14,824 research outputs found
Evolution of magnetic properties in F-doped brownmillerite Ca 2 Fe 2 O 5
No full textBrownmillerite structure is derived from an ordered arrangement of oxygen vacancies within a perovskite structure, where alternating stacked layers of tetrahedral and octahedral units are formed with oxygen vacancy channels aligned in the tetrahedral layer. Naturally it becomes an important proposition to attempt anion engineering with such structures. Accordingly, we have synthesized oxyfluoride of brownmillerite Ca 2 Fe 2 O 5 by low temperature fluorination technique with poly(vinylidene fluoride) as a fluorinating agent. The global as well as local structural changes and the magnetic properties upon fluorination were investigated. Extended X-Ray Absorbtion Fine Structure Spectroscopy (EXAFS) confirmed substitution of oxygen by fluorine at the tetrahedral sites, thereby changing the oxidation state of Fe which in turn tunes the magnetic properties converting the parent G-type antiferromagnet structure to a spin glass-like one at low temperature
Identifying the nature of dielectric anomalies in SrFeO3−
No full textEarlier comprehensive study of SrFeO3−δ series of compounds showed interplay between the oxygen vacancy and the magnetic and transport properties of the system. We have synthesized a powder sample of SrFeO3−δ series with oxygen vacancy in the range . X-ray powder diffraction confirms the tetragonal crystal structure of the compound while the magnetic data indicates the presence of response from both orthorhombic (Sr4Fe4O11) and tetragonal (Sr8Fe8O23) phases which is common for the series with intermediate compositions. The temperature dependent dielectric measurements have been carried out in the 10 kHz–1 MHz frequency range. The dielectric constant and loss are found to be nearly temperature independent for around room temperature. This temperature independent response could possibly be referred to as ‘flat loss’ corresponding to a low loss behavior, making the compound a potential candidate to capacitor industry. The anion stoichiometry has been further manipulated by introducing a more electronegative fluoride anion in the vacant oxygen sites where the structure changes to cubic one at room temperatur
[Letter from Alex Bradford to Lieutenant and Mrs. Ray Starner - November 4, 1940]
No full textLetter from Alex Bradford to Lieutenant and Mrs. Ray Starner describing the the current state of affairs that the author was experiencing, including: the London blitz, the moral of the troops on the ground, and the collective company of men opposing the Nazi regime
Re-examining the nature of ordering in CaMnO2 : The role of Mn-O covalency in the local structure
No full textFluorinated hexagonal 4H SrMnO 3 : a locally disordered manganite
No full textThe structural, magnetic and dielectric properties of the hexagonal four-layered form of SrMnO 3 along with its fluorinated counterpart have been investigated. Among these, the fluorinated hexagonal four-layered compound is being reported for the first time. Although the bulk crystal structure does not change with fluorination, there are clear spectroscopic evidences of local chemical fluorination having disordered Mn-F rich phases in the fluorinated compound. Fluorination of bulk antiferromagnetic SrMnO 3 changes the magnetic properties considerably where a spin- or cluster-glass-like state appears below 42 K with a large exchange bias. In addition to that, the F-doped SrMnO 3 indicates development of polarization and the system exhibits ferroelectric nature which has been confirmed through the strain-electric field (S-E) hysteresis loop
The student's guide to completing an author study
No full textThe 'Student's guide to completing an author study' emerged during the early development of the school library resource center program at Glen Stewart Elementary School in Stratford Canada on Prince Edward Island. This research process centered on an author study, with direct teaching and clear assignment. The resulting model has been adapted to various grade levels and subject areas in different schools.Source type: Electronic(1)http://proquest.umi.com/pqdweb?did=49237063&Fmt=7&clientId=65345&RQT=309&VName=PQ
Active X-ray optics for the next generation of X-ray space telescopes
No full textDescribed within is the design, manufacture, metrology and X-ray testing of an active X-ray
prototype intended for the next generation of X-ray telescopes. One of the challenges faced by
the X-ray telescope community is how to combine high resolution and high sensitivity into one
system, as weight limitations place constraints on the optics that can be launched. Therefore the
mandate of the active X-ray prototype is to provide high sensitivity through the ability of the optics
to be nested and to deliver high angular resolution through the active control of the optic’s form.
Piezoelectric unimorph actuators provide the active component: it is intended that they will correct
for figure errors within the optic and therefore increase the angular resolution capability.
The prototype’s design is based upon an ellipsoidal segment which provides point-to-point
focussing of an X-ray source. The prototype itself is composed of an electroformed nickel optic
where the non-reflective surface is populated with 30 piezoelectric actuators and it is the production
of the prototype that is the core of the presented research. Metrology of the actuators’ influence
functions is presented and highlight the prototype’s ability to deform its optic surface by microns.
In addition, the measured influence functions are compared against finite element models and a
distinct similarity between the functions is observed.
The prototype was tested at an X-ray beamline facility in November 2008 and the results
showed the prototype’s ability to correct the optic to achieve an improved angular resolution: from
0.786 arc-minutes to 0.686 arc-minutes in terms of full width half maximum. Finally, difficulties
in the manufacture of the prototype and X-ray testing shall be presented alongside future work in
conclusion to this thesis
Metal-insulator transition in Ba3Fe1−xRu2+xO9: Interplay between site disorder, chemical percolation, and electronic structure
No full textPerovskites containing barium metal at the A site often take up unusual hexagonal structures having more than one type of possible sites for the B cation to occupy. This opens up various different B−B- or B-O-B-type connectivities and consequent physical properties which are naturally missing in cubic perovskites. BaRuO3 is one such system where doping of Ru (4d4) by other transition metals (Mn+) creates similar conditions, giving rise to various M-Ru interactions. Interestingly, the 6H hexagonal structure of doped barium ruthenate triple perovskite (Ba3MRu2O9) seems to possess some internal checks because within the structure M ion always occupies the 2a site and Ru goes to the 4f site, allowing only M-O-Ru 180∘ and Ru-O-Ru 90∘ interactions to occur. The only exception is observed in the case of the Fe dopant, which allows us to study almost the full Ba3Fe1−xRu2+xO9 series of compounds with wide ranges of x because here Fe ions have the ability to freely go to the 4f sites and Ru readily takes up the 2a positions. Therefore, here one has the opportunity to probe the evolution of electronic and magnetic properties as a function of doping by going from BaRuO3 (paramagnetic metal) to BaFeO3 (ferromagnetic insulator). Our detailed experimental and theoretical results show that the series does exhibit a percolative metal-insulator transition with an accompanying but not coincidental magnetic transition as a function of x
Author, Geraldine Brooks at the National Library of Australia for the 2009 Ray Mathew Lecture, Canberra, 23 October 2009 [picture] /
No full textTitle from acquisitions documentation.; Part of the collection: Portraits of author, Geraldine Brooks during her visit to the National Library of Australia for the 2009 Ray Mathew Lecture, Canberra, 23 October 2009.; Acquired in digital format; access copy available online.; Mode of access: Internet via World Wide Web.; Photographed by a staff member of the National Library of Australia
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
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