Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe2O4 spinel oxides. Moessbauer spectra for NiFe2O4 at various temperatures (79 ≤ T ≤ 900 K) are well fitted by two sextets associated with 57Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point TC was deduced by zero velocity Moessbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation Bhf(T)=Bhf(0)[1-(T/TC)n]βn, where n = 1 (based on the Landau-Ginzburg theory) and n = 2 (based on the Stoner theory). A systematic decrease of the Moessbauer spectrum shift with increasing temperature is observed.