The anisotropy constant of nonstoichiometric iron oxide powder has been studied by Mössbuer spectroscopy. The diameter of the powder was about 10 nm. At room temperature

the area ratio of the paramagnetic fraction to the ferromagnetic fraction of Mössbauer spectra before and after aging treatment was almost unchanged

but the quadrupole value was drastically changed

which suggested that the anomalous increasing of coercivity in nonstoichiometric powder should be due to the additional anisotropy induced by lattice distortion after partial oxidation of samples.