J. Herrero-Martín

Strong coupling of Sm and Fe magnetism in SmFeAsO as revealed by magnetic x-ray scattering

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element-specific x-ray resonant and nonresonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively, according to the same magnetic representation Gamma(5) and the same propagation vector (1 0 1/2). Below 5 K, the magnetic order of both sublattices changes to a different magnetic structure, and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice, as well as a change in the magnetic structure below 5 K, provides clear evidence of a surprisingly strong coupling between the two sublattices, and indicates the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds.

Structural distortion, charge modulation and local anisotropies in magnetite below the Verwey transition using resonant X-ray scattering.

The pattern of charge modulations and local anisotropies below the Verwey transition has been determined and quantified in high-quality Fe(3)O(4) single crystals and thin films grown on MgO by using resonant X-ray scattering at the Fe K-edge. The energy, polarization and azimuthal angle dependencies of an extensive set of reflections with potential sensitivity to charge or local anisotropy orderings have been analyzed to explore their origins. A charge disproportion on octahedral B sites of 0.20 ± 0.05 e(-) with [0 0 1] and [1 1 0] cubic periodicities has been confirmed, while no significant charge disproportion has been obtained with [0 0 1/2] cubic periodicity. Additional charge modulations in the monoclinic a-b plane are also present. In addition, the occurrence of new forbidden (1, 1, 0) and (0, 0, 2n + 1/2) cubic reflections that arise from the anisotropy of the local structure around different tetrahedral and octahedral Fe atoms is shown. This complex pattern of weak charge modulations and local anisotropies is fully compatible with the low-temperature crystal structure refined in the non-polar C2/c space group and disproves any bimodal charge disproportion of the octahedral Fe atoms.

Spin state and structural changes at the metal-insulator transition in YBaCo2O5.5 by synchrotron x-raysa)

Synchrotron x-ray diffraction and absorption, together with magnetic and thermoelectric measurements, have been used to investigate the mechanism driving the metal to insulator transition in YBaCo2O5.50. Results show a structural and magnetic transition concomitant with the metal to insulator transition. Absorption spectroscopy at the Co L3 edge as a function of temperature reveals minor changes across the transition. The set of experimental results are hardly conciliative with a high to low spin state change of Co ions at octahedral sites in this cobaltite.

Structural properties and singular phase transitions of metallic Pr0.50Sr0.50CoO3 cobaltite.

The Pr0.50Sr0.50CoO3 perovskite exhibits unique magnetostructural properties among the rest of the ferromagnetic/metallic Ln0.50Sr0.50CoO3 compounds. Existing reports are largely controversial. We have determined and described its structural evolution, which follows the Pm3̅m → R3̅c → Imma → I4/mcm transformations. The structural changes have been thoroughly described. The results are confronted with distinct nonconventional properties and spin-lattice coupling effects in another half-doped cobaltite based on praseodymium, Pr0.50Ca0.50CoO3. The Imma → u2009I4/mcm symmetry change is responsible for the unexpected second magnetic transition.