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Spin-polarised probing visually renders magnetism

Posted: 08 Aug 2014 ?? ?Print Version ?Bookmark and Share

Keywords:iron telluride? magnetic structure? superconductivity?

Scientists from Max Planck Institute for Solid State Research inched closer to developing more energy-efficient superconducting technology, having visually rendered on an atomic scale the magnetic structure of so-called strongly correlated electron system of iron telluride.

Iron telluride is the non-superconducting parent compound of the iron chalcogenide superconductors. Prior to this work, information about the magnetic structure was provided only by neutron diffraction, which produced an imprecise image. The researchers now hope to be able to apply the method to materials which exhibit both superconducting and magnetic properties in order to find out more about the relationship between magnetism and superconductivity.

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Magnetic order of iron tellurides, imaged with a low-temperature scanning tunnelling microscope. The enlarged section shows the atomic structure. Source: Peter Wahl, University of St Andrews & MPI for Solid State Research

Substances such as copper oxide ceramics or iron arsenic compounds are deemed to be high-temperature superconductors: they do not have to be cooled as much as other materials in order to render them superconducting. Why is this? To date there are hypotheses, but no proven description of the precise processes.

A key question which many research groups are now posing is the one about the relationship between magnetic and superconducting properties of these materials, explained Peter Wahl from the Max Planck Institute for Solid State Research and University of St Andrews. Can both effects occur at one and the same location? Or are they mutually exclusive? Physicists think it is possible that the magnetic properties of the materials could even be the cause of their superconductivity.

In order to examine this, researchers have long been looking for a procedure that allows characterisation of the magnetic structures in these strongly correlated electronic materials on the atomic scale. The method of neutron diffraction has so far been the tool of choice for investigating the magnetic order, but it provides only spatially averaged insights into the magnetic structure.

Spin-polarised probing

The Stuttgart-based Max Planck researchers have made use of a spin-polarised scanning tunnelling microscope, which can image the orientation of the magnetic moments of individual atoms. The method is not new, but has so far mostly been applied to metal surfaces and nanostructures. What was not clear until now was whether the method could be used to clarify the magnetic structure of a strongly correlated system such as iron telluride. This is because the top layer of this material consists of tellurium, an element which itself is not magnetic.

The scientists have now shown that the spin-polarised scanning tunnelling microscope can also be applied to strongly correlated electron materials despite their complex chemistry. The iron lattice below most likely exerts too great an influence. Narrow longitudinal stripes, which result from the anti-ferromagnetic order in the iron telluride, can be recognised in the image taken by the scanning tunnelling microscope. Within the stripes, all magnetic moments have the same orientation; on the adjacent stripes, it is in the opposite direction.

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