Correlation between oxygen octahedral tilts and functional behaviors of SrRuO3 and BiFeO3 film

Abstract

Crystal functionalities are directly connected with the equilibrium position of atoms in a unit cell. For example, ferroelectricity, ferromagnetism, and piezoelectricity. The repositioning atoms can be caused by the intentional tuning of the energy landscape. The functionalities of perovskites (ABO3) are good model systems to prove this relation. The energy state of perovskite materials is controlled using mechanical energy from external sources accommodating various clamping substrates. Consequently, the changes of their functionalities are induced. However, undesired lattice distortion, displacement of B atoms, and/or tilting of oxygen octahedra of perovskite crystals are occurred in this approach. In order to understand the effects of both lattice distortion and displacement of B atoms, collaborations between experimental and theoretical studies have been performed. The control the position of oxygen atom is essential to investigate the effect of oxygen octahedral tilting. The change of position of oxygen atoms leads to manipulate of functionality in perovskite. Here, we report the artificial manipulation of oxygen octahedral tilt angles with metallic SrRuO3 (SRO) films and consequently multiferroic BiFeO3 (BFO) thin films on the top of SRO film. First, we fabricated octahedral tilted monoclinic SRO (MSRO) and octahedral non-tilted tetragonal SRO (TSRO) thin films on SrTiO3 (STO) substrates using control oxygen atmosphere in PLD growth process. To investigate the octahedral tilting and the phase transformations of SRO thin films, we used in-situ synchrotron X-ray scattering to MSRO and TSRO thin films on STO substrates with temperature variation. The octahedral tilted MSRO thin films were highly crystalline and the monoclinic distortion angle was 0.45°. The phase transition from the MSRO to TSRO phase occurred at approximately 200℃. Conversely, no phase transformations of the TSRO thin film occurred within the temperature range from 30 to 250℃. We showed that the octahedral RuO6 tilt angle originated from oxygen deficiency was strongly affected to the phase transformation in the SRO thin films. Second, we present a strategy to control simultaneous magnetic and ferroic orders in a multiferroic BFO thin film by artificially tuning octahedral tilt angles using strong oxygen octahedral coupling with bottom SRO layers. We show that changing the octahedral structural coherency in bottom monoclinic/tetragonal SRO platform layers can metastasize through the stoichiometric BFO thin film over 25 nm—not confined as an interface phenomenon, resulting in controllable ferromagnetic and ferroelectric properties. This is the first demonstration of octahedra-derived multiferroic properties that can be stabilized in a thin film form without the help of complex chemical modifications. The complex interplay between octahedral tilts and polar/magnetic orders is examined by advanced STEM, synchrotron X-ray scattering, and magnetic measurements, revealing that the tilt symmetry is critical to tailoring the multiferroicity. Our approach provides a new platform to manipulate lattice-coupled ferroic order parameters including the dramatic enhancement of ferroelectricity and the realization of unprecedented magnetization in a thin film form.