Emerging dilute ferromagnetism in high-Tc superconductors driven by point defect clusters

Our results reveal the coexistence of ferromagnetic interactions much closer to high Tc superconductivity than previously suspected and open a new playground for exploring interactions between the two antagonistic phenomena, ferromagnetism and superconductivity. https://doi.org/10.1002/advs.201500295

Left panel, Z-contrast atomic resolution image of the YBCO lattice with an Y124 intergrowth imaged along the [010] direction. The dimmer contrast in the double Cu (2VCu ) chain 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 (=0o) for a YBCO thin-film.

Our work is able to connect a dilute magnetic behavior, a macroscopic phenomenon, explored using XMCD measurements with atomic-scale structure and chemistry obtained using STEM imaging and spectroscopy, both of which are tied up through DFT calculations.

We report, for the first time, the existence of a dilute ferromagnetic system in the superconducting state of YBa2Cu3O7−δ (YBCO) cuprates. We demonstrate that the appearance of this dilute ferromagnetic state is associated to a network of vacancy clusters

Our work presents fundamental new physical insights. The combination of the advanced techniques used in this work is a step forward to underscore new phenomena probing nanoscale functionalities such as magnetism at the atomic-scale and its influence to macroscopic phenomena such as diluted ferromagnetism and Superconductivity.

Also, YBa2Cu3O7 (Y123) is the most studied among High Temperature superconductors, where pining mechanisms are determined by defects. The ability to predict the physics of vortex pining in High Temperature superconductors depends on accurate characterization of nanoscale structural defects like the one reported here.

Experiment and techniques

Amongst the various nano-scale defects reported in YBCO, the Cu-O double chain is the most common one, regardless of the growth technique of the thin films. This is the so-called Y2Ba4Cu8-γO16-d (Y124) intergrowth, which is essentially an extra Cu-O atomic plane within the YBCO, what poses a stoichiometric challenge. In this work we unveil the complex nature of this defect. First, using aberration-corrected scanning transmission electron microscopy (STEM) imaging and spectroscopy, we showed how the system solves the local off-stoichiometry induced by the extra Cu-O chain, removing half of the Cu atoms in selected chains. Indeed, no technique other than aberration corrected STEM could have ever come with such remarkable observation and characterization of a point defect cluster like the one presented in this work. Secondly, by combining the STEM results with density functional theory (DFT) calculations, we predict these defect clusters within the superconducting material to have robust magnetic moments with ferromagnetic ordering. Finally, we used X-ray magnetic circular dichroism (XMCD) spectroscopy, which provides evidence of Cu magnetic moments and the presence of a dilute network of magnetic defects within the high Tc superconducting state.

Researchers

J. Gazquez1, R. Guzman1, R. Mishra2,3,4, E. Bartolomé5, J. Salafranca6,3, C. Magén7, M. Varela6,3, M. Coll1, A. Palau1, S. M. Valvidares8, P. Gargiani8, E. Pellegrin8, J. Herrero-Martin8, S. J. Pennycook9, S.T. Pantelides2,3, T. Puig1, X. Obradors1

1Institut de Ciència de Materials de Barcelona, Spain

2Department of Physics and Astronomy, Vanderbilt University

3Materials Science and Technology Division, Oak Ridge National Laboratory

4Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63130, USA

5Escola Universitària Salesiana de Sarrià (EUSS), Barcelona, Spain

6Universidad Complutense de Madrid. Madrid 28040, Spain

7Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragón – ARAID, Universidad de Zaragoza, Zaragoza 50018, Spain

8ALBA Synchrotron Light Source, Barcelona, Spain

9Department of Materials Science and Engineering, National University of Singapore