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. Mössbauer 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 T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0){1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.