Effect of thermal annealing under vacuum on the crystal structure, size, and magnetic properties of ZnFe2O4 nanoparticles

In this paper, we report the variations in the crystal structure, average particle size, and magnetic properties of ZnFe2O4 nanoparticles on thermal annealing, using in situ high temperature x-ray diffraction (XRD). Fine powder of ZnFe2O4 nanoparticles with an average particle size of 9.3 nm, prepared through coprecipitation technique, has been used in these studies. The powder is heated from room temperature to 1000 degrees C, under vacuum in steps of 100 degrees C and the XRD pattern is recorded in situ. A sudden drop in the lattice parameter from 8.478 to 8.468 A is observed at 800 degrees C, above which it increases with increasing temperature. After annealing at 1000 degrees C, the lattice parameter reduces from 8.441 to 8.399 A and the magnetization value increases from 5 to 62 emu/g, suggesting the possibility of a conversion of the cubic structured ZnFe2O4 from normal to inverse spinel structure due to canting of ions between the tetrahedral and octahedral interstitial sites. During annealing, the Zn2+ ions move from tetrahedral site to octahedral site whereas Fe3+ ions redistribute within the octahedral and tetrahedral sites in order to reduce the strain. The increase in the average particle size from 9 to 27 nm, after the thermal annealing at 1000 degrees C, can be attributed to coalescence phenomenon, which starts at 600 degrees C. The estimated value of the activation energy of ZnFe2O4 nanoparticles during the growth is 18.207 kJ/mol. (C) 2007 American Institute of Physics.