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

Abstract

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°C, under vacuum in steps of 100°C and the XRD pattern is recorded in situ. A sudden drop in the lattice parameter from 8.478 to 8.468 Å is observed at 800°C, above which it increases with increasing temperature. After annealing at 1000°C, the lattice parameter reduces from 8.441 to 8.399 Å 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°C, can be attributed to coalescence phenomenon, which starts at 600°C. The estimated value of the activation energy of ZnFe₂O₄ nanoparticles during the growth is 18.207 kJ/mol.