Bruce Ravel

Magnetic, structural, and transport properties of thin film and single crystal Co2MnSi

The magnetic, structural, and transport properties of the Heusler alloy Co2MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L21 structure for all films grown between 573 and 773 K. Films grown at 773 K display a four-fold decrease in the resistivity relative to those grown at lower temperatures and a corresponding 30% increase in the residual resistivity ratio (ρ300u200aK/ρ5u200aK). We show that the higher growth temperature results in lattice constants, room temperature resistivities, and magnetic properties that are comparable to that of the bulk single crystal.

Oxidation of iron in iron/gold core/shell nanoparticles

Recently, reverse micelles were used to produce core/shell nanoparticles of iron and gold of highly uniform size distribution and reported to contain metallic iron. A composite structure of this sort is attractive in biosensor and bioactuator applications; the metallic iron core provides high magnetic sensitivity and the goldcoating is easily functionalized. With iron/gold nanoparticles prepared by this method, we directly measure the chemical state of the iron component using x-ray absorption spectroscopy(XAS) and interpret our results along with the measuredmagnetization of the particles. In earlier reports, metallicity of the iron core was inferred from magnetization measurements rather than directly measured. In contrast, XAS directly measures the structural environment of the iron atoms. Because XAS interpretation is independent of symmetry or periodicity, it is an ideal tool for characterizing nanoparticles. Using XAS, we demonstrate that the iron component of the iron/gold nanoparticles is fully oxidized. We suggest that oxidation is a persistent problem in core/shell nanoparticles of this variety and recommend that characterization by XAS be a standard part of nanoparticle preparation.

Atomic disorder in Heusler Co2MnGe measured by anomalous x-ray diffraction

Heusler alloys Co2MnX, where X is Ge or Si, are predicted by the band theory to be a half-metallic ferromagnets, i.e., metals with all conduction electrons of the same spin state. To date, Heusler alloys have been measured with spin polarizations of 50%–60%, higher than Fe (∼40%), but not a true half metal. Structural defects involving site swapping between the Co and Mn sublattices have been proposed as a mechanism to diminish spin polarization. In this work, we present an anomalous x-ray diffraction measurement on Co2MnGe to determine antisite disorder in thin films with a high precision.

Determining the Grain Composition of the Interstellar Medium with High-Resolution X-Ray Spectroscopy

We investigate the ability of high-resolution X-ray spectroscopy to directly probe the grain composition of the interstellar medium. Using iron K-edge experimental data of likely ISM dust candidates taken at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory, we explore the prospects for determining the chemical composition of astrophysical dust and discuss a technique for doing so. Focusing on the capabilities of the Astro-E2 XRS microcalorimeters, we assess the limiting effects of spectral resolution and noise for detecting significant X-ray absorption fine structure signal in astrophysical environments containing dust. We find that given sufficient signal, the resolution of the XRS will allow us to definitively distinguish gas-phase from dust-phase absorption and certain chemical compositions.

Dense medium plasma synthesis of carbon/iron-based magnetic nanoparticle system

Using dense medium plasma technology, hybrid iron and iron oxide/carbon-based nanoparticle composites were synthesized under room temperature and atmospheric pressure conditions. Based on results from electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, Raman microscopy, atomic force microscopy and scanning electron microscopy, we conclude that the material is composed of spherical particles, 40–60 nm in diameter, which are a graphitic carbon host structure embedded with iron and iron oxide. Thermal gravimetry/differential thermal gravimetry analysis indicates that these composites are stable up to temperatures as high as 600u200a°C. Ferromagnetic resonance spectroscopy (FMR) and extended x-ray absorption fine-structure spectroscopy suggest that the bulk of the FMR signal in question is due to metallic Fe. Magnetite or maghemite is present, and the metallic content of the metal particles is 58(8)% and the remainder is oxidized with a sixfold oxygen coordination shell similar t...

Prospects for determining the grain composition of the interstellar medium with Chandra and Astro E2

The ability of high resolution X‐ray spectroscopy to directly probe the grain composition of the ISM is discussed. Using experimental data taken at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory, current and near‐future mission prospects for (1) distinguishing gas from dust phase absorption, and (2) determining the chemical composition of interstellar grains, through the details of X‐ray absorption fine structure is assessed. Details, in‐depth discussions and complete references can be found in Lee & Ravel 2005.

NSLS course on EXAFS data collection and analysis

The three-day course was divided into two mornings of lectures, two afternoons of hands-on data collection using six different NSLS spectroscopy beamlines (X9B, X11A, X18B, X19A, X23B, and X26A), and one full day of data analysis. The instructors on the NSLS beamlines were beamline scientists Nebojsa Marinkovic, Kaumudi Pandya, Wolfgang Caliebe, Syed Khalid and Tony Lanzirotti as well as Scott Calvin from NRL.