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Magnetic decoupling in sputtered Fe/Si superlattices and multilayers

Description: A series of sputtered Fe/Si superlattices was grown to study the magnetic coupling between ferromagnetic Fe layers (30{Angstrom} thick) for Si spacer-layer thicknesses (t{sub Si}) between 10 and 40{Angstrom}. The material is ferromagnetically coupled for t{sub Si}<13{Angstrom} and antiferromagneticly coupled for 13{Angstrom}<t{sub Si}<17{Angstrom}. For t{sub Si}>17{Angstrom} the magnetic Fe layers are uncoupled. X-ray analysis indicates that the system is well layered, but that the crystal structure remains coherent only for t{sub Si}<17{Angstrom}. These results, along with our Moessbauer investigation, strongly suggest that the Si layer is crystalline for t{sub Si}<17{Angstrom}, and is iron silicide in nature. For thicker spacers, Si becomes amorphous. We propose a model of the layering that is consistent with the known properties of Fe-silicide.
Date: September 1, 1992
Creator: Fullerton, E. E.; Mattson, J. E.; Lee, S. R.; Sowers, C. H.; Huang, Y. Y.; Felcher, G. et al.
Partner: UNT Libraries Government Documents Department

Low temperature Moessbauer thermometry

Description: In Moessbauer low temperature thermometry the temperature is determined from the relative intensities of the various hyperfine lines as these are affected by the temperature dependent Boltzman populations of the hyperfine levels in the resonant nuclei. One can perform either source experiments or those that make use of the nuclear polarization in the absorber. The second method is more advisable because the radioactive heating is much smaller. The present work is concerned with a fundamental difficulty in absorber thermometry which arises from the fact that the line intensity in a Moessbauer spectrum is also dependent on the absorber thickness, the dependence on that is not a linear one. It is demonstrated that if this thickness effect is not analyzed properly, the experimental results will yield erroneous temperature values, destroying the absolute nature of the Moessbauer thermometer. (WHK)
Date: January 1, 1975
Creator: Maletta, H. & Shenoy, G.K.
Partner: UNT Libraries Government Documents Department

Vibrational Spectroscopy of Fe(OH)2 at High Pressure: Behavior of the O-H Bond

Description: Infrared and Raman spectra of Fe(OH){sub 2}, ''white rust'', were measured between 7 GPa and 21 GPa at ambient temperature. The frequency of the infrared-active A{sub 2u} and of the Raman-active A1g stretching modes of the O-H group decrease linearly with pressure with slopes of -1.3 {+-} 0.1 cm{sup -1}/GPa and -4.9 {+-} 0.2 cm{sup -1}/GPa, respectively. The peak widths of both the infrared-active and Raman-active modes increase non-linearly with pressure, with a discontinuous increase of in broadening between 10 and 12.5 GPa. The overall broadening of the A{sub 2u} and of the A{sub 1g} stretching modes is approximately four-fold in the examined pressure range. The results of this spectroscopic study are compatible with the trends observed in recent neutron diffraction studies in the isostructural Co(OH){sub 2}. Progressive pressure-induced H disordering could be a viable model to interpret both the broadening of the OH stretching mode and the changes in oxidation state of Fe recently observed by Moessbauer spectroscopy.
Date: November 9, 2004
Creator: Speziale, S; Jeanloz, R; Milner, A; Pasternak, M P & Zaug, J
Partner: UNT Libraries Government Documents Department

Introduction to nuclear resonant scattering with synchrotron radiation

Description: In recent years, the use of synchrotron radiation has enjoyed increasing interest in applications to topics of Moessbauer spectroscopy. The development was initiated by the pioneering experimental work of Gerdau et al. following the original proposal of Ruby to use synchrotron radiation for the excitation of low energy nuclear resonances. From the early experiments it was clear that synchrotron radiation experiments with nuclear resonances would only succeed if familiar energy resolved measurements were replaced with a new time resolved technique. During the last decade, the authors experienced the refinement of this novel method for obtaining hyperfine parameters. This exciting development-materialized because of more intense synchrotron radiation sources at the European Synchrotron Radiation Facility (ESRF) and at the Advanced Photon Source (APS), powerful new avalanche photo diode detectors, and improved high energy resolution monochromators. Simultaneously the tools for evaluation of the novel time spectra were created, e.g., Sturhahn and Gerdau developed extensive computer codes based on the theoretical descriptions of Hannon and Trammel. Many beautiful demonstrations of the basic features of the coherent elastic scattering channel using Bragg- and Laue-reflections from single crystals deepened the understanding of nuclear resonant scattering. The concepts leading to the application of synchrotron radiation to elastic and inelastic nuclear resonant scattering are discussed. The resulting new experimental techniques are compared to conventional Moessbauer spectroscopy. A survey of situations that favor experiments with synchrotron radiation is offered.
Date: August 1, 1997
Creator: Sturhahn, W.; Alp, E.E.; Toellner, T.S.; Hession, P.; Hu, M. & Sutter, J.
Partner: UNT Libraries Government Documents Department

Inelastic scattering of synchrotron radiation from electrons and nuclei for lattice dynamics studies.

Description: The inelastic scattering of x-rays, one of the first applications of x-rays to the field of condensed matter physics, has been rejuvenated in the last decade. The availability of synchrotrons radiation from wiggler and undulator sources combined with advances in monochromatization of the incident beam and analysis of the scattered beam with meV resolution led to the measurement of phonon dispersion relations. In addition, the use of Moessbauer nuclei as scatterer and analyzers has led to the discovery of the inelastic nuclear resonant scattering technique. This new method allows extraction of partial phonon density of states from amorphous materials, thin films, multilayers and interfaces, and liquids.
Date: December 3, 1999
Creator: Alp, E. E.; Sturhahn, W.; Sinn, H.; Toellner, T.; Hu, M.; Sutter, J. et al.
Partner: UNT Libraries Government Documents Department

Moessbauer effect study of the Fe spin structure in exchange-bias and exchange-spring systems.

Description: Theoretical and experimental issues connected with exchange-bias and exchange-spring behavior are briefly reviewed, and the potential of conversion electron Moessbauer spectroscopy (CEMS), including the {sup 57}Fe probe layer technique, to reveal the spin structure in layered systems is emphasized. First experimental results are reported for the new exchange-bias system FeSn{sub 2}/Fe and for the exchange-spring system Sm-Co/Fe.
Date: March 14, 2002
Creator: Keune, W.; Kuncser, V. E.; Doi, M.; Askin, M.; Spies, H.; Sahoo, B. et al.
Partner: UNT Libraries Government Documents Department

Pressure effect on the electronic structure of iron in (Mg,Fe)(Al,Si)O3 perovskite: A combined synchrotron M?ssbauer and x-ray emission spectroscopy study up to 100 GPa

Description: We investigated the valence and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample, (Mg{sub 0.88}Fe{sub 0.09})(Si{sub 0.94}Al{sub 0.10})O{sub 3}, at 300 K and up to 100 GPa, using diamond-anvil cells and synchrotron Moessbauer spectroscopy techniques. Under elevated pressures, our Moessbauer time spectra are sufficiently fitted by a ''three-doublet'' model, which assumes two ferrous (Fe{sup 2+}) iron types and one ferric (Fe{sup 3+}) iron type with distinct hyperfine parameters. At pressures above 20 GPa, the fraction of the ferric iron, Fe{sup 3+}/{Sigma}Fe, is about 75% and remains unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. Between 20 and 100 GPa, the quadruple splittings of all three iron types do not change with pressure, while the isomer shift between the Fe{sup 3+} types and the Fe{sup 2+} type increases continuously with increasing pressure. In conjunction with previous x-ray emission data on the same sample, the unchanging quadruple splittings and increasing isomer shift suggest that Fe{sup 2+} undergoes a broad spin crossover towards the low-spin state at 100 GPa, while Fe{sup 3+} remains in the high-spin state. The essentially constant quadruple splittings of Fe{sup 2+} can also be taken as an indication for strong resistance against further distortion of the local iron environment after initial compression.
Date: January 23, 2006
Creator: Li, J; Sturhahn, W; Jackson, J; Struzhkin, V V; Lin, J F; Zhao, J et al.
Partner: UNT Libraries Government Documents Department

Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD

Description: Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was &gt; 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrent with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can ...
Date: June 22, 2009
Creator: Pearce, C. I.; Henderson, C. M. B.; Telling, N. D.; Pattrick, R. A.D.; Vaughan, D. J.; Charnock, J. M. et al.
Partner: UNT Libraries Government Documents Department

TABLES AND GRAPHS FOR COMPUTING DEBYE-WALLER FACTORS IN MOSSBAUER EFFECT STUDIES

Description: A table of functions is presented that simplifies the evaluation of the Debye-Waller factor which gives the fraction of recoil-free gamma ray processes in Mbssbauer effect studies. A graph of the recoil-free fraction, f, as a function of the ratio of the Debye temperature to the crystal temperature for selected recoil energy to Debye temperature ratios is included for rapid estimation of f. (auth)
Date: January 15, 1962
Creator: Muir, A. H., Jr.
Partner: UNT Libraries Government Documents Department

Techniques for inelastic x-ray scattering with {mu}eV resolution.

Description: We introduce a novel type of spectrometer that provides a {micro}eV bandpass together with a tunability over a few meV. The technique relies on nuclear resonant scattering (Moessbauer effect) of synchrotrons radiation at the 14.4-keV resonance of {sup 57}Fe. Energy tuning is achieved by the Doppler effect in high speed rotary motion. The resonantly scattered monochromatic radiation is extracted by a polarization filtering technique or by spatial separation due to the ''nuclear lighthouse effect''.
Date: October 23, 1998
Creator: Rohlsberger, R.
Partner: UNT Libraries Government Documents Department

Ferrocyanide safety program: FY 1995 report on Mossbauer spectroscopy tank activities

Description: This report summarizes FY 1995 activities on the Mossbauer Spectroscopy task. The National Aeronautics and Space Administration has developed a miniaturized Mossbauer spectrometer that is small enough to perform elevation scans in the Hanford Site waste tank liquid observation wells. Mossbauer spectroscopy is a sensitive and selective method that can detect and distinguish between different iron-based compounds in many types of chemical environments. Iron is major constituent of ferrocyanide waste and information about its location and composition in the tanks supports interim safe storage of the waste and final resolution of the Ferrocyanide Safety Issue. Results obtained from studies of ferrocyanide waste simulants and those from the first test in a hot cell environment using radioactive tank waste are presented
Date: October 2, 1995
Creator: Riedel, F.R.
Partner: UNT Libraries Government Documents Department

Phonon density of states in epitaxial Fe/Cr(001) superlattices

Description: Incoherent nuclear resonant absorption of synchrotron radiation at the 14.413 keV nuclear resonance of {sup 57}Fe was employed to measure directly the Fe-projected (partial) photon density of states (DOS) in epitaxial [Fe(8.7ML)/Cr(8ML)]{sub 200} superlattices and alloy films MBE-grown on MgO(001). Isotopically depleted {sup 56}Fe was used which gives no resonance signal. 0.7 monolayers (ML) thick {sup 57}Fe-probe layer (1{angstrom}) of 95.5% enrichment were placed either at the {sup 56}Fe-on-Cr interfaces or at the center of the {sup 56}Fe layers, thus providing a nuclear resonance signal from different places in the films. In addition, the authors prepared an epitaxial film which contains only a 1{angstrom}-thick {sup 57}Fe submonolayer in Cr(001) and no {sup 56}Fe layers. Moreover, they prepared a 7000 {angstrom}-thick epitaxial {sup 57}Fe{sub 0.03}Cr{sub 0.97}(001) alloy film. The measurements were performed at 300 K with 2.3 meV energy resolution around 14.413 keV. The phonon DOS of the center site was found to be very similar to that of bulk bcc Fe. Compared to the center site, the DOS of the other samples show distinct differences. In particular, longitudinal vibrations of Fe atoms are suppressed at the Fe/Cr interfaces.
Date: October 21, 1999
Creator: Ruckert, T.; Keune, W.; Sturhahn, W.; Hu, M. Y.; Sutter, J. P.; Toellner, T. S. et al.
Partner: UNT Libraries Government Documents Department