Argonne National Laboratory scientists have discovered that only half the atoms in some iron-based superconductors are magnetic, providing the first conclusive demonstration of the wave-like properties of metallic magnetism.

The discovery allows for a clearer understanding of the magnetism in some compounds of iron, the iron arsenides, and how it helps induce superconductivity, the resistance-free flow of electrical current through a solid-state material, which occurs at temperatures up to 138 degrees Kelvin, or minus -135º C.

Magnetic order in (Sr,Na)Fe2As2: The crystal structure contains planes of iron atoms (shown as red spheres). Half the iron sites have a magnetization (shown as red arrows), which points either up or down, but the other half have zero magnetization. This shows that the magnetism results from the constructive and destructive interference of two magnetization waves, a clear sign that the magnetic electrons are itinerant, which means they are not confined to a single site. The same electrons are responsible for the superconductivity at lower temperature. Image Credit: Argonne National Laboratory. Click image for the largest view.

Magnetic order in (Sr,Na)Fe2As2: The crystal structure contains planes of iron atoms (shown as red spheres). Half the iron sites have a magnetization (shown as red arrows), which points either up or down, but the other half have zero magnetization. This shows that the magnetism results from the constructive and destructive interference of two magnetization waves, a clear sign that the magnetic electrons are itinerant, which means they are not confined to a single site. The same electrons are responsible for the superconductivity at lower temperature.  Image Credit: Argonne National Laboratory. Click image for the largest view.

Argonne senior physicist Raymond Osborn, one of the project’s lead researchers explained, “In order to be able to design novel superconducting materials, one must understand what causes superconductivity. Understanding the origin of magnetism is a first vital step towards obtaining an understanding of what makes these materials superconducting. Given the similarity to other materials, such as the copper-based superconductors, our goal was to improve our understanding of high-temperature superconductivity.”

Argonne senior physicist Stephan Rosenkranz, the project’s other lead researcher offered that from an applied perspective, such an understanding would allow for the development of magnetic energy-storage systems, fast-charging batteries for electric cars and a highly efficient electrical grid.

Superconductors reduce power loss. The use of high-temperature superconducting materials in the electrical grid, for example, would significantly reduce the large amount of electricity that is lost as it travels though the grid, enabling the grid to operate more efficiently.

The researchers were able to show that the magnetism in these materials was produced by mobile electrons that are not bound to a particular iron atom, producing waves of magnetization throughout the sample. They discovered that, in some iron arsenides, two waves interfere to cancel out, producing zero magnetization in some atoms. This quantum interference, which has never been seen before, was revealed by Mössbauer spectroscopy, which is extremely sensitive to the magnetism on each iron site.

Researchers also used high-resolution X-ray diffraction at the Advanced Photon Source (APS) and neutron diffraction at Oak Ridge National Laboratory’s Spallation Neutron Source (SNS) to determine the chemical and magnetic structures and to map the electronic phase diagram of the samples used. The APS and SNS are DOE Office of Science User Facilities.

Rosenkranz said, “By combining neutron diffraction and Mössbauer spectroscopy, we were able to establish unambiguously that this novel magnetic ground state has a non-uniform magnetization that can only be produced by itinerant electrons. These same electrons are responsible for the superconductivity.”

The research has been published in the journal Nature Physics.

Next, Rosenkranz and Osborn plan to characterize the magnetic excitations, or fluctuations of iron-based superconductors, to determine how they to relate to and possibly cause superconductivity.

The surprise is half of all those atoms in the research test samples are not in on the magnetism. Its qualifies as a shock and a wake up call for process engineers and designers to get busy up rating their materials.

On the other hand the test equipment for learning what material compositions are better are far between and few in number. But the benefits of better materials now is known in part and is a huge challenge with a gargantuan payoff.

Rosenkranz’s list of applications is rather short. The potential list would include virtually everything using a magnetic field. This team’s work is a much more important thing than just a press release.


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