M¨ossbauer Studies on Superexchange Interactions in Fe3O4

Fe3O4 has been studied by using M¨ossbauer spectroscopy and X-ray diffraction. The crystal isfound to have a cubic spinel structure with the lattice constant a0 = 8.3970 ± 0.0005 °A. The temperaturedependence of the magnetic hyperfine fields at 57Fe nuclei at the tetrahedral (A) and theoctahedral (B) sites is analyzed by using the N´eel theory of ferrimagnetism. The dominant superexchangeinteraction is found to be the antiferromagnetic intersublattice A − O − B superexchangeinteraction and its strength is JA−B = −23.4 kB. The intrasublattice A−O−A superexchange interactionis also antiferromagnetic with a strength of JA−A = −14.6 kB. The weakest superexchangeinteraction is the ferromagnetic B −O −B interaction: JB−B = 1.23 kB. The Debye temperaturesof the tetrahedral and the octahedral sites are found to be A = 351 ± 5 K and B = 322 ± 5 K,respectively.