Formation of ultrathin cobalt ferrite films by interdiffusion of Fe3O4/CoO bilayers

Abstract

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (CoxFe3-xO4) films by interdiffusion of Fe3O4/CoO bilayers. Bilayer samples with different Fe3O4/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped SrTiO3(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between 300 degrees C and 600 degrees C have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from 300 degrees C annealing temperature and is completed for temperatures above 500 degrees C. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric CoFe2O4 film with partial inversion has been formed exhibiting homogeneously distributed Co2+ and mainly Fe3+ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.