Magnetic and transport properties of YBa2Cu3O7 - La0.7Ca0.3MnO3 heterostructures

Abstract

The exploration of interface properties in complex oxide heterostructures and superlattices is one of the new exciting fields in condensed matter sciences. This is particularly originating from the technological advances in synthesizing heterostructures with atomic scale precision by advanced thin film deposition techniques. There is a plethora of novel achievements culminating in unexpected results, such as generating artificial multifunctional materials with the prominent example of the appearance of interface electrical conductivity and even superconductivity in between insulating films (SrTiO3 - LaAlO3). In this thesis a special case of heterostructures is treated. Here, heterostructures composed of superconducting YBa2Cu3O7 and half-metallic ferromagnetic La2/3Ca1/3MnO3 are investigated and the interplay of the two long-range antagonistic ordering principles - superconductivity and ferromagnetism - is intended to be studied. Whereas the physics of such structures with the CuO2 planes of the superconducting YBa2Cu3O7 oriented parallel to the substrate plane ( i.e. the short coherence length of the superconductor, ξc ~ 0.1 nm is facing the interface perpendicular) has been explored in great detail, little is known in the case of the CuO2 planes oriented perpendicular to the substrate plane and thus ab ~ 1.6 nm is pointing perpendicular to the interface. In the former case, the properties of the heterostructures and superlattices are determined by an interplay of charge transfer and orbital reconstruction, but the mechanisms occurring in the latter case are unknown so far. Prior to elaborated experiments to study the interface properties at an atomistic scale, the technology of fabricating such structures has to be accomplished and their macroscopic properties (structure, transport and magnetic properties) have to be investigated. It is the goal of this thesis to prepare the ground for the atomistic studies by developing the technological prerequisites for the growth of (110)-oriented YBCO-LCMO heterostructures and characterize their structural, electric and magnetic macroscopic properties. Due to the sensitivity of the macroscopic properties of such structures to the crystallographic perfection of the interfaces a substantial part of this thesis is devoted to the corresponding enabling technology. Advanced PLD techniques are used to fabricate single layer (110)-oriented YBCO and LCMO films, bilayers as well as twin-free (103)-oriented LCMO-YBCO-LCMO trilayers and (110)-oriented YBCO-LCMO-YBCO structures with ultrathin LCMO films (nominally 1-2 nm ) in between 50 nm YBCO. These (110)-oriented trilayers serve as a precursor for a prototype planar Josephson junction technology. A comparison of the experimental results for (001) and (110) - oriented heterostructures reveals distinct changes in the ordering temperatures Tc and TCurie giving a fingerprint of different microscopic mechanisms taking place at the interfaces. Furthermore, in the twin-free (103)-oriented trilayer samples a novel positive Meissner effect has been observed which is ascribed to the magnetic domain arrangement of the LCMO.