Year

2014

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

A hundred years passed since the discovery of superconductivity, but it is only during last decades superconductors transformed from exotic object for physical experiments to practically promising materials. During this time new theoretical concepts, measurement techniques and fabrication methods were invented in order to study these materials. New types of superconductors were discovered. It is hard to overestimate current impact of superconductivity in modern life: the most sensitive devices (SQUIDs) based on Josephson effect, loss-free cables for electric networks in Tokyo and New York, high-speed levitating trains in China and Japan, high-field magnets for magneto-resonance imaging and power generators, and other applications have become available.

One of the most popular and well studied superconducting material is a YBa2Cu3O7 (YBCO) high-temperature superconductor (HTSC). High working temperatures, high transport currents and technological effectiveness enable a potential of YBCO application in all possible areas from industrial power applications to microelectronics. Hence, a technological control or enhancement of YBCO performance is of current importance. Moreover, well studied YBCO is a good model material to study some physical aspects of HTSC behaviour which are not answered or not quantified.

Enhancement of superconducting properties or even novel physical effects can be obtained combining superconducting and ferromagnetic materials (so-called hybrids). FePt ferromagnetic material is of a particular interest for application in highfield hybrids because of high coercive force and magnetic anisotropy. In addition FePt is promising for “traditional” applications of ferromagnets, such as magnetic recording media, micro-electro mechanical systems or high energy product magnets. However, high degree of technological control is required in order to reach necessary performance of FePt.

Functional materials, such as YBCO or FePt, demonstrate their best performance or reveal certain physical effects in form of thin films. In this work large degree of control on structure of quasi-single crystal superconducting YBCO thin films and hard ferromagnetic L10-FePt thin films is demonstrated simply by varying laser frequency of pulsed laser deposition. Significant structural modifications have been obtained for the films of constant thicknesses. These modifications are shown to dramatically affect corresponding physical properties of thin films.

In particular, critical temperature of YBCO thin films is shown to decrease with laser frequency. Moreover, a strong dependence on the laser frequency is discovered for the critical current density behaviour as a function of the applied magnetic field (Jc(Ba)) of YBCO films with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications of YBCO films and corresponding properties variations are discussed.

In case of FePt thin films the experimental dependence of structure on laser frequency demonstrates the opposite growth development to the trend reported for the modulated flux in the literature. A theoretical model based on the mean field approach is developed, which quantitatively describes the structural changes obtained experimentally. Structure modulation of FePt thin films obtained by varying laser frequency result in modification of demagnetization mechanism and subsequent 20-fold increase of coercive field. A sensitivity of physical properties to structure variation provides an instrumental ability for tuning the practical characteristics of these films by changing the laser frequency of their deposition.

One of the most practically important characteristics of any superconductor is a critical current dependence on applied magnetic field (Jc(Ba)). Relatively high working temperature of YBCO modifies the concepts of critical current and vortex pinning, making description of Jc(Ba) more sophisticated. A new model of critical current density for high-quality YBCO thin films has been proposed, combining thermally activated flux creep with a vortex pinning on columnar defects. The pinning for thermally activated vortices has been described as a strong pinning on chains of individual edge dislocations that form low-angle domain boundaries in pulsed laser deposited high-quality YBCO thin films. An electric field criteria (Ecr) introduced into the model allows to apply it for description of critical current obtained by different measurement techniques. In addition, the dependence of the irreversibility field on the Ecr criterion can be obtained within the model.

The dependences (Jc(Ba) for high-quality YBCO thin films over the entire applied magnetic field range were obtained using MPMS, VSM PPMS and four-probe method. Thus, the measurements of Jc(Ba) were done employing Ecr varied in 9 order of magnitude range. Transport current and quasi-equilibrium magnetization measurement data were successfully fitted by the developed model with appropriate electric field criteria. Hence, for the first time the model consolidates substantially different Jc(Ba) dependences of YBCO thin films obtained by different measurement techniques. At the same time the dynamic magnetization measurements of the Jc(Ba) obtained using VSM strongly depend on instrumentally defined parameters, introducing inconsistencies in the experimental results. The model calculations are able to explain the Jc(Ba) curves only if the instrumental vibrations affecting vortex behaviour are minimised.

Further, a systematic study on the behaviour of superconductors with varied VSM settings (frequency f and amplitude A of vibration) is presented. It was shown that the vibration affects critical currents of all possible types of superconductors during their measurement employing VSM magnetometer regardless geometry of the sample, vortex pinning properties or measurement temperature. Enhancement of vibration frequency or amplitude leads to a progressive reduction of critical current density with applied magnetic field. The vibration effect is more intense for thin films, it is responsible for development of unexpected kinks on Jc(Ba) curves in thin films. The study of magnetic moment dependence of YBCO film on applied magnetic field measured at different VSM settings revealed the asymmetry of the vibration effect on ascending and descending magnetic field branches of magnetization, indicating uneven states of vortex lattice in these branches. In addition, certain thermal response of magnetization to vibration with high f and A was detected. Investigation of magnetization relaxation process of YBCO film showed that the vibration effect in general can be interpreted as a suppressed pinning potential where the thermal response is one of the sources for pinning reduction. Critical current measurements in YBCO film using VSM with magnetic field sweep rate scaled with vibration frequency (f/(dBa/dt)) revealed a strong nonthermal effect of vibration on dynamics of vortices. The vibration effect, reducing the Jc(Ba), decreases irreversibility field of YBCO thin films. It was demonstrated how a technically defined irreversibility field B*irr can be a useful characteristic that reflects the vibration effect on entire Jc(Ba) dependence.

It was discussed that the effect of vibration on magnetization, relaxation of magnetization, critical current and irreversibility field is governed by thermal and magnetic fluctuations of vortex structure in oscillating superconducting sample. The role of thermal fluctuations is relatively small: estimated enhancement of temperature induced by vibration does not exceed 1 K. The major impact is expected from magnetic fluctuations, which promote relaxation of vortex lattice in sweeping magnetic field, reduce effectively the pinning potential and degrade the critical current. The mechanism of forced relaxation is expected to be similar to “vortex shaking” mechanism where magnetic periodic perturbations create local ac currents and “shake” vortices out of pinning centres.

In order to understand a microscopic nature of vibration effect on YBCO film, a behaviour of vortices in 300 nm YBCO film in 1 T and 2 T of applied magnetic field and 77 K were simulated employing Langeving molecular dynamic simulation. In general, results of simulations confirmed a dominating role of out-of-plane dislocations of relatively low density in establishing the critical current of YBCO films; weak pinning centres of much higher density have only marginal effect. Depinning currents obtained in simulated magnetic fields without vibration are in a good agreement with critical currents measured by VSM. Vibration at 1 T makes interaction of vortices with weaker pinning centres completely negligible, while pinning on out-of-plane dislocations still is effective. Shaking of vortices in 2 T decreases depinning current more substantially then at 1 T, assisting to depinning from all kinds of pinning centres including dislocations. In addition, shaking induces a peak effect behaviour of I-V characteristic. The origin of peak effect (vortex lattice orderdisorder transition vs. matching effect) is determined by applied magnetic field. This difference in vortex behaviour is responsible for switching theJc(Ba) trend at magnetic field of the kink developed by vibration of YBCO film.

Finally it was noticed, that vibration of Nb film subjected to flux jumps reduce magnetization (Mfj) and enhance the field second magnetization peak (BSMP ). The increase of BSMP implies that vibration induces transition from critical state to undercritical flux jump mediated state. Behaviour of Mfj and BSMP together with other evidences presented justify the self-organised criticality nature of flux jumps in Nb films during magnetometery (represented by a sandpile) instead of commonly assumed thermo-magnetic instability origin. A simple expression for fitting Mfj(Ba) affected by vibration was designed considering avalanche process in a flow-like regime. Fitting of Mfj(Ba) allowed to estimate a nonuniformity of magnetic field in a sample space (approx. 10−5).

FoR codes (2008)

020404 Electronic and Magnetic Properties of Condensed Matter; Superconductivity, 020406 Surfaces and Structural Properties of Condensed Matter, 091205 Functional Materials, 100706 Nanofabrication, Growth and Self Assembly

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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.