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. Credit: Image courtesy of DOE/Argonne National Laboratory
Scientists have discovered only half the atoms in some iron-based superconductors are magnetic, providing the first conclusive demonstration of the wave-like properties of metallic magnetism. It allows for a clearer understanding of the magnetism in iron arsenides, and how it helps induce superconductivity which occurs at up to 138K, or minus -135 deg Celsius.
“In order to be able to design novel superconducting materials, one must understand what causes superconductivity,” said Argonne senior physicist Raymond Osborn. “Given the similarity to other materials, such as the copper-based superconductors, our goal was to improve our understanding of high-temperature superconductivity.” This would allow for development of magnetic energy-storage systems, fast-charging batteries for electric cars and a highly efficient electrical grid. 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.
They showed 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, 2 waves interfere to cancel out, producing 0 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.
They also used high-resolution Xray 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.
“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,” Rosenkranz said.
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. http://www.anl.gov/articles/new-magnetism-research-brings-high-temp-superconductivity-applications-closer
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