Search Results (27 total)

We have measured core-level x-ray-absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Gd-L_2,3 edges to characterize the low-lying Gd-5d derived conduction-band states in thin films of the 4f ferromagnet GdN with a unit-cell volume 8.6% above that of bulklike layers. The nonequilibrium structure is obtained by N⁺ plasma-assisted reactive sputter deposition at room temperature. The Curie temperature T_C, a key quantity for magnetism, amounts to only half the bulk value of ∼60 K indicating a significant reduction of the effective exchange interaction between the 4f states. An intricate observation is that the ratio of the dichroic signal amplitudes in the lattice-expanded layers, ∣L₃∕L₂∣, is up to three times higher than the value expected from the degeneracy of the 2p_3∕2 and 2p_1∕2 core states, which is observed for the bulklike layers. This is mainly due to a reduced L₂ XMCD amplitude. We suggest that the effect may be related to the different weight the crystal-field split Gd-5d final states (t_2g and e_g) have in the absorption process at the L₂ and L₃ edges, and to the special electronic band structure of this strongly correlated material and its modification upon lattice expansion. This hypothesis is supported by the observation that the L₂ absorption edge is shifted to lower energies upon ferromagnetic ordering while the L₃-edge position remains inert.

X-ray magnetic circular dichroism (XMCD) measurements have been performed at the L_II,III edges of Er in the intermetallic compounds ErMn₂, ErFe₂, ErCo₂, ErNi₂, and ErAl₂, as well as in the ionic compound Er₂(SO₄)₃,8H₂0, in order to study the evolution of the XMCD spectra as a function of the 5d states filling and∕or their hybridization with magnetic or nonmagnetic electronic states. Almost all the spectra present the same general features, confirming that the 4f-5d intra-atomic coupling is a key point to explain XMCD spectra. Thus, we analyze the models based on this interaction, proposed to account for the unexpected sign of E1-XMCD measurements, as well as the nonstatistical branching ratio between XMCD at the L_II and L_III edges. We underline the impossibilities of reproducing our measurements using these models. Moreover, we point out the special role played by Fe which leads, at the L_II edge of Er, to a significant modification of the shape of XMCD spectrum and an unexpected temperature dependence. The XMCD revealed an unexpected behavior of the 5d magnetic polarization in these compounds which is not visible in macroscopic measurements. This also demonstrates the non-negligible role played by the 5d-3d hybridization.

The transition of the magnetization of multilayers [CeH₂(x Å)/Fe(16 Å)]×n, x=10, 16, 25, from planar to perpendicular orientation at low temperatures is explained on a microscopic basis by performing angle- and temperature-dependent measurements of x-ray magnetic circular dichroism at the 2p absorption edge of Fe and at the 2p and 3d absorption edges of Ce. The 3d orbital magnetic moment in the Fe sublayers is considerably enhanced with respect to bulk bcc Fe and distinctly larger parallel to the layer normal than perpendicular to it. The Ce 4f states in these structures are well localized. The 4f magnetic moment is oriented along the layer normal due to a strong single-ion anisotropy resulting from crystal-field effects. The spin-split 3d states of Fe induce magnetic order on the Ce 5d states via hybridization and spin-orbit coupling, even on ions more distant from the interface. By intra-atomic 5d-4f exchange coupling the 4f states become magnetically polarized, with increasing strength toward low temperature. Together with the 5d-mediated 4f-3d coupling and the crystal-field induced single-ion 4f anisotropy this leads to a perpendicular orientation of the Fe 3d moment at low temperature. Hence the Ce 4f states are the motor of the reorientation transition of the multilayer magnetization. The 4f-5d exchange interaction in the hydrided Ce sublayers affects dramatically the spectral shape of the dichroic Ce 5d spectra and, at low temperature, the ratio of the integrated signals at the L₂ and L₃ edges.

The x-ray magnetic circular dichroism (XMCD) has been measured at the Co K edge in hcp-Co and R-Co compounds (R=La,Tb,Dy). The structure of the experimental XMCD spectra in the near-edge region has been observed to be highly sensitive to the magnetic environment of the absorbing site. Calculations of the XMCD have been carried out at the Co K edge in Co metal, LaCo₅, and TbCo₅ within the multiple-scattering framework including the spin-orbit coupling. In the three systems, the XMCD spectra in the near-edge region are well reproduced. The possibility to separate the local effects from those due to the neighboring atoms in the XMCD cross section makes possible a more physical understanding of the spectra. The present results emphasize the major role played by the d states of the Tb ions in the XMCD spectrum at the Co K edge in the TbCo₅ compound.

Experiments are reported of x-ray absorption and x-ray magnetic circular dichroism at the L_2,3(2p→5d) and M_4,5(3d→4f) absorption edges of Ce and La in the multilayers [La/Ce(10Å)/La/Fe(30Å)]×n and [Ce/La(10Å)/Ce/Fe(30Å)]×n. In these layers, Ce adopts an α-phase-like electronic structure. Strong 3d-5d and 3d-4f hybridization is effective at the interfaces. It induces an ordered magnetic moment on the 5d and 4f states in the ground state of Ce and of the 5d states of La at room temperature, in antiparallel orientation to the Fe moment. By choosing the special laminar structure of the two lanthanides intercalated in between the Fe layers, the spatial extension of the Fe-induced magnetic order is directly probed. The 4f polarization of Ce and the 5d polarization of La are limited to the direct interface with Fe. In contrast, the polarization of Ce-5d states extends deeply into the layers. There is evidence that the 5d states of α-like Ce in these layered structures are very close to a ferromagnetic instability. Remarkably, magnetic order on the Ce-5d states can exist without a polarization of the 4f states. Apparently the 4f-(5d,6s) hybridization wins over the intraatomic 4f-5d exchange interaction, and stabilizes a nonmagnetic 4f ground state. Comparative studies on alloys Ce_xFe_1-x confirm this observation: near x=0.34 the polarization of the 4f states gets lost, but the 5d states are magnetically ordered. The special electronic and magnetic properties of Ce and La in the multilayer systems are reflected in the reversal of the macroscopic magnetization studied by the magneto-optical Kerr effect.

X-ray magnetic circular dichroism (XMCD) experiments have been performed at the L_2,3 edges of gadolinium in the amorphous Gd-Ni-Co and Gd-Ni alloys in order to observe the effect of the transition metal on the polarization of the rare-earth 5d band. The magnetic properties of the Gd-Ni-Co alloys, weak anisotropy or perpendicular easy axis of magnetization, are very well suited to XMCD experiments and very large XMCD signals are observed at the Gd L_2,3 edges. The signals corrected from the circular polarization rate (P_c =0.65) can reach up to -22% at the L₂ and +11% at the L₃ for some samples. The V-like behavior of the XMCD signal at the Gd L edges near the compensation temperature is understood in terms of distribution of the Co concentration in the Gd-Ni-Co sputtered films. Within the spin-dependent radial matrix model, the evolution of the XMCD intensity in the Gd-Ni-Co films as a function of the Co concentration qualitatively accounts for the variation of the spin-dependent final-state density Δρ/ρ monitored by the Co concentration.

Quadrupolar ( E2) transitions in x-ray magnetic circular dichroism at the L_III edge of Yb in YbFe₂ are evidenced through temperature and angular dependences. The choice of Yb is particularly relevant since E2 transitions come as a single peak. The width, position, and intensity of the E2 contribution are determined. The E2 peak appears in the middle of the dipolar signal. These measurements allow for the determination of dipole and octupole moments of the 4 f shell, whose angular and temperature dependences were found to be in agreement with calculation.

This systematic study of R(Ni_xCo_1-x)₅ compounds (R=La, Gd, and Tb) has allowed us to investigate the combined effects of the 3d-5d hybridization and 4f-5d exchange on the rare-earth L_2,3-edge magnetic circular x-ray dichroism (MCXD) signal. In La compounds, where the 5d polarization results only from the magnetic transition metals via the hybridization of 5d and 3d bands, the MCXD signal scales with the 5d moment and can be explained by a one-particle model for the absorption process. When the 4f-5d exchange intervenes, like in the Gd and Tb compounds, its effects on the MCXD sign, amplitude, and the amplitude variations are very important. A description of the L_2,3-edge MCXD spectra taking into account the spin-dependence of the radial matrix elements explains qualitatively the results of this study.

We present a combined x-ray-absorption-spectroscopy (XAS) and x-ray-circular-magnetic-dichroism (XCMD) study performed on the CeFe₁₁TiH_x systems. Looking at the changes of the local structure around the metal atoms by EXAFS (extended x-ray-absorption fine structure), the crystallographic position occupied by hydrogen in these compounds has been inferred. Moreover, the influence of hydrogen into the intermediate-valence state of Ce and the electronic localization is discussed. Both XAS and XCMD results evidence the strong correlation between the structural and magnetic changes associated to the modification of the hybridization between the Fe (3d) and Ce (5d) bands.

Magnetic circular x-ray dichroism on Co /Cu and Fe /Cu multilayers at the K edge of copper shows (i) that the p band of copper is significantly spin polarized by the adjacent Co or Fe atoms, (ii) that the spin polarization of the copper layers strongly depends on the adjacent magnetic layer, and (iii) that the magnetic polarization is not restricted to the interface layer, i.e., it departs from a simple 1/t_Cu dependence.

An experimental study of conventional x-ray absorption and of magnetic circular x-ray dichroism has been performed on La/Fe and on Ce/Fe multilayers at the L₂ and L₃ edges of La and Ce and at the K edge of Fe to characterize the impact of the interaction of La or Ce and Fe on the local electronic and magnetic structure of the interfaces. The spectra reveal that the interaction is particularly unusual for Ce/Fe, where two effects have been observed. First, on a depth scale of at least 15 Å near the interface, the Ce atoms adopt the electronic structure of the α phase, with itinerant 4f states. Second, these α-phase-like Ce atoms are magnetically polarized and carry an ordered magnetic 5d spin moment. This moment is antiferromagnetically coupled to the Fe 3d moment. Interface mixing on a length scale of the magnetic polarization can be excluded. For the La/Fe system, a similar magnetic polarization is effective, but in contrast to Ce/Fe it is restricted to the immediate interface with Fe. The results are discussed with reference to recent measurements of the macroscopic magnetization and Mössbauer spectra. Comparison with intermetallic compounds of rare-earth and late transition-metal elements points to the importance of hybridization effects for the Fe and Ce or La electronic states near the interfaces in the multilayers.

In this paper we discuss the specific features of K-edge magnetic circular x-ray-dichroism (MCXD) spectra of Fe and Co in bcc Fe-Co alloys and Fe/Co multilayers. In the alloys, the systematic variations of shape and amplitude of the near-edge MCXD signals as a function of concentration can be correlated with the changes in local atomic environment. The difference between the Co K-edge and Fe K-edge spectra of the same alloy shows that the signal is central atom dependent. Well above the absorption edge (E-E₀>80 eV) the extended x-ray-absorption fine structure (EXAFS)-like oscillations in the MCXD spectra clearly reflect the local bcc structure and appear not to depend on the absorbing atom. In both alloys and multilayers, the magnetic EXAFS intensity can be correlated to the average magnetization in the neighborhood of the absorbing atom.

Hydrogen insertion in the Ce₂Fe₁₇H_x system leads to spectacular changes of the physical properties. A relative increase of the Curie temperature of almost 100% is observed as well as the highest volume increase. In addition to previous neutron diffraction measurements which pointed out an increase of the local iron moments, we present in this paper absorption and magnetic circular x-ray dichroism (MCXD) experiments performed at the L_2,3 edges of Ce and the K edge of iron. These measurements allow us to study the evolution of the Ce electronic structure upon hydrogen insertion. Values for the Ce valence have been derived from both absorption and MCXD signals. The MCXD data are analyzed for their shape, intensity, and sign, and compared to the macroscopic magnetic measurements. It is shown that in these compounds the Ce atoms bear a 5d magnetic moment, which is coupled antiferromagnetically to that on iron. There is no orbital contribution to this 5d moment which is thus solely of spin origin. The values of the 5d moment are deduced as a function of each composition in the Ce₂Fe₁₇H_x system at low and room temperature. At the K edge of iron, the shape and intensity of the dichroic signal have been related to the filling of the majority 3d band.

Magnetic circular x-ray dichroism (MCXD) studies at L_2,3 absorption edges in intermediate-valence (IV) compounds CeFe₂, Ce(Fe_0.8Co_0.2)₂, CeCo₅, Ce₂Co₁₇, and trivalent 4f¹ systems (CeRu₂Ge₂, Ce₃Al₁₁, CePd, CeFe₂H_3.8) at the Ce L_2,3 edges are presented. The comparison with reference compounds (La, Lu, Hf, Gd) confirms the existence, in such IV systems, of an ordered 5d magnetic moment on the Ce site antiferromagnetically coupled with the 3d Fe (Co) moment. Moreover, this study suggests, provided the 4f orbital moment is almost quenched, the existence of a simple proportionality between the MCXD intensity and the magnetic moment of the probed atom. For instance the MCXD method leads to a Ce 5d magnetic moment in CeFe₂ of about 0.35μ_B antiferromagnetically coupled with the 3d Fe moment, in good agreement with band-structure calculations. The MCXD applications in Ce(Fe,Co)₂ and CeFe₂H_x permit us to define precisely the electronic structure of Ce in such compounds. Particularly we will show that MCXD experiments at the L_2,3 absorption edges of Ce are able to yield valuable information on the degree of hybridization of the 4f electrons in the ground state. Indeed, systematic comparison between the MCXD signals of IV and normal trivalent Ce compounds reveals the extreme sensitivity of the L_2,3 MCXD experiments to the degree of localization of the 4f electrons.

This paper reports experimental evidence of multielectron excitations in magnetic circular x-ray dichroism (MCXD) spectra, measured at the L_I,II,III edges of rare earths (RE) in several RE–transition-metal (Fe, Co) compounds. These multielectron features involve transitions from the 2p and 4d levels and exhibit a very strong magnetic enhancement compared to normal MCXD.

It has been shown that the one-electron approach is not sufficient to understand core-level photoexcitation in narrow-band systems, such as the 4f of the rare-earth elements or the 3d of the late transition-metal elements. Refinements that take into account the strong Coulomb interaction of the core hole with all localized or quasilocalized electrons in the final state have been introduced; this approach still leaves the photoelectron as a spectator in the full relaxation process. In this paper we show the need to take into account the role of the slow photoelectron in the many-body problem. Thanks to two sets of polarization-dependent K-edge experiments on single crystals of La₂CuO₄ and Pr₂NiO₄, the role of the photoelectron is shown to be anisotropic for cuprates but not anisotropic for nickelates. This different behavior is expected because the relaxation leads to the electron transfer O 2p→Cu 3d in cuprates, where only one channel with x²-y² symmetry is available, whereas in nickelates two quasiequivalent channels x²-y² and 3z²-r² are opened with almost equal probability.

In this paper we present an experimental investigation of the magnetic properties of metallic multilayers (here Nd-Fe) by magnetic x-ray dichroism (MXD). The magnetic absorption cross sections have been measured, using a dispersive optics, on both the L_II edge of Nd and the K edge of Fe in several Nd-Fe multilayers (Λ≊40–100 Å), for temperatures ranging from 77 to 300 K. We will show that the intensity of the magnetic absorption can be roughly correlated to the ordered magnetic moments present either on Nd or Fe, and thus the MXD allows direct information to be obtained on the local magnetic properties on a given atom. Moreover, by performing systematic studies as a function of the modulation length Λ, we discuss the efficiency of such experiments to yield direct information about the magnetic properties of the interfaces themselves. MXD experiments also give the sense of the coupling (ferromagnetic in the present case) between the magnetic moments of Nd and Fe in those systems. MXD results are compared to those given by bulk magnetization measurements and Mössbauer experiments on ⁵⁷Fe.

The pressure dependence of the interatomic distances and their mean-square deviations (Debye-Waller factors) in SrTiO₃ are studied by x-ray-absorption spectroscopy in a diamond anvil cell up to 15 GPa. The pressure dependence of Sr-O and Sr-Ti interatomic distances have been determined and compared with x-ray-diffraction data. The comparison confirms the occurrence of a structural phase transition induced by pressure. In the low-pressure phase the Sr-O and the Sr-Ti bond compressibilities are very close to those deduced from x-ray-diffraction data, and a Debye model can describe the pressure dependence of the oxygen and titanium Debye-Waller factors. On the contrary, in the high-pressure phase, the mean Sr-O and Sr-Ti bond compressibilities become smaller, and the Debye-Waller factors increase with pressure, showing an increase of the oxygen and titanium static disorder due to various Sr-O and Sr-Ti distances.

X-ray-absorption near-edge spectra (XANES) of high-T_c bismuth superconductors (2:2:0:1, 2:2:1:2, and 2:2:2:3) have been recorded at the Bi L_III, L_I, and Cu K edges. With respect to the reference Bi₂O₃ [Bi(iii) valence state], edge shifts towards low energies have been found in the spectra of all the superconducting cuprates at the Bi L_III edge. This has been interpreted in terms of a reduced bismuth valency smaller than Bi (iii). This result is a characteristic of the superconducting intergrowths, since other layer compounds with Bi cations in similar environments, Bi₂SrNb₂O₉ and Bi₂Sr₂CaFe₂O₉, do not present any shift with respect to Bi₂O₃. The copper formal valency has been deduced either from charge-balance considerations assuming a well-known oxygen stoichiometry or edge shifts at the copper K edge. High copper valencies have been found, especially in the 2:2:0:1 compound, but again, as was observed before in thallium and lead-substituted thallium cuprates, critical temperatures increase when the copper valency decreases, i.e., when the number of holes per copper in an oxygen-like p band decreases, provided this number stands above a critical value estimated previously to be 0.17.

High-pressure behavior of GaP has been studied by x-ray-absorption spectroscopy in a diamond-anvil cell up to 36 GPa. The room-temperature equation of state of phase I has been determined. The transition from a fourfold (zinc-blende) to a sixfold (β-tin) coordination scheme is observed near 26 GPa. Analysis of the data obtained on a sample quickly depressurized from 36 GPa shows that the transition is not reversible and the recovered sample is mainly amorphous.

High-pressure properties of krypton were investigated by energy-dispersive extended x-ray absorption fine-structure spectroscopy in a diamond anvil cell at room temperature. The equation of state agrees very well with x-ray diffraction data. The pressure dependence of the Debye-Waller factor was determined and compared with calculations using pair potentials. The analysis of the x-ray absorption near-edge structure part of the spectrum shows the possibility of measuring the pressure in bubbles of krypton implanted in metallic matrices.

Taking advantage of the high sensitivity of the Ce valence to the local environment, we investigated the early stage of the solid-state reaction in Ce/Ni multilayers through x-ray absorption spectroscopy. Ce L_III-edge and Ni K-edge spectra have been compared to distributions of experimental spectra obtained from amorphous Ce-Ni alloys. Two possible diffusion profiles have been examined. It is found that the experimental data are compatible with the formation of a homogeneous amorphous-alloy interlayer instead of a Fickian profile.

Electrochemically synthesized polythiophene is a very promising conducting polymer able to support tiny metallic particles which could be useful in various applications (e.g., conducting leads, catalysis,...). Time-resolved in situ investigation of the process of the inclusion of metallic aggregates has been achieved by dispersive x-ray absorption spectroscopy. The unique capability of this new structural tool comes from the association of the properties of synchrotron radiation with a photodiode array used as a position-sensitive detector.

The rate of formation of extended defects by heavy energetic ions in track-forming minerals shows a pronounced nonlinear dependence on the linear ionization rate. It is suggested that the formation of an extended defect—as recorded by small-angle x-ray scattering—requires a certain threshold ionization density. As a consequence, the mean number of observable defects per ion track is related more directly to the fluctuation in ionization density than to its mean value.

Diffuse x-ray measurements carried out on AuFe single crystals of 14.4, 15.4, and 19 at.% Fe show evidence for short-range order quite different in nature from that found in AgMn and CuMn alloys. This short-range order is seen as (1) a strong small-angle scatterings suggesting a tendency towards segregation, (2) a system of diffuse streaks extending in a discontinuous manner along 〈11/20〉 axes and equivalents, and (3) an asymmetry of the diffuse streaks about each Bragg spot. We interpret this ensemble of facts by the existence of Fe-rich platelets oriented parallel to {420} planes, about two-planes thick and extending 30 Å parallel to their plane.