Oxygen deficiency and migration mediated electric polarization in Fe,Co-substituted SrTiO${}_{3-\delta }$

Abstract

We use density functional theory (DFT) calculations to show that oxygen vacancies ($v_{\mathrm{O}}$) and mobility induce noncentrosymmetric polar structures in SrTi${}_{1-x-y}$Fe${}_x$Co${}_y$O${}_{3-\delta }$ ($x=y=0.125$) with $\delta =\{0.125,0.25\}$, enhance the saturation magnetization and give rise to large changes in the electric polarization $|\Delta P|$. We present an intuitive set of rules for SrTiFeCoO${}_{3-\delta }$ (STFC), which are based on the interplay between (Co/Fe)-$v_{\mathrm{O}}$ defects, magnetic cations coordination and topological vacancy disorder. STFC structures convey layered crystals with sheets of linear organized O${}_{4,5,6}$-coordinated Fe-Co pairs, sandwiched with layers of O${}_5$-coordinated Ti. Co,Fe-$v_{\mathrm{O}}$ defects are the source of the crystal distortions, cations off-centering and bending of the oxygen octahedra, which added to the charge redistribution mediated by $v_{\mathrm{O}}$, the cations electronegativity and valence states trigger an effective electric polarization. Oxygen migrations for $\delta =0.125$ provides us with $|\Delta \mathbf{P}|$ $>\sim 10\mu$C/cm${}^2$ due to a quantum-of-polarization differences between $\delta =0.125$ structures. Increasing the deficiency to $\delta =0.25$ yields $|\Delta \mathbf{P}|$ whose O-migration resolved polarization for $\delta =0.25$ is $>\sim 3\mu$C/cm${}^2$ in the worst case scenario. Magnetism is dominated by the Fe,Co spin states for $\delta =0.125$ while there is a raid of Ti magnetic moments ($\sim 1{\mu }_B$) for $\delta =0.25$. Magnetic and electric order parameters change for variations of $\delta$ or oxygen migrations for a given deficiency. Our results capture characteristics observed in the end-members of the series SrTi(Co,Fe)O${}_3$, and suggest the existence of a broader set of rules for oxygen deficient multiferroic oxides.

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Cited by in corpus (1)

• First-principles based Monte Carlo modeling of oxygen deficient Fe-substituted SrTiO${}_3$ experimental magnetization