Structural features of epitaxial NiFe_2O_4 thin films grown on different substrates by direct liquid injection chemical vapor deposition

摘要

NiFe_2O_4 (NFO) thin films are grown on four different substrates, i.e., Lead Zinc Niobate-Lead Titanate (PZN-PT), Lead Magnesium Niobate-Lead Titanate (PMN-PT), MgAl_2O_4 (MAO) and SrTiO_3 (STO), by a direct liquid injection chemical vapor deposition technique (DLI-CVD) under optimum growth conditions where relatively high growth rate (～20nm/min), smooth surface morphology and high saturation magnetization values in the range of 260-290 emu/ cm~3 are obtained. The NFO films with correct stoichiometry (Ni:Fe=1:2) grow epitaxially on all four substrates, as confirmed by energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. While the films on PMN-PT and PZN-PT substrates are partially strained, essentially complete strain relaxation occurs for films grown on MAO and STO. The formations of threading dislocations along with dark diffused contrast areas related to antiphase domains having a different cation ordering are observed on all four substrates. These crystal defects are correlated with lattice mismatch between the film and substrate and result in changes in magnetic properties of the films. Atomic resolution HAADF imaging and EDX line profiles show formation of a sharp interface between the film and the substrate with no inter diffusion of Pb or other elements across the interface. Antiphase domains are observed to originate at the film-substrate interface.