arXiv subjects

S. B. Wilkins

Publications and source records attributed to S. B. Wilkins.

35 records · Page 2Linked to original sources

arXiv2008

A critical re-examination of resonant soft x-ray Bragg forbidden reflections in magnetite

Magnetite, Fe$_3$O$_4$, displays a highly complex low temperature crystal structure that may be charge and orbitally ordered. Many of the recent experimental claims of such ordering rely on resonant soft x-ray diffraction at the oxygen K and iron L edges. We have re-examined this system and undertaken soft x-ray diffraction experiments on a high-quality single crystal. Contrary to previous claims in the literature, we show that the intensity observed at the Bragg forbidden (001/2)$_c$ reflection can be explained purely in terms of the low-temperature structural displacements around the resonant atoms. This does not necessarily mean that magnetite is not charge or orbitally ordered, but rather that the present sensitivity of resonant soft x-ray experiments does not allow conclusive demonstration of such ordering.

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arXiv2008

Ordering of localized electronic states in multiferroic TbMnO3: a soft X-ray resonant scattering study

Soft X-ray resonant scattering (XRS) has been used to observe directly, for the first time, the ordering of localized electronic states on both the Mn and Tb sites in multiferroic TbMnO$_3$. Large resonant enhancement of the X-ray scattering cross-section were observed when the incident photon energy was tuned to either the Mn $L$ or Tb $M$ edges which provide information on the Mn 3d and Tb $4f$ electronic states, respectively. The temperature dependence of the XRS signal establishes, in a model independent way, that in the high-temperature phase (28 K $\leq$ T $\leq$ 42 K) the Mn 3d sublattices displays long-range order. The Tb $4f$ sublattices are found to order only on entering the combined ferroelectric/magnetic state below 28 K. Our results are discussed with respect to recent hard XRS experiments (sensitive to spatially extended orbitals) and neutron scattering.

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arXiv2008

Determination of magnetic order of the rare-earth ions in multiferroic TbMn2O5

We have employed resonant x-ray magnetic scattering to specifically probe the magnetic order of the rare-earth ions in multiferroic $\mathrm{TbMn_2O_5}$. Two energy resonances were observed, one originated from the E1-E1 dipolar transition and the other from the E2-E2 quadrupolar transition. These resonances directly probe the valence 5d band and the partially occupied 4f band, respectively. First, full polarization analysis, which is a measurement of the scattered polarization as a function of incident polarization, confirmed a spin polarization of the terbium valence states (probed by the E1-E1 transition) by the $\mathrm{Mn^{4+}}$ spin density in the commensurate phase. Second, full polarization analysis data were collected in the low-temperature incommensurate and commensurate phases when tuned to the E2-E2 resonance. By employing a least-squares fitting procedure, the spin orientations of the terbium ion sublattice were refined.

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arXiv2008

Disentangling multipole resonances through a full x-ray polarization analysis

Complete polarization analysis applied to resonant x-ray scattering at the Cr K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994 eV). The physical mechanism behind this phenomenon is a subtle interference effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions, leading to a phase shift between the respective scattering amplitudes. This effect may be exploited to disentangle two close-lying resonances that appear as a single peak in a conventional energy scan, in this way allowing to single out and identify the different multipole order parameters involved.

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arXiv2007

X-ray Resonant Scattering Study of the Order Parameters in Multiferroic TbMnO$_3$

We report on an extensive investigation of the multiferroic compound TbMnO$_3$. Non-resonant x-ray magnetic scattering (NRXMS) revealed a dominant $A$-type domain. The temperature dependence of the intensity and wavevector associated with the incommensurate magnetic order was found to be in good agreement with neutron scattering data. XRS experiments were performed in the vicinity of the Mn $K$ and Tb $L_3$ edges in the high-temperature collinear phase, the intermediate temperature cycloidal/ferroelectric phase, and the low-temperature phase. In the collinear phase resonant $E1-E1$ satellites were found at the Mn $K$ edge associated with $A$-type but also $F$-type peaks. The azimuthal dependence of the $F$-type satellites (and their absence in the NRXMS experiments) indicates that they are most likely non-magnetic in origin. We suggest instead that they may be associated with an induced charge multipole. At the Tb $L_3$ edge resonant $A$- and $F$-type satellites ($E1-E1$) were observed in the collinear phase. These we attribute to a polarisation of the Tb 5$d$ states by the ordering of the Mn sublattice. In the cycloidal/ferroelectric phase a new set of resonant satellites appear corresponding to $C$-type order. These appear at the Tb $L_3$ edge only. In addition to a dominant $E1-E1$ component in the $σ-π^\prime$ channel, a weaker component is found in the pre-edge with $σ-σ^\prime$ polarization. Calculations of the XRS were performed using the $FDMNES$ code showing that the unrotated $σ-σ^\prime$ component of the Tb $L_3$ $C$-type peaks appearing in the ferroelectric phase contains a contribution from a multipole that is odd with respect to both space and time, known in various contexts as the anapole.

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arXiv2007

Influence of static Jahn-Teller distortion on the magnetic excitation spectrum of PrO2: A synchrotron x-ray and neutron inelastic scattering study

A synchrotron x-ray diffraction study of the crystallographic structure of PrO2 in the Jahn-Teller distorted phase is reported. The distortion of the oxygen sublattice, which was previously ambiguous, is shown to be a chiral structure in which neighbouring oxygen chains have opposite chiralities. A temperature dependent study of the magnetic excitation spectrum, probed by neutron inelastic scattering, is also reported. Changes in the energies and relative intensities of the crystal field transitions provide an insight into the interplay between the static and dynamic Jahn-Teller effects.

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arXiv2007

Resonant x-ray scattering study of NpRhGa_5 and NpCoGa_5

We report a resonant x-ray scattering (RXS) study of antiferromagnetic neptunium compounds NpCoGa_5 and NpRhGa_5 at the Np M_4 and Ga K-edges. Large resonant signals of magnetic dipole character are observed below the Neel temperatures at both edges. The signals at the Np edges confirm the behaviour determined previously from neutron diffraction, i.e. the moments along [001] in NpCoGa_{5} and in NpRhGa_5 a reorientation of the moments from the c-axis direction to the ab plane. In the latter material, on application of magnetic field of 9 Tesla along the [010] direction we observe a change in the population of different [110]-type domains. We observe also a magnetic dipole signal at the Ga K-edge, similarly to the reported UGa_3 case, that can be interpreted within a semi-localized model as an orbital polarization of the Ga 4p states induced via strong hybridization with the Np 5f valence band. Quantitative analysis of the signal shows that the Ga dipole on the two different Ga sites follows closely the Np magnetic moment reorientation in NpRhGa_5. The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.

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arXiv2007

Resonant soft x-ray magnetic scattering from the 4f and 3d electrons in DyFe(4)Al(8)

Soft x-ray resonant scattering has been used to examine the charge and magnetic interactions in the cycloidal antiferromagnetic compound \dyfeal. By tuning to the Dy $M_4$ and $M_5$ absorption edges and the Fe L(2) and L(3) absorption edges we can directly observe the behavior of the Dy 4f and Fe 3d electron shells. Magnetic satellites surrounding the (110) Bragg peak were observed below 60 K. The diffraction peaks display a complex spectra at the Dy M(5) edge, indicative of a split 4f electron band. This is in contrast to a simple resonance observed at the Fe L(3) absorption edge, which probes the Fe 3d electron shell. Temperature dependant measurements detail the ordering of the magnetic moments on both the iron and the dysprosium antiferromagnetic cycloids. The ratio between the intensities of the Dy M(4) and M(5) remained constant throughout the temperature range, in contrast to a previous study conducted at the Dy L(2,3) edges. Our results demonstrate the ability of soft x-ray diffraction to separate the individual magnetic components in complicated multi-element magnetic structures.

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arXiv2006

Multi-k magnetic structures in USb_{0.9}Te_{0.1} and UAs_{0.8}Se_{0.2} observed via resonant x-ray scattering at the U M4 edge

Experiments with resonant photons at the U M4 edge have been performed on a sample of USb_{0.9}Te_{0.1}, which has an incommensurate magnetic structure with k = 0.596(2) reciprocal lattice units. The reflections of the form , as observed previously in a commensurate k = 1/2 system [N. Bernhoeft et al., Phys. Rev. B 69 174415 (2004)] are observed, removing any doubt that these occur because of multiple scattering or high-order contamination of the incident photon beam. They are clearly connected with the presence of a 3k configuration. Measurements of the reflections from the sample UAs_{0.8}Se_{0.2} in a magnetic field show that the transition at T* ~ 50 K is between a low-temperature 2k and high-temperature 3k state and that this transition is sensitive to an applied magnetic field. These experiments stress the need for quantitative theory to explain the intensities of these reflections.

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arXiv2006

Determination of the Antiferroquadrupolar Order Parameters in UPd3

By combining accurate heat capacity and X-ray resonant scattering results we have resolved the long standing question regarding the nature of the quadrupolar ordered phases in UPd_3. The order parameter of the highest temperature quadrupolar phase has been uniquely determined to be antiphase Q_{zx} in contrast to the previous conjecture of Q_{x^2-y^2} . The azimuthal dependence of the X-ray scattering intensity from the quadrupolar superlattice reflections indicates that the lower temperature phases are described by a superposition of order parameters. The heat capacity features associated with each of the phase transitions characterize their order, which imposes restrictions on the matrix elements of the quadrupolar operators.

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arXiv2006

Evidence of Orbital Ordering in Jahn-Teller Undistorted LaSr$_{2}$Mn$_{2}$O$_{7}$

Resonant soft x-ray diffraction has been used to probe the temperature dependent orbital and magnetic structure of $\mathrm{LaSr_{2}Mn_{2}O_7}$. Previous crystallographic studies have shown that this material has almost no MnO$_{6}$ oxygen displacement due to Jahn-Teller distortions at low temperatures. Within the low-temperature A-type antiferromagnetic phase, we found strong intensity at the $({1/4},{1/4},0)$ orbital and $LaSr_{2}Mn_{2}O_7$ magnetic reflections. This shows that even in the near absence of Jahn-Teller distortion, this compound is strongly orbitally ordered. A fit to the Mn $L$-edge resonance spectra demonstrates the presence of orbital ordering of the Mn$^{3+}$ ions with virtually no Jahn-Teller crystal field in addition to possible Mn$^{3+}$ and Mn$^{2+}$~like valence fluctuations.

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arXiv2005

Direct observation of electric-quadrupolar order in UO$_{2}$

We report direct experimental evidence for long-range antiferro ordering of the electric-quadrupole moments on the U ions. Resonant x-ray scattering experiments at the uranium $M_{4}$ absorption edge show a characteristic dependence in the integrated intensity upon rotation of the crystal around the scattering vector. Although quadrupolar order in uranium dioxide was advocated already in the 1960s no experimental evidence for this phenomenon was provided until now. We conclude with a possible model to explain the phase diagram of the solid solutions of UO$_{2}$ and NpO$_{2}$. We suggest that in the region $0.30 < x < 0.75$ neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.

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arXiv2004

Orbital Bi-Stripes in High Doped Bilayer Manganites

We present high resolution high energy and resonant x-ray diffraction results from La2-2xSr1+2xMn2O7 for x=0.55,0.575 and 0.60. These compounds show superlattice reflections at wavevectors of (h+-δ,k+-δ,l) and (h+-2δ,k+-2δ,l), arising from orbital ordering with associated Jahn-Teller distortions and charge ordering respectively. We observe a phase transition between the x=0.55 and x=0.575 doping levels. Samples with x=0.55 display structural characteristics similar to those previously reported for x=0.5. Compared to this, the long range order in samples with x=0.55 and x=0.6 have a distinct change in wavevector and correlation. We attribute this to a new orbital bi-stripe phase, accompanied by weak, frustrated, charge ordering. The observed azimuthal dependence of the orbital order reflections supports the model proposed for this new phase.

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arXiv2004

Resonant Soft X-ray Scattering Investigation of Orbital and Magnetic Ordering in $\rm La_{0.5}Sr_{1.5}MnO_4$

We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn $L_{\rm 2,3}$ edges in \lsmo. The orderings display complex energy features close to the Mn absorption edges. Systematic modeling with atomic multiplet crystal field calculations was used to extract meaningful information regarding the interplay of spin, orbital and Jahn-Teller order. These calculations provide a good general agreement with the observed energy dependence of the scattered intensity {for a} dominant orbital ordering of the $ d_{x^2 - z^2} / d_{y^2 - z^2} $ type. In addition, the origins of various spectral features are identified. The temperature dependence of the orbital and magnetic ordering was measured and displays a strong interplay between the magnetic and orbital order parameters.

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arXiv2004

Resonant X-ray Scattering Study of Magnetic and Electric-quadrupole Order in U$_{0.75}$Np$_{0.25}$O$_2$

We have used element specific X-ray resonant scattering to investigate the M edge resonances in a single crystal of \unpo. Earlier neutron diffraction and Mössbauer studies had shown the presence of long-range AF order below T$_{N}$ = 19 K, with sizeable magnetic moment both on the U and the Np ions. RXS results confirm the presence of an ordered dipole magnetic moment on the Np ions, but also reveal the presence of an anti-ferro arrangement of the electric quadrupole moments on both U and Np ions, with the same propagation vector that defines the magnetic structure. From the azimuthal dependence of the intensities we are able to determine the exact configuration of the quadrupolar ordering. The intensities of Bragg peaks associated with magnetic dipole and electric quadrupole order have different temperature dependences. On cooling below T$_{N}$, the magnetic dipole order develops faster on the uranium ions, with magnetic order on the Np ions increasing at a lower rate. At the same temperature, quadrupolar order on both the U and Np ions occurs along with an internal Jahn-Teller lattice distortion.

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arXiv2003

Multi-$\vec{k}$ Configurations

Using resonant x-ray scattering to perform diffraction experiments at the U M$_{4}$ edge novel reflections of the generic form $ $ have been observed in UAs$_{0.8}$Se$_{0.2}$ where $\vec{k} = < k00>$, with $k = {1/2}$ reciprocal lattice units, is the wave vector of the primary (magnetic) order parameter. The $ $ reflections, with $10^{-4}$ of the $ $ magnetic intensities, cannot be explained on the basis of the primary order parameter within standard scattering theory. A full experimental characterisation of these reflections is presented including their energy, azimuthal and temperature dependencies. On this basis we establish that the reflections most likely arise from the electric dipole operator involving transitions between the core 3d and partially filled $5f$ states. The temperature dependence couples the $ $ peak to the triple-$\vec{k}$ region of the phase diagram: Below $\sim 50$ K, where previous studies have suggested a transition to a double-$\vec{k}$ state, the intensity of the $ $ is dramatically reduced. Whilst we are unable to give a definite explanation of how these novel reflections appear, this paper concludes with a discussion of possible ideas for these reflections in terms of the coherent superposition of the 3 primary (magnetic) order parameters.

cond-mat↗

arXiv2003

Direct observation of orbital ordering in La$_{0.5}$Sr$_{1.5}$MnO$_4$ using soft x-ray diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La$_{0.5}$Sr$_{1.5}$MnO$_4$, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the Manganese $K$ edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese $L_{III}$ and $L_{II}$ edges which probes the orbital ordering directly. Energy scans at constant wavevector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering, direct orbital ordering and strong cooperative Jahn - Teller distortions of the Mn$^{3+}$ ions.

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