Superexchange interactions in Ni0.5Co0.5Fe2O4

Abstract

Slowly cooled Ni0.5Co0.5Fe2O4 has been investigated over a temperature range from 13 to 880 K using the Mössbauer technique. X-ray diffraction shows that it has an inverse spinel structure and a lattice constant of a=8.346±0.005 Å. The Mössbauer spectra that are laid less overlapping at low temperature have been fitted with two sextets in the ferrimagnetic state. The iron ions at both A (tetrahedral) and B (octahedral) sites are found to be in ferric high-spin states. Its Néel temperature TN is found to be 865±3 K. As the temperature increases toward TN a more overlapping effect of the six-line patterns in A and B sites is observed and interpreted to have originated from different temperature dependencies of the magnetic hyperfine fields at A and B iron sites. Assuming that A–B magnetic interactions are stronger than A–A and B–B interactions, we have calculated the hyperfine fields at the A and B iron sites in ionic distribution (Fe)A[Ni0.5Co0.5Fe]BO4 as a function of temperature. The theoretical Mössbauer spectra from the above calculated hyperfine fields are obtained and compared with experimental spectra.