Microstructural analysis and paramagnetic to ferromagnetic phase transition of chemically synthesized nanoparticles of Tb-doped ZnO

Tb³⁺-doped zinc oxide was prepared by the co-precipitation method. The as-dried sample was annealed at 80, 300, 500, 700, and 1000 °C. Rietveld analysis of the X-ray diffraction patterns of the samples annealed up to 300 °C showed that all the Tb³⁺ ions were entered in the ZnO lattice. But a fraction of Tb³⁺ ions could not enter in the ZnO lattice above 300 °C and this fraction increases with the increase of annealing temperature. The crystallite size and the internal strain due to substitution of bigger size R-ions in the ZnO lattice of the samples were estimated by using the Hall–Williamson plot. Results extracted from high resolution transmission electron microscopy are in agreement with those obtained from the XRD analysis. Magnetic susceptibility (χ) in the range of 300–14 K and magnetization as a function of magnetic field in the range of 300–5 K of the sample annealed at 80 °C were measured by Faraday and SQUID magnetometers, respectively. Values of χ in the paramagnetic region were analyzed by invoking the crystal field interaction of the Tb³⁺ ions with its diamagnetic neighbors. Paramagnetic to ferromagnetic phase transition has been observed at low temperature and the saturation magnetization measured at 5 K is quite high compared to the pristine sample.