Only recently discovered, the iron-based pnictide superconductors have extended the established class of copper-based high-temperature superconductors and attracted tremendous interest. Researchers expect that the study of their characteristics, particularly in comparison to those of the more traditional copper-based compounds, may provide important clues to the understanding of superconductivity at high temperatures. Researchers from Kyoto University in Japan1 have now developed a theoretical model that correctly explains the magnetic properties of iron pnictides.

Fig. 1: Crystal structure and magnetic ordering in iron-based pnictide superconductors. (Upper) Crystal structure. (Lower) Antiferromagnetic order with alternating magnetic moments (orange arrows) of in-plane iron atoms.

The iron pnictide superconductors are the first superconductors to be discovered that contain an element (iron) that is also magnetic. This is significant, as magnetism typically suppresses superconductivity. At temperatures above the threshold of superconductivity in these materials, however, the pnictides do show antiferromagnetism (Fig. 1). The driving force behind this antiferromagnetic ordering has been the subject of intense debate, as the theoretical models developed to explain it have so far not been able to account for all the phenomena observed, such as the iron pnictides’ optical absorption.

It had been assumed that the antiferromagnetic order is mediated by strong coupling of the individual atomic magnetic moments. The new model now takes a different approach in modelling the electronic properties by explicitly assuming a much weaker coupling of those magnetic moments. “Our calculations based on weak coupling are consistent with optical data and also suggest smaller individual magnetic moments than previously predicted,” explains Takami Tohyama regarding the findings of his research team.

In addition, the researchers calculated the electrical resistance of these pnictides in the presence of a magnetic field. The dependence of the electrical resistance on the direction of measurement was found to differ depending on whether strong or weak coupling was assumed. Such experiments are therefore an opportunity to confirm the theoretical model experimentally, and, according to Tohyama, are already underway.

More importantly, this theoretical model points directly towards the important role of collective wave-like movements of the magnetic moments in the pnictides, which facilitate magnetic coupling. “Our theoretical model shows that the spin-density waves are a good starting point to study not only antiferromagnetism but also the mechanism of superconductivity in the pnictides,” says Tohyama.