Imaging polar gradients in multiferroic SrMnO3 thin films

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).Engineering strain in oxides is a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties, but presently remains challenging. Promising intrinsic multiferroics are some manganites with the cubic ABO3 perovskite structure. For instance, previous works have shown that the antiferromagnetic paraelectric SrMnO3 (SMO) could become ferroelectric upon artificial expansion of the unit cell by epitaxial strain. In this case, the induced lattice distortion is expected to induce ferroelectricity by the off-centering of the magnetic cation Mn4+. In this work we explore the existence of polar gradients induced by epitaxial strain in 10-nm thick SMO thin films grown by pulsed laser deposition on LSAT (100) substrates, whose polar nature was predicted theoretically, and recently observed experimentally. Aberration-corrected scanning transmission electron microscopy (STEM) experiments have been carried out to determine the strain fields and the local atomic displacements within the unit cell from Annular Bright Field (ABF) images with atomic resolution. As a result, we map the polar rotation of the ferroelectric polarization at atomic resolution, both far from and near the domain walls and find flexoelectricity resulting from vertical strain gradients (see Fig.1). The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.Peer reviewe