D. L. Knies

Magnetic and transport properties of radiation damaged La0.7Ca0.3MnO3.0 thin films

Magnetization and resistivity measurements are reported for a series of radiation damaged La0.7Ca0.3MnO3 pulsed laser deposited thin films. When plotted as a function of activation energy, trends in the electrical transport properties are similar to those exhibited in the magnetic properties. A sharp drop in both Tc and Tp in samples with activation energies greater than ∼110 meV suggests a “decoupling” of the magnetic and transport properties. The results suggest the magnetic order is no longer sufficient to delocalize the system of the extra disorder induced by the radiation damage.

A report on the Naval Research Laboratory AMS facility

Abstract A large portion of the novel trace element AMS (TEAMS) facility at the Naval Research Laboratory (NRL) is now installed. Vacuum and beam optics hardware is in place, and testing has begun with a single-cathode ion source in place of our commercial secondary ion mass spectrometer source. This more intense source simplifies diagnostic testing and our initial research efforts. We have received a portion of our 12 position-sensitive-detector modules for the focal plane of the spectrograph, and their testing has begun. For our initial research, post-acceleration stripping was explored as a means to remove the interfering 32S isobar in 32Si measurements, and measurements were performed on a test bench to evaluate the feasibility of neutral beam injection for species like Ar which cannot form negative ions. Programmatically, NRL is actively involved in the study of gas hydrates present under the ocean floor, which includes plans to analyze cycling between various carbon pools present there. Since 14C analysis is an important part of this work, a graphitization facility to process various sources of carbonaceous material has been constructed. In addition, the TEAMS design will be modified to include a multi-cathode ion source and a switching electrostatic analyzer (ESA) to choose between the two different ion sources.

A NEW ACCELERATOR MASS SPECTROMETER FOR TRACE ELEMENT ANALYSIS AT THE NAVAL RESEARCH LABORATORY

Abstract A new accelerator mass spectrometer (AMS) facility is under construction at the Naval Research Laboratory for trace element analysis of electronic, biological, and geological materials. The design provides for parallel mass analysis over a broad mass range for conducting and insulating samples, and offers 10 μm lateral image resolution, depth profiling, and sensitivity down to tens of ppt of trace impurities. The facility will use a modified commercial secondary ion mass spectrometer as the source of secondary ions. A Pretzel magnet will act as a unique recombinator to simultaneously transmit from 1 to 200 amu ions, but attenuate intense matrix-related beams. After acceleration, a single charge state will be selected by a 3° electrostatic bend, then the selected ions will be energy analyzed by a 2.2 m radius, 30° spherical electrostatic analyzer ( E ΔE ≈ 800 ). Finally, a split pole mass spectrograph with a 1.5 m long focal plane will provide parallel analysis over a broad mass range ( M max M min ≈ 8 ) with high mass resolution ( M ΔM ≈ 2500 ).

Epitaxial Pb–Fe–O film with large planar magnetic anisotropy on (0001) sapphire

Epitaxial films (1.7 μm thick) with the composition PbFe12.9O22.9 and hexagonal lattice parameters a=5.12 A and c=23.67 A have been pulsed laser deposited at 600 °C in 50 mTorr of O2 onto single-crystal (0001) sapphire substrates. Epitaxy was determined using standard powder x-ray diffraction (XRD) and grazing incidence XRD. The films were deposited using a single-phase polycrystalline PbFe12O19 target. The composition of the films is PbFe12.9O22.9, which was measured using Rutherford backscattering spectrometry. Static magnetic measurements were performed using a vibrating sample magnetometer and SQUID magnetometer in order to measure magnetic anisotropy, magnetic remanence (Mr), coercive field (Hc), and saturation magnetization (4πMs) of the films. The PbFe12.9O22.9 films exhibit magnetically isotropic behavior in the film plane with remanence ratio (Mr/Ms) and Hc values of 88±2.9% and 2500±97 Oe, respectively. However, the films are anisotropic with respect to the film normal such that the c axis is a ...

Sample distillation/graphitization system for carbon pool analysis by accelerator mass spectrometry (AMS)

Abstract A facility at the Naval Research Laboratory (NRL), Washington, DC, has been developed to extract, trap, cryogenically distill and graphitize carbon from a suite of organic and inorganic carbon pools for analysis by accelerator mass spectrometry (AMS). The system was developed to investigate carbon pools associated with the formation and stability of methane hydrates. However, since the carbon compounds found in hydrate fields are ubiquitous in aquatic ecosystems, this apparatus is applicable to a number of oceanographic and environmental sample types. Targeted pools are dissolved methane, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), solid organic matrices (e.g., seston, tissue and sediments), biomarkers and short chained (C1–C5) hydrocarbons from methane hydrates. In most instances, the extraction, distillation and graphitization events are continuous within the system, thus, minimizing the possibility of fractionation or contamination during sample processing. A variety of methods are employed to extract carbon compounds and convert them to CO2 for graphitization. Dissolved methane and DIC from the same sample are sparged and cryogenically separated before the methane is oxidized in a high temperature oxygen stream. DOC is oxidized to CO2 by 1200 W ultraviolet photo-oxidation lamp, and solids oxidized in sealed, evacuated tubes. Hydrocarbons liberated from the disassociation of gas hydrates are cryogenically separated with a cryogenic temperature control unit, and biomarkers separated and concentrated by preparative capillary gas chromatography (PCGC). With this system, up to 20 samples, standards or blanks can be processed per day.

1-200 AMU TUNABLE PRETZEL MAGNET NOTCH-MASS-FILTER AND INJECTOR FOR TRACE ELEMENT ACCELERATOR MASS SPECTROMETRY

Abstract A novel recombinator and injector has been designed for the trace element accelerator mass spectrometer (TEAMS) at the Naval Research Laboratory. It will allow parallel analysis of a broad range of impurities in electronic, biological, and geological materials. The recombinator consists of a tunable Pretzel magnet that serves as a notch mass filter [1]. The mass filter has near-amu mass resolution over the range of 1–200 amu for 40 keV ions. Masks positioned along the Pretzels symmetry axis block intense matrix-related beams, (e.g. Si, SiO, Si 3 , etc.) while passing beams of interest into the accelerator. The primary and secondary columns of a Physical Electronics model 6300 quadrupole secondary ion mass spectrometer (SIMS) form the key components of the ion source for the injector. This commercially proven instrument facilitates depth profiling, imaging, and the analysis of both conducting and insulating substrates at pressures as low as 10 −10 Torr.

Radiocarbon and Stable Carbon Isotope Analysis to Confirm Petroleum Natural Attenuation in the Vadose Zone

CO2 and CH4 radiocarbon and stable carbon isotope ratios were used to assess natural attenuation at a fuel-contaminated soil site at the Norfolk Navy Base, Norfolk, VA (USA). Soil gas samples were collected spatially over a monitoring network in October 2002 and in March 2003. CO2 and CH4 from regions with high petroleum concentrations were 14C-depleted relative to uncontaminated areas. 14C-depleted methane suggested methanogenic hydrocarbon degradation. The difference in CO2 age between background and plume-influenced areas indicated that approximately 90% of the CO2 at the latter was petroleum derived, making contaminant the primary source of carbon for the microbial assemblage.

Virtual mesa and spoiler midinfrared angled-grating distributed feedback lasers fabricated by ion bombardment

It is demonstrated that the suppression of parasitic Fabry–Perot-like lasing modes substantially enhances the beam quality and brightness of wide-stripe angled-grating distributed feedback lasers emitting in the midwave infrared. The direct facet-to-facet gain path is blocked by loss regions that are created by ion bombardment with 900 keV silicon ions. Both virtual mesa structures, in which loss regions bound both sides of the 300-μm-wide angled gain path, and spoiler structures, in which loss is induced only near the facets, decrease the etendue of the output by nearly an order of magnitude, and increase the brightness by up to a factor of 3.

ION-BEAM-ASSISTED DEPOSITION OF AU NANOCLUSTER/NB2O5 THIN FILMS WITH NONLINEAR OPTICAL PROPERTIES

Abstract Gold nanocluster thin films (∼ 200 nm thickness) consisting of metal clusters ∼ 5 nm in size embedded in a matrix of Nb2O5 were deposited by ion beam-assisted deposition (IBAD) by coevaporation of Au and Nb with O2+ ion bombardment. The microstructure and optical characteristics of these films were examined as-deposited and after annealing at 600°C. Annealing crystallized the amorphous oxide matrix and ripened the nanoclusters. A strong linear absorption at the wavelength of the surface plasmon resonance for Au developed as a result of annealing. The linear optical behavior was modeled using Mie scattering theory. Good agreement was found between the nanocluster sizes predicted by the theory and the particle sizes observed experimentally using transmission electron microscopy (TEM). The nonlinear optical (NLO) properties of the nanocluster films were probed experimentally using degenerate four wave mixing and nonlinear transmission. The wavelength was near the peak of the surface plasmon resonance as measured by VIS/UV spectroscopy. Values of |χxxxx(3)| were 7.3 × 10−8 and 3.0 × 10−10 esu for annealed and unannealed samples, respe The dominant mechanism for the nonlinear response was change in dielectric constant due to the generation of a distribution of hot, photoexcited electrons.

Determination of 32Si by AMS at the US Naval Research Laboratory

Abstract Measurement of 32 Si by accelerator mass spectrometry (AMS) at relatively low energy was undertaken at the Naval Research Laboratory. Measurement of this naturally occurring radioisotope is difficult due to the low level of production of the radioisotope and the ubiquitous nature of the stable isobaric interference, 32 S. The isobar suppression was enacted using a new chemical separation and the differential energy loss profile of the 32 Si and 32 S ions after their transmission through a thin carbon foil. Determination of the energy loss and energy straggle profiles of the transmitted ion beam were made and compared to the predictions made using TRIM code.