Nanosized superparamagnetic precipitates in cobalt-doped ZnO

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc... These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution x-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.Comment: 20 pages, 6 figures; details about background subtraction added to section III. (XMCD

M3,2-Edge X-Ray Absorption Near Edge Structure of 5d Metals

We report M3,2 edge XANES measurements for a series of 5d metals, Hf, Ta, W, Re, Ir, Pt and Au using a total electron yield technique. It was found that the M3,2 XANES for most 5d metals exhibit intense resonances at the threshold (whitelines). The area under the whitetine correlates with the unoccupied densities of d states of the metal. The M3,2 XANES features and their systematic are comparable to their L3,2 counterparts. These results clearly suggest that M3,2 edge XANES can be ased as an alternative probe to the conventional L3,2 -edge XANES for the unoccupied densities of d states of the 5d metals and their compounds

XAFS Studies of Self-Aligned Platinum Silicide Thin Films at the Pt M3,2 Edge and the Si K-Edge

Pt-Si thin films with the thickness of several hundred Å prepared on n-type Si(100) by UHV sputter-deposition procedures and subsequent annealing have been studied with X-ray absorption fine structure spectroscopy at the Pt M3,2 edge and the Si K-edge. It is found that, under favourable conditions, single phase PtSi films can be obtained. These films exhibit the same XAFS characteristics as those of bulk samples. The M3,2 edge exhibits XANES features very similar to those of the Pt L3,2 edge obtained from the samples. The analysis of the Pt M3,2 edge whiteline and the Si K-edge results show significant charge redistribution at both Pt and Si sites upon silicidation

Angular Dependent XAFS Studies of a MoSi2 Single Crystal

We report angular dependent X-ray absorption measurements at the Mo L3,2 edge and the Si K-edge for a MoSi. single crystal. The crystal was oriented so that an azimuthal rotation of the crystal about the surface normal (direction of the incident photons) would align the c-axis of the crystal anywhere between parallel and perpendicular to the polarization of the photons. It was found that while the Mo L3,2 edge XANES shows little angular dependence, the Si K-edge XANES shows a very strong polarization dependence of which the intensity exhibits a two-fold symmetry. This angular dependence of the Si K-edge XANES is attributed to the high densities of unoccupied states localized perpendicular to the Si-Si bond

Synchrotron radiation studies of platinum silicide thin films

PtSi thin films prepared by UHV sputter-deposition procedures on n-type Si(100) wafers have been studied with the following techniques: (a) X-ray absorption fine structure spectroscopy at the Si K-edge, Si L2,3-edge, and Pt L3-edge; (b) X-ray reflectivity at photon energies below and above the Pt L3-edge threshold and (c) Photoemission. These techniques provide valuable information about the electronic structure, morphology, local structure, thickness, density and roughness, and surface and interface properties of the films. Preliminary results from the application of these techniques to the study of several PtSi thin films (with thickness from several hundreds to thousands of \uc5) are reported.NRC publication: Ye