Cation redistribution in ultrafine zinc ferrite subjected to high energy ball milling

Santhosh D.Shenoy^$, P.A.Joy*, Alok Banerjee⁺ and M.R.Anantharaman^$

^$Department of Physics, Cochin university of Science and Technology, Cochin, 682 022

^*Physical Chemistry Division, National Chemical Laboratory, Pune 411 008

⁺Inter university consortium for DAE Facilities, Khandwa road, Indore-452 017

Zinc ferrite in the micron regime is a normal spinel with no net magnetisation. However, zinc ferrite in the fine regime has been reported to be exhibiting ferrimagnetic properties and has been a subject of intense research recently. Ultrafine zinc ferrite particles (ZnFe₂O₄) with a very small amount of alpha ferric oxide (aFe₂O₃) prepared by low temperature chemical coprecipitation technique were subjected to high energy ball milling under dry conditions. These samples were milled for several hours and subjected to structural and magnetic studies at various stages of milling. XRD analysis indicates that the particles are nanomertric in size. aFe₂O₃ phase was found to be increasing with milling time. Strain calculations indicates a sharp increase in the lattice strain with milling time. Room temperature magnetization curves show a typical superparamagnetic feature for unmilled sample but with milling, hysteresis was observed. The saturation magnetization (M_s) attains a higher value of 16.41 emu/gm for 10 hours milled sample while the initial coprecipitated ZnFe₂O₄ exhibits an M_s of 2.196 emu/gm. AC magnetic susceptibility studies at 133.33Hz and 2 Oe field indicates a blocking temperature (T_B) shift towards higher temperature with milling. Below the blocking temperature the magnetization direction within each nanoparticle aligns along the easy axis of the nanoparticle. But the random arrangement of easy axes of nanoparticles causes the magnetic susceptibility to decrease steadily below the blocking temperature. For unmilled sample the T_B is well below liquid nitrogen temperature while for 10 hours milled sample this rises to around 230⁰K. X-ray diffraction, room temperature VSM and low temperature AC susceptibility studies suggests cation reversal that might have been facilitated by the presence of aFe₂O₃ in the initial sample. The local temperature and pressure developed during high energy ball milling would have contributed to the migration of Zn²⁺ cation from the tetrahedral (A) site to the octahedral (B) site.