We study the interaction between superconductors and ferromagnets in two systems: a ferromagnet-superconductor bilayer, and a thin superconducting film with a periodic array of magnetic dots upon it, with spontaneous vortices appearing in the systems. We show that the superconducting phase transition is of the first order in a ferromagnet-superconductor bilayer and of the second order in the superconducting film with a periodic array of magnetic dots upon it. The shift of the transition temperature, (delta)Tc, due to the presence of a ferromagnetic layer may be positive or negative in the ferromagnet-superconductor bilayer and is always negative in the superconducting film with a periodic array of magnetic dots upon it. The dependence of (delta)Tc on geometrical factors and the external magneticfield is found. The theory is extended to multilayer structures. Next, we study the anisotropy dependence of the critical current in a thin superconducting film with a periodic array of magnetic dots with magnetization perpendicular to the film with spontaneous vortices and antivortices. The phase diagrams for the appearance of spontaneous vortices and antivortices are given for the square arrays of circular and square F dots respectively when the direction of the magnetization is parallel to the superconducting film.

There are many parameters that significantly affect the electrical performance of ceramic-core superconducting composite wire and tapes, which remain ambiguous and require more labor on their optimization. BSCCO 2212 has not been paid the attention and investment it deserves. In this regard, all optimization efforts were made for BSCCO 2223. In our work, a practical and inexpensive manufacturing method, thermally and mechanically optimized for Pb doped BiSrCaCuO 2212 superconducting multifilament (38 filaments) wires and tapes, was successfully employed. Optimized parameters can be classified under material, mechanical (deformation) and heat treatment (thermal) subgroups. Parameters involved with materials included investigation of deformation behavior of two different sheath metals; pure silver and 0.02% magnesia dispersion reinforced silver alloy. Pb doped BiSrCaCuO 2212 ceramic superconductor powder, the other source of material related parameters, was synthesized following the "Thermal Co-decomposition or Wet Mix" method. Fabrication of mono and multifilament wires with Oxide Powder in Tube (OPIT) method followed next. Optimization of drawing deformation was practiced to achieve the best ceramic grain alignment and smoother ceramic powder core/metal sheath interface in order to avoid "sausaging" in wires. Rolling of the wire products into tapes by following different deformation regimes was the other manufacturing stage of the project. Variable and constant reduction-per-pass deformation paths were employed to reveal their effects on our composites with distinct sheath material and filament formation. The search for the best (optimum) heat treatment schedule for our Bi2212 superconductor composites, a modified version of "step solidification partial melting", was employed successfully. A solution through tried recipes for the bubbling problem that occurred with our tapes was also addressed. Electrical performance tests of fully reacted wires were carried out in our laboratory and very promising results were attained.

Discovery of high-temperature superconductors aroused a great interest in studies on these materials. In 1987 an yttrium-based compound YBCO (Yttrium-Barium- Copper-Oxide) was synthesised with Tc at 93 K. It was immediately evident that the BCS theory does not explain the pairing mechanism in these oxide-based superconductors, which predicted that a maximum Tc of about 30 K could be achieved. Layered copper oxides were for a long time the most studied high-Tc superconductors. With the discovery, at the beginning of 2008, by Kamihara of a new non-cuprate high-Tc superconductor a different path of studies had opened. This family of quaternary oxyarsenides LaO1-xFxFeAs was surprising due to the presence of Fe, which should lead to long-range magnetic order suppressing superconductivity. Although the parent compound LaFeAsO is not superconducting, electron doping (by partial substitution of O2- with F-) leads to superconductivity with Tc of 26 K. Since these discoveries there has been extensive research on the family of rare-earth quaternary oxypnictides with general formula REFaAsO (RE = La, Sm, Ce, Nd, Pr, Gd, Tb, Dy) and with Tc reaching 55 K for electron doped SmFeAsO. A study of new NdFeAsO family with Sr doping on Nd site was conducted. Synthesis of Nd1-xSrxFeAsO compounds and their further characterisation revealed that hole doping in these materials is a successful route to achieve superconductivity. Partial substitution of Nd3+ by Sr2+ with small doping level (x = 0.05) shows semiconducting type behaviour, while increasing doping level leads to the emergence of metallic conductivity and with level of doping 20% obtaining superconductivity at Tc = 13.5 K. These materials adopt a simple tetragonal crystal structure (space group P4/nmm), which undergoes a structural phase transition on cooling to orthorhombic symmetry (space group Cmma). The changes of structural, electronic and magnetic properties with doping level show very different behaviour – non-symmetric to the electron doped side of the phase diagram. The family of Fe-As based superconductors has expanded further by the discoveries of A1-xA’xFe2As2 (A = Alkaline Earth, A’ = Alkali metal), LiFeAs and NaFeAs phases, Sr3Sc2Fe2As2O5. A series of iron-based oxyarsenides with general formula Sr4M12O6M2 2As2 (M1 = Sc, Cr, V; M2 = Fe, Co, Ni) was synthesised and their structure was studied using powder X-ray and neutron diffraction. These materials adopt tetragonal crystal structure (space group P4/nmm) in analogy with REFeAsO systems. The Fe-As planes are sandwiched between perovskite K2NiF4-type oxide layers of Sr4M2O6 (MI = Sc, Cr, V), the large size of which creates a large distance between Fe-As planes. In spite of structural analogy with REFeAsO these new materials show a different behaviour from REFeAsO, which is absence of a structural phase transition on cooling. Both Sr4V2O6Fe2As2 and Sr4Sc2O6Ni2As2 compounds are superconducting without doping with Tc at 28 K and 2.5 K respectively. Neither Sr4Sc2O6Fe2As2 nor Sr4Cr2O6Fe2As2 show these properties, but are good parent compounds for doping, which may lead to producing compounds that achieve superconductivity. Further research was made with synthesising doped Sr4Cr2O6Fe2As2 with doping on Fe site with Co. The structure and magnetic and electrical properties of these compounds were studied and proved to show superconducting behaviour with Tc up to 17.5 K for 5% Co doping.

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