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ABSTRACT: Through powder x-ray diffraction we have investigated the structural behavior
of SmVO3, in which orbital and magnetic degrees of freedom are believed to be
closely coupled to the crystal lattice. We have found, contrary to previous
reports, that SmVO3 exists in a single, monoclinic, phase below 200 K. The
associated crystallographic distortion is then stabilized through the
magnetostriction that occurs below 134 K. The crystal structure has been
refined using synchrotron x-ray powder diffraction data measured throughout the
structural phase diagram, showing a substantial Jahn-Teller distortion of the
VO6 octahedra in the monoclinic phase, compatible with the expected G-type
orbital order. Changes in the vanadium ion crystal field due to the structural
and magnetic transitions have then been probed by resonant x-ray diffraction.
08/2012;
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ABSTRACT: Soft X-ray resonant diffraction is a new technique and there exist several examples of large resonant enhancements of charge
and magnetic scattering that can be obtained at the L-edges of 3d transition metal oxides. Here we have employed resonant soft X-ray scattering at the manganese L edges which provide a direct measurement of the orbital ordering. We have studied the layered manganite La0.5Sr1.5MnO4 that displays charge, spin, and orbital ordering. Energy scans at constant wavevector show that there are two separate contributions
to the observed scattering, direct Goodenough orbital ordering and strong cooperative Jahn–Teller distortions of the Mn3+ ions. Finally we will show how that the charge, spin and orbital degrees of freedom are strongly correlated in these materials.
KEY WORDS:orbital ordering–manganites–X-ray scattering–synchrotron radiation
Journal of Superconductivity 04/2012; 18(5):687-691.
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T A W Beale,
S B Wilkins,
R D Johnson,
S R Bland,
Y Joly,
T R Forrest,
D F McMorrow,
F Yakhou,
D Prabhakaran,
A T Boothroyd,
P D Hatton
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ABSTRACT: We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wave vector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.
Physical Review Letters 08/2010; 105(8):087203. · 7.37 Impact Factor
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ABSTRACT: We report the design and construction of a novel soft x-ray diffractometer installed at Diamond Light Source. The beamline endstation RASOR is constructed for general users and designed primarily for the study of single crystal diffraction and thin film reflectivity. The instrument is comprised of a limited three circle (theta, 2theta, and chi) diffractometer with an additional removable rotation (phi) stage. It is equipped with a liquid helium cryostat, and post-scatter polarization analysis. Motorized motions are provided for the precise positioning of the sample onto the diffractometer center of rotation, and for positioning the center of rotation onto the x-ray beam. The functions of the instrument have been tested at Diamond Light Source, and initial test measurements are provided, demonstrating the potential of the instrument.
The Review of scientific instruments 07/2010; 81(7):073904. · 1.52 Impact Factor
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T. A. W. Beale,
S. B. Wilkins,
R. D. Johnson,
S. R. Bland,
Y Joly,
T. R. Forrest,
D. F. McMorrow,
F. Yakhou,
D. Prabhakaran,
A. T. Boothroyd,
P. D. Hatton
[show abstract]
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ABSTRACT: We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wavevector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization. Comment: 5 pages, 4 figures
06/2010;
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ABSTRACT: Magnetite is one of the oldest known magnetic materials, but questions still surround both its crystal and electronic structures at low temperature. The most debated of these low temperature properties regard the presence, or lack of, of charge and orbital order. Using resonant x-ray diffraction at the iron K-edge to probe the long range order present on the iron sites, we have studied (0 0 )C type reflections. By using the technique of full linear polarisation, we have shown that the key features of the reflections can be described merely using the simplified Pmca structure, without invoking orbital order.
Journal of Physics Conference Series 02/2010; 211(1):012009.
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ABSTRACT: Resonant x-ray scattering measurements have been performed at the (011) Bragg forbidden reflection of YVO3 at the vanadium K-edge. Data were taken above and below the orbital, magnetic and structural transition occurring at 77 K. Energy spectra calculated by the FDMNES code are shown to be in excellent agreement with our experimental data, conclusively showing the signal to originate from anisotropic tensor of susceptibility scattering; ie. solely due to distortions of the lattice and hence the crystal field. We thus resolve the ambiguous origin of the resonant energy spectra in the literature.
Journal of Physics Conference Series 02/2010; 200(1):012073.
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ABSTRACT: Despite being one of the oldest known magnetic materials, and the classic mixed valence compound, thought to be charge ordered, the structure of magnetite below the Verwey transition is complex and the presence and role of charge order is still being debated. Here, we present resonant x-ray diffraction data at the iron K-edge on forbidden (0, 0, 2n+1)(C) and superlattice [Formula: see text] reflections. Full linear polarization analysis of the incident and scattered light was conducted in order to explore the origins of the reflections. Through simulation of the resonant spectra we have confirmed that a degree of charge ordering takes place, while the anisotropic tensor of susceptibility scattering is responsible for the superlattice reflections below the Verwey transition. We also report the surprising result of the conversion of a significant proportion of the scattered light from linear to nonlinear polarization.
Journal of Physics Condensed Matter 12/2009; 21(48):485601. · 2.55 Impact Factor
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S B Wilkins,
T R Forrest, T A W Beale,
S R Bland,
H C Walker,
D Mannix,
F Yakhou,
D Prabhakaran,
A T Boothroyd,
J P Hill,
P D Hatton,
D F McMorrow
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ABSTRACT: The magnetic structures which endow TbMnO(3) with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L(2) edge, Mn 3d moments; Tb M(4) edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures.
Physical Review Letters 11/2009; 103(20):207602. · 7.37 Impact Factor
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S. B. Wilkins,
T. R. Forrest, T. A. W. Beale,
S. R. Bland,
H. C. Walker,
D Mannix,
F. Yakhou,
D. Prabhakaran,
A. T. Boothroyd,
J. P. Hill,
P. D. Hatton,
D. F. McMorrow
[show abstract]
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ABSTRACT: The magnetic structures which endow TbMnO$_3$ with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn $L_2$ edge, Mn 3d moments; Tb M$_4$ edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a non-collinear magnetic structure, as has previously been supposed, it is shown to occur between two non-collinear structures. Comment: Submitted to Physical Review Letters
10/2009;
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ABSTRACT: 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.
12/2008;
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T. R. Forrest,
S. R. Bland,
S. B. Wilkins,
H. C. Walker, T. A. W. Beale,
P. D. Hatton,
D. Prabhakaran,
A. T. Boothroyd,
D Mannix,
F. Yakhou,
D. F. McMorrow
[show abstract]
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ABSTRACT: 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.
10/2008;
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ABSTRACT: 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. Comment: 10 pages, 11 figures, accepted in Physical Review B
09/2008;
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Physical Review B 05/2007; 75(17). · 3.69 Impact Factor
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ABSTRACT: 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.
02/2007;
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Phys. Rev. B. 07/2006; 74(4).
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ABSTRACT: Resonant soft x-ray diffraction has been used to probe the temperature dependent orbital and magnetic structure of LaSr2Mn2O7. Previous crystallographic studies have shown that this material has almost no MnO6 oxygen displacements due to Jahn–Teller distortions at low temperatures. Within the low-temperature A-type antiferromagnetic phase, we found strong intensity at the () orbital and (0, 0, 1) magnetic reflections. This shows that even in the near absence of Jahn–Teller distortions, this compound is strongly orbitally ordered. A fit to the Mn L-edge resonance spectra demonstrates the presence of orbital ordering of the Mn3+ ions with virtually no Jahn–Teller crystal field in addition to possible Mn3+ and Mn2+-like valence fluctuations.
Journal of Physics Condensed Matter 06/2006; 18(24):L323. · 2.55 Impact Factor
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ABSTRACT: The use of softer-energy X-rays produced by synchrotron radiation for diffraction is an area of current interest. In this paper, experiments exploiting resonant scattering at the L absorption edges of 3d transition metal elements are reported. Such energies, typically 500-1000 eV, are at the extreme limit of soft X-ray diffraction where absorption effects are so severe that the sample and diffractometer must be placed in a windowless high-vacuum vessel. In addition, the Ewald sphere is so small as to likely contain, at most, only a single Bragg reflection. Advantages of using such radiation for the study of weak diffraction effects such as anomalous scattering, charge ordering, magnetic diffraction and orbital ordering are reported.
Journal of Synchrotron Radiation 08/2005; 12(Pt 4):434-41. · 2.73 Impact Factor
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ABSTRACT: 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.
01/2005;
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ABSTRACT: 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+-\delta,k+-\delta,l) and (h+-2\delta,k+-2\delta,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. Comment: 8 pages, 9 figures
12/2004;
