Monitoring B-site ordering and strain relaxation in NiFe2O4 epitaxial films by polarized Raman spectroscopy

Polarized Raman spectra of NiFe2O4 (NFO) films of varying thickness and growth temperature are investigated and discussed. We find that the relaxed films obtained at higher temperatures on MgAl2O4 (MAO) substrate exhibit spectra identical to those of single crystals and provide strong indications for ordering of Ni2+ and Fe3+ at the octahedral sites. There is evidence for a certain degree of B-site ordering even for the thinnest film and the ones grown at the lowest temperature. The variations of Raman mode frequencies and lattice parameters with growth temperature and film thickness provide evidence that the volume of the unit cell decreases under the compressive strain of the NFO film-MAO substrate mismatch. In general, we conclude that the film relaxation and the B-site ordering are more sensitive to the growth temperature rather than to the film thickness. Even relatively thin films grown at high temperatures show almost-relaxed lattice parameters and enhanced B-site ordering unlike the low-temperature films, which remain strained even when they are thick.