Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

Abstract

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe 2 O 4 spinel oxides. Mössbauer spectra for NiFe 2 O 4 at various temperatures (79 ≤ T ≤ 900 K) are well fitted by two sextets associated with 57 Fe 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 equationwhere 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. KeywordsFerrites · Mössbauer spectroscopy · Hyperfine fields Ferrites are a group of oxides with the spinel crystal structure AB 2 O 4 , where A and B cations are contained in the tetrahedral (A) and octahedral (B) sites. The magnetic order depends on different superexchange interactions mediated by intervening oxygen ions. The superexchange interactions are characterized by the exchange integrals J AB (A-O-B), J AA (A-O-A) and J BB (B-O-B). Ferrimagnetic ordering takes place when the intersublattice interaction (A-O-B) is stronger than