Although defects may be seen as detrimental, they play essential roles in many materials. In some complex oxides for instance, they present an opportunity to enhance particular properties, or even engineer new ones. An archetypal example is the high-Tc superconductor YBa₂Cu₃O_7−δ (YBCO), where defects are indispensable to immobilize quantized vortices in the presence of magnetic fields. Therefore, determining the atomic structure of defects is critical to unravel and control their effect on the physical properties. In this talk, we will do precisely that with an unforeseen complex point-defect that leads to an unexpected formation of ferromagnetic clusters embedded within a superconductor. The commonest YBCO defect comprises the doubling of Cu-O chains between Ba-O planes, a ubiquitous intergrowth regardless of the deposition technique. Here, we will unclothe the true nature of this defect using a combination of experiments and theory to provide a complete picture of the structure and chemistry of these intergrowths at the atomic-scale and their effect on the electronic and magnetic properties of YBCO. First, we will show, by means of scanning transmission electron microscopy (STEM), how the system solves the local off-stoichiometry induced by the extra Cu-O chain, removing half of the Cu atoms in selected chains, and the distortions induced by the vacancies. Secondly, we will show using density functional theory (DFT) how the complex structure of these intergrowths affects the electronic properties, and yields a magnetization density that extends into the neighboring Cu-O planes. Finally, we will present X-ray magnetic circular dichroism (XMCD) spectroscopy results, which provide evidence of the theoretically predicted Cu magnetic moments and the presence of a dilute network of magnetic defects within the high-Tc superconducting state, see Figure 1. See for more details “Emerging dilute ferromagnetism in high-Tc superconductors driven by point defect clusters”, Adv. Sci. 2016, 1500295.

Figures:

Figure 1. Left panel, Z-contrast image of the YBCO lattice with an Y2Ba4Cu8-γO16-d (Y124) intergrowth imaged along the [010] direction. The dimmer contrast in the double Cu chain (yellow arrow) is due to the presence of Cu vacancies (2VCu). The inset shows the isosurface plot showing the spin density associated with a 2VCu+3VO defect along with an integrated magnetization/area profile along the z-axis. Right panel, Cu L2,3 edge (top) background-subtracted XAS and (bottom) XMCD spectra measured at 6 T, 1.6 K in normal incidence (q=0º) for a YBCO thin-film.

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EMC Abstracts - https://emc-proceedings.com/abstract/point-defect-driven-ferromagnetism-in-yba2cu3o7-x-superconductor/