Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Fedoseev, Sergey, Investigation of superconducting thin films and multilayered structures for electronic applications, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4363
The grand challenge in twenty-first-century physical and material sciences is related to enabling continued advances in information processing and data storage beyond conventional silicon based electronics. Recent researches in oxide electronics suggest that it may be plausible to implement complex systems at the device level, thereby drastically increasing computational density and power efficiency. High temperature superconducting (HTS) ceramics YBa2Cu3O7-δ (YBCO) is currently one of the most promising candidates for the creation of sophisticated cryo-electronic devices such as: Josephson junctions, microwave filters, single photon detectors, and etc. One of the important problems in YBCO thin film technology is an enhancement of superconducting properties of epitaxial films which allows the creation of reliable cryo-electronic devices with high performances. Heteroepitaxial structures consisting, for example, of YBCO layers and layers of different superconducting cuprates having a very similar crystal lattice are likely to have enhanced microstructural properties and possibly better performances of Josephson junctions. The same multilayering approach can be successfully implemented for the fabrication of superconducting single photon detectors on the base of extremely thin heteroepitaxial structures grown on monocrystalline substrates.
The doped perovskite manganite La1-xCaxMnO3 (LCMO) is one of most extensively studied material for future oxide electronics due to the interactions between the electronic, magnetic and crystal lattices, and the wide range of phases that can coexist. This is very attractive ferromagnetic oxide because of its colossal magnetoresistance (CMR) and so the great application potential for magnetic memory and spintronics devices. Due to increasing demands for high quality manganite thin films with good electrical and magnetic properties, there is an important issue to grow epitaxial films of LCMO on conventional substrates as well as the creation of multilayered heterostructures and superlattices.
The interplay between superconductivity (S) and ferromagnetism (F) in hybrid structures is one of the fascinating fields of modern research. Such artificially made bilayers, multilayers, and superlattices could have clear advantage over naturally grown layered crystals due to the fact that the thickness of magnetic and superconducting components can be tailored individually. The oxide based doped Mott insulators, like high temperature superconductor (YBCO) and ferromagnetic manganite (LCMO) are of particular interest for making S/F hybrids.
This thesis is focused on fundamental studies of both the fabrication by Pulsed Laser Deposition (PLD) and characterization of: YBCO based multilayered device structures for practical applications, LCMO thin films and LCMO/STO superlattices, as well as YBCO/LCMO hybrid structures and superlattices with thin insulating interlayer.
During this work, a few additional related research and technical problems have been addressed. An emphasis was made onto automation and optimization of PLD technique (to prepare thin films of the highest quality especially for longue size samples l = 8 cm). Optimal deposition conditions for thin films of YBCO, LCMO and their hybrids were defined that has significantly increased the productivity of PLD system and the samples reproducibility.
Multilayered approach has been employed for fabrication of step-edge YBCO/NdBCO multilayered Josephson junction. The results demonstrate significant enhancement of the tunnelling current in the multilayered junctions compared to the single layered ones. We attribute the result observed to the formation of a more suitable microstructure in YBCO/NdBCO multilayered films, which reveals (i) smoother surfaces, (ii) fewer and shallower voids, (iii) a larger density of extended defects that provide stronger pinning, (iv) a smaller number of facets at the boundary, and (v) thinner and more uniform barrier at the junction boundary, providing more homogeneous tunnelling properties across the junction. As a result, the multilayered junctions show higher Ic and IcRn values, and an uniform distribution of the Josephson current.
The properties of multilayered YBCO/SmBCO/YBCO thin films structures with reduced thicknesses (< 90 nm) were investigated. It was found that as the film’s thickness decreased there was a decrease in superconducting characteristics (Tc, Jc) of the structure. The properties obtained were rationalized using SEM and MOI visualization techniques. It was demonstrated that the thinner samples had very poor structural homogeneity, and thus magnetic flux could easily penetrate the sample along many defects. If an epitaxial growth of initial layers of YBCO and SmBCO is improved, the multilayering approach may enhance structural and electromagnetic properties of the films at very thin (tens of nm) thickness.
High quality epitaxial LCMO films on different substrates were manufactured by PLD method and their magnetic and transport properties were investigated. It was shown that the Curie temperature for LCMO samples is strongly dependent on film thickness and substrate type having the trend of significant reduction as the film thickness decreases. LCMO films thicker than 150 nm have demonstrtated bulk-like behaviour with metal-insulator transition at Tp ≈ 250 K. All LCMO thin film samples demonstrated CMR with negative magnetoresistance minimum in the vicinity of metal-insulator transition temperature.
Magnetic and transport properties of multilayered hybrid structures based on YBCO/STO/LCMO multilayers with thin insulating interlayer were investigated. Although measurements show the coexistence of ferromagnetism at high temperatures (T > Tc) and superconductivity at low temperatures (T < Tc) in hybrid structures, they demonstrate no enhancement in the critical current compared to single YBCO films, but hybrids with YBCO as the first grown layer always have better superconducting properties. In the hybrid structure of S/I/F type (YBCO micro-bridge on top of buffered LCMO film) the possibility to control the microbridge resistance by injection of an in plane current through LCMO layer was demonstrated. The application of injection current dramatically changes the shape of R(T) curve, it leads to the decreasing in Tc, demonstrate the drop in resistance before Tp, and the reduction in resistance at room temperature. The tri-layers hybrid structure of F/I/S type (LCMO on top of buffered YBCO micro-bridge) also demonstrated the unusual R(T) behaviour, with a sharp dip in resistance below Tc. It was also found that electroresistance and magnetoresistance around TRmin can be tuned by electric current, which may have applications in controlling the properties of such YBCO/LCMO hybrid based devices.
We have managed to establish an acceptable technology for combination of manganite films exhibiting the CMR effect with HTS (YBCO) films for novel hybrid functionalities and devices, so that the relevant magnetic properties of ferromagnetic layer would be exhibited below the superconducting transition temperature. So the LCMO/STO superlattices with very thin sub-layers (~3-7 nm) were grown by PLD on different substrates and their properties were investigated by means of magnetic and transport measurements. We have obtained TCurie ≈ 75 K which is even below liquid nitrogen temperature (and Tc = 91 K for YBCO films). Importantly, the resistivity vs temperature curves of superlattices below the metal-insulator temperature have remarkable sharp features exhibiting about two order of magnitude drops of resistivity, which result in the positive peak of magnetoresistance in excess of 1800% at 75 K. This behaviour was explained in the framework of the phase separation theory for manganites for the well-organized 3D structures, which experience dimensional transitions of their spin-charge transport properties. The significant result is that because the structure of the superlattices can be easily controlled, it can enable the prescribed tuning of the magnetoresistance properties required for applications.
Four series of cuprate/insulator/manganite superlattices with STO, PBCO, and CeO2 as insulating layer between YBCO and LCMO sub-layers were fabricated by PLD. It was shown that superconducting properties of such superlatices may be significantly improved to be compared with single layer YBCO film by the introduction of an STO insulator with thickness di > 1 nm. All samples (even with thinnest LCMO sub-layer) demonstrated the suppression of superconductivity by means of critical temperature and critical current. Superlattices of type (YBCO/PBCO/LCMO)20 have a quite different shape of resistance vs temperature R(T) curves for two mirror samples depending on LCMO or YBCO was the first grown layer. If the LCMO layer was grown first the superconductivity is depressed significantly and ferromagnetic properties (LCMO-like behaviour) become more pronaunced. The similar behaviour of the mirror sample with LCMO as a first grown layer and even well-defined re-entrant resistance peak at Tre ≈ 46 K (for the current I = 10 μA) was observed in the superlattices of type (YBCO/CeO2/LCMO)20. It was shown that the re-entrant resistance is stable and it can be controlled by the external magnetic field and applied current which is interesting for possible practical applications.