Abstract
Mössbauer absorption spectra of 57Fe in ultrafine powders of NiFe2O4 were taken in external magnetic fields from 0 to 20 kOe at room temperature. A transition from a spectrum with no hyperfine field, Hhpf, to a Zeeman split spectrum of Hhpf=500 kOe has been observed. Comparison of the results with magnetization measurements suggest that the drastic effect of the external field is due to the polarization of the superparamagnetic particles by the field.