A research team led by the US Department of Energy’s (DOE’s) Argonne National Laboratory (CSA CSM) has discovered that only half the atoms in some iron-based superconductors are magnetic, providing a conclusive demonstration of the wave-like properties of metallic magnetism in these materials.

The discovery allows for a clearer understanding of the magnetism in some compounds of iron (the iron arsenides) and how this magnetism helps induce superconductivity, the resistance-free flow of electrical current through a solid-state material that occurs at temperatures up to 138K, or -135°C.

“In order to be able to design novel superconducting materials, one must understand what causes superconductivity,” says Argonne senior physicist Raymond Osborn, one of the project’s lead researchers. “Understanding the origin of magnetism is a first vital step toward 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.”

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

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 not bound to a particular iron atom, producing waves of magnetization throughout the sample. The team discovered that, in some iron arsenides, two waves interfere to cancel out, producing zero magnetization in some atoms. This never-seen-before quantum interference was revealed by Mössbauer spectroscopy, a technique that is extremely sensitive to the magnetism on each iron site.

Researchers also used high-resolution X-ray diffraction at the Advanced Photon Source and neutron diffraction at Oak Ridge National Laboratory’s Spallation Neutron Source (CSA CSM) to determine the chemical and magnetic structures and to map the electronic phase diagram of the samples used.

“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 says.

Next up, 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 team’s current research is available from Nature Physics.