Strain-induced tetragonal distortions and multiferroic properties in polycrystalline Sr1−xBaxMnO3 ( x=0.43−0.45 ) perovskites

Abstract

We report the structure-property phase diagram of unique single-ion type-1 multiferroic pseudocubic ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ perovskites. Employing a specially designed multistep reduction-oxidation synthesis technique, we have synthesized ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ compositions in their polycrystalline form with a significantly extended Ba solubility limit that is only rivaled by a very limited number of crystals and thin films grown under nonequilibrium conditions. Understanding the multiferroic interplay with structure in ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ is of great importance as it opens the door wide to the development of newer materials from the parent $(A{A}^{\ensuremath{'}})(B{B}^{\ensuremath{'}}){\mathrm{O}}_{3}$ system with enhanced properties. To this end, using a combination of time-of-flight neutron and synchrotron x-ray scattering techniques, we determined the exact structures and quantified the Mn and oxygen polar distortions above and below the ferroelectric Curie temperature ${T}_{\mathrm{C}}$ and the N\'eel temperature ${T}_{\mathrm{N}}$. In its ferroelectric state, the system crystalizes in the noncentrosymmetric tetragonal $P4mm$ space group, which gives rise to a large electric dipole moment ${P}_{\mathrm{s}}$, in the $z$ direction, of 18.4 and $29.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ for $x=0.43$ and 0.45, respectively. The two independently driven ferroelectric and magnetic order parameters are single-handedly accommodated by the Mn sublattice leading to a novel strain-assisted multiferroic behavior in agreement with many theoretical predictions. Our neutron diffraction results demonstrate the large and tunable suppression of the ferroelectric order at the onset of AFM ordering and confirm the coexistence and strong coupling of the two ferroic orders below ${T}_{\mathrm{N}}$. The refined magnetic moments confirm the strong covalent bonding between Mn and the oxygen anions, which is necessary for stabilizing the ferroelectric phase.