Effect of annealing on the microstructure of NiFe1.925Dy0.075O4 thin films

Dysprosium-doped nickel-ferrite (NiFe1.925Dy0.075O4) thin films were fabricated using RF sputter-deposition. Structural studies indicate that the effect of post-deposition annealing is significant on structural evolution in NiFe1.925Dy0.075O4 films. As-grown NiFe1.925Dy0.075O4 films were amorphous. Annealing (Ta) in air at 450–1000 °C results in the formation of nanocrystalline NiFe1.925Dy0.075O4 films, which crystallize in the inverse spinel structure. The average grain size (L) increases from 5 to 40 nm with increasing Ta from 450 to 1000 °C. Lattice constant of NiFe1.925Dy0.075O4 films is higher compared to that of NiFe2O4 due to partial substitution of Dy3+ ions for Fe3+ ions. The lattice parameter increases from 8.353 to 8.362 Å with increasing Ta from 450 to 1000 °C which is attributed to the lattice-strain developed in the NiFe1.925Dy0.075O4 films with increasing Ta. The corresponding density of NiFe1.925Dy0.075O4 films increases from 3.2 to 3.9 g/cm3 with increasing annealing temperature. Magnetization measurements indicate the ferromagnetic behavior of all the films while the coercive field values at 300 K are found to be 0.0134 T and 0.0162 T for as grown and Ta = 1000 °C films, respectively.