Mixed spinel structure in nanocrystalline NiFe₂O₄

Nanocrystalline NiFe₂O₄ spinel has been synthesized with various grain sizes by high-energy ball milling. From the high-field magnetization studies and extended x-ray-absorption fine-structure, and Mössbauer measurements in an external magnetic field of 5 T applied parallel to the direction of gamma rays, we could observe that Ni²⁺ ions occupy tetrahedral sites on grain-size reduction due to milling. The Fe³⁺ spins have a canted structure and the canting angle increases with grain-size reduction. It is possible that the core Fe³⁺ spins are also canted because of the magnetocrystalline anisotropy introduced by the occupancy of the Ni²⁺ ions in the tetrahedral sites.