Browsing by Author "Lawrence, SM"

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    Circularly polarized soft x-ray diffraction study of helical magnetism in hexaferrite
    (American Physical Society, 2010-03-01) Mulders, AM; Lawrence, SM; Princep, AJ; Staub, U; Bodenthin, Y; García-Fernández, M; Garganourakis, M; Hester, JR; Macquart, RB; Ling, CD
    Magnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral. © 2010, American Physical Society
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    Direct observation of charge order and an orbital glass state in multiferroic LuFe2O4
    (American Physical Society, 2009-08-14) Mulders, AM; Lawrence, SM; Staub, U; García-Fernández, M; Scagnoli, V; Mazzoli, C; Pomjakushina, E; Conder, K; Wang, YD
    Geometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe2+ and Fe3+. The polarization and azimuthal angle dependence of the superlattice reflections demonstrate the absence of differences in anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state. © 2009, American Physical Society
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    Doping and temperature dependence of Mn 3d states in A-site ordered manganites
    (American Physical Society, 2010-12-06) García-Fernández, M; Staub, U; Bodenthin, Y; Pomjakushin, V; Mirone, A; Fernández-Rodríguez, J; Scagnoli, V; Mulders, AM; Lawrence, SM; Pomjakushina, E
    We present a systematic study of the electronic structure in A-site ordered manganites as function of doping and temperature. The energy dependencies observed with soft x-ray resonant diffraction (SXRD) at the Mn L2,3 edges are compared with structural investigations using neutron powder diffraction as well as with cluster calculations. The crystal structures obtained with neutron powder diffraction reflect the various orbital and charge ordered phases, and show an increase in the Mn-O-Mn bond angle as function of doping and temperature. Cluster calculations show that the observed spectral changes in SXRD as a function of doping are more pronounced than expected from an increase in bandwitdh due to the increase in Mn-O-Mn bond angle and are best described by holes that are distributed at the neighboring oxygen ions. These holes are not directly added to the Mn 3d shell but centered at the Mn site. In contrast, the spectral changes in SXRD as function of temperature are best described by an increase of magnetic correlations. This demonstrates the strong correlations between orbitals and magnetic moments of the 3d states. © 2010, American Physical Society
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    Resonant soft x-ray powder diffraction study to determine the orbital ordering in a-site-ordered SmBaMn2O6
    (American Physical Society, 2008-02) García-Fernández, M; Staub, U; Bodenthin, Y; Lawrence, SM; Mulders, AM; Buckley, CE; Weyeneth, S; Pomjakushina, E; Conder, K
    Soft x-ray resonant powder diffraction has been performed at the Mn L-2,L-3 edges of A-site-ordered SmBaMn2O6. The energy and polarization dependences of the (1/4 1/4 0) reflection provide direct evidence for a (x(2) - z(2))/(y(2) - z(2))-type orbital ordering in contrast to single-layer manganite. The temperature dependence of the reflection indicates an orbital reorientation transition at approximate to 210 K, below which the charge- and orbital-ordered MnO2 sheets show AAAA-type of stacking. The concurring reduction of the ferromagnetic superexchange correlations leads to further charge localization. © 2008, American Physical Society