Influence of evolving microstructure on electrical and magnetic characteristics in mechanically synthesized polycrystalline Ni-ferrite nanoparticles

NiFe2O4 nanoparticles were synthesized via mechanical activation of NiO and Fe2O3 powders in a high energy planetary ball mill. The results indicated that after 30 h of ball milling the NiO/Fe2O3 reacted with a solid-state reaction mode producing nickel ferrite particles with 12 nm in size. Differential scanning calorimetry (DSC) on the 30-h activated sample showed nickel ferrite's Curie temperature to be around 573 degrees C. The 30 h-activated compacted powders were sintered from 900 to 1300 degrees C with 100 degrees C increments. From the XRD patterns results, however, increasing sintering temperature to 1300 degrees C led to an increase of the Ni-ferrite's crystallite sizes; the sizes remained in nano scale and no phase change was observed. A critical grain size of 211 nm was found from plotting the grain size against coercivity, suggesting a single-domain particles area was reduced and multi-domain particles area was increased by increasing sintering temperature. The grain growth activation energy was found to be 64.43 kJ/mol. The electrical resistivity obtained was similar to 10(6) Omega cm at 900 degrees C and its value decreased to similar to 10(5) Omega cm at 1300 degrees C sintering. VSM studies showed saturation-magnetization increase from 8.5 emu/g in the 30 h-activated sample to 52.6 emu/g in those sintered at 1300 degrees C. Two different shapes of hysteresis loops were observed in the B-H curves. (C) 2015 Elsevier B.V. All rights reserved.