A new type of magnetism unveiled in an iconic material

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Scientists have made a breakthrough discovery in strontium ruthenate – with potential for new applications in quantum electronics.

Since the discovery of superconductivity in Sr2RuO4 in 1994, hundreds of studies were published on this compound, which suggested that Sr2RuO4 is a very special system with unique properties. These properties make Sr2RuO4 a material with great potential, for example, for the development of future technologies, including superconducting spintronics and quantum electronics, due to its ability to simultaneously carry lossless electric currents and magnetic information. An international research team led by scientists from the University of Konstanz has now been able to answer one of the most interesting open questions about Sr2RuO4: Why does the superconducting state of this material exhibit certain characteristics that are typically found in materials known as ferromagnets, which are considered to be antagonists of superconductors? The team found that Sr2RuO4 hosts a new form of magnetism, which can coexist with superconductivity and also exists independently of superconductivity. The results were published in the latest issue of Communication Nature.

After a research study that spanned several years and involved 26 researchers from nine different universities and research institutes, the missing piece of the puzzle seems to have been found. Alongside the University of Konstanz, the universities of Salerno, Cambridge, Seoul, Kyoto and Bar Ilan as well as the Japan Atomic Energy Agency, the Paul Scherrer Institute and the Centro Nazionale delle Ricerche took part in the study.

So far not the right tool to find evidence

“Despite decades of research on Sr2RuO4, there was no evidence for the existence of this unusual type of magnetism in this material. A few years ago, however, we wondered if the reconstruction that occurs in this material on the surface, where the crystalline structure shows some small changes at the atomic scale level, could also lead to an electronic order with properties magnetic. Following this intuition, we realized that this question had probably not been addressed because no one had used the “right tool” to find evidence of this magnetism, which we thought could be extremely weak and only limited to few atomic layers of the surface of the material,” says the leader of this international research study, Professor Angelo Di Bernardo of the University of Konstanz, whose research focuses on spintronic and quantum superconducting devices based on innovative materials.

To carry out the experiment, the team used high-quality single crystals of Sr2RuO4 prepared by the group of Dr. Antonio Vecchione of the Centro Nazionale delle Ricerche (CNR) Spin in Salerno. “Make large crystals of Sr2RuO4 without any impurities was a tall order but crucial for the success of the experiment, as the defects would have given a signal similar to the magnetic signal we were looking for,” explains Dr. Vecchione.

The right tool is a muon beam

The special “tool” the researchers used to unveil the new magnetism is a beam of particles called muons that are produced in a particle accelerator in Switzerland at the Paul Scherrer Institute (PSI). “At PSI, we have the only facility in the world to produce implantable muons with an accuracy of a few nanometers. These particles, which can be used to detect extremely small magnetic fields, could be stopped very close to the surface of Sr2RuO4which was crucial for the success of the experiment,” says Dr Zaher Salman who coordinated the experiment at the PSI muon facility.

“It was a very nice experience to perform measurements in an international beam time facility like PSI and to interact with such a large group of inspiring scientists from all over the world, from the very beginning of my PhD in Konstanz”, says Roman Hartmann, a PhD researcher who also contributed as first author to the study.

The authors also developed a theoretical model suggesting the origin of this hidden surface magnetism. “Unlike conventional magnetic materials whose magnetic properties derive from the quantum mechanical property of an electron known as spin, a cooperative swirling motion of interacting electrons, generating circulating currents at the nanometer scale, sub- tends the magnetism discovered in Sr2RuO4says Dr. Mario Cuoco from CNR-spin who developed the theoretical model together with Dr. Maria Teresa Mercaldo and other colleagues from the University of Salerno.

New perspectives for basic and applied research

As pointed out by Professor Jason Robison from the University of Cambridge, the results confirm that “physical properties can be dramatically changed at the surface of complex material and at interfaces within thin-film heterostructures, and these modifications can be exploited to discover new science for basic and applied research, including the design and development of quantum devices.”

Project co-authors also include Professor Yoshiteru Maeno of Kyoto University, the scientist who discovered superconductivity in Sr2RuO4 in 1994 and who has contributed to some of the most important studies of this material published in the last 30 years.

“This discovery not only solves a long-standing puzzle and makes the material iconic Sr2RuO4 even more interesting than before, but could also trigger new investigations that could possibly help answer other striking open questions in materials science,” says Professor Elke Scheer of the University of Konstanz, another of the leaders of the project and leader of the mesoscopic systems research team.

The new type of magnetism discovered in Sr2RuO4 is essential to better understand also the other physical properties of Sr2RuO4 including its unconventional superconductivity. The fundamental discovery may also lead to the search for this new form of magnetism in other materials similar to Sr.2RuO4 as well as trigger new studies to better understand how such magnetism can be manipulated and controlled for new quantum electronics applications.

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