Interface control of a morphotropic phase boundary in epitaxial samarium modified bismuth ferrite superlattices

Abstract

Interfacial control of a polar-(rhombohedral) to-non-polar (orthorhombic) phase transition in (001)-oriented epitaxial $\mathrm{BiFe}{\mathrm{O}}_{3}\text{/}(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{m}}_{x})\mathrm{Fe}{\mathrm{O}}_{3}$ superlattices is presented. We demonstrate controlling the composition at which a polar phase transformation takes place by tuning the strength of the interlayer interactions while holding the average composition constant. It is shown that the thickness of the superlattice layers has a strong influence on the interlayer polar coupling, which in turn changes the phase transition. For the shortest periods studied (layers 5- and 10-nm thick) the onset of the phase transition is suppressed along with a significant broadening (as a function of $\mathrm{S}{\mathrm{m}}^{3+}$ concentration) of an incommensurately modulated phase determined by two-dimensional x-ray diffraction mapping. Consequently, a ferroelectric character with robust polarization hysteresis and enhanced dielectric constant is observed even for substitution concentration of $\mathrm{S}{\mathrm{m}}^{3+}$ which would otherwise lead to a leaky paraelectric in single-layer $(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{m}}_{x})\mathrm{Fe}{\mathrm{O}}_{3}$ films. The experimental results are fully consistent with a mean-field thermodynamic theory which reveals that the strength of the interlayer coupling is strongly affected by the polar-polar interaction across the interface.