Iron disilicide in a bulk form is practically nonmagnetic. In contrast, nanoparticles of FeSi 2 exhibit superparamagnetism with blocking temperatures ranging from 8 K ͑15 nm͒ to 34 K ͑55 nm͒. Their relatively low saturation magnetization suggests that the magnetic behavior is associated with only a small fraction of Fe ions, which have a sufficient number of other Fe as nearest neighbors. The chemical disorder is presumably induced in the formation of nanoparticles. A spin glass-type anomaly below 10 K observed in specific heat data gives a further evidence for the compositional heterogeneity.

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