characterizing electron transport, which can be derived from reflection electron energy loss spectroscopic (REELS) studies of given materials. There are, however, a plenty of opportunities available how to improve the sensitivity, selectivity, and information depth of these techniques. Among these, the coincidence techniques help to identify the physical processes leading to specific structures in the experimental electron spectra, clean up the spectra from unwanted contributions of interfering processes, and limit the depth of analytical information. The resonant excitation can yield unprecedented chemical state selectivity and can greatly improve the detection limit for particular species while providing unique information on the electronic structure in the proximity of the excited atom. High‐energy‐resolution spectroscopy of high‐energy electrons induced by hard X‐rays from solids allows to get an insight into deeper subsurface regions owing to the much increased information depth for energetic electrons, and in addition to the possibility for collecting information on the bulk chemical and electronic structures without interfering effects because of the presence of the surface, this spectroscopy provides a nondestructive access to the chemical state‐resolved composition at deeply buried interfaces. This article intends to give a brief review on selected electron–electron coincidence techniques, resonant Auger electron spectroscopic methods, and high‐energy electron spectroscopic methods, namely, the hard X‐ray photoelectron spectroscopy (HAXPES), focusing on the principle and specific instrumentation of the techniques, the underlying physics of the fundamental processes utilized, the analytical information provided, and important fields of applications. These highly sensitive, selective, and uniquely informative electron spectroscopic methods are expected to be used increasingly in studies of sophisticated novel materials of great practical importance, especially in fields of nanotechnology, micro‐ and nanoelectronics, nano‐biotechnology, nanomedicine, and development of novel solar cells.

High energy photoelectron spectroscopy (PES) is an essential tool to overcome the high surface sensitivity of the conventional photoelectron spectroscopy for the study of bulk electronic structures of strongly correlated electron systems, which often have many different electronic structures in the surface and the bulk. The potential of the soft X-ray ARPES (SX-ARPES) in the several hundred eV region for bulk Fermiology of such systems is first demonstrated. Higher bulk sensitivity is achieved by hard X-ray PES (HAXPES) beyond several keV. Deconvolution of the surface, subsurface and bulk is feasible by combining SX-PES and HAXPES spectra measured at several very different h ν. Recoil effects of photoelectrons are observed in HAXPES not only in core spectra but also in valence band spectra of some materials with light elements. The present status and future possibility of high energy PES are discussed.

Application of hard X-ray photoelectron spectroscopy to electronic structure measurements for various functional materials

The development of magnetic Heusler compounds, specifically designed as materials for spintronic applications, has made tremendous progress in the very recent past [1–21]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% [1]. These materials are exceptionally well suited for applications in magnetic tunnel junctions acting, for example, as sensors for magnetic fields. The tunnelling magneto-resistance (TMR) effect is the relative change in the electrical resistance upon application of a small magnetic field. Tunnel junctions with a TMR effect of 580% at 4 K were reported by the group of Miyazaki and Ando [1], consisting of two Co2MnSi Heusler electrodes. High Curie temperatures were found in Co2 Heusler compounds with values up to 1120 K in Co2FeSi [2]. The latest results are for a TMR device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a TMR effect of 174% [3].

Soft X-ray ARPES and Fermiology of strongly correlated electron systems and PES by hard X-ray and extremely low energy photons

Heusler compounds at a glance.- New Heusler compounds and their properties.- Crystal structure of Heusler compounds.- Substitution effects in double Perovskites: How the crystal structure influences the electronic properties.- Half-metallic ferromagnets.- Correlation and chemical disorder in Heusler compounds: a spectroscopical study.- Theory of the half-metallic Heusler compounds.- Electronic structure of complex oxides.- Local structure of highly spin polarized Heusler compounds revealed by nuclear magnetic resonance spectroscopy.- New materials with high spin polarization investigated by X-ray magnetic circular dichroism.- Hard X-ray photoelectron spectroscopy of new materials for spintronics.- Characterization of the surface electronic properties of Co2Cr1-xFexAl.- Magneto-optical investigations and ion beam-induced modification of Heusler compounds.- Co2Fe(Al1-xSix) Heusler alloys and their applications to spintronics.- Transport properties of Co2(Mn,Fe)Si thin films.- Preparation and investigation of interfaces of Co2Cr1-xFexAl thin films.- Tunnel magnetoresistance effect in tunnel junctions with Co2MnSi Heusler alloy electrode and MgO barrier.

Photoemission spectroscopy (PES) is a suite of experimental tools to learn about the electronic and chemical structure of materials and surfaces. Normally implemented in a surface-sensitive manner, the research performed under this grant focused on pushing PES into less explored regimes, to reveal bulk electronic structure, to reveal tomographic (layer-resolved) chemistry and electronic structure of layered materials and heterostructures, and to reveal interface phenomena at the junction of two different materials. Standing wave (SW) spectroscopies have also been applied to PdCoO2, a material of interest due to its high conductivity and electron-hydrodynamic tendencies. This material can be modeled as an alternating layered structure consisting of metallic Pd layers and insulating CoO2 layers. Using SW XPS, the total electronic structure has been decomposed into contributions from the two layers, and computations highlighted the different many-body interactions in the two layers (Comm. Phys. 4, 143 (2021)). We have also used hard-x-ray angle-resolved photoemission spectroscopy (ARPES), to investigate LaB6, a technologically important material with widespread application as a cathode material for electron microscopes. We measured the bulk electronic structure of this material and found that the one-step model of photoemission better captured the electronic structure and correlations. This model treats the three steps of the photoemission process--excitation, transport of the photoelectron to the crystal surface, and escape into the vacuum—as a single quantum mechanically coherent process (Phys. Rev. Mater. 5, 055002 (2021)). We also applied x-ray photoelectron spectroscopy, implemented in a near total reflection grazing incidence geometry to elucidate technologically relevant interfaces, such as those between a substrate and photoresist (J. Phys. D: Appl. Phys. 54 464002 (2021)).

This is the third cluster issue of Journal Physics D: Applied Physicsdevoted to half-metallic Heusler compounds and devices utilizing this class of materials. Heusler compounds are named after Fritz Heusler, the owner of a German copper mine, the Isabellenhütte, who discovered this class of materials in 1903 [1]. He synthesized mixtures of Cu2Mn alloys with various main group metals Z = Al, Si, Sn, Sb, which became ferromagnetic despite all constituents being non-magnetic. The recent success story of Heusler compounds began in 1983 with the discovery of the half-metallic electronic structure in NiMnSb [2] and Co2MnZ [3], making these and similar materials, in particular PtMnSb, also useful for magneto-optical data storage media applications due to their high Kerr rotation.

Surface and bulk electronic structures of Heusler-type (L21 type) Fe2VAl have been investigated by photoelectron spectroscopy, in particular, for the valence band and V 2p core level regions, in order to elucidate the changes in the valence band electronic structures for the surface and bulk regions. In the valence band spectrum, the intensity at the Fermi level EF is increased for the surface-sensitive low photon energy excitation in comparison to the bulk-sensitive high photon energy excitation. It is also found that the intensity around a binding energy of 0.4eV is enhanced for large photoelectron takeoff angles for the bulk-sensitive photoelectron spectrum. The V 2p core level spectrum shows a surface-derived shoulder structure on the low binding energy side of the main feature, which suggests that the valence electron concentration around V may be large in the surface layers in comparison to the bulk. These facts suggest that a pseudogap is formed around EF in the bulk electronic structure, as predicted by band calculations, and that it is destroyed in the surface layers by the V 3d states as well as the Fe 3d states emerging in midpseudogap of the bulk electronic structure.