Year

2021

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

At this time, the significance of superconductivity is increasing as a result of our increased need for clean electrical energy. It has become important to find innovative solutions for the production of energy and for reducing the losses due its transportation. This requirement makes it attractive to use the special properties of high-temperature superconductors. Products such as sensitive superconducting quantum interference (SQUID) devices based on the Josephson Effect, high-speed floating trains, low loss cables, highly efficient motors, high-field magnets for magneto-resonance imaging, and power generators are now arriving on the market. The requirement for a high-temperature superconductors material that has a high critical current and can tolerate high fields needed for all of these. The YBa2Cu3O7-x (YBCO) superconductor has been considered as one of most popular and thoroughly studied materials for the past two decades.

This is because of its high critical temperature (Tc), which is above 91 K, its high transport currents, and its technological effectiveness, which makes it possible for an YBCO application to work in all areas of microelectronics and industrial power applications. On the other hand, a high electrical current is needed for most of the practical applications of YBCO that require transport under a magnetic field with low losses, which means that a high in-field critical current density (Jc) is required. The factors that control the current carrying capacity of this material impose criteria for its successful technological development, and there is a need to clear understand them.

Consequently, much research has been carried out that was mainly focused on the Jc improvement of YBCO thin films. The use of a magnetic environment is one of the most effective ways. An investigation has been carried out in this thesis work on a hybrid system that consists of superconducting YBCO thin films enclosed by ferromagnetic iron. A magnetization measurement was incorporated, and current densities of the thin films were extracted. A comparison between these characteristics and the transport current values was also made. It is evident from the measurements that the maximum critical current density (known as the overcritical current density) can surpass the critical current density obtained in the same thin films without the iron environment. The results indicate that the critical current density is highly dependent upon the iron environment’s location, the dimensions of the iron magnets, the distances between the thin films and the magnets and the orientation of the outer field. The current density enhancement is attributed to the magnetic interaction among the superconducting thin films and the soft ferromagnetic iron environment. This result of this interaction is likely to be a rearrangement of supercurrents within the films, eliminating excessive stray fields near the corners of the thin films and thus resulting in a more homogeneous supercurrent distribution. The entrance of magnetic flux in the form of vortices is prevented through this, and therefore, the critical current densities that are determined by vortex pinning turn out to be less relevant. It is shown that the choice of the iron environment affects the manipulation of the critical current density.

Furthermore, a study has been carried on the angular dependence of the magnetization measurements in YBCO in the angular range of 90o to 30o between the sample surface and the magnetic field for various frequencies ranging from 10 to 60 Hz. When a vibrating sample magnetometer (VSM) was employed, it was seen that an effect was exerted on Jc during these measurements with increasing angle and frequency. As a function of applied magnetic field (Ba), the increase in the angle and vibration frequency leads to a progressive reduction of Jc. There is a significantly stronger effect of the vibrations that increases the angle, irrespective of temperature changes. This frequency develops kinks on the Jc(Ba) curves that indicate some specific vortex state/motion disturbance, and as the angle and frequency increase, the kinks become more pronounced.

Through analyzing the magnetization loops, it was observed that there was an asymmetry of the vibration frequency effect in descending and ascending Ba, which is due to the variations in the free energy of the resultant vortex structures. It was demonstrated through the critical current Jc(Ba) measurements with the magnetic field sweep rate proportional to the frequency of vibration that a scaling behavior of the vibration induces suppression of the critical current, even with no thermal induction.

FoR codes (2008)

0203 CLASSICAL PHYSICS, 0204 CONDENSED MATTER PHYSICS, 0206 QUANTUM PHYSICS, 0912 MATERIALS ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.