Growth of nickel ferrite thin films using pulsed-laser deposition

Epitactic thin films of nickel ferrite spinel (NiFe2O4) have been grown on (0 0 0 1) oriented sapphire substrates using pulsed-laser deposition. Three different methods were used to produce the films. The first was direct deposition using a stoichiometric NiFe2O4 target. In the other two methods, a solid-state reaction between NiO and Fe2O3 was utilized to produce the spinel. In one case, the spinel was formed in situ while film growth occurred in the deposition system; in the other case, a NiO/Fe2O3 heterostructure was reacted ex situ at elevated temperatures in air. The resulting ferrite films were analyzed in cross section using both conventional and high-resolution transmission electron microscopy. In general, the predominant defects found in the spinel films were twin boundaries. However, the topology of the twin boundaries in each of the three films was different. These differences were attributed to, and correlated with, the growth method and reaction processes used to produce the spinel film.