TEM investigation of growth mechanisms and microstructure of model YBCO coated conductor architectures deposited by metalorganic decomposition

Descripció del recurs: el 21-08-2008Consultable des del TDXTítol obtingut de la portada digitalitzadaThis thesis is divided in six chapters. In the first one, we expose an amenable brief introduction to the YBCO compound, as well as to the CSD method. Then, in the second chapter we describe briefly the experimental techniques used for the physical characterization of the CSD thin films presented in this work. The results, chapters 3, 4 , 5 and 6, are presented in two parts: Part I reports many aspects governing the growth mechanisms of CSD films, in particular the identification of those factors controlling the evolution of microstructures, which remain poorly established in comparison with vacuum deposited films. We give first an account of the evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 films deposited from chemical solutions (chapter 3), and second the microstructural evolution of YBCO from trifluoroacetate precursors, which follow a complex compositional trajectory (chapter 4). In part II the microstructure of TFA-YBCO thin films with high critical current densities is analysed. We depict intrinsic structural defects occurring within YBCO films, focusing in extended defects which can lead to strong flux-pinning and high critical current density (chapter 5). The microstructure of TFA-YBCO films with artificial defects has been also studied (chapter 6). The inclusion of BaZrO3 results in strong increase of the critical current density, demonstrating that chemical solution growth is a very flexible methodology to nanostructure YBCO films and coated conductors. Finally, we present the general conclusions of this study

Epitaxial stabilization of ε-Fe2O3 (00l) thin films on SrTiO3 (111)

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Thin films of the metastable and elusive ε-Fe2O3 have been epitaxially stabilized on SrTiO3 (111) substrates. The ε-Fe2O3 films present a (001) orientation perpendicular to the substrate and three in-plane domains measuring a few nanometers and showing atomically sharp interfaces. We argue that this domain structure, rather than the epitaxial-strain, plays an essential role in stabilizing the ε-Fe2O3 by minimizing the energy of (100) surfaces. The ε-Fe2O3 films show a large in-plane coercivity ∼ 8 kOe which combined with the magnetoelectric character claimed for this oxide may lead to novel applications in spintronics

Engineering two-dimensional superconductivity and Rashba spin-orbit coupling in LaAlO/SrTiO quantum wells by selective orbital occupancy

The discovery of two-dimensional electron gases (2DEGs) at oxide interfaces-involving electrons in narrow d -bands-has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s - and p - electrons. There is a growing consensus that emerging properties at these novel quantum wells-such as 2D superconductivity and magnetism-are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO₃/SrTiO₃ wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin-orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO₃/SrTiO₃ interfaces. Two-dimensional electron gases at oxide interfaces induce exotic behaviours. By studying samples with different crystal orientation, Herranz et al. show that the extension and anisotropy of the oxide quantum well properties can be controlled through selective sub-band filling via orientational tuning

Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling

In this paper, we demonstrate that in Ta/MgO/IrMn tunneling junctions, containing no ferromagnetic elements, distinct metastable resistance states can be set by field cooling the devices from above the Néel temperature (TN) along different orientations. Variations of the resistance up to 10% are found upon field cooling in applied fields, in-plane or out-of-plane. Well below TN, these metastable states are insensitive to magnetic fields up to 2 T, thus constituting robust memory states. Our work provides the demonstration of an electrically readable magnetic memory device, which contains no ferromagnetic elements and stores the information in an antiferromagnetic active layer

Epitaxial stabilization of ε-Fe2O3 (00l) thin films on SrTiO3 (111)

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Thin films of the metastable and elusive ε-Fe2O3 have been epitaxially stabilized on SrTiO3 (111) substrates. The ε-Fe2O3 films present a (001) orientation perpendicular to the substrate and three in-plane domains measuring a few nanometers and showing atomically sharp interfaces. We argue that this domain structure, rather than the epitaxial-strain, plays an essential role in stabilizing the ε-Fe2O3 by minimizing the energy of (100) surfaces. The ε-Fe2O3 films show a large in-plane coercivity ∼ 8 kOe which combined with the magnetoelectric character claimed for this oxide may lead to novel applications in spintronics

Two-dimensional electron gases at LaAIO_3/SrTiO_3 interfaces: orbital symmetry and hierarchy engineered by crystal orientation

Recent findings show the emergence of two-dimensional electron gases (2DEGs) at LaAIO_3/SrTiO_3 interfaces along different orientations; yet details on band reconstructions have remained so far unknown. Via x-ray linear dichroism spectroscopy, we demonstrate that crystal symmetry imposes distinctive 2DEG orbital hierarchies on (001)-and (110)-oriented quantum wells, allowing selective occupancy of states of different symmetry. Such orientational tuning expands the possibilities for electronic engineering of 2DEGs and opens up enticing opportunities to understand the link between orbital symmetry and complex correlated states at LaAIO_3/SrTiO_3 quantum wells

Monolithic integration of room-temperature multifunctional BaTiO 3 -CoFe 2 O 4 epitaxial heterostructures on Si(001)

Altres ajuts: Beatriu de Pinós postdoctoral scholarship (2011 BP-A 00220 and 2011 BP-A_2 00014)The multifunctional (ferromagnetic and ferroelectric) response at room temperature that is elusive in single phase multiferroic materials can be achieved in a proper combination of ferroelectric perovskites and ferrimagnetic spinel oxides in horizontal heterostructures. In this work, lead-free CoFe 2 O 4 /BaTiO 3 bilayers are integrated with Si(001) using LaNiO 3 /CeO 2 /YSZ as a tri-layer buffer. They present structural and functional properties close to those achieved on perovskite substrates: the bilayers are fully epitaxial with extremely flat surface, and exhibit robust ferromagnetism and ferroelectricity at room temperature

Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling

In this paper, we demonstrate that in Ta/MgO/IrMn tunneling junctions, containing no ferromagnetic elements, distinct metastable resistance states can be set by field cooling the devices from above the Néel temperature (TN) along different orientations. Variations of the resistance up to 10% are found upon field cooling in applied fields, in-plane or out-of-plane. Well below TN, these metastable states are insensitive to magnetic fields up to 2 T, thus constituting robust memory states. Our work provides the demonstration of an electrically readable magnetic memory device, which contains no ferromagnetic elements and stores the information in an antiferromagnetic active layer