Temperature and thickness evolution and epitaxial breakdown in highly strained BiFeO ₃ thin films

We present the temperature- and thickness-dependent structural and morphological evolution of strain-induced transformations in highly strained epitaxial BiFeO ₃ films deposited on LaAlO ₃ (001) substrates. Using high-resolution x-ray diffraction and temperature-dependent scanning-probe-based studies, we observe a complex temperature- and thickness-dependent evolution of phases in this system. A thickness-dependent transformation from a single, monoclinically distorted, tetragonal-like phase to a complex mixed-phase structure in films with thicknesses up to ∼200 nm is the consequence of a strain-induced spinodal instability in the BiFeO ₃/LaAlO ₃ system. Additionally, a breakdown of this strain-stabilized metastable mixed-phase structure to nonepitaxial microcrystallites of the parent rhombohedral structure of BiFeO ₃ is observed to occur at a critical thickness of ∼300 nm. We further propose a mechanism for this abrupt breakdown that provides insight into the competing nature of the phases in this system.