Magnetic skyrmions are topologically protected spin textures with great technological potential. These topologically non-trivial non-coplanar spin textures give rise to a topological Hall effect, enabling the purely electronic detection of magnetic skyrmions. We report a clear topological Hall effect in thin films of the the Heusler alloy Mn2CoAl, a ferromagnetic spin-gapless semiconductor, capped by a thin layer of Pd. We exploit the strong thickness- and temperature-dependence of the anomalous Hall effect in this system, tuning it to zero to enable the unambiguous measurement of the topological Hall effect, which is observed for temperatures between 3 K and 280 K. The topological Hall effect is evidence of skyrmions, and we demonstrate the simultaneous coexistence of opposite polarity skyrmions using a novel method involving minor field loops of the Hall effect.

Additive manufacturing (AM) is rapidly expanding in many industrial applications because of the versatile possibilities of fast and complex fabrication of added value products. This manufacturing process would significantly reduce manufacturing time and development cost for nuclear components. However, the process leads to materials with complex microstructures, and their structural stability for nuclear application is still uncertain. This study focuses on 316L stainless steel fabricated by selective laser melting (SLM) in the context of nuclear application, and compares with a cold-rolled solution annealed 316L sample. The effect of heat treatment (HT) and hot isostatic pressing (HIP) on the microstructure and mechanical properties is discussed. It was found that after HT, the material microstructure remains mostly unchanged, while the HIP treatment removes the materials porosity, and partially re-crystallises the microstructure. Finally, the tensile tests showed excellent results, satisfying RCC-MR code requirements for all AM materials.

The effect of stacking faults (SF) on flux pinning and critical current (Ic) in rare earth based coated conductors was investigated. The SF density in YBa2Cu3O7−δ (YBCO) films with and without Dy addition, produced by metal organic deposition, was modified by altering the oxygenation temperature. A detailed microstructural analysis of the coated conductors was performed by x-ray diffraction, scanning and transmission electron microscopy and energy dispersive spectroscopy, and the observed defect population was correlated with both the self-field and in-field Ic. We report that the best self-field Ic was obtained for samples having a low SF density, in spite of the SF being effective flux pinning defects at 77 K for magnetic fields applied within the ab plane. We also show that the SF have no observable flux pinning effect at low temperatures. This study demonstrates that for devices operated at low temperatures, the elimination of SF in the conductor wires is essential to attain higher Ic.

We examine the temperature, field and field angle dependence of the critical current of a SmBa2Cu3Oy coated conductor produced by reactive co-evaporation. A transmission electron microscopy based microstructural analysis shows the film contains extended c-axis defects, stacking faults, and two different species of inclined defects. By applying a maximum entropy decomposition of the field angle dependent critical current Ic(θ) datasets we are able to identify the individual contributions of these defects to the critical current even though they do not produce distinct peaks but rather an anisotropy in Ic(θ). We are able to confirm the structure property relationships by determining the matching fields where each of the individual defect contributions are a maximum and showing that these are consistent with the observed microstructure. For a critical current component having a maximum magnitude at an intermediate temperature we propose a model of thermally activated depinning to explain the behaviour.

No abstract

<p>High-temperature superconductors are of great interest because they can transport electrical current without loss. For real-world applications, the amount of current, known as the critical current Ic, that can be carried by superconducting wires is the key figure of merit. Large Ic values are necessary to off-set the higher cost of these wires. The factors that improve Ic (microstructure/performance relationship) in the state-of-the-art coated conductor wires based on YBa₂Cu₃O₇ (YBCO) are not fully understood. However, microstructural defects that immobilise (or pin) tubes of magnetic flux (known as vortices) inside the coated conductors are known to play a role in improving Ic. In this thesis, the vortex-defect interaction in YBCO superconductors was investigated with high-end transmission electron microscopy (TEM) techniques using two approaches.  First, the effect of dysprosium (Dy) addition and oxygenation temperature on the microstructure and critical current were investigated in detail. Changing only the oxygenation temperature leads to many microstructural changes in pure YBCO coated conductors. It was found that Dy addition reduces the sensitivity of the YBCO to the oxygenation temperature, in particular it lowers the microstructural disorder while maintaining the formation of nanoparticles, which both contribute to the enhancement of Ic.  In the second approach, two TEM based techniques (off-axis electron holography and Lorentz microscopy) were used to study the magnetic flux vortices. Vortex imaging was attempted with a TEM operated at 300 kV on both a YBCO crystal as well as a YBCO coated conductor. Many challenges were encountered including sample preparation, inhomogeneity, and geometry, in addition to the need to perform measurements at cryogenic temperatures. Although vortices were not able to be observed in the coated conductors, tentative observation of vortices in a YBCO crystal was made using Lorentz microscopy. Improvements for future electron holography experiments on YBCO at low voltage are suggested. This work represents a pioneering step towards directly imaging vortices in YBCO using more widely available microscopes with the aim of better understanding flux pinning to ultimately boost Ic in superconducting wires.</p>

<p>High-temperature superconductors are of great interest because they can transport electrical current without loss. For real-world applications, the amount of current, known as the critical current Ic, that can be carried by superconducting wires is the key figure of merit. Large Ic values are necessary to off-set the higher cost of these wires. The factors that improve Ic (microstructure/performance relationship) in the state-of-the-art coated conductor wires based on YBa₂Cu₃O₇ (YBCO) are not fully understood. However, microstructural defects that immobilise (or pin) tubes of magnetic flux (known as vortices) inside the coated conductors are known to play a role in improving Ic. In this thesis, the vortex-defect interaction in YBCO superconductors was investigated with high-end transmission electron microscopy (TEM) techniques using two approaches.  First, the effect of dysprosium (Dy) addition and oxygenation temperature on the microstructure and critical current were investigated in detail. Changing only the oxygenation temperature leads to many microstructural changes in pure YBCO coated conductors. It was found that Dy addition reduces the sensitivity of the YBCO to the oxygenation temperature, in particular it lowers the microstructural disorder while maintaining the formation of nanoparticles, which both contribute to the enhancement of Ic.  In the second approach, two TEM based techniques (off-axis electron holography and Lorentz microscopy) were used to study the magnetic flux vortices. Vortex imaging was attempted with a TEM operated at 300 kV on both a YBCO crystal as well as a YBCO coated conductor. Many challenges were encountered including sample preparation, inhomogeneity, and geometry, in addition to the need to perform measurements at cryogenic temperatures. Although vortices were not able to be observed in the coated conductors, tentative observation of vortices in a YBCO crystal was made using Lorentz microscopy. Improvements for future electron holography experiments on YBCO at low voltage are suggested. This work represents a pioneering step towards directly imaging vortices in YBCO using more widely available microscopes with the aim of better understanding flux pinning to ultimately boost Ic in superconducting wires.</p>