FIB plan view lift-out sample preparation for TEM characterization of periodic nanostructures obtained by spinodal decomposition in Co1.7Fe1.3O4 thin films

Abstract

There is a miscibility gap in the CoFe₂O₄–Co₃O₄ phase diagram. In this miscibility gap, the oxides can be subjected to a spinodal transformation. It has already been observed in oxides consisting of crystals greater than or equal to 100 nm that spinodal decomposition leads to the formation of two alternating iron-rich and cobalt-rich spinel phases. The pseudo-periodic alternation occurs approximately every 5 nm. In the miscibility gap, thin films of pure iron cobaltites, consisting of crystallites of the order of 10 nm in diameter and around 300 nm in thickness, undergo transformation when they are treated at 600 °C for several hours. X-ray diffraction and Raman spectroscopy clearly reveal this transformation, which is accentuated as a function of the treatment time. An electron microscopy study of the cross-sections (view of the films along their thickness), confirms the progressive separation of the former spinel oxide in iron-rich and cobalt-rich spinel phases, without however revealing a pseudo-periodic organization of these phases, whatever the time of treatment. In an attempt to reveal this organization, a specific method of preparation has been implemented to extract the upper part of the films parallel to their basic plane and to observe the crystallites in plan view. The alternation of the iron- and cobalt-rich phases could, however, only be found in the largest crystallites. It seems that the nanometric size of the crystallites prevents the establishment of a pseudo-periodic organization of the phases during the periodic transformation. The observation of compositional anomalies in the grain boundaries seems to support the hypothesis related to a nanometric effect of the crystallization.