Polarized X-ray Absorption Spectra Research Articles

Robust ferromagnetism in a cubic perovskite oxide with Curie temperature above 600 K

Robust ferromagnetism with high Curie temperatures (TC), especially above room temperature, in perovskite oxides is extremely rare because the oxygen-mediated antiferromagnetic superexchange generally dominates in these nearly cubic perovskite lattices. Fe4+ ions show potential to promote magnetism responses due to more apparent hybridization between Fe4+ and oxygen orbitals with Jahn-Teller distortion, while this distortion also sacrifices the stability of Fe4+-based oxides. Here, by engineering oxygen vacancies, we report robust ferromagnetism with TC above 600 K in cubic perovskite BaFeO3-δ films epitaxially grown on KTaO3 substrates. A mixture of Fe4+ and Fe3+ cations, mediated by the oxygen vacancies, was identified to be responsible for the robust magnetism as revealed by atomic-scale studies and element-selective circularly polarized X-ray absorption spectra. This study indicates that spin-state ordering could be manipulated in a simple perovskite oxide by engineering oxygen stoichiometry and would be of significance toward future technological applications.

Polarization analysis by means of individual soft x-ray absorption spectra of rare earths

X-ray magnetic circular dichroism (XMCD), which by virtue of the sum rules provides element-specific spin and orbital moments, is obtained from the difference between two polarized spectra by reversing the direction of either the light helicity or the applied magnetic field. Usually, it is tacitly assumed that these two spectra are obtained using the same absolute degree of light and magnetic polarization. This is, however, not always possible and depends on circumstances that can be beyond control. First, we recapitulate the conventional XMCD sum rule method to obtain the values of the moments and emphasize some of the complications in the case of the rare-earth ${M}_{4,5}$ edges, such as the presence of strong core-hole $jj$ overlap, linear dichroism, and magnetic dipole term $\ensuremath{\langle}{T}_{z}\ensuremath{\rangle}$. Instead, we propose an alternative method. Using the individual polarized x-ray absorption spectra obtained at the Ho and Dy ${M}_{5}$ edges, where each of the $\mathrm{\ensuremath{\Delta}}J=\ensuremath{-}1,0$, and $+1$ transitions are separated by $\ensuremath{\sim}2$ eV in photon energy, we are able to determine independently the degree of circular dichroism in a single spectrum. Since light is a transverse wave, we need to include, apart from the circular dichroism, also a linear dichroism contribution in order to fit the circularly polarized spectra. In the measurements on paramagnetic rare-earth dopants it was found that reversing the field produces the same degree of circular dichroism, while reversing the helicity yields a $\ensuremath{\sim}$20% difference in the degree of circular dichroism.

High thermal stability of anti-ferromagnetic coupled molecules with FeCo layers

We propose the optimization of the magnetic remanence and the thermal stability of Mn phthalocyanine coupled with a ferromagnetic substrate, by exploiting interlayer exchange coupling within an advanced organic spin interface architecture, constituted by a FeCo film covered by a graphene membrane, hosting the MnPc molecular layer. The challenge to obtain magnetic remanence for molecular systems stable up to room temperature has been accomplished thanks to a super-exchange path, mediated by the π orbital of the organic ligands of the molecule and of the graphene sheet, favoring an antiferromagnetic (AFM) alignment for the MnPc molecules with the FeCo film. This spin interface with a strong AFM coupling mediated by a graphene spacer is optimized against thermal fluctuations, presenting a well defined remanence even at room temperature, as demonstrated by the persistent dichroic signal in temperature-dependent circularly polarized x-ray absorption spectra.

Non-empirical calculation of X-ray magnetic circular dichroism in lanthanide compounds.

Affordable calculations of X-ray magnetic circular dichroism and X-ray linear dichroism spectra of lanthanide ions purely based on structural input are difficult to achieve. Here we report on the successful application of ligand-field density-functional theory to obtain an exquisite reproduction of experimental spectra. As a testbed we use TbPc2 single-molecule magnets on a flat substrate.

On the Calculation of an X-Ray Natural Circular Dichroism Signal

The sequential derivation of an expression that describes an X-ray natural circular dichroism (XNCD) signal in the most general case is performed. The obtained expression is used as a basis to consider XNCD signals and their structural origins in copper metaborate CuB2O4 crystals at an incident radiation energy near the K absorption edge of copper and lanthanum–gallium silicate La3Ga5SiO14 crystals at an incident radiation energy near the K absorption edge of gallium. It has been demonstrated that the measurements of circularly polarized X-ray absorption spectra in combination with corresponding first-principle calculations provide an efficient method for studying the contributions of crystallographically non-equivalent atoms to the X-ray optical activity of crystals.

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spectroscopy

We present here an x-ray absorption study of the $R$Fe${}_{2}$O${}_{4}$ series (LuFe${}_{2}$O${}_{4}$, YFe${}_{2}$O${}_{4}$, YbFe${}_{2}$O${}_{4}$, and LuCoFeO${}_{4}$) at the Fe $K$-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal $c$ axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe${}^{2+}$ and Fe${}^{3+}$ does not fit to the experimental XANES of either LuFe${}_{2}$O${}_{4}$, YFe${}_{2}$O${}_{4}$, or YbFe${}_{2}$O${}_{4}$, discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 \ifmmode\pm\else\textpm\fi{} 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions.

Polarization-dependent soft X-ray absorption of over-doped superconducting Sr2CuO3 + δ single crystal

In order to study the factors affecting superconductivity of Sr2CuO3+δ, polarized X-ray absorption spectra (XAS) were measured at Cu L23 edge and O K edge of an over-doped single crystal specimen. This polarized XAS study reveals a distinctive anisotropic local electronic structure of Cu coordination: the unoccupied states are dominated by the in-plane components of Cu 3dx2−y2 and O 2px,y. However, the signal from the unoccupied out-of-plane O 2pz and Cu 3dz2 states also appear. Furthermore, the detailed analysis of Cu L3-edge XAS at 300K and 17.3K indicates that the conductive carrier partly shifts from out-of-plane to in-plane in the superconducting state. Theoretical calculation of Cu L23-edge core level spectra based on Charge Transfer Multiplet theory is fairly consistent with above experimental results.

Intra- and intersite electronic excitations in multiferroicTbMnO3probed by resonant inelastic x-ray scattering

$\text{Mn}\text{ }3d$ valence states and elementary electronic excitations in single-crystalline ${\text{TbMnO}}_{3}$ were probed with resonant inelastic x-ray scattering (RIXS) (or resonant x-ray emission spectroscopy, RXES), polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized $\text{Mn}\text{ }K$-edge x-ray absorption spectra of ${\text{TbMnO}}_{3}$ crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The $1s3p$-RXES spectra obtained at the $\text{Mn}\text{ }K$ edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of $1s\ensuremath{\rightarrow}3d$ transitions, indicating a relatively delocalized character of unoccupied $\text{Mn}\text{ }3d$ states. The additional $\text{K}\ensuremath{\beta}$ emission profile at energy loss $\ensuremath{\sim}62\text{ }\text{eV}$ is attributed to the off-site dipole $\text{Mn}\text{ }1s{\text{-Mn}}^{\ensuremath{'}}\text{ }3d$ and/or $\text{Mn}\text{ }1s\text{-Tb}\text{ }5d$ transitions, originating from hybridization between $\text{Mn}\text{ }4p$ states and neighboring ${\text{Mn}}^{\ensuremath{'}}\text{ }3d/\text{Tb}\text{ }5d$ states. The off-site dipole transition makes a considerable contribution to the pre-edge region of $\text{Mn}\text{ }K$-edge spectrum. Three prominent RIXS features at $\ensuremath{\sim}2.9$, 7, and 11 eV were observed. Based on $\text{GGA}+U$ calculations, the 7 eV region corresponds to transitions from $\text{O}\text{ }2p$ states to unoccupied minority $\text{Mn}\text{ }3d$ states, whereas the 11 eV band is ascribed to transitions from the $\text{O}\text{ }2p$ band to the empty $\text{Tb}\text{ }5d$ band. The broad 2.9 eV band is attributed to the coexistence of on-site $\text{Mn}\text{ }3d\text{-Mn}\text{ }3d$ and off-site $\text{Mn}\text{ }3d{\text{-Mn}}^{\ensuremath{'}}\text{ }3d$ transitions.

Polarized x-ray absorption spectra ofLa1−xSr1+xMnO4: Electronic state of Mn atoms

We have measured polarized x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectra at the Mn $K$ edge on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ samples with $x=0$, 0.3, and 0.5 at different temperatures from $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Both fluorescence and total electron yield modes were used for polarized measurements, whereas nonpolarized absorption spectra have also been recorded in transmission mode on powder samples for $x=0$, 0.3, 0.45, 0.5, and 0.55. The experiment discards the presence of distinct ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ integer valence states for any of the mixed-valence samples $(x=0.3,0.5)$ in the whole temperature range. The local structure (geometrical and electronic) around the Mn atom is anisotropic in all samples. Moreover, there is no significant change in the Mn local geometric and electronic structure of compounds with $0.45\ensuremath{\leqslant}x\ensuremath{\leqslant}0.55$ crossing the charge-ordering transition $({T}_{\mathrm{C}\mathrm{O}}=230\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The anisotropy in the absorption spectra is then explained in terms of the tetragonal distortion of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedron. It decreases as $x$ increases, the oxygen environment being nearly isotropic for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$. Anisotropy is also observed at the prepeaks. The intensity of the in-plane structures strongly increases with the Sr content, indicating that the doped holes mainly go into the ab plane. We conclude that the electronic state of Mn atoms can be only described in terms of an intermediate valence, even for anisotropic samples such as two-dimensional $\mathrm{La}\ensuremath{-}\mathrm{Sr}$ manganites.

Covalency in the uranyl ion: A polarized x-ray spectroscopic study

Polarized oxygen Kα x-ray absorption and emission spectra, near 530 eV, of a single crystal of Cs2UO2Cl4 are reported. With the aid of density functional theory calculations and the absorption data, the relative energies of the empty molecular orbitals having primarily uranium 5f and 6d character are assigned. The emission spectra give access to the energy of excitations to these orbitals from the various filled valence-shell orbitals. These energies support the conclusion from the optical spectra that valence excitations from the σu occupied valence orbitals occur at substantially lower energies than those from the σg, πg, and πu orbitals. This latter group of molecular orbitals have much larger oxygen-2p character. The participation of the pseudocore 6p shell in the covalent bonding is established directly by the presence of a charge-transfer transition in emission. With the aid of previous work on the polarized uranium L1- and L3-edge absorption spectra, the order of the empty metal-centered antibonding valence orbitals is established as 5fπ<5fσ∼6dδ∼6dσ<6dπ. The greater antibonding character of 6dπ compared with 6dσ can be explained by the relatively small overlap of the latter orbital with oxygen 2p. However, the relative energies of 5fπ and 5fσ cannot be due to differences in overlap and are attributed instead to a strong hybridization between 5fσ and 6pσ. This superposition is confirmed by the linear dichroism in the U L1-edge absorption spectrum.

Polarized X-ray absorption spectroscopy of the low-temperature photoproduct of carbonmonoxy-myoglobin

Visible light can break the Fe—CO bond in Fe(II) carbonmonoxy-myoglobin (MbCO) giving an unligated product (Mb*) that is almost stable at T < 30 K. Fe K-edge polarized X-ray absorption spectra (P-XAS) of the photoproduct (T = 20 K) of an oriented single crystal (0.2 × 0.2 × 0.3 mm) of sperm whale MbCO (space group P21) have been collected. By rotating the crystal the X-ray photon polarization vector has been oriented almost parallel (with an angle α = 23°) or perpendicular (α = 86°) to the heme normal of each myoglobin molecule. The crystal was continuously illuminated by a white-light source during the data collection. The polarized data give novel information on the Fe-heme electronic/structural rearrangement following photolysis. The XANES (X-ray absorption near-edge structure) spectrum polarized in the direction close to the Fe—CO bond changes dramatically after photolysis, exhibiting a shift of ∼2 eV, due to electronic relaxation of empty states of pz symmetry, while more subtle changes are observed in the spectrum polarized along the heme plane, sensitive to the heme-plane geometry. Changes in the pre-edge region can be interpreted to provide insight into the electronic structure of the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) in the MbCO → Mb* photochemical reaction at low temperature.

Pre-edge structure analysis of Ti K-edge polarized X-ray absorption spectra in TiO2 by full-potential XANES calculations.

The pre-edge region of Ti N-edge polarized XANES spectra in TiO2-mtile is investigated by full-potential calculations based on the finite-difference method. Both dipolar and quadrupolar transitions are considered. The use ofnon muffin-tin potential allows a clear interpretation of the pre-edge features. The results are consistent with Full-potential LAPW band structure calculations, and are also compared with multiple-scattering calculations.

Polarized x-ray absorption spectroscopy study of the symmetry of unoccupied electronic states near the Fermi level in the system

A study of the electronic states near the Fermi level in high- superconductors, which is so important to understanding the mechanism of superconductivity, has been carried out on well characterized single crystals of superconducting . A large number of polarized x-ray absorption spectra have been recorded with the E vector parallel to the a - b plane and perpendicular to it at the Dragon beam line at the Brookhaven Light Source using the fluorescence mode of detection. These bulk spectra of and O K are then analysed to obtain the density of the unoccupied and orbitals from the spectra and the density of the and orbitals from the O K spectra. The results clearly show the presence of orbitals in a substantial amount although the same cannot be said of the orbitals. Tight-binding calculations of the holes are also reported and compared with experimental results.

Polarized X-ray absorption spectra and electronic structure of molybdenite (2H-MoS2)

Polarized S K- and L-edge, Mo L3- and L2-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS2) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4dz 2, and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t 2g − /t 2g + and e g − /e g + sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4dx 2 — y2 and 4dxy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S pz- and px,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 px,y-like state is lower than that of the S 3pz state. Polarized S K-edge XANES also reveal the features of antibonding S pz- and px,y-like states in molybdenite. The feature assigned to the S 3 pz-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 px,y-like states.

Angular-dependence analysis of polarized X-ray absorption spectra

The linearly polarized Cu K and Fe K pre-edge absorption spectra of single-crystal Bi 2Sr 2CaCu 2O 8 and FeCO 3 are investigated in the transmission geometry. Using the orientation-dependent selection rules for 1s → 3d quadrupole transitions, the angular dependence of the spectra are analyzed and unoccupied m-resolved densities of 3d-like states are determined.

X-ray absorption spectroscopy of oriented cytochrome oxidase

The polarized X-ray absorption spectra of the copper, iron and zinc sites of mitochondrial cytochrome oxidase in oriented membrane multilayers have been examined. The copper X-ray absorption edge spectra indicate the presence of a tetragonal copper, which we assign as Cu B, oriented with the long axis approximately orthogonal to the membrane normal. We have also detected the presence of a relatively long (2.6 Å) Cu-S or Cu-Cl interaction, which we assign to a copper-thioether (probably Met 210) coordination at the Cu A site, with the bond oriented along the membrane normal. The coordination of the zinc, the iron and the Cu B heme a 3 binuclear site are discussed.

Electronic structure of La 2CuO 4 by joint analysis of O K and Cu K and L 3 edge X-ray absorption spectra

The symmetry of the conduction bands over a range of 15 eV above the Fermi level in La 2CuO 4 has been studied by the joint analysis of the polarized X-ray absorption spectra at several edges: O K-edge, Cu K-edge, and Cu L 3-edge. The partial density of states of the conduction bands D X ( l, m l ) projected on the atomic site X (Cu and O) with selected orbital angular momentum l, and its projection m l along the c-axis have been obtained. The oxygen K-edge X-ray absorption near edge structure (XANES) spectrum has been calculated by multiple scattering approach. A good agreement between the one-electron calculations in the fully relaxed final state potential and the experimental spectra for the final states in the high energy conduction bands has been found.

Symmetry of the itinerant 3d9 ligand hole induced by doping in high Tc superconductors by polarized Cu L3 XAS

An extensive study of electronic structure of YBa2Cu3O∼7vhas been carried out by means of X-ray spectroscopy. Polarized Cu L3 X-ray absorption spectra of single crystals have been measured and compared with the analogous spectra of polycrystalline samples with different oxygen content. From this analysis we have found that the electronic holes L∗ are associated with the d hole in the Cu 3d3z2-r2 orbital in the 3d9L∗ configuration.

Polarized X-ray absorption spectra of halogen-doped polyacetylene

The local structure and chemical states of halogens doped in stretch-oriented trans -polyacetylene are studied by polarized X-ray absorption spectroscopy. Polarized EXAFS and near-edge spectra are measured for (CHBr y) x and (CHI y) x on the Br K- and I L-edges over a wide range of dopant concentration. The Fourier transform of EXAFS oscillations for (CHBr y) x indicate that in the lightly doped region (0.01<y<0.2), polybromine ions are formed which are highly oriented along the c-axis while bromines also form covalent bonds with (CH) x which increases on going to heavily doped region (y>0.2). It is expected that bromines are incorporated as isolated ions in the early stage of doping (y<0.01). The Fourier transform analysis (CHI y) x indicate that iodines form also polyions which are chemically less reactive.

Polarized X-ray absorption spectra of Langmuir-Blodgett films: Local structure studies of merocyanine dyes

Polarized X-ray absorption spectroscopy has been applied to study the local structure and bonding states in Langmuir-Blodgett films of a merocyanine dye diluted with arachidic acid, using synchroton radiation and a fluorescence detection technique. Both in-plane and out-of-plane anisotropies, which decreased after annealing, were observed in the X-ray absorption near-edge structure spectra with respect to the electrical field vector. These results imply that (1) the chromophores are highly oriented with respect to the axis parallel to the dipping direction and plane and (2) these preferential orientations degrade when heat treated. The radial distribution around a chalcogen atom, obtained from the Fourier transform of the extended X-ray absorption fine structure, is interpreted in terms of the intramolecular structure of merocyanine.