Superconductors are materials without electrical resistance. That means that they can conduct electricity without losing energy. However, they need to be cooled down to extremely low temperatures (near the absolute zero, -273ºC) in order to show these properties. In 1986, a breakthrough took place with the discovery of the high temperature superconductors: the copper oxides, also known as cuprates, which can keep the superconducting state at higher temperatures (up to -135ºC, compatible with liquid nitrogen).
These materials have been one of the hottest research topics for the last 30 years but, still today, there are many aspects to discover about them. For instance, it was believed that superconductivity and magnetism were excluding phenomena. But now a group of researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC), have reported the existence of a dilute ferromagnetic system in YBCO cuprates (YBa2Cu3O7−δ). This discovery not only allows understanding better these materials but also enhance their physical properties.
Using a combination of techniques, including ALBA's X-rays, they have been able to identify a new magnetic behaviour within the superconductor and to correlate it to a network of copper and oxygen vacancies clusters. Therefore, contrary to what was previously suspected, these results show that ferromagnetic interactions do also happen in high temperature superconductors and offer new insights about the fundamental characteristics of these materials.
This is a step forward in understanding magnetism at the atomic scale, contributing to a better knowledge and control of their properties. It may have a great impact on vortex pinning in high temperature superconductors, enabling their use in superconducting magnets for research, energy or transport, among others.
A combination of advanced techniques
To understand the complex nature of these materials, researchers have combined different techniques during the study. Scanning transmission electron microscopy (STEM) imaging and spectroscopy let scientists observe and characterize the defects clusters of the analysed materials.
These results were also validated by density functional theory (DFT) calculations, showing that they present magnetic moments with ferromagnetic ordering.
Finally, at the BOREAS beamline, X-ray magnetic circular dichroism (XMCD) spectroscopy was key to demonstrate the existence of copper magnetic moments and the presence of magnetic defects in these materials.
The future of high temperature superconductors
The aim of the researchers is to continue exploring new defects landscapes designing nano-scale defects to strongly pin magnetic pinning and making these materials more efficient in the presence of high magnetic fields.
The Institute of Materials Science of Barcelona (ICMAB-CSIC) has a broad experience and knowledge about superconductors - initiated 25 years ago – that are addressed to understand the physical properties, the development and integration of high temperature superconductors into power systems.
"This work has only been possible bringing together different expertise. Our results connect a magnetic behavior at the atomic-scale using synchrotron's XMCD measurements, with atomic structure and chemistry obtained using electron microscopy, both of which are connected through theoretical calculations. This research presents both fundamental new physical insights and startling observations in one of the most studied compounds in Materials Science's history since its discovery in 1986", according to Jaume Gazquez, one of the researchers of the study.
Figure: Left panel, atomic resolution image of the superconductor lattice showing the planar defect where the Cu and O vacancies are located (yellow arrow). The spots represent the atomic columns of YBCO. The dimmer contrast of the atomic columns, therefore, signals the presence of Cu vacancies. The inset shows, in yellow, the region where the vacancies’ surrounding Cu and O atoms show a ferromagnetic behaviour. Medium panel, sketch of the superconductor structure with Cu and O atoms (in yellow) showing ferromagnetism. Right panel, Cu L2,3 edge (top) background-subtracted XAS and (bottom) XMCD spectra measured at 6 T, 1.6 K in normal incidence for a YBCO thin-film.
