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Grain boundary structures in La{sub 2/3}Ca{sub 1/3}MnO{sub 3} thin films

Description: As with many other oxide-based compounds that exhibit electronic behavior, structural defects have a strong influence on the electronic properties of the CMR manganites. In this work, the authors have studied the effect of grain boundaries on the transport properties and on the local orientation of magnetization. Thin films of the perovskite-related La{sub 2/3}Ca{sub 1/3}MnO{sub 3} compound were deposited onto bicrystal substrates using pulsed laser deposition. Transport measurements showed some enhancement of magnetoresistance across the grain boundary. The structure of the boundary was evaluated by electron microscopy. In contrast with the highly meandering boundaries typically observed in bicrystals of high temperature superconductors, the boundaries in these films are relatively straight and well defined. However, magneto-optical imaging showed that the local magnetization was oriented out of the plane at the grain boundary while it was oriented within the plane in the grains on either side. This coordinated reorientation of local magnetization near the grain boundary leads to enhanced magnetoresistance across the boundary in low fields.
Date: October 26, 1999
Creator: Miller, D. J.; Lin, Y.-K.; Vlasko-Vlasov, V. & Welp, U.
Partner: UNT Libraries Government Documents Department

Meissner holes in remagnetized superconductors

Description: Flux distributions on different faces of YBCO single crystals remagnetized by unidirectional and rotating fields are studied using advanced magneto-optical techniques. Unusual structures corresponding to the appearance of strong current concentration along certain fronts are found. At such fronts the vortex lines bend into closed loops which then collapse and form flux free cylinders. Strong magnetization currents along these Meissner holes result in essential flux redistribution and development of dynamical instabilities in the samples.
Date: April 1996
Creator: Vlasko-Vlasov, V. K.; Welp, U. & Crabtree, G. W.
Partner: UNT Libraries Government Documents Department

Meisnner holes and turbulent flux structures in high-T{sub c} superconductors

Description: The magnetic flux structure in HTSC single crystal plates during remagnetization in unidirectional and rotating fields is imaged using advanced magneto-optical techniques. it is found that bending of the flux lines is crucial for remagnetization scenario even in the case of thin plates in parallel fields. Flux bending results in formation of flux free cylinders (Meissner holes) surrounded by closed vortex loops. Essential increase of the current along the holes stipulates instabilities and appearance of unusual turbulent current and flux patterns.
Date: February 1997
Creator: Kabanov, V. V.; Nikitenko, V. I.; Vlasko-Vlasov, V. K.; Welp, U. & Crabtree, G. W.
Partner: UNT Libraries Government Documents Department

Observation of the structural phase transition in manganite films by magneto-optical imaging.

Description: A high-resolution magneto-optical imaging technique is used to reveal the formation of twins occurring during a martensitic phase transition at {approximately}105K in LCMO films grown on STO substrates. The magnetic contrast arises due to the magneto-elastic tilts of the Mn - magnetic moments in the twins. Different magnetic structures are found in LCMO films grown on MgO, NGO, and LAO substrates showing the importance of the substrate material for the manganite film properties.
Date: August 31, 1999
Creator: Crabtree, G. W.; Lin, Y.; Miller, D. J.; Nikitenko, V. I.; Vlasko-Vlasov, V. K. & Welp, U.
Partner: UNT Libraries Government Documents Department

Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.
Date: August 31, 1999
Creator: Berger, A.; Gray, K. E.; Miller, D. J.; Mitchell, J. F.; Vlasko-Vlasov, V. K. & Welp, U.
Partner: UNT Libraries Government Documents Department

Vortices in dense self-assembled hole arrays.

Description: We present a study of the upper critical field and pinning strength from the resistivity and magnetization of a Nb film containing a dense array of 45 nm diameter holes on a hexagonal lattice with a spacing of 101 nm. The holes were formed by self-assembly in anodic aluminum oxide (AAO) using an electrochemical procedure. Confinement effects and Little-Parks oscillations are seen above 6 K, and strong pinning with matching field effects is seen below 6 K. Above the first matching field interstitial vortices coexist with vortices trapped in the hole array. Pinning in the Nb films with hole arrays is enhanced by two orders of magnitude over that in continuous Nb films. At low temperature, flux avalanches are observed and imaged using the magneto-optical Faraday effect.
Date: October 9, 2002
Creator: Crabtree, G. W.; Welp, U.; Xiao, Z. L.; Jiang, J. S.; Vlasko-Vlasov, V. K.; Bader, S. D. et al.
Partner: UNT Libraries Government Documents Department

Nano-patterning of Co/Pt multilayers.

Description: One of the most promising new systems for both basic research and technical development are the periodic arrays of nano-magnetic elements, as shown. Such arrays show remarkably rich and novel magnetic behavior. Periodic magnetic arrays not only provide excellent opportunities for new science but also constitute a promising candidate for new applications such as high-density, patterned magnetic memory for advanced computer designs in the next decade. In the presentation they will focus on patterned [Co4 {angstrom}/Pt10 {angstrom}]n multilayers, which demonstrate an unusually strong out-of-plane magnetic anisotropy and sensitivity to structural modification. This strong out-of-plane anisotropy makes Co/Pt ML the favorite candidates for perpendicular magnetic recording. The patterned Co/Pt ML are representatives of a new generation of magnetic nanostructures with lateral dimensions in 100 nm range, and individual layer thickness approaching a monolayer scale (e.g., 4 {angstrom} of Co and 10 {angstrom} of Pt). In addition to traditional lithography, they are developing a new approach to tailoring the local magnetic properties of Co/Pt ML. In this process the local magnetic properties are modified not through conventional modulation of the chemical composition, but through structural modification induced by low energy ion implantation.
Date: July 24, 2001
Creator: Metlushko, V.; Crabtree, G.; Welp, U.; Vlasko-Vlasov, V.; Baldo, P.; Rehn, L. E. et al.
Partner: UNT Libraries Government Documents Department

The interaction of vortices with twin boundaries

Description: Twin boundaries provide a convenient defect for investigating vortex dynamics in superconductors. They occur naturally in YBa{sub 2}Cu{sub 3}O{sub 7}, with well defined planar geometries. They are a strong pinning defect, and at the temperature of the melting transition they are the dominant pinning sites in clean crystals. Twin boundaries are easily seen in polarized light, making their location and number directly observable without special equipment. Finally, they present a highly anisotropic pinning potential to the vortices, creating the possibility of interesting new behavior. In this paper, we describe experiments using twin boundaries to probe two effects in vortex dynamics: a new peak effect in the critical current occurring in the solid state just below the melting transition, and anisotropic pinning by planar defects.
Date: May 1, 1995
Creator: Crabtree, G. W.; Kwok, W. K.; Welp, U.; Fendrich, J.; van der Beek, C. J.; Vlasko-Vlasov, V. K. et al.
Partner: UNT Libraries Government Documents Department

Magneto-optical imaging of transport current densities in superconductors

Description: Direct imaging of the paths of transport currents in superconductors creates many new possibilities for exploring the basic features of vortex pinning mechanisms and for improving the performance of superconducting materials. A technique for imaging the path and magnitude of the transport current density flowing in superconductors is described. Results are given for a 37-filament BSCCO 2223 powder-in-tube wire, showing a highly inhomogeneous current path within the filaments.
Date: December 31, 1995
Creator: Crabtree, G.W.; Welp, U.; Gunter, D.O.; Zhong, W.; Balachandran, U.; Haldar, P. et al.
Partner: UNT Libraries Government Documents Department

Vortex pinning in Nb films patterned with nano-scale hole-arrays.

Description: Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide (AAO) as substrate. Pronounced matching effects in the magnetization are seen in fields up to 9 kOe. Flux pinning in the patterned samples is enhanced by two orders of magnitude as compared to unpatterned reference samples in applied fields exceeding 5 kOe. The matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the nano-scale spacing of the holes.
Date: September 17, 2002
Creator: Welp, U.; Xiao, Z. L.; Jiang, J. S.; Vlasko-Vlasov, V. K.; Bader, S. D.; Crabtree, G. W. et al.
Partner: UNT Libraries Government Documents Department

Practical superconductor development for electrical power applications - annual report for FY 2003.

Description: Most large-scale applications of high-critical-temperature superconductors will require conductors that can carry large currents in the presence of applied magnetic fields. This report describes progress at Argonne National Laboratory (ANL) in the research and development of practical superconducting components and devices. These efforts primarily focus on the use of Y-Ba-Cu-O system in second-generation conductors, but they also include investigations of Bi-Pb-Sr-Ca-Cu-O systems for use in first-generation conductors. Results are presented in the areas of processing first-generation superconductors and second-generation (2G) superconductors with several different architectures, applying Raman microscopy to the characterization of 2G conductors, studying the role of oxygen doping in the grain boundary transport of 2G conductors, and evaluating the mechanical properties of 2G conductors.
Date: October 18, 2004
Creator: Balachandran, U.; Baurceanu, R.; Clauss, H.; Dorris, S. E.; Emerson, J. E.; Erck, R. A et al.
Partner: UNT Libraries Government Documents Department