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Electronic and local structure of CaBaCo4-xMxO7 (M= Fe, Zn) revealed by X-ray absorption spectroscopy

AuthorsBlasco, Javier; Cuartero, Vera; Subías, G.; García, Joaquín; Rodríguez-Velamazán, J. A.; Ritter, C.
Issue Date2018
Citation17th International Conference on X-Ray Absorption Fine Structure (2018)
AbstractMagnetoelectric materials attract a great interest due to their wide applications in spintronics. Most of them are based on transition metal oxides with polar structures showing magnetic frustration. This is the case for CaBaCo4O7, which is ferrimagnetic below Tc=64K and also shows a linear magnetoelectric coupling below this temperature. Ab-initio calculations proved that the material is pyroelectric, and the large pyroelectric currents observed were ascribed to exchange-striction effects. It adopts an orthorhombic unit cell (space group Pbn21). The crystallographic structure of CaBaCo4O7 consists of a stacking of alternating triangular (T) and kagomé (K) layers of CoO4 tetrahedra along the c axis. There are four different crystallographic sites for Co: Co1 stays at the T layer, while Co2, Co3, and Co4 are in the K layers and ferrimagnetism appears because the magnetic moments at the Co1 and Co4 sites are larger than those at the Co2 and Co3 sites. The stoichiometric formula corresponds to CaBaCo22+Co23+O7 and therefore the nominal oxidation state for Co is 2.5+. Furthermore, the substitution of Co with another transition metal may be useful to increase the transition temperature and magnetic coupling. To fully characterize the tetrahedral distortion on CaBaCo4O7 and the effects of doping with Fe and Zn on Co sublattice, we performed XAS measurements as a function of temperature on Co, Fe and Zn K edges at BM23 at the ESRF (Grenoble, France). CaBaCo4-xFexO7 (x=0.5, 1, 1.5, 2) and CaBaCo4-x-yFexZnyO7 (x=0, 1, 2 and y=1) samples were synthesized by solid state chemistry reactions and the resulting powders were pressed into pellets mixed with cellulose for optimized XAS transmission measurements. From XANES, we conclude that a mixed-valence oxidation state is found for Co atom in all the studied CaBaCo4-xMxO7 (M=Fe or Zn) samples, with a combination of Co2+ and Co3+ in their formal ionic species. Fe and Zn are incorporated as Fe3+ and Zn2+ in the whole series. The analysis of EXAFS measurements show that the largest Debye-Waller factors are found for the intermediate Co3+/2+ valences, indicating either a larger distortion or the occupation of different crystallographic sites (disordered distribution). EXAFS measurements as a function of temperature show the presence of a local disorder highly localized in the CoO4 tetrahedra that remains unchanged for all the samples except for the parent CaBaCo4O7 compound. A small but appreciable increase in the local distortion of the CoO4 tetrahedra is observed at the magneto-electric transition temperature for this sample. This reveals the occurrence of a local magneto-elastic coupling at the ferrimagnetic phase that may be related to the observation of the pyroelectric effect in this composition.
DescriptionResumen del trabajo presentado a la 17th International Conference on X-Ray Absorption Fine Structure (XAFS), celebrada en Kraków (Polonia) del 22 al 27 de julio de 2018.
Appears in Collections:(ICMA) Comunicaciones congresos
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