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Structural study of Sr{sub 2}CuO{sub 3+delta} by neutron powder diffraction

Description: Average crystal structures of superconducting Sr{sub 2}CuO{sub 3+{delta}} synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen deficient La{sub 2}CuO{sub 4}-type tetragonal T structure with O vacancies located in the CuO{sub 2} planes, not in the Sr{sub 2}O{sub 2} layers. This raises important questions about the superconductivity in Sr{sub 2}CuO{sub 3+{delta}} reported to be a 70 K superconductor.
Date: November 1, 1996
Creator: Shimakawa, Y.; Jorgensen, J.D.; Mitchell, J.F.; Hunter, B.A.; Shaked, S.; Hinks, D.G. et al.
Partner: UNT Libraries Government Documents Department

Electronic/ionic conductivity and oxygen diffusion coefficient of Sr-Fe-Co-O system

Description: Oxides in the system Sr-Fe-Co-O exhibit both electronic and ionic conductivities. Recently, Sr-Fe-Co-O system attracted great attention because of the potential to be used for oxygen permeable membranes that can operate without the electrodes or external electrical circuitry. Electronic and ionic conductivities at various temperatures have been measured on two compositions in Sr-Fe-Co-O system named SFC-1 and SFC-2. The electronic transference number is much greater than the ionic transference number in SFC-1 sample, while the electronic and ionic transference numbers are very close in SFC-2 sample. At 800{degrees}C, the electronic conductivity and ionic conductivity are {approx}76 S{center_dot}cm-1 and =4 S-cm-1, respectively, for SFC-1. While, for SFC-2, the electronic and ionic conductivities are =10 S-cm-1 and {approx}7 S-cm-1, respectively. By a local fitting to {sigma}{center_dot}T = A exp(-E{sub {alpha}}/{kappa}{Tau}), we found that the oxide ion activation energies are 0.92 eV and 0.37 eV respectively for SFC-1 and SFC-2 samples. Oxygen diffusion coefficient of SFC-2 is {approx}{times}10{sup {minus}7} cm{sup 2}/sec at 900C.
Date: March 1, 1995
Creator: Ma, B.; Park, J.H.; Balachandran, U. & Segre, C.U.
Partner: UNT Libraries Government Documents Department

High-temperature magnetic anomalies in Sr-doped La manganite structures

Description: The temperature dependence of the magnetization M, susceptibility {chi}, and magnetoresistance MR for 3 perovskite-variant manganite structures were studied: monoclinic (x=0.075) and orthorhombic (x=0. 125) La{sub 1-x}Sr{sub x}MnO{sub 3}, and tetragonal layered La{sub 2- 2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x=0.4) with x also indicating the nominal fraction of Mn{sup 4+}. In each case, evidence is found for unusual magnetic states at temperatures T above their primary magnetic transitions. In the first case, the high-T {chi} deviates from Curie-Weiss expectations, in the second case the MR extends to high T, and in the last, M and {chi} exhibit short-range anomalies at high T. This suggests that a key feature of these systems is the existence of multiple magnetic energy scales, independent of structure, dimensionality, or doping levels.
Date: November 1996
Creator: Potter, C. D.; Swiatek, M.; Mitchell, J. F.; Hinks, D. G.; Jorgensen, J. D.; Bader, S. D. et al.
Partner: UNT Libraries Government Documents Department

Defect structure of the mixed-conducting Sr-Fe-Co-O system

Description: Electrical conductivity of the mixed-conducting Sr-Fe-Co-O system was investigated at high temperatures and various oxygen partial pressures (pO2). The system exhibits not only high combined electrical and oxygen ionic conductivities but also structural stability in both oxidizing and reducing environments. Conductivity of SrFeCo{sub 0.5}O{sub x} increases with temperature and pO2, within the experiment pO2 range (1-10{sup -18} atm). p-type conduction was observed, the activity energy of which decreases with pO2. A model of the defect chemistry in the Sr-Fe-Co-O system is proposed. The pO2- dependent conducting behavior can be understood by considering the trivalent-to-divalent transition of the transition metal ions in the system.
Date: November 1, 1996
Creator: Ma, B.; Balachandran, U.; Chao, C.-C. & Park, J.-H.
Partner: UNT Libraries Government Documents Department

Synthesis, tailored microstructures and `colossal` magnetoresistance in oxide thin films

Description: We have grown La{sub 1-x}Sr{sub x}MnO{sub 3} films, using both pulsed laser deposition and a polymeric sol-gel route. These two growth techniques result in different microstructures, but in both cases the texture (epitaxy or polycrystallinity) can be controlled by choice of substrates and growth conditions. The crystallography and microstructure of these films were studied using XRD and high- resolution TEM. The magnetic/magnetotransport properties of these films are discussed in context of their growth and microstructural parameters.
Date: September 1, 1996
Creator: Krishnan, K.M.; Modak, A.R.; Ju, H. & Bandaru, P.
Partner: UNT Libraries Government Documents Department

A{sub 2}Zn{sub 3}As{sub 2}O{sub 2}(A = Ba, Sr): a rare example of square planar zinc

Description: Purpose of this study was to explore synthesis of novel solid-state compounds that could potentially exhibit interesting or useful physical properties. Goal was to extend the classes of compounds A{sub 2}Mn{sub 3}Pn{sub 2}O{sub 2} (A=Sr,Ba; Pn=P,As,Sb,Bi) to include a transition metal other than Mn. High-temperature fluxes were used to produce these new compounds.
Date: June 1994
Creator: Keane, P. M. & Burdett, J. K.
Partner: UNT Libraries Government Documents Department

Two-dimensional intrinsic and extrinsic ferromagnetic behavior of layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} single crystals

Description: The low-field magnetization M and susceptibility {chi} are reported for the two-layered Ruddleson-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} for x = 0.4 (denoted La{sub 1.2}Sr{sup 1.8}Mn{sub 2}O{sub 7}) with an in-plane magnetic easy axis. As T approaches the Curie temperature (T{sub C} = 116 K) on cooling, where the metal-insulator transition occurs in zero-field, {chi} diverges as H{sup {minus}1/{delta}{prime}}, with {delta}{prime} = 21 {+-} 5. Also, near an extrinsic Curie transition attributed to {approximately} 0.1% volume fraction of intergrowths at T* = 2.47T{sub C}, M scales as (1 {minus} T/T*){sup {beta}}, with {beta} = 0.25 {+-} 0.02. These results can be understood within the context of 2D XY models, and provide a new perspective of the layered manganites.
Date: June 6, 1997
Creator: Potter, C. D.; Swiatek, M.; Bader, S. D.; Argyriou, D.N.; Mitchell, J. F.; Miller, D. J. et al.
Partner: UNT Libraries Government Documents Department

Neutron scattering study in the spin-1/2 ladder system: Sr{sub 14}Cu{sub 24}O{sub 41}

Description: Inelastic neutron scattering measurements were performed on the S=1/2 quasi-one-dimensional system Sr{sub 14}Cu{sub 24}O{sub 41}, which has both simple chains and two-leg ladders of copper ions. We have observed that both the chain and the ladder exhibit a spin gap, which originates from a dimerized state.
Date: October 1, 1996
Creator: Matsuda, M.; Katsumata, K.; Shapiro, S.M. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Short range spin correlations in the CMR material La{sub 1.41}Sr{sub 1.6}Mn{sub 2}O{sub 7}

Description: The (La{sub 1{minus}x}Sr{sub x}){sub 2}Mn{sub 3}O{sub 7} compounds are layered materials that exhibit higher magneto-resistance than the corresponding 3D manganite perovskites. Quasi-elastic neutron scattering on a polycrystalline sample of La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} shows that the spin fluctuation spectrum of these layered CMR materials is qualitatively similar to those found in the perovskite manganites (La,Ca)MnO{sub 3}; their concentration, lifetime, and coherence length increase as T decreases to {Tc}. Unlike the perovskites the authors found a lower spin-diffusion constant above {Tc} of {approximately} 5 meV {angstrom}{sup 2}.
Date: December 1, 1997
Creator: Kelley, T.M.; Argyriou, D.N.; Robinson, R.A.; Nakotte, H.; Mitchell, J.F.; Osborn, R. et al.
Partner: UNT Libraries Government Documents Department

Vacancy related defects in La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} thin films

Description: Laser ablated La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} thin films have been studied by Doppler-broadening-detected positron annihilation using a variable-energy positron beam. The oxygen partial pressure during cooling from the growth temperature was altered through the range 760 torr to 10{sup -5} torr to change the oxygen non-stoichiometry of the films. The measured Doppler broadened lineshape parameter S was found to increase with increasing oxygen nonstoichiometry. For films cooled with an oxygen partial pressure of {le} 10{sup -4} Torr positron trapping to monovacancy type defects is inferred. For the film cooled in 10{sup -5} torr oxygen the magnitude of the increase in S, with respect to that measured from the film cooled in 760 Torr oxygen, showed positron trapping to vacancy cluster defects was occurring.
Date: 1997
Creator: Keeble, D. J.; Krishnan, A. & Nielsen, B.
Partner: UNT Libraries Government Documents Department

Structural and magnetic states in layered manganites: An expanding view of the phase diagram

Description: Colossal magnetoresistive (CMR) manganites display a spectacular range of structural, magnetic, and electronic phases as a function of hole concentration, temperature, magnetic field, etc. A1though the bulk of research has concentrated on the 3-D perovskite manganites, the ability to study anisotropic magnetic and electronic interactions made available in reduced dimensions has accelerated interest in the layered Ruddlesden-Popper (R-P) phases of the manganite class. The quest for understanding the coupling among lattice, spin, and electronic degrees of freedom (and dimensionality) is driven by the availability of high quality materials. In this talk, the authors will present recent results on synthesis and magnetic properties of layered manganites from the La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} series in the Mn{sup 4+}-rich regime x >0.5. This region of the composition diagram is populated by antiferromagnetic structures that evolve from the A-type layered order to G-type ''rocksalt'' order as x increases. Between these two regimes is a wide region (0.7 < x < 0.9) where an incommensurate magnetic structure is observed. The IC structure joins spin canting and phase separation as a mode for mixed-valent manganites to accommodate FM/AF competition. Transport in these materials is dominated by highly insulating behavior, although a region close to x = 0.5 exhibits metal-nonmetal transitions and an extreme sensitivity to oxygen content. They suggest two possible explanations for this transport behavior at doping just above x = O.5: localization by oxygen defects or charge ordering of Mn{sup 3+}/Mn{sup 4+}sites.
Date: January 5, 2000
Creator: Mitchell, J. F.; Millburn, J. E.; Ling, C.; Argyriou, D. N. & Bordallo, H. N.
Partner: UNT Libraries Government Documents Department

Structure--properties relationships for manganese perovskites

Description: By combining the results of dc magnetization, ac susceptibility, magnetoresistivity, magnetostriction, and x-ray and neutron powder diffraction data for stoichiometric La{sub 1{minus}x}Sr{sub x}MnO{sub 3} the authors have constructed a phase diagram that describes the magnetic, transport, and structural properties and the relationships among them as a function of composition and temperature. Correlations among physical and structural properties have been observed that are consistent with a competition between ferromagnetism and JT distortion. A metallic state occurs below the Curie temperature when both coherent and incoherent JT distortions are suppressed.
Date: October 14, 1999
Creator: Dabrowski, B.; Xiong, X.; Chmaissem, O.; Bukowski, Z. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Raman study of phonons in Sr{sub 3}Mn{sub 2}O{sub 6.54} and Sr{sub 3}Mn{sub 2}O{sub 7} layered manganites

Description: Polycrystalline Sr{sub 3}Mn{sub 2}O{sub 6.54} and Sr{sub 3}Mn{sub 2}O{sub 7} layered manganites were studied at room temperature using Raman spectroscopy. While the phonon spectrum of the stoichiometric Sr{sub 3}Mn{sub 2}O{sub 7} sample is consistent with the group theoretical analysis for the 14/mm structure, three additional modes are observed in the phonon spectrum of the oxygen deficient Sr{sub 3}Mn{sub 2}O{sub 6.54} sample. Based on an analysis of the temperature dependence of the Raman spectrum of Sr{sub 3}Mn{sub 2}O{sub 6.54} and a comparison with the Raman spectrum of LaSr{sub 2}Mn{sub 2}O{sub 7}, the authors conclude that the extra modes should be viewed as disorder induced Raman scattering.
Date: April 27, 2000
Creator: Guedes, I.; Mitchell, J. F.; Argyriou, D. & Grimsditch, M.
Partner: UNT Libraries Government Documents Department

Measurement of the critical current and transient characteristics of a high-temperature superconductor tube using a pulsed current supply.

Description: The transient response of a melt-cast-processed BSCCO-2212 superconductor tube is investigated by using a pulsed current source. It was found that (1) the maximum induced current and the excitation current at field penetration increase with the maximum excitation current, and (2) there is a time delay between peak excitation current and peak magnetic field inside the superconductor. These observations can be explained by the concept of magnetic diffusion. The ac steady-state critical current of the superconductor was found to depend on the magnitude of the current increment. The critical current determined by using the pulsed current system agrees fairly well with the ac steady-state critical current determined by using relatively large current increment.
Date: August 27, 1998
Creator: Cha, Y. S.
Partner: UNT Libraries Government Documents Department

Microstructures and electrical properties of SrRuO{sub 3} thin films on LaAlO{sub 3} substrates

Description: Conductive SrRuO{sub 3} thin films have been deposited using pulsed laser deposition. Structure and microstructure of this system were studied using XRD, SEM, and scanning tunneling microscopy. Electrical properties of these films were measured. The film deposited at 250{degree}C is amorphous-like, showing semiconductor-like temperature dependence of electric conductivity. Film deposited at 425{degree}C is crystalline with very fine grain size (100 {approximately} 200{angstrom}), showing both metallic and semiconductor-like temperature dependence of the electrical conductivity. Film deposited at 775{degree}C shows reisistivity of 280 {mu}{Omega}{center_dot}cm at RT and resisdual resistivity ratio of 8.4. Optimized deposition conditions were determined. Possible engineering applications of SrRuO{sub 3} thin films are discussed. Bulk and surface electronic structures of SrRuO{sub 3} are calculated using a semi-empirical valence electron linear combination of atomic orbitals, and they are used to understand the electrical properties of the films.
Date: March 1, 1996
Creator: Chu, F; Mitchell, T.E.; Hawley, M; Landrum, G; Jia, Q.X. & Wu, X.D.
Partner: UNT Libraries Government Documents Department

Intrinsic and extrinsic magnetic properties of the naturally layered manganites

Description: Structural and magnetic properties of the two-layered Ruddlesden-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} with x = 0.3--0.5 are highlighted. Intrinsic properties of these naturally layered manganites include a colossal magnetoresistance, a composition-dependent magnetic anisotropy, and almost no remanence. Above the Curie temperature there is a non-vanishing extrinsic magnetization attributed to intergrowths (stacking faults in the layered structure). These lattice imperfections consist of additional or missing manganite layers, as observed in transmission electron microscopy. Their role in influencing the properties of the host material is highlighted.
Date: November 30, 1999
Creator: Berger, A.; Mitchell, J. F.; Miller, D. J.; Jiang, J. S. & Bader, S. D.
Partner: UNT Libraries Government Documents Department


Description: Recent instrumentation developments in photoemission are providing new insights into the physics of complex materials. With increased energy and momentum resolution, it has become possible to examine in detail different contributions to the self-energy or inverse lifetime of the photohole created in the photoexcitation process, Employing momentum distribution and energy distribution curves, a detailed study of the optimally doped cuprate, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub g+{delta}}, shows that the material behaves like a non-Fermi liquid with no evidence for the quasi-particles characteristic of a Fermi liquid.
Date: October 13, 1999
Creator: JOHNSON,P.D.
Partner: UNT Libraries Government Documents Department

Charge delocalization and structural response in layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}: Enhanced Jahn-Teller distortion in the metallic regime

Description: Temperature dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the metal-insulator transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.
Date: November 1996
Creator: Mitchell, J. F.; Argyriou, D. N. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department