Jaru Jutimoosik

Determination of miscibility in MgO-ZnO nanocrystal alloys by x-ray absorption spectroscopy

The local structure of MgxZn1−xO nanocrystals is studied using synchrotron x-ray absorption near edge structures (XANES) over the full range of composition, from x = 0 to 1. Mg and Zn K-edges XANES measurements allow us to selectively study the local environments around Mg and Zn atoms in these nanocrystalline samples. Our results indicate that, for MgO-ZnO alloys, Zn is highly miscible in the rocksalt domain (i.e., up to ∼50 at. %) while the miscibility of Mg in the wurtzite domain is much less but is still substantial (i.e., up to ∼20 at. %). The simulated XANES spectra, based on first principles methods, are consistent with the observed spectra, confirming our finding. Because it is short-ranged and element-specific, the technique is useful for local structure and crystal phase determination of nanostructures, quantum dots, and mixed-phase alloys in general.

Enhancement of thermoelectric properties of CoSb3-based skutterudites by double filling of Tl and In

Thermoelectric (TE) generators can directly generate electrical power from waste heat, and thus could be an important part of the solution to future power supply and sustainable energy management. The main obstacle to the widespread use of TE materials in diverse industries, e.g., for exhaust heat recovery in automobiles, is their low efficiency in converting heat to electricity. The conversion efficiency of TE materials is quantified by the dimensionless figure of merit, ZT, and the way to enhance ZT is to decrease the lattice thermal conductivity (κlat) of the material, while maintaining a high electrical conductivity, i.e., to create a situation in which phonons are scattered but electrons are unaffected. Here, we report skutterudites filled by Tl and In, Tl0.1InxCo4Sb12, which allow a dramatic reduction of κlat, yielding a ZT of 1.2 at 700 K. We demonstrate that the reduction of κlat is due to the effective phonon scattering induced both by the rattling of Tl and In and by the naturally formed In2O3 n...

Lattice positions of Sn in Cu2ZnSnS4 nanoparticles and thin films studied by synchrotron X-ray absorption near edge structure analysis

Lattice positions of Sn in kesterite Cu2ZnSnS4 and Cu2SnS3 nanoparticles and thin films were investigated by XANES (x-ray absorption near edge structure) analysis at the S K-edge. XANES spectra were analyzed by comparison with simulations taking into account anti-site defects and vacancies. Annealing of Cu2ZnSnS4 nanoparticle thin films led to a decrease of Sn at its native and defect sites. The results show that XANES analysis at the S K-edge is a sensitive tool for the investigation of defect sites, being critical in kesterite thin film solar cells.

Self-organization of Fe clusters on mesoporous TiO2 templates

Fe layers with thicknesses between 5 and 100 nm were sputtered on mesoporous nanostructured anatase TiO2 templates. The morphology of these hybrid films was probed with grazing-incidence small-angle X-ray scattering and X-ray reflectivity, complemented with magnetic measurements. Three different stages of growth were found, which are characterized by different correlation lengths for each stage. The magnetic behavior correlates with the different growth regimes. At very small thicknesses the TiO2 template is coated and a porous Fe film results, with in-plane and out-of-plane magnetization components. With increasing thickness, agglomeration of Fe occurs and the magnetization gradually turns mostly in plane. At large thicknesses, the iron grows independently of the template and the magnetization is predominantly in plane with a bulk-like characteristic.

Exotic exchange bias at epitaxial ferroelectric-ferromagnetic interfaces

Multiferroics in spintronics have opened up opportunities for future technological developments, particularly in the field of ferroelectric (FE)-ferromagnetic (FM) oxide interfaces with functionalities. We find strong exchange bias shifts (up to 84 Oe) upon field cooling in metal-oxide (Fe/BaTiO3) films combining FM and FE layers. The saturation magnetic moment of the FM layer is also significantly higher than in bulk (3.0 ± 0.2 μB/atom) and the reversal mechanism occurs via a domain nucleation process. X-ray absorption spectroscopy at the Fe K-edge and Ba L3-edge indicate presence of few monolayers of antiferromagnetic FeO at the interface without the formation of any BaFeO3 layer. Polarized neutron reflectometry corroborates with our magnetization data as we perform depth profiling of the magnetic and structural densities in these bilayers. Our first principles density functional calculations support the formation of antiferromagnetic FeO layers at the interface along with an enhancement of Fe magnetic ...

On preferred Mn site in multiferroic BiFeO3: A view by synchrotron x-ray absorption near edge structure spectroscopy

Synchrotron X-ray Near Edge Structure (XANES) experiments were performed on Mn-doped BiFeO3 samples and compared with ab initio XANES calculations. The Fe and Mn K-edge XANES measurements were carried out on Mn-doped BiFeO3 powders (with 5–40 Mn mole%). Both XANES spectra were obtained in a fluorescent mode at ambient temperature with a Ge(111) double crystal monochromator. From the XANES results, it was clearly identified that the oxidation state of both Fe and Mn ions in BiFeO3 structure was +3. The features of the measured Mn K-edge XANES were consistent with ab initio XANES of Mn on the Fe site and inconsistent with Mn on other sites. The clear agreement between measured and ab initio theoretical XANES spectra was the strongest evidence of Mn substituting for Fe in BiFeO3 structure for low Mn content. More interestingly, at higher Mn content, the presence of a second phase precipitation of BiMnO3 and BiMn2O5 was evident. This clearly indicated the Mn solubility limit in the BiFeO3 structure, while the two trace compounds could also be responsible for the relevant properties reported in Mn-doped BiFeO3 materials.

Temperature dependent local structure in BaTiO3 single crystal

ABSTRACT The temperature dependent local structure of BaTiO3 single crystal was investigated by Synchrotron X-ray Absorption Spectroscopy (XAS) technique. From the X-ray Absorption Near-Edge Structure (XANES) measurements at Ti K-edge, it was seen that the local structure around Ti atom gradually changed with increasing temperature, showing the phase transition from tetragonal to cubic. A comparison between the measured and calculated XANES spectra revealed the gradual change of local structure as a result of the decreasing c/a ratio.

Synchrotron X-Ray Absorption Study of Cu and Mn Doped BiFeO3-BaTiO3 Multiferroic Ceramics

In this work, an experimental X-ray Absorption Spectroscopy (XAS) measurement was employed to determine the local structure and valency of Cu and Mn in BiFeO3-BaTiO3. Synchrotron x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) experiments were performed on Cu-doped and Mn-doped BiFeO3-BaTiO3 samples. The Cu- and Mn-doped BiFeO3-BaTiO3 ceramic samples were used for the XAS experiment. XAS spectra at the Cu and Mn K-edge were recorded in flurorescent mode with 13-element Ge detector. The spectra were collected at ambient temperature with a Ge(111) double crystal monochromator and recorded after performing an energy calibration. The features of the measured Cu and Mn K-edge XANES was consistent with Cu and Mn on the Fe/Ti site and inconsistent with Cu and Mn on other sites. The clear agreement was the strongest evidence of Cu and Mn substituting for B-site in BiFeO3-BaTiO3 materials. In addition, the valency of both Cu and Mn ions in BiFeO3-BaTiO3 materials was also confirmed.

Effect of electric field on local structure of PZT single crystal studied by X-ray absorption spectroscopy technique

ABSTRACT The aim of this work is to investigate the local structure of PbZr1−xTixO3 (PZT) single crystal with x = 0.44 under the application of electric field employing Synchrotron X-ray Absorption Near-Edge Structure (XANES) technique. In this experiment, PZT single crystal was subjected to 0.35 kV/mm and 0.70 kV/mm electric fields. The Ti K-edge XANES spectra reflect the significant change of the local structure of PZT unit cell before and after static electrical loading. From the XANES results, it was clearly seen that XANES spectral intensity gradually changed when the electrical load was applied. This can be caused by the shift of Ti atoms in the unit cell. The clear agreement between the measured and simulated XANES spectra was the supporting evidence of Ti off-center displacement in the PZT unit cell.

Phase formation investigation in PZT materials by Synchrotron X-ray absorption spectroscopy techniques

ABSTRACT PbZr1-xTixO3 (PZT) powders (x = 0.45–0.54) were synthesized by conventional solid state reaction method. The phase information was investigated by a combination of X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. The XRD measurements indicated the global phase formation of the prepared powders was the mixture of rhombohedral and tetragonal phases. However, the local phase formation around titanium atom was revealed using X-ray Absorption Near-Edge Structure (XANES), indicating the existence of monoclinic phase within the compositional range studied.