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Domain topology of micro-scale, ferromagnetic pancakes

Description: Planar, micron-scale ellipses patterned from 700A-thick Co films exhibit nearly complete suppression of hysteresis when magnetized in- plane along their short axes. Using a combination of Magnetic Force Microscopy and Vibrating Sample Magnetometry, we find that the suppression of hysteresis is associated with the continuous deformation of a dipole field configuration. The presence of hysteresis for in-plane, long-axis magnetization is associated with transitions between topologically inequivalent configurations.
Date: February 1, 1998
Creator: Bedrossian, P. J., LLNL
Partner: UNT Libraries Government Documents Department

Application of MXCD to magnetic thin-film sensors

Description: While Magnetic X-ray Circular Dichroism (MXCD) has been applied extensively to the extraction of elemental magnetic moments in various magnetic multilayers, the configuration of actual devices imposes certain constraints on the application of MXCD to devices. Using a set of real, thin-film spin valve devices with varying Cu spacer layer thicknesses, we demonstrate the correlation between MXCD and R-H measurements on those devices as well as the restrictions on the interpretation of MXCD data imposed by both the device topology and the formulation of realistic error estimates.
Date: May 17, 1996
Creator: Bedrossian, P.J.; Tobin, J.G. & Jankowski, A.F.
Partner: UNT Libraries Government Documents Department

Modeling of ion implantation and diffusion in Si

Description: Classical molecular dynamics simulations are used to study damage produced during implantation of semiconductors with different ion masses and energies between 1-25 keV. The time scale for these simulations is only on the order of ns, and therefore problems like transient enhanced diffusion of dopants or formation of extended defects can not be studied with these models. Monte Carlo simulations, including as input the results obtained from molecular dynamics calculations, are used to extend the simulation time, and in particular, to study processes like ion implantation and defects diffusion in semiconductors. As an example, we show results for diffusion of the damage produced by implantation of Si with 5 keV Xe ions at low doses. Results of the simulations are compared with experiments in order to validate the model.
Date: September 1, 1996
Creator: Caturla, M-J; Diaz de la Rubia, T. & Bedrossian, P.J.
Partner: UNT Libraries Government Documents Department

Demonstration of lithographic patterning in measurements of general and localized corrosion on alloy 22

Description: We have demonstrated a new technique capable of detecting generalized corrosion of metallographically-polished materials with nanometer-scale precision. After exposing a lithographically-patterned coupon of Alloy 22 to an electrolyte in a potentiostatically-controlled cell for twenty-four hours, we detected the loss of up to 130nm of metal. In addition, ''wormholes'' were detected at certain points of intersection of three grain boundaries.
Date: July 1, 1999
Creator: Bedrossian, P J & Farmer, J C
Partner: UNT Libraries Government Documents Department

Surface topographies of one-year weight-loss coupons of alloy C-22 from long-term corrosion testing

Description: We have used an atomic force microscope (AFM) to characterize the surface topographies of weight-loss coupons of Alloy C-22 which had been exposed to two different environments in the Long-Term Corrosion Test Facility at LLNL for one year. We have observed a silicate deposit on these coupons, with the most extensive coverage occurring on the coupon immersed in an acidified bath. We have not detected localized corrosion on these coupons.
Date: June 11, 1999
Creator: Bedrossian, P J
Partner: UNT Libraries Government Documents Department

Observation of Localized Corrosion of Ni-Based Alloys Using Coupled Orientation Imaging Microscopy and Atomic Force Microscopy

Description: We present a method for assessing the relative vulnerabilities of distinct classes of grain boundaries to localized corrosion. Orientation imaging microscopy provides a spatial map which identifies and classifies grain boundaries at a metal surface. Once the microstructure of a region of a sample surface has been characterized, a sample can be exposed to repeated cycles of exposure to a corrosive environment alternating with topographic measurement by an atomic force microscope in the same region in which the microstructure had been mapped. When this procedure is applied to Ni and Ni-based alloys, we observe enhanced attack at random grain boundaries relative to special boundaries and twins in a variety of environments.
Date: November 24, 1999
Creator: Bedrossian, P. J.
Partner: UNT Libraries Government Documents Department

Magnetic force microscopy of single-domain cobalt dots patterned using interference lithography

Description: We have fabricated arrays of Co dots of diameters 100 and 70 nm using interference lithography. Density of these arrays is 7.2x10{sup 9}/in{sup 2}. Magnetic force microscopy indicate that the Co dots are single domain with moments that can be controlled to point either in-plane or out-of-plane. Interference lithography is a process that is easily scaled to large areas and is potentially capable of high throughput. Large, uniform arrays of single-domain structures are potentially useful for high-density, low-noise data storage.
Date: March 20, 1996
Creator: Fernandez, A.; Bedrossian, P.J.; Baker, S.L.; Vernon, S.P. & Kania, D.R.
Partner: UNT Libraries Government Documents Department

Modeling the corrosion of high-level waste containers: CAM-CRM interface

Description: A key component of the Engineered Barrier System (EBS) being designed for containment of spent-fuel and high-level waste at the proposed geological respository at Yucca Mountain, Nevada is a two-layer canister. In this particular design, the inner barrier is made of a corrosion resistant material (CRM) such as Alloy 825, 625 or C-22, while the outer barrier is made of a corrosion-allowance material (CAM) such as A516 or Monel 400. At the present time, Alloy C-22 and A516 are favored. This publication addresses the development of models to account for corrosion of Alloy C-22 surfaces exposed directly to the Near Field Environmental (NFE), as well as to the exacerbated conditions in the CAM-CRM crevice.
Date: June 1, 1998
Creator: Farmer, J.C.; Bedrossian, P.J. & McCright, R.D.
Partner: UNT Libraries Government Documents Department

Modeling the corrosion of high-level waste containers: CAM-CRM interface

Description: A key component of the Engineered Barrier System (EBS) being designed for containment of spent-fuel and high-level waste at the proposed geological repository at Yucca Mountain, Nevada is a two-layer canister. In this particular design, the inner barrier is made of a corrosion resistant material (CRM) such as Alloy 825, 625 or C-22, while the outer barrier is made of a corrosion-allowance material (CAM) such as A5 16 or Monel 400. At the present time, Alloy C-22 and A516 are favored. This publication addresses the development of models to account for corrosion of Alloy C-22 surfaces exposed directly to the Near Field Environment (NFE), as well as to the exacerbated conditions in the CAM-CRM crevice. [5]. Haynes International has published corrosion rates of Alloys 625 and C-22 in artificial crevice solutions (5-10 wt. % FeCl,) at various temperatures (25, 50 and 75 C) [6,7]. In this case, the observed rates for Alloy C-22 appear to be due to passive dissolution. It is believed that Alloy C-22 must be at an electrochemical potential above the repassivation potential to initiate localized corrosion.
Date: June 1, 1998
Creator: Bedrossian, P J; Farmer, J C & McCright, R D
Partner: UNT Libraries Government Documents Department

Coupling Automated Electron Backscatter Diffraction with Transmission Electron and Atomic Force Microscopies

Description: Grain boundary network engineering is an emerging field that encompasses the concept that modifications to conventional thermomechanical processing can result in improved properties through the disruption of the random grain boundary network. Various researchers have reported a correlation between the grain boundary character distribution (defined as the fractions of special and random grain boundaries) and dramatic improvements in properties such as corrosion and stress corrosion cracking, creep, etc. While much early work in the field emphasized property improvements, the opportunity now exists to elucidate the underlying materials science of grain boundary network engineering. Recent investigations at LLNL have coupled automated electron backscatter diffraction (EBSD) with transmission electron microscopy (TEM) and atomic force microscopy (AFM) to elucidate these fundamental mechanisms. This investigation provides evidence that grain boundary network engineering and the formation of annealing twins disrupt the connectivity of the random grain boundary network and is likely responsible for the experimentally observed improvement in properties. This work illustrates that coupling of automated EBSD with other microstructural probes such as TEM and AFM provides data of greater value than any single technique in isolation. The coupled techniques have been applied to aid in understanding the underlying mechanisms of grain boundary network engineering and the corrosion properties of individual boundaries.
Date: January 26, 2000
Creator: Schwartz, A.J.; Kumar, M.; Bedrossian, P.J. & King, W.E.
Partner: UNT Libraries Government Documents Department

Magnetic coupling in spin-valves from magnetic circular dichroism

Description: The authors have applied magnetic x-ray circular dichroism (MXCD) to extract elemental magnetic moments for real, thin-film spin valve devices and demonstrated the correlation between MXCD and R-H measurements on those devices. The spin valves consisted of NiFe/Co/Cu/Co/NiFe sandwiches with variable Cu spacer-layer thickness, grown on a thick NiO film, as described previously. One permalloy layer is therefore magnetically pinned to the substrate which it contacts. Spin-polarized, X-ray absorption spectra were collected using beamline 8-2 at the Stanford Synchrotron Radiation Laboratory. The Poynting vector of the incident beam was aligned at grazing incidence to the films, either parallel or antiparallel to their remanent magnetization. For each spin valve, four absorption spectra were collected, With the remanent magnetization initially parallel to the incident beam direction, a pair of spectra were recorded, one with the helicity of the incident photons parallel to the remanent magnetization of the device, and one with their helicity antiparallel to the remanent magnetization. Next, the samples were rotated in order to align their remanent magnetization antiparallel to the incident beam direction, and a pair of absorption spectra was similarly obtained from beams polarized with opposite helicity. Finally, each absorption spectrum was normalized to the incident photon flux.
Date: December 18, 1995
Creator: Bedrossian, P.J.; Tobin, J.G.; Jankowski, A.F.; Waddill, G.D.; Anthony, T.C. & Brug, J.A.
Partner: UNT Libraries Government Documents Department

Potential Biogenic Corrosion of Alloy 22, A Candidate Nuclear Waste Packaging Materials, Under Simulated Repository Conditions

Description: The U.S. Department of Energy has been charged with assessing the suitability of a geologic nuclear waste repository at Yucca Mountain (YM), NV. Microorganisms, both those endogenous to the repository site and those introduced as a result of construction and operational activities, may contribute to the corrosion of metal nuclear waste packaging and thereby decrease their useful lifetime as barrier materials. Evaluation of potential Microbiological Influenced Corrosion (MIC) on candidate waste package materials was undertaken reactor systems incorporating the primary elements of the repository: YM rock (either non-sterile or presterilized), material coupons, and a continual feed of simulated YM groundwater. Periodically, both aqueous reactor efflux and material coupons were analyzed for chemical and surfacial characterization. Alloy 22 coupons exposed for a year at room temperature in reactors containing non-sterile YM rock demonstrated accretion of chromium oxide and silaceous scales, with what appear to be underlying areas of corrosion.
Date: January 12, 2000
Creator: Horn, J.M.; Martin, S.I.; Rivera, A.J.; Bedrossian, P.J. & Lian, T.
Partner: UNT Libraries Government Documents Department

Passive films and blistering of titanium

Description: Coupons of titanium alloys under consideration as components of the Engineered Barrier System in the proposed repository at Yucca Mountain have been evaluated for their passive film composition and stability. Oxide depths and compositions on specimens exposed in long-term corrosion testing for one year were determined with x-ray photoemission spectroscopy. The specimens removed from long-term testing, as well as separate coupons polarized cathodically in an electrochemical cell, exhibited blistering associated with hydride formation in both scanning electron microscopy and atomic force microscopy.
Date: December 1, 1998
Creator: Bedrossian, P J; Estill, J C; Farmer, J C; McCright, R D & Phinney, D L
Partner: UNT Libraries Government Documents Department