In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present Letter, we report a distinct and unexpected surface magnetic phase transition at a lower temperature than the Néel temperature. Employing grazing incidence x-ray resonant magnetic scattering, we have observed the near-surface behavior of uranium dioxide. UO2 is a noncollinear, triple-q, antiferromagnet with the U ions on a face-centered cubic lattice. Theoretical investigations establish that at the surface the energy increase-due to the lost bonds-is reduced when the spins near the surface rotate, gradually losing their component normal to the surface. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic XY character and are decoupled from the bulk. The structure factor of a finite two-dimensional XY model fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class.

Symmetry analysis is combined with x-ray scattering experiments to investigate the lattice modulation associated with the incommensurate magnetic structure in the case of a double-k structure. The expansion of the free energy shows that the components of the magnetic structure with propagation vectors k(1) and k(2) can couple via components of lattice modulations. It is shown that the classical diffraction peaks reflecting a 2k propagation vector, associated with magneto-elastic effects in single-k structures, will coexist with diffraction peaks with propagation vectors k(1) - k(2) or k(1) + k(2). The existence of these latter peaks can be considered as a signature of a double-k magnetic structure. In the case of the double-k modulated structure of CeAl2, group theory is applied directly to the study of the charge modulation. An x-ray scattering study of the 2k satellites shows that the lattice displacements of the two Ce sites of the structure are antiparallel to each other, and perpendicular to the direction of the magnetic modulation. We also confirm experimentally the existence of k(1) + k(2) satellites.

A magnetic Bragg reflection corresponding to the wave vector k13 = (2pi/a)[1/2,1/2,1/2] of the antiferro-quadrupolar ordering is found in CeB6 in zero magnetic field below the Neel temperature TN. Its intensity is two orders of magnitude weaker than those due to the basic magnetic structure [O. Zaharko et al., Phys. Rev. B 68, 214401 (2003)]. The peak has a width of the other Bragg reflections below TN, but widens abruptly at T = TN with simultaneous increase of intensity. Correlation length just above TN is of the order of 70 A. The peak intensity decreases to zero at T = 7 K with no visible anomaly at the antiferro-quadrupolar ordering temperature TQ = 3.3 K. The features of this magnetic ordering are typical for the itinerant magnetism with 5d electron of Ce3+ [Yu.S. Grushko et al., phys. stat. sol. (b) 128, 591 (1985)] being involved.

An asymmetric shift in the position of the magnetic Bragg peak with respect to the fiducial lattice has been observed by resonant x-ray scattering in a diverse series of antiferromagnetic compounds. A possible explanation is given in terms of a generalized Berry phase correction.

X-ray magnetic scattering experiments have been performed in CeSb at the Ce and Sb L edges. In the non-resonant regime, we observe charge satellites reflecting the lattice modulation associated with the periodicity of paramagnetic Γ8 planes. At the Ce L2 edge we observe strong magnetic resonances, due to the antiferromagnetic stacking of the ferromagnetic Γ7. The study at the Sb L1 edge shows a magnetic dipole resonance, which supports the model of strong p–f mixing, used to explain the origin of the long-range magnetic order of CeSb.

Resonant x-ray magnetic scattering is widely used as an element selective probe of magnetism in solids. The present work deals with a different, less frequently addressed aspect: the electronic shell selectivity. Due to the complexity of the atomic effects inherent to the resonant process and at the origin of the electronic shell selectivity, the data are generally considered on a qualitative basis. Here, we try to extend the arguments to a semiquantitative level. We show, through a detailed spectroscopic study of the resonance at the L 2 and L 3 edges of samarium in a single-crystal epitaxial film, how the exploitation of the atomic effects can lead to a deeper understanding of long-range magnetic order in this material. At the L 2 , 3 edges of rare earths, dipole resonances carry information on the polarization of the 5d band, whereas quadrupole resonances reflect the polarization of the 4f shell, The narrow width of the 4f band permits the interpretation of the quadrupole resonance below the L 3 edge using atomic considerations. A systematic study of the dependence of the 4f quadrupole resonance on wave-vector transfer shows that, within our resolution, the magnitude of the 4f moments in samarium is independent of the local environment (cubic or hexagonal). On the other hand, the energy dependence of the dipole resonance at and above the L 2 and L 3 absorption edges shows two maxima and is interpreted in the framework of an extended density of 5d states, Finally, the relative temperature dependences of the dipole and quadrupole resonances sheds light upon the respective contributions of the 4f and 5d levels to the long-range magnetic order in samarium.

We report on X-ray magnetic diffraction studies of the spin density wave antiferromagnetism formed in the conduction electron band of chromium. Non-resonant X-ray magnetic scattering was used to directly determine that chromium has zero orbital magnetisation. Furthermore, the azimuthal dependence of this scattering provides unique evidence that chromium forms a linearly polarised wave. In the vicinity of the K absorption edge, resonant X-ray magnetic scattering was observed. A consistent model of the magnetic scattering has been derived from the resonant and non-resonant magnetic amplitudes. The enhancement of the magnetic intensity arises primarily from dipole transitions from the core 1s level to 4p states. Quadrupole transitions to the magnetic 3d states are essentially non-existent due to their sensitivity to (and the absence of) orbital moment. This effect is predicted from atomic considerations of the 3d5 (\(\) = 0) transition metal ions.

Synchrotron experiments with uranium antiferromagnetic compounds have discovered large ( >1000) enhancements of the magnetic scattering intensities at the K edges of nominally nonmagnetic anions, e.g., Ga and As. The width in energy, the position with respect to the white line, and the azimuthal and polarization dependencies permit one to associate the signal with transitions of E1 dipole symmetry from 1s to 4p states. In momentum space, the signal exhibits long-range order at the antiferromagnetic wave vector. We discuss possible channels capable of generating the observed enhancements.

Resonant X-ray scattering experiments have been performed in 2.8% Cr-doped V2O3 single crystal at the Vanadium K-photoabsorption edge. Using linear polarization analysis and comparing the angular dependence of scattered photons with structure factor calculations we can discriminate the nature of the different resonant X-ray processes involved in forbidden lattice reflections enhanced by resonances. We present an experimental method to extract information on local properties of edge-atom such as the anisotropy of the local atomic environment, the atomic magnetic moment orientation and orbital ordering.

Frozen-in phason fluctuations in single grains of icosahedral Al-Pd-Mn quasicrystals have been studied by high-resolution coherent X-ray scattering. Bragg peak widths scale as expected for a distribution of uniform phason strains. Strong relaxation of the uniform phason strain is observed after sample annealing. Large intensity fluctuations or speckle patterns are observed in the Bragg peaks due to the uniform phason strain distribution in the sample. Speckle patterns are also observed in the diffuse scattering located close to the Bragg reflections and are related to long-wavelength phason fluctuations taking place in the sample.

A number of superstructure reflections (h/2k/2l/2) with h,k,l=2n+1 are observed in the intermediate phase II of CeB6 by resonant X-ray scattering at the LII and LIII absorption edges of Ce. This gives evidence of a zero-field ordering with wave vector k0=2π/a[1/21/21/2]. The intensities of the reflections (5/21/21/2), (5/23/23/2) and (7/21/21/2) in nonresonant conditions, where reliable calculations can be made, are consistent neither with the Jahn–Teller pair distortion nor with the Γ3 quadrupolar ordering proposed in the literature as possible zero-field ordering mechanisms. Apparently, the Γ5 quadrupolar ordering accompanied by displacements of the boron atoms should also be considered, but the experimental data do not allow to draw definite conclusions.

Non-resonant magnetic X-ray scattering (NRXMS) is a unique tool allowing the separation of the spin and orbital moment density contributions to the total magnetization density. This method has been successfully applied to the simple transition metal oxides: MnO, CoO and CuO. It is common habit to consider the orbital moment of these oxides to be quenched by the cubic crystal electric field. We do, however, observe non-vanishing orbital moments in the case of CoO and CuO. The partial re-establishment of the orbital moment is related to spin-orbit coupling.

The zero-field magnetic phases of NdCu2 have been investigated using non-resonant X-ray scattering and the results are compared to those from neutron scattering. The reduced scattering volume of X-ray scattering as opposed to the bulk average measured by neutron scattering allowed us to investigate the magnetic properties in the near-surface region. As in the neutron scattering experiment, three magnetic phases AF1, AF2 and AF3 have been observed, and they exist in the same degree of order as the crystal lattice. In the near-surface region, the large magnetic unit cell present in AF2 is slightly different as compared to the bulk. Our results validate the first order of the phase transitions between the magnetic phases and allow us to localize the stabilization of the magnetic phase AF2 in real space. The modulation of the low-temperature structures AF1 and AF2 are squared up. In addition, we find a magnetically induced modulation of the electronic distribution in the low-temperature phase AF1 which is consistent with a lattice distortion (magneto-elastic coupling).

K-edge resonant x-ray magnetic scattering experiments have been performed on antiferromagnetic NiO, The observation of two resonances at the K edge allows the construction of models to compare the electronic properties of NiO and the observed resonant magnetic x-ray scattering. From the polarization analysis of the scattered beam, a quadrupolar transition (1s - 3d) and a dipolar transition (1s - 4p) are identified. While the quadrupolar transition can be modeled using an atomic picture for the 3d electrons, the dipolar transition is associated to a broadband structure of p electrons and its energy profile is compared to electronic band-structure calculations.

We report on resonant magnetic X-ray scattering studies of a samarium epitaxial film at the samarium L3 edge. We observe one quadrupolar resonance below the edge, reflecting the polarization of the 4f electrons, and two dipolar resonances above the edge, related to the polarization of the 5d band. We demonstrate, by following the thermal evolution of resonant magnetic intensities of both types, that the polarization of the 4f and 5d electrons present exactly the same temperature dependence, even very close to the ordering temperature, in agreement with the RKKY model for long-range magnetic order in rare earths.

A number of superstructure reflections (h/2 k/2 l/2) with h, k, l = 2n + 1 are observed in the intermediate phase II of CeB6 by resonant X-ray scattering at the L-II and L-III absorption edges of Ce. This gives evidence of a zero-field ordering with wave vector k(0) = 2 pi /a [1/2 1/2 1/2]. The intensities of the reflections (5/2 1/2 1/2), (5/2 3/2 3/2) and (7/2 1/2 1/2) in nonresonant conditions, where reliable calculations can be made, are consistent neither with the Jahn-Teller pair distortion nor with the Gamma (3) quadrupolar ordering proposed in the literature as possible zero-field ordering mechanisms. Apparently, the rs quadrupolar ordering accompanied by displacements of the boron atoms should also be considered, but the experimental data do not allow to draw definite conclusions. (C) 2001 Elsevier Science B.V. All rights reserved.

Among the materials exhibiting metal–insulator transition, vanadium sesquioxide V2O3 and its Cr-alloys have received most attention from the theoretical and experimental point of view. It is now widely agreed that the unusual magnetic properties can be explained by taking both orbital and spin degrees of freedom into account. Whereas many experiments give direct information on the magnetic structure, only indirect evidence suggests the existence of orbital order in cases like V2O3, where it is not accompanied by cooperative Jahn–Teller distortion. We have demonstrated the existence of orbital ordering in V2O3 by means of X-ray resonant scattering at the vanadium K-edge using tunable synchrotron radiation. The observed orbital structure confirms the interplay between orbital and magnetic order, and provides an illustration of a new experimental method to extract information on the electronic orbital occupancy in solids.

X-ray magnetic scattering from ordered Cu spins has been observed in a high-${T}_{c}$ compound. The measurements were made on the anomalous cuprate ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.92}$ with x-ray photon energies tuned in the vicinity of the Cu K edge. The high wave-vector resolution enabled us to observe an incommensurate double-$Q$ Cu spin structure below ${T}_{\mathrm{Pr}}=19$ K that forms as a result of coupling between the magnetically ordered Cu and Pr sublattices. Above ${T}_{\mathrm{Pr}},$ the Cu ordering is commensurate, ruling out static spin-charge stripe order as an explanation for the absence of superconductivity in this material.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation G. H. Lander, D. Mannix, F. Wastin, J. Rebizant, D. Mannix, E. Lidström, C. Vettier, R. Caciuffo, N. Bernhoeft, P. Normile, W. G. Stirling, A. Hiess, C. Vettier; X-ray magnetic scattering from transuranium systems. AIP Conf. Proc. 7 July 2000; 532 (1): 35. https://doi.org/10.1063/1.1292191 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search