Emerging Diluted Ferromagnetism in High-Tc Superconductors Driven by Point Defect Clusters.

Jaume Gazquez,Anna Palau,Javier Herrero‐Martin,Sokrates T Pantelides,Roger Guzman,Stephen J Pennycook,Mariona Coll,Teresa Puig,S Manuel Valvidares,Cesar Magén,Xavier Obradors,Juan Salafranca,Pierluigi Gargiani,Rohan Mishra,Eric Pellegrin,Maria Varela,Elena Bartolomé

doi:10.1002/advs.201500295

Abstract

Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high‐temperature superconductor YBa2Cu3O7‐δ (Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration‐corrected scanning transmission microscopy has been used for exploring, on a single unit‐cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect‐driven ferromagnetism, and X‐ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects.

Highlights

Superconductivity and ferromagnetism are considered to be naturally exclusive phenomena, and they can coexist in high-quality oxide interfaces between SrTiO3 and other nonmagnetic oxides,[1] and in heavy fermions systems and ruthenocuprates,[2,3,4] they usually show a competing behavior, like in superconducting/ ferromagnetic heterostructures
Since the precursors or targets used for the growth of the films have a stoichiometry of Y123, the extra amount of Cu needed for the formation of Y124 intergrowths would lead to a local Cu off-stoichiometry, a situation that worsens in the case of Y123 nanocomposites where the number of Y124 intergrowths multiplies
We have unveiled the complexity of the so-called Y124 intergrowths, the most common defect in Y123 thin films

Summary

Introduction

Superconductivity and ferromagnetism are considered to be naturally exclusive phenomena, and they can coexist in high-quality oxide interfaces between SrTiO3 and other nonmagnetic oxides,[1] and in heavy fermions systems and ruthenocuprates,[2,3,4] they usually show a competing behavior, like in superconducting/ ferromagnetic heterostructures. Mishra Department of Mechanical Engineering and Materials Science Washington University in St. Louis St. Louis, MO 63130, USA. Varela Universidad Complutense de Madrid Madrid 28040, Spain. High-temperature superconductor Y123 is among the most relevant playgrounds for the investigation of the physics of magnetic flux lines, in which nanoscale defects are essential to pin vortices and prevent resistive losses in the presence of magnetic fields.[10] significant efforts have been made in recent years toward nanoengineering Y123 films, such as the assembly of nonsuperconducting phases within the Y123 matrix, which have proven to enhance their performance in power applications.[11,12,13,14,15,16,17,18,19,20] In chemical solution deposition (CSD) derived Y123 films displaying enhanced vortex pinning efficiency, the introduction of secondary phases within the superconducting matrix renders a huge increase of Y124 intergrowths[19,21] while keeping critical temperature (Tc) of ≈90 K. The density of intergrowths may vary significantly with the growth method and the processing and in the present study we used CSD derived films in which these intergrowths have higher concentrations

Results and Discussion

Conclusions

Experimental Section