Proximity Effect And Vortex Matter In Heterostructures Consisting Of Oxide Superconductor And Ferromagnet

Due to the antagonism between superconductivity and ferromagnetism, the interaction between them is one of the most fascinating and challenging topics in modern condensed matter physics. Heterostructures consisting of high-temperature superconducting (HTS) cuprates and colossal-magnetoresistance (CMR) ferromagnetic manganites not only provide an object to investigate the complicated interplay between superconductivity and ferromagnetism, but also present a good basis in structure to realize new functional devices such as superconducting spin valve, Josephson superconductingÏ€junction etc. In particular, the strong electronic correlations of the two materials lead to the coupling of charge, spin, orbital ordering superimposed by lattice effects, which combines with the high degree of spin polarization in CMR materials and the low carrier density in HTS materials, giving rise to rich physical mechanism and extensive potential application.In this dissertation, we firstly give an introduction to the characteristics of the epitaxial growth and the microstructures of HTS/CMR hybrids. Considerable research work has been done to understand the interaction between the superconductivity and ferromagnetism. By artificial design of the epitaxial stress between the film and the substrate, the components and thicknesses of the constituent layers, as well as the total structure, we manipulate the spatial homogeneity and the relative strength of the two order parameters at the interface in a controlled way. This thesis not only deals with many physical effects of the electronic proximity at the interface such as the coexistence between superconductivity and ferromagnetism, the diffusion of the spin-polarized quasiparticles, spin scattering at the ferromagnetic interface, etc., but also involves the pure electromagnetic coupling between both subsystems through their respective magnetic stray fields. The corresponding results are listed as follows. 1) By using PLD technique, we prepare a few types of HTS/CMR heterostructures and hybrids. Due to the similar crystalline structure, well matching lattice parameters, and good chemical compatibility, all the samples exhibit perfect c-axis orientation, highly good in-plane texture, and flat surface morphology and interface, thus providing a prerequisite for studying the interaction between superconductivity and ferromagnetism.2) A novel method to create non-uniform magnetization of the ferromagnet is introduced, in which the inhomogeneous ferromagnetic order is caused by the mismatched stress between the film and the substrate. Using Raman spectroscopy and x-ray diffraction, we analyse the stress evolution and lattice distortion induced by thickness variation and lattice misfit in LSMO films, and establish the correlation between the microstructures and the macroscopic magnetism of the film. It is disclosed the characteristic of ferromagnetic/antiferromagnetic (F/AF) phase separation in a strained film due to the partially released stress and the high disorders.3) YBa₂Cu₃O_7-δ/La_0.67Sr_0.33MnO₃ (YBCO/LSMO) bilayers are designed to investigate the magnetic-dependent vortex pinning effect. It is revealed that the competition between the core pinning and the magnetic pinning is controlled by the temperature, demonstrating the inhomogeneous magnetization induced by the stress which modulates the local field thus can be used to pin the vortices. The magnetic pinning appears at high temperatures and extends to intermediate magnetic fields, showing promising power application for HTS films and coated conductors at high running temperature. Furthermore, we observe giant paramagnetic response in the LSMO/YBCO/LSMO trilayer with extremely inhomogeneous magnetic layers, increasing monotonically with enhancing applied field strength during the field-cooled (FC) process and demonstrating obvious irreversibility against the cycling of the temperature. It is believed that the paramagnetic moments originate from the highly compressed flux tuned by the magnetic pinning in the FC process, again disclosing the magnetic response of the vortex matter to the inhomogeneously local field.4) By means of the macroscopic measurements of the electromagnetic transport, we study the three most important electronic processes at the interface nearby, i.e. the superconducting proximity effect, the local pair breaking at the ferromagnetic boundary and the diffusion of the spin-polarized quasiparticles. Firstly, the magnetic measurements of the YBCO/LSMO multilayers disclose the interfacial competition between the superconducting and ferromagnetic order parameters subject to the thicknesses of the constituent layers. In the multilayer with extremely thin individual layers, the superconducting and ferromagnetic components are both observed which implies the possible coexistence of the two antagonistic long-range orders. Secondly, in the YBCO/LCMO quasi-multilayers with insulating ferromagnet of La_0.88Ca_0.12MnO₃ inserting periodically, the suppressed superconductivity is attributed to the local pair breaking of the exchange field at the ferromagnetic boundary, as the choosed insulator prevents the penetration of the electrons. Finally, in the designed YBCO/LSMO bilayers grown on STO substrates, it is revealed that the diffusion of the spin-polarized quasiparticles plays a critical role in depressing the superconcutivity. Based on the measurements of the magnetotransport and the low field magnetoresistance, direct evidence of field-suppressed interfacial spin scattering is given.All studies above are of very importance not only for understanding the properties of the pair-breaking by the exchange field and the interaction of different length scales at the oxide S/F interface, but also for designing new electronic devices in the future.