arXiv subjects

J. O'Connell

Publications and source records attributed to J. O'Connell.

8 recordsLinked to original sources

arXiv2025

Structure and Magnetic Properties of Vacuum-Annealed CoFeB Thin Films: From Amorphous Alloy to Metastable (Co,Fe)23B6 tau-Boride

Controlled crystallization of amorphous alloys offers a powerful route to tailor magnetic and structural properties at the nanoscale. Thin films of CoFeB alloy are essential for the development of various spintronic devices. The crystallization mechanisms of CoFeB during the annealing process have been thoroughly investigated in earlier studies, demonstrating that boron diffuses from the amorphous film, allowing the remaining CoFe to form a body-centered cubic lattice. Here, a distinct transformation pathway in pulsed-laser-deposited amorphous Co40Fe40B20 films is revealed, where vacuum annealing drives the formation of a metastable tau-boride phase, (Co,Fe)23B6. Comprehensive structural characterization - combining X-ray diffraction, transmission electron microscopy, and compositional analysis - proves that tau-boride forms with high crystalline quality and minimal boron loss. Following the transition from amorphous CoFeB films to crystalline (Co,Fe)23B6, an improvement of magnetic properties is observed, with corresponding increases in such values as saturation magnetization, coercivity, loop squareness, and average magnetic moment. The reproducible stabilization of a boron-rich metastable phase in CoFeB thin films expands the known crystallization landscape of this technologically important alloy system. These findings provide new insight into phase engineering in transition-metal borides and open opportunities for designing nanostructured magnetic materials with tunable functionality for next-generation spintronic and nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

First In-Beam Demonstration of a hybrid LaBr3/CeBr3/BGO array to measure radiative capture resonance energies in an extended gas target using a novel time of flight technique

We have deployed a new hybrid array of LaBr3, CeBr3, and BGO scintillators for detecting $γ$ rays at the DRAGON recoil separator at TRIUMF. The array was developed to improve $γ$-ray timing resolution over the existing BGO array. This allows the average position of resonant capture in an extended gas target to be determined with $\sim$15 mm precision or better, even with five or fewer detected capture events. This, in turn, allows determination of resonant capture energies with statistical uncertainties below ${\sim} 1\%$. Here we report the results of a first in-beam demonstration of the array, measuring the $E_{cm} = 0.4906(3)$ MeV resonance in the ${}^{23}\mathrm{Na}(p,γ){}^{24}\mathrm{Mg}$ reaction, focusing on the timing properties of the array and its anticipated performance in future experiments with radioactive beams.

physics.ins-det↗

arXiv2022

Investigating a predicted metallicity [Fe/H] variation in the Type II Globular Cluster NGC 362

NGC 362 is a non-common Type II Galactic globular cluster, showing a complex pseudo two-color diagram or 'chromosome map'. The clear separation of its stellar populations in the color-magnitude diagram and the distribution of the giant stars in the chromosome map strongly suggests that NGC 362 could host stars with both cluster-nominal as well as enhanced heavy-element abundances, and one of them could be iron. However, despite previous spectroscopic observations of NGC 362, no such iron variation has been detected. Our main goal is to confirm or disprove this result by searching for any internal variation of [Fe/H] which would give us insight into the formation and evolution of this interesting globular cluster. In this paper, we present the abundance analysis for a sample of 11 red giant branch members based on high-resolution and high S/N spectra obtained with the MIKE echelle spectrograph mounted at the Magellan-Clay telescope. HST and GAIA photometry and astrometry has been used to determine atmospheric parameters and membership. We obtained T$_{\text{eff}}$, log(g) and v$_{\text{t}}$ for our target stars and measured the mean iron content of the sample and its dispersion with three different methods, which lead to [Fe/H]$_1$=-1.10$\pm0.02$, [Fe/H]$_2$=-1.09$\pm0.01$ and [Fe/H]$_3$=-1.10$\pm0.01$, while the internal dispersion turned out to be $σ_{[\text{Fe/H}]_1}$=0.06$\pm0.01$, $σ_{[\text{Fe/H}]_2}$=0.03$\pm0.01$ and $σ_{[\text{Fe/H}]_3}$=0.05$\pm0.01$ respectively. The error analysis gives an internal dispersion due to observational error of 0.05 dex. Comparing the observed dispersion with the internal errors, we conclude that NGC 362 does not show any trace of an internal iron spread.

astro-ph.GA↗

arXiv2021

Exploring the s-process history in the Galactic disk: Cerium abundances and gradients in Open Clusters from the OCCAM/APOGEE sample

The APOGEE Open Cluster Chemical Abundances and Mapping (OCCAM) survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce II lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra (BACCHUS) in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for Ages $<$ 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/$α$-element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]-Age plane (where X can be H, Fe, and the $α$-elements O, Mg, Si, or Ca). These metallicity-dependant relations result in [Ce/Fe] and [Ce/$α$] ratios with age that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d[Ce/H]dR$_{GC}$ being negative, while d[Ce/Fe]/dR$_{GC}$ is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by $\sim$+0.009 dex-kpc$^{-1}$-Gyr$^{-1}$. Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from AGB stars, $\sim$Gyr time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.

astro-ph.GA↗

arXiv2018

dockChain: A Solution for Electric Vehicles Charge Point Anxiety

This paper addresses Charge Point Anxiety surrounding electric vehicles (EVs), an issue preventing the mass adoption of this greener mode of transport. We discuss the design and implementation of a charge point adapter called \textit{dockChain} that will help mitigate Charge Point Anxiety. The key feature of the dockChain is that it allows multiple EVs to connect simultaneously to a single charge by connecting the adapters together in a chain resulting in additional charging opportunities - without the need for infrastructural changes. We describe the operation of the network of adapters, the hardware components and charging policies for the adapter. A distributed algorithm that can detect the length of the chain in a dockChain network is also presented.

eess.SY↗

arXiv2013

Calibrations of Atmospheric Parameters Obtained from the First Year of SDSS-III APOGEE Observations

The SDSS-III Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a three year survey that is collecting 100,000 high-resolution spectra in the near-IR across multiple Galactic populations. To derive stellar parameters and chemical compositions from this massive data set, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) has been developed. Here, we describe empirical calibrations of stellar parameters presented in the first SDSS-III APOGEE data release (DR10). These calibrations were enabled by observations of 559 stars in 20 globular and open clusters. The cluster observations were supplemented by observations of stars in NASA's Kepler field that have well determined surface gravities from asteroseismic analysis. We discuss the accuracy and precision of the derived stellar parameters, considering especially effective temperature, surface gravity, and metallicity; we also briefly discuss the derived results for the abundances of the alpha-elements, carbon, and nitrogen. Overall, we find that ASPCAP achieves reasonably accurate results for temperature and metallicity, but suffers from systematic errors in surface gravity. We derive calibration relations that bring the raw ASPCAP results into better agreement with independently determined stellar parameters. The internal scatter of ASPCAP parameters within clusters suggests that, metallicities are measured with a precision better than 0.1 dex, effective temperatures better than 150 K, and surface gravities better than 0.2 dex. The understanding provided by the clusters and Kepler giants on the current accuracy and precision will be invaluable for future improvements of the pipeline.

astro-ph.SR↗

arXiv2010

The PHASES Differential Astrometry Data Archive. I. Measurements and Description

The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) monitored 51 sub-arcsecond binary systems to determine precision binary orbits, study the geometries of triple and quadruple star systems, and discover previously unknown faint astrometric companions as small as giant planets. PHASES measurements made with the Palomar Testbed Interferometer (PTI) from 2002 until PTI ceased normal operations in late 2008 are presented. Infrared differential photometry of several PHASES targets were measured with Keck Adaptive Optics and are presented.

astro-ph.IM↗

arXiv2010

The PHASES Differential Astrometry Data Archive. II. Updated Binary Star Orbits and a Long Period Eclipsing Binary

Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems have been combined with lower precision single-aperture measurements covering a much longer timespan (from eyepiece measurements, speckle interferometry, and adaptive optics) to determine improved visual orbits for 20 binary stars. In some cases, radial velocity observations exist to constrain the full three-dimensional orbit and determine component masses. The visual orbit of one of these binaries---alpha Com (HD 114378)---shows that the system is likely to have eclipses, despite its very long period of 26 years. The next eclipse is predicted to be within a week of 2015 January 24.

astro-ph.SR↗