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Abstract

We report resonant inelastic x-ray scattering (RIXS) excited by circularly polarized x-rays on Mn-Zn ferrite at the Mn L2,3-resonances. We demonstrate that crystal field excitations, as expected for localized systems, dominate the RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms of spin-resolved partial density of states, which has been the standard approach for RIXS dichroism. We observe large dichroic RIXS at the L2-resonance which we attribute to the absence of metallic core hole screening in the insulating Mn-ferrite. On the other hand, reduced L3-RIXS dichroism is interpreted as an effect of longer scattering time that enables spin-lattice core hole relaxation via magnons and phonons occurring on a femtosecond time scale.

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... To achieve a good comparison between theory and experiment, calculated XAS/XES spectra within the one-electron approach often need to be fitted to experimental L 3 /L 2 or M 3 /M 2 and t 2g -e g branching ratios, as well as experimental spin-orbit splitting values and to an additional broadening for the L 2 emission lines due to the Coster-Kronig decay process. A possible solution to this kind of empirical procedure should be tested using multiplet theory [38,50] or through many-body perturbation theory by solving the Bethe-Salpeter equation (BSE) [51]. ...
... Hence, besides the existing controversial theoretical interpretations about which type of AFM coupling dominates (i.e., in-plane or out-of-plane), the evident disagreement between theory (AFM) and experiment (FM) in determining the lower energy magnetic ordering in Cr 2 GeC calls for the need of further experimental and theoretical efforts [72,155,17,116,156]. Experimentally, an alternative and more bulk-sensitive spectroscopic technique than XMCD [50], such as resonant magnetic X-ray scattering (XRMS) [157] utilizing constructive interference at suitable Bragg scattering angles [21] would be more useful for measuring weak magnetic signals in nanolaminates. ...
Article
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This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... In this paper, we investigate the magnetism in a highquality single-crystal Cr 2 GeC (0001) thin film sample, using element-specific, polarization-, and temperaturedependent magnetic circular dichroism (XMCD) [23]. By changing the circular polarization from right to lefthanded, and the direction of the applied magnetic field, the magnetic Cr moments are probed. ...
... We further believe that the relatively small spin magnitudes of the C atoms (C 1 and C 2 ) shown in Table II stabilize the ferromagnetic coupling of the Cr atoms within each layer, while the individual spins of the Ge atoms (Ge 1 and Ge 2 ) are essential for establishing a Ge-mediated super-exchange coupling between the vertically piled layers. This interpretation is consistent with findings in other two-dimensional materials e.g., metalcoordinated networks and ferrites [23,44]. Considering all possibilities of spin distortions modulated by a 3D spin-wave distribution, the unit cell is thus not a perfect anti-ferromagnet with a null total spin but, instead exhibits residual magnetic components both along the ab-basal plane and along the c-axis with a total magnetization of 0.02 and 0.05 µ B /cell, respectively. ...
... In this paper, we investigate the magnetism in a highquality single-crystal Cr 2 GeC (0001) thin film sample, using element-specific, polarization-, and temperaturedependent magnetic circular dichroism (XMCD) [23]. By changing the circular polarization from right to lefthanded, and the direction of the applied magnetic field, the magnetic Cr moments are probed. ...
... We further believe that the relatively small spin magnitudes of the C atoms (C 1 and C 2 ) shown in Table II stabilize the ferromagnetic coupling of the Cr atoms within each layer, while the individual spins of the Ge atoms (Ge 1 and Ge 2 ) are essential for establishing a Ge-mediated super-exchange coupling between the vertically piled layers. This interpretation is consistent with findings in other two-dimensional materials e.g., metalcoordinated networks and ferrites [23,44]. Considering all possibilities of spin distortions modulated by a 3D spin-wave distribution, the unit cell is thus not a perfect anti-ferromagnet with a null total spin but, instead exhibits residual magnetic components both along the ab-basal plane and along the c-axis with a total magnetization of 0.02 and 0.05 µ B /cell, respectively. ...
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The magnetism in the inherently nanolaminated ternary MAX-phase Cr$_{2}$GeC is investigated by element-selective, polarization and temperature-dependent, soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The measurements indicate an antiferro-magnetic Cr-Cr coupling along the $c$-axis of the hexagonal structure modulated by a ferromagnetic ordering in the nanolaminated $ab$-basal planes. The weak chromium magnetic moments are an order of magnitude stronger in the nanolaminated planes than along the vertical axis. Theoretically, a small but notable, non-spin-collinear component explains the existence of a non-perfect spin compensation along the $c$-axis. As shown in this work, this spin distortion generates an overall residual spin moment inside the unit cell resembling that of a ferri-magnet. Due to the different competing magnetic interactions, electron correlations and temperature effects both need to be considered to achieve a correct theoretical description of the Cr$_{2}$GeC magnetic properties.
... To achieve a good comparison between theory and experiment, calculated XAS/XES spectra within the one-electron approach often need to be fitted to experimental L 3 /L 2 or M 3 /M 2 and t 2g -e g branching ratios, as well as experimental spin-orbit splitting values and to an additional broadening for the L 2 emission lines due to the Coster-Kronig decay process. A possible solution to this kind of empirical procedure should be tested using multiplet theory [38,50] or through many-body perturbation theory by solving the Bethe-Salpeter equation (BSE) [51]. ...
... Hence, besides the existing controversial theoretical interpretations about which type of AFM coupling dominates (i.e., in-plane or out-of-plane), the evident disagreement between theory (AFM) and experiment (FM) in determining the lower energy magnetic ordering in Cr 2 GeC calls for the need of further experimental and theoretical efforts [72,155,17,116,156]. Experimentally, an alternative and more bulk-sensitive spectroscopic technique than XMCD [50], such as resonant magnetic X-ray scattering (XRMS) [157] utilizing constructive interference at suitable Bragg scattering angles [21] would be more useful for measuring weak magnetic signals in nanolaminates. ...
Article
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... Thus, RIXS-MCD has the potential to identify the d orbitals related to magnetism induced by DMI. Furthermore, RIXS-MCD provides a chance to extract information about the ground-state spin state [24][25][26][27] and to describe the mechanism of DMI. ...
Article
Fe L2,3-edge x-ray absorption spectra (XAS) and magnetic circular dichroism (MCD) in resonant inelastic x-ray scattering (RIXS) of α−Fe2O3 were measured to identify the electronic structure responsible for its weak ferromagnetism caused by the Dzyaloshinskii-Moriya interaction (DMI) at room temperature. In contrast to negligible MCD in XAS, MCD in RIXS (RIXS-MCD) was clearly observed in the dd excitation at 1.8 eV via excitation to charge-transfer states. Furthermore, RIXS-MCD showed a crystal orientation dependence, indicating that the observed RIXS-MCD originated from DMI. The observed RIXS-MCD is well described by ab initio charge-transfer multiplet calculations, revealing that the RIXS-MCD derives from spin flip excitations at delocalized eg orbitals. By the combination of the experiments and calculations, RIXS-MCD has unraveled that the origin of DMI in α−Fe2O3 is the eg orbitals, which are strongly hybridized with the 2p orbitals of oxygen atoms. The results demonstrate the importance of RIXS-MCD for identifying the electronic structure related to DMI.
... In RIXS at the Cu L-edge of the La 2−x Sr x CuO 4 family [18], of Bi 2 Sr 2 CaCu 2 O 8+ı [19] as well as of Sr 14 Cu 24 O 41 [20] dispersive magnetic modes have been observed at similar energies. Here we will attempt to describe the Cu L-RIXS within a ligand field multiplet model [21] that projects the local contributions. Fig. 3 shows a close up of the resonant L 3 spectra measured at 20 • incidence angle in comparison to calculated model spectra with 3d 9 (Cu 2+ ) final states in D 4h and C 4 symmetries. ...
Article
We present a study on the high-Tc superconductor (HTSC) YBa2Cu3O7-x (YBCO) using polarization-dependent X-ray absorption and resonant inelastic X-ray scattering. High-resolution measurements using synchrotron-radiation are compared with calculations using a quasi-atomic multiplet approach performed at the Cu 2p3/2-edge of YBCO. We use a multiplet approach within the single impurity Anderson model to reproduce and understand the character of the localized low-energy excitations in YBCO. We observe a distinct peak at about 0.5 eV in O K RIXS. This peak shows dependence on doping, incident energy, and momentum transfer that suggests that it has a different origin than the previously discussed cuprate bi-magnons. Therefore, we assign it to multi-magnon excitations between the Zhang Rice bands and/or the Upper Hubbard bands, respectively.
... The XAS spectra obtained for the Mn L edge and the Fe L edge correspond very well to those published in literature for bulk and thin film ferrites. 4,18,25,27,39,40 In order to obtain more detailed information, XMCD spectra have been measured at both edges. These data are also presented in Fig. 2 together with results of simulations, in particular, for the Fe L edge in Fig. 2(c). ...
Article
A comprehensive study on growth of ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) films on single crystalline strontium titanate(001) (SrTiO3) substrates was carried out. Under the optimized conditions, a thin film with a layer thickness of 200 nm was deposited, and the structural properties were investigated. Contrary to data published in literature, no buffer layer was necessary to achieve epitaxial growth of a poorly lattice-matched layer. This was confirmed for Mn0.5Zn0.5Fe2O4(001) on SrTiO3(001) by x-ray diffraction and the adjoined phi scans, which also revealed a lattice compression of 1.2% of the manganese zinc ferrite film in the out-of-plane direction. Using x-ray photoelectron spectroscopy, the near surface stoichiometry of the film could be shown to agree with the intended one within the uncertainty of the method. X-ray absorption spectroscopy showed an electronic structure close to that published for bulk samples. Additional x-ray magnetic circular dichroism investigations were performed to answer detailed structural questions by a comparison of experimental data with the calculated ones. The calculations took into account ion sites (tetrahedral vs. octahedral coordination) as well as the charge of Fe ions (Fe2þ vs. Fe3þ). Contrary to the expectation for a perfect normal spinel that only Fe3p ions are present in octahedral sites, hints regarding the presence of additional Fe2p in octahedral sites as well as Fe3þ ions in tetrahedral sites have been obtained. Altogether, the layer could be shown to be mostly in a normal spinel configuration.
... Spin-resolved valence band excitations were also studied using magnetic circular dichroism and magnetic linear dichroism Yablonskikh et al., 2001;Miyawaki et al., 2017;Zimmermann et al., 2018;Elnaggar et al., 2020) in RIXS with an external magnetic field for magnetization of the sample (Magnuson et al., 2006;Elnaggar et al., 2019;Umetsu et al., 2019). ...
Article
An overview of both experimental and theoretical results in the field of resonant scattering of tunable soft and hard x-ray radiation is presented, with a main focus on the closely related processes of resonant inelastic x-ray scattering (RIXS) and resonant Auger scattering (RAS). The review starts with an overview of fundamental dynamical aspects of RIXS illustrated for different systems. A detailed analysis of case studies with increasing complexity, considering both gas-phase and condensed matter (liquids and solids) applications, is given. In the review, the most important achievements in investigations of coupled electron-nuclear dynamics and structural aspects in studies of liquids and solids over the last two decades are outlined. To give a perspective on the insights from RIXS and RAS, the x-ray results are discussed against the background of complementary experimental techniques like vibrational infrared absorption and Raman spectroscopy, as well as small-angle x-ray and neutron scattering. Finally, recent achievements in time-resolved studies based on x-ray free-electron lasers are described.
... Note that the M 3 /M 2 branching ratio is lower (0.8:1) for measurements with polarization along the c-axis than in the basal ab-plane (1:1). The trend in XES branching ratios in the transition-metal compounds is a signature of the degree of ionicity in the systems [40], due to the additional Coster-Kronig process [24,41]. The lower 3p 3/2 /3p 1/2 peak ratio along the c-axis is thus an indication of higher ionicity (resistivity) than in the basal ab-plane. ...
Article
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
The electronic structure of nanolaminate Ti2AlC and Ti2AlN thin films, so-called MAX-phases, were investigated by soft X-ray emission spectroscopy. These nanolaminated carbide and nitride compounds represent a class of layered materials with a combination of properties from both metals and ceramics. The bulk-sensitive soft X-ray emission technique is particularly useful for detecting detailed electronic structure information about internal monolayers and interfaces. The Ti-Al bonding is manifested by a pronounced peak in the Ti L-emission of Ti2AlC and Ti2AlN that is not present in the binary TiC system. The spectral shape of Al L-emission in the MAX-phase is strongly modified in comparison to metallic Al. By replacing or partly exchanging C with N, a change of the electron population can be achieved causing a change of covalent bonding between the laminated layers, which enables control of the material properties.
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The microstructure, electronic structure and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrC(x) in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at.% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding.
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The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to those of ScN and Sc metal, have been investigated by bulk-sensitive soft-x-ray emission spectroscopy. The measured Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated spectra using first-principles density-functional theory including dipole transition-matrix elements. The main Sc 3d–N 2p and Sc 3d–Al 3p chemical bond regions are identified at −4 and −1.4 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states in the Al L2,3 emission from Sc3AlN in comparison to that for pure Al metal is found, which reflects the Sc 3d–Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structures of Sc3AlN, ScN, and Sc metal are discussed in relation to the change in the conductivity and elastic properties.
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We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O K resonance of NiO showing a rich excitation spectrum. We perform multisite Ni6O19 cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4-5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also dd excitations at 1 eV are observed. Finally, we point out that O K RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy delta and the Hund coupling energy J(H).
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The resonant inelastic x-ray scattering (RIXS) cross section at the L and M edges of transition-metal compounds is studied using an effective scattering operator. The intensities of the elastic peak and for spin-flip processes are derived. It is shown how the polarization dependence can be used to select transitions. Comparisons are made with experiment. A detailed analysis of the polarization and angular dependence of L- and M-edge RIXS for divalent copper compounds, such as the high-Tc superconductors, is given.
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Total and partial atomic-level widths of the K-, L-, M-, and N-levels of the elements (covering Z up to 120 for K- and L-levels) are presented in graphic form. Graphs are based on theoretical predictions; total widths are natural level widths as governed by the rates of the various deexcitation processes for a single hole in that level. The drawings make possible, throughout the periodic table, a quick survey of the magnitude and trends of the total level widths, rates and yields of radiative, Auger, and Coster-Kronig processes,and widths of x-ray, Auger, and Coster-Kronig lines.
Article
The disordered ferrite system, Mn1-xZnxFe2O4 for 0≤x≤0.95 has been studied by the Mössbauer-effect technique at 4.2 K in applied magnetic fields of 50 and 90 kOe. In these fields a ferrimagnetic structure is implied for the entire family of compounds. For x=0 the cation distribution is (Mn0.82Fe0.18)[Mn0.18Fe1.82]O4; for larger x the iron concentration on A (tetrahedral) sites decreases, and becomes less than 2 at.% by x=0.6. For x≥0.6, some of the Fe ions on B (octahedral) sites have their spin moments reversed. Within the experimental error, the magnetic moments of the iron ions are collinear for x<0.5 and noncollinear for x>0.5. The canting angles for normal B-site cations are found to increase with increasing zinc content, and to depend on the applied field. The sample with x=0.80 has the most stable structure. The hyperfine fields and isomer shifts show only a small variation with sample composition.
Article
We present experimental and theoretical results on CoO resonant Raman scattering (RRS) with excitation at many values in the whole Co2+-L2,3 region, and with a final state containing a Co-3s hole. The theory includes the final-state interaction between the configurations with one 3s hole and that with two 3p holes [configuration interaction (CI)]. A cluster calculation with CI reproduces the experimental results with excitation spanning the whole L3 region. In the L2 region the theory reproduces the evolution of the RRS doublet at higher outgoing energy. On this basis the role of CI and of the ligand hole effects are discussed vs the excitation energy. At least 5 eV above the L3 threshold, the experimental results show also a nondispersive component, with the intensity rapidly increasing with the excitation energy. The position and shape of this component are well reproduced by a model calculation accounting for all possible partitions between the transitions to the continuum and to the 3d shell. However, the intensity of the calculated nondispersive component is about an order of magnitude smaller than the measured one. The implications of this fact in terms of the dynamics of the intermediate state are outlined.
Article
We present a theory of radiative resonant x-ray Raman scattering by solids and polymers beyond the Born-Oppenheimer approximation. The role of electron-phonon coupling of the intermediate core excited and final electronic states is analyzed in detail for π-electron systems and solids assuming the frozen-orbital approximation. The momentum exchange processes between valence electrons and phonons and with x-ray photons are analyzed, showing the similarities and differences between the two processes. The resonant inelastic x-ray scattering by a π-electron system is quenched up to zero in the Born-Oppenheimer approximation with frozen orbitals if the momentum exchange between valence electrons and x-ray photons is neglected. The electron-phonon and electron-photon interactions open the scattering channels for all occupied states in π systems. The frequency dependence of these effects is analyzed, showing that when the duration of the scattering is shortened by a large detuning of the excitation frequency, the role of electron-phonon coupling of both core-excited and final states is suppressed, depleting the cross section for π systems up to zero. The detuning quenches the symmetry breaking of the core-excited electronic states and results in a restoration of the selection rules and a conservation of electron momentum. Specific selection rules for the zero-phonon line in x-ray Raman spectra of linear polyenes are found. A detailed investigation of the narrowing, or collapse, of the electron-vibrational bands is given. When the detuning is large, the spectral profile is described by a joint density of states. It is predicted that the singularities of this joint density of states follows the Raman-Stokes dispersion law, something that allows a mapping of the band structure. We found that the phonon broadening of these singularities is completely quenched by detuning. A detailed investigation of the spectral shape versus detuning of the so-called excitonic band is given.
Article
Both the interpretation of atomic spectra and the application of atomic spectroscopy to current problems in astrophysics, laser physics, and thermonuclear plasmas require a thorough knowledge of the Slater-Condon theory of atomic structure and spectra. This book gathers together aspects of the theory that are widely scattered in the literature and augments them to produce a coherent set of closed-form equations suitable both for computer calculations on cases of arbitrary complexity and for hand calculations for very simple cases.
Article
Spin-polarized photoemission has developed into a versatile tool for the study of surface and thin film magnetism. In this review, we examine the methodology of the technique and its application to a number of different problems, including both valence band and core level studies. After a detailed review of spin-polarization measurement techniques and the related experimental requirements we consider in detail studies of the bulk properties both above and below the Curie temperature. This section also includes a discussion of observations relating to unique metastable phases obtained via epitaxial growth. The application of the technique to the study of surfaces, both clean and adsorbate covered, is reviewed. The report then examines, in detail, studies of the spin-polarized electronic structure of thin films and the related interfacial magnetism. Finally, observations of spin-polarized quantum well states in non-magnetic thin films are discussed with particular reference to their mediation of the oscillatory exchange coupling in related magnetic multilayers.
Article
Wenn ein Atom äußerer monochromatischer Strahlung von der Frequenz ν ausgesetzt ist, sendet es nicht nur sekundäre monochromatische Kugelwellen von der Frequenz ν aus, die mit der einfallenden Strahlung kohärent sind, sondern das Korrespondenzprinzip verlangt, daß im allgemeinen auch noch Kugelwellen von anderen Frequenzen ausgesandt werden. Diese Frequenzen sind alle von der Form ∣ν±ν*∣, wo hν* den Energieunterschied des Atoms im betrachteten und in irgend einem anderen Zustand bezeichnet. Die nicht kohärente Streustrahlung entspricht zum Teil gewissen Prozessen, die kürzlich von Smekal ins Auge gefaßt wurden anläßlich Betrachtungen, die an die Vorstellung von Lichtquanten anknüpfen. In der Abhandlung wird gezeigt, wie sich eine wellentheoretische Analyse der streuenden Wirkung des Atoms an der Hand des Korrespondenzprinzips m ungezwungener und scheinbar eindeutiger Weise durchführen läßt. Die Ausführungen bauen durchaus weiter auf der Auffassung der Verbindung der Wellenstrahlung des Atoms mit den stationären Zuständen, die in einer neuen Arbeit von Bohr, Kramers und Slater vertreten ist, und die Folgerungen, wenn sie sich bestätigen sollten, dürften eine interessante Stütze für diese Auffassung bilden.
Article
We have fabricated and studied the structure and magnetic properties of high quality single crystalline (Mn,Zn)Fe 2 O 4 , NiFe 2 O 4 , and CoFe 2 O 4 films. Although (Mn,Zn)Fe 2 O 4 and NiFe 2 O 4 films grown directly on SrTiO 3 and MgAl 2 O 4 show mediocre structural and magnetic properties, these same films grown on SrTiO 3 and MgAl 2 O 4 buffered with CoCr 2 O 4 or NiMn 2 O 4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the underlying layer. X ray, Rutherford backscattering spectroscopy, atomic force microscopy, and transmission electron microscopy analysis provide a consistent picture of the structural properties of these ferrite films. © 1996 American Institute of Physics.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
We report on instrumentation developments concerning a subset of ESRF beamlines: BL6 & 26 (linear, circular dichroism); BL8 (energy dispersive XAFS); BL18 (multipurpose energy scanned XAFS studies). We review in more detail the instrumentation of BL6 & 26 which are now under commissioning. A wave plate was successfully used at 2.8 keV in the direct undulator beam. High-quality CMXD spectra have already been recorded in the fluorescence detection mode.
Article
I present spin-resolved valence band studies of magnetic 3d-transition metal materials using magnetic circular dichroism in soft X-ray emission (SXEMCD). The non-resonantly excited SXEMCD of Fe, Co, and Ni, as well as several compounds with ferromagnetically aligned Mn atoms are compared. A simple model picture accounts for the basic principles involved. Quantitative estimates show that it is likely that substantial valence-band relaxation due to the intermediate-state core hole takes place in itinerant systems. A z+1 approximation shows that the magnitude of SXEMCD is proportional to the magnetic moment of such a core hole impurity. Magnetic circular dichroism in resonant inelastic X-ray scattering (RIXS) is discussed. For strongly localized 3d-electrons as found in some Mn compounds RIXS probes dd-excitations and the corresponding MCD. For more itinerant systems, such as iron, it is argued that RIXS leads to the observation of a convolution of the occupied and (parts of ) the unoccupied 3d-bands. Finally, an overview of possible future developments in helicity-resolved SXES is given.
Article
We demonstrate that soft X-ray emission excited at the L3 threshold of Fe metal maps the spin-resolved local partial density of occupied states. In contrast to earlier predictions, the spin resolution is achieved primarily by selective excitation. Circular magnetic dichroism in the X-ray emission process is also shown to have an influence on the final state spin contribution to the spectra. The importance of taking reflection, self-absorption of the incoming and outgoing radiation and the Auger decay channels into consideration in the data analysis is emphasized.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
The magnetic linear dichroism (MLDAD) in Fe 2p photoemission spectra of an epitaxial ultrathin iron film has been determined. The experiment reveals multiplet related spectra, which allow a detailed characterization of the photoemission process in a simple final state model, that emphasizes the core-valence interaction in Fe 2p photoemission. The same model was used to describe the Cr 2p photoemission spectrum of Cr adsorbates on a Fe surface. The importance of the investigations for the discussion of the 2p photoelectron spectra of 3d metals is pointed out.
Article
High-resolution x-ray fluorescence spectroscopy has been performed on magnetically oriented polycrystalline iron samples. Circularly polarized synchrotron radiation was used to excite the Fe L2,3 x-ray emission, either directly from a bending magnet (white radiation), or from an asymmetric wiggler followed by a monochromator tuned to the L2 edge. Large differences in the magnetic circular dichroism (MCD) were observed. Spin-polarized relativistic linear muffin-tin orbital calculations and a simple qualitative model demonstrate that the L3 x-ray emission dichroism is partially canceled by the L2-L3M4,5 Coster-Kronig process. The Coster-Kronig process is large enough to make MCD x-ray absorption experiments, performed in the fluorescence mode, unreliable.
Article
We measured the Mn {ital L}{sub {alpha},{beta}} x-ray fluorescence spectra of MnO excited by selected photon energies near the {ital L}{sub 2,3} absorption edges. The resulting resonant inelastic x-ray scattering spectra probe low-lying electronic excited states, due to {ital dd} and charge-transfer excitations. Using a two-step model and a purely atomic approximation, we reproduce both energies and varying intensities of {ital dd} excitations relative to the electronic recombination peak. Our results show that strongly varying line shapes in resonant x-ray emission need not be due to channel interference effects. {copyright} {ital 1996 The American Physical Society.}
Article
Unoccupied energy bands of iron are mapped by inverse photoemission from Fe(100), Fe(110), and Fe(111). The ferromagnetic exchange splitting deltaEex of the uppermost d band is measured for the H'25 point, where the minority- and majority-spin subbands are both empty (deltaEex=1.8 eV with H'25↓ at 1.9 eV and H'25↑ at 0.12 eV above EF). Several other critical points are determined, such as the minority-spin Gamma12 and P3 points, the majority-spin N3 point, and the higher-lying H15,H1 points of s,p character. Critical points and exchange splitting are compared with first-principles, local-density calculations. The real part of the self-energy is obtained from this comparison, and the imaginary part by measuring the liftime broadening. In the d-band region, the self-energy causes a 10% compression of the bands and a linear broadening Gamma(E)~0.6\|E-EF\|.
Article
We report experiments demonstrating circular dichroism in the x-ray-fluorescence spectra of magnetic systems, as predicted by a recent theory. The data, on the L2,3 edges of ferromagnetic iron, are compared with fully relativistic local spin density functional calculations, and the relationship between the dichroic spectra and the spin-resolved local density of occupied states is discussed.
Article
We point out that dichroism in x-ray-emission spectra can be studied in fluorescence experiments without resolving the chirality of the emitted radiation, provided the incident photons are circularly polarized. We argue that such experiments would be unique probes of metallic magnetism. The effect is demonstrated in iron using a first-principles calculation based on a fully relativistic multiple-scattering theory.
Article
We show that the 2p core-level line shapes of transition metal (TM) compounds are strongly influenced by the presence of more TM ions. Using calculations of clusters, involving more than one TM ion we explain the double peaked main line of the Ni 2p spectrum of NiO as well as the difference in line shape compared with Ni impurities in MgO and show the sensitivity of the line shape in NiO to surrounding defects. This intersite charge transfer screening also explains the wide asymmetric shape of the main line of the Cu 2p spectrum of CuO and the high-Tc compounds.
Article
Spin-resolved studies of Ni core-level photoemission are reported. The results indicate that the final-state Ni 3 d electron configuration in not purely 3d10, in contradiction with early theories emphasizing complete 3d-electron core-hole screening. A model is described in which extra-atomic screening involving unoccupied pure 3d states and unpolarized and hybridized 3d- 4sp states leads qualitatively to a final-state 3d valence configuration of ~50% 3d9 and ~50% 3d10. The relationship of these results to more recent theory employing the Anderson impurity model is discussed.
Article
A new class of undulator capable of producing linear and/or helical polarization is described. The magnetic field, power, spectral flux, brilliance and interactions with the electron beam of such undulators are discussed. The case of Helios, an undulator presently installed on the ESRF, is discussed in detail.
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