Perovskite phase stabilization and induced polar ordering in strained (Sr1-xBax)MnO3 epitaxial thin films

Abstract

The possibility of controlling the magnetization (polarization) by an electric (magnetic) field in materials displaying ferroelectric and magnetic order in the same phase, so-called multiferroic materials, has triggered great amount of research in the last few years. However, very few of them have been proved to show strong magnetoelectric coupling, mainly due to the different mechanisms of ferroelectricity and magnetism in these compounds. Exception can be found, though, in perovskite AMnO3 system (A = Ca, Sr, Ba), in which Mn4+ is expected to be able to drive both the magnetic order and the required non-centrosymmetric distortion for ferroelectric order. Yet ferroelectricity is allowed in this system, solely, when the unit cell volume is large enough to promote Mn off-centring, which may be achieved by using increasingly larger A cation, as partially replacing Sr with Ba, or artificially expanding the lattice parameters by epitaxial strain engineering. However, increasing the size of the A-cation destabilizes the perovskite structure, becoming different non-ferroelectric hexagonal polymorphs the ground state phase. In this work, we report on the synthesis of the perovskite phase of (Sr1-xBax)MnO3 solid solution for Ba content ranging 0.2 ≤ x ≤ 0.5 in thin films [5]. Moreover, we have managed to grow epitaxial tensile-strained (Sr1xBax)MnO3 films, ranging from 0% to 4% strain. In bulk, high hydrostatic pressures are used for synthesizing the metastable pseudocubic phase. Here, instead, we use an alternative powerful mechanism for films, the so-called epitaxial stabilization. Still, we demonstrate that the stability of the perovskite phase over the non-ferroelectric hexagonal phase is only found in a very delicate balance of growth conditions, which severely worsens on increasing Ba content. Finally, using Scanning Transmission Electron Microscopy and electrical measurements based on impedance spectroscopy we have assessed the polar character of (Sr1-xBax)MnO3 films as a function of Ba-content and epitaxial strain, covering a wide range in this 2D phase diagram. The possibility of controlling the magnetization (polarization) by an electric (magnetic) field in