Synopsis: Magnetic order is no match for the lattice

Density-functional calculations provide a comprehensive picture of how magnetic order evolves with doping in two iron pnictide compounds.
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Illustration: Alan Stonebraker

Understanding magnetic order in the two major families of iron-based superconductors with FeAs layers—namely, electron-doped LaFeAsO1-xFx and hole-doped Ba1-2yK2yFe2As2—is important because magnetism seems to be intimately tied with superconductivity. At about 150K, the undoped compounds (x and y = 0) acquire a ferromagnetic stripe ordering along the shorter axis of the square Fe sublattice, while displaying antiferromagnetic ordering along the longer axis and between the Fe layers. Doping with enough carriers suppresses the magnetic order and induces superconductivity in both compounds, though in Ba1-2yK2yFe2As2 magnetism and superconductivity coexist for 0.10<y<0.15.

Although density-functional calculations overestimate the value of the Fe moment, they generally reproduce the observed magnetic structure for undoped pnictides. In an article appearing in Physical Review B, Alexander Yaresko, Guo-Qiang Liu, Viktor Antonov, and Ole Krogh Andersen at the Max-Planck Institute in Stuttgart, Germany, present comprehensive calculations on experimentally observed crystal structures of LaFeAsO1-xFx and Ba1-2yK2yFe2As2 to determine the magnetic behavior as a function of doping. They find that electron doping above x>0.1 in LaFeAsO1-xFx destabilizes the stripe magnetic order and leads to an incommensurate spin spiral order, whereas hole-doping in Ba1-2yK2yFe2As2 leaves the magnetic stripe order intact up to y=0.25. The authors find that in both compounds, the classical Heisenberg model with nearest and next-nearest neighbor spin interactions is inadequate to describe the magnetic order and may require additional terms to accurately compute the energy. – Sarma Kancharla


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