Two types of magnetic configurations—antiferromagnetism (AFM) and ferromagnetism (FM)—have been found to compete with each other, thwarting the rise of superconductivity in iron pnictides, a class of high-temperature superconductors. In an AFM fluctuation electron spins temporary align in an alternating checkboard pattern while electron spins align temporary in the same direction in a FM fluctuation. While FM fluctuations have been suggested in the iron pnictide superconductors by theoretical calculations, the possible existence of FM fluctuations has not yet been examined from a experimental point of view. To root out FM fluctuations, scientists used nuclear magnetic resonance (NMR), a technique similar to the MRI procedure used in hospitals, which probes the structure and composition of materials. They found clear evidence for FM fluctuations in various species of iron pnictide superconductors. The findings suggest that these FM fluctuations compete with superconductivity, and this competition between FM and AFM fluctuations may be a key ingredient in understanding the temperature at which superconductivity arises in the iron pnictides.
Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR