INTERFACE PHENOMENA IN COMPLEX NANO-OXIDES

Progress in nanotechnology requires new approaches to materials synthesis that make it possible to induce new functionalities down to the smallest scales. An objective of materials research is to achieve an enhanced control over the physical properties of materials such as ferromagnets, ferroelectrics and superconductors. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of the atomic structures can produce large variations in their physical macroscopic responses and result in multiple functionalities. Thin films oxides often present radically different physical properties than bulk materials, frequently due to epitaxial strain, and interface effects are paramount to understand these new properties. A particularly interesting strategy is strain engineering, which can modify the interatomic distances and even the chemistry of the film to produce novel physical phenomena. In the case of complex oxides, due to the strong correlation between chemical composition, lattice and electronic structure, modifying the growth conditions, epitaxial strain and interfase phenomena can induced multiple exciting properties such as ferromagnetism, ferroelectricity, metallicity, multiferroicity or superconductivity among others. To confront this challenge, advanced Transmission Electron Microscopy techniques provides structural and chemical information with atomic resolution to unveil the microscopic origin of this interface phenomena in complex oxides at the nanoscale.

For instance, ferroic oxides often contain ferroelastic domains. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material’s properties. Actually, large local stresses induces a high density of domain walls that promote the formation of new structural and chemical phases. In this case we have characterized by aberration-corrected STEM a new two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbium manganite (TbMnO3), grown in thin layers under epitaxial strain on strontium titanate (SrTiO3) substrates. A comprehensive characterization of the structure and chemical composition of this novel ferromagnetic phase by means of atomically resolved HAADF and EELS have evidenced that this new phase, yet to be created by standard chemical routes, is characterized by the selective substitution of alternate Tb atoms in the perovskite structure by Mn atoms. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.

a) HAADF-STEM image of the TbMnO3–SrTiO3 interface. b) Domain wall of TbMnO3 close to the interface with the SrTiO3 substrate, with the proposed atomic model superimposed. c–f) STEM-EELS spectrum image of the domain wall; c) HAADF signal, d) Tb signal, e) Mn signal, f) Color map of Tb (green) and Mn (red).

Engineering strain is a particularly promising route to synthesize thin films of multiferroic materials, aimed at exhibiting strong magnetoelectric coupling at room temperature. Precise control of the strain gradient has shown a great potential at custom tailoring multiferroicity, but presently remains challenging. Among the most intriguing multiferroics are some manganites with the ABO3 perovskite structure. Some of them are bulk multiferroics, while others only show multiferroicity as epitaxially strained thin films. For instance, the antiferromagnetic SrMnO3 becomes ferroelectric upon artificial expansion of the unit cell [1,2]. In this case, the induced lattice distortion is expected to induce ferroelectricity by the off-centering of the magnetic cation Mn4+.

Atomic-resolution observation of polar states in 10-nm-thick epitaxial SrMnO3 films grown by pulsed laser deposition on SrTiO3 (100). Aberration-corrected Scanning Transmission Electron Microscopy experiments have been carried out to determine the strain fields and the local atomic displacements within the unit cell through the quantitative analysis of Annular Bright Field (ABF) images with atomic resolution. ABF enables the simultaneous imaging of heavy and light atoms, which enable the visualization of the oxygen octahedra distortions in perovskite oxides as well as the off centering of the cations. As a result, we map the cell-by-cell electric polarization of SrMnO3 at atomic resolution, both far from and near the domain walls, and observe polar rotations along the growth directions resulting from vertical strain gradients [3]. This phenomena, the so-called flexoelectricity, is originated by a gradual distribution of oxygen vacancies across the film thickness and the subsequent change of the out-of-plane lattice parameter, according to electron energy loss spectroscopy. Thus, this work present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.

Figure 2. Measurement of the polar displacements in SrMnO3 films by aberration corrected STEM-ABF imaging in cross sectional specimens along [110] (right). Oxidation state determined by analysis of the EELS fine structure of the O K edge.
Based on SrMnO3, the perovskite Sr1-xBaxMnO3 (SBMO) system is also an ideal candidate for tailoring the magnetoelectric properties of multiferroics, in this case by combining Ba doping and epitaxial strain due to the strong coupling between polar instability, spin order, and lattice. In this work, we demonstrate the epitaxial growth of highly Ba-doped (x > 0.3) SBMO films grown on TbScO3 (100), together with the onset of polar order on tensile strained films by aberration corrected Scanning Transmission Electron Microscopy (STEM) using Annular Bright Field (ABF) imaging, which we can correlate with the macroscopic dielectric behavior. In particular, polar films evidence a strong dependence of the capacitance as a function of temperature, which is a common feature of ferroelectric materials, and sharp anomalies at the magnetic (Neel temperature) ordering temperature evidencing a strong magnetoelectric coupling.
Figure 3. Annular Bright Field (ABF) STEM [110] cross sectional images of a) Sr0.7Ba0.3MnO3 and b) Sr0.6Ba0.4MnO3 films grown on TbScO3 (100) evidencing the different displacements of the oxygen octahedra (strong in x = 0.3, weak in x = 0.4) as a function of the combined epitaxial strain and chemical doping on the parent SrMnO3.

HIGHLIGHTS

Artificial chemical and magnetic structure at the domain walls of an epitaxial oxide. S. Farokhipoor, C. Magén, S. Venkatesan, J. Íñiguez, C.J.M. Daumont, D. Rubi, E. Snoeck, M. Mostovoy, C. de Graaf, A. Müller, M. Döblinger, C. Scheu and B. Noheda. Nature 515, 379–383 (2014).

Polar-Graded Multiferroic SrMnO3 Thin Films.
Roger Guzmán, Laura Maurel, Eric Langenberg, Andrew R. Lupini, Pedro A. Algarabel, José A. Pardo, and Ceśar Mageń.
Nano Letters 16, 2221-2227 (2016).

Controlling the Electrical and Magnetoelectric Properties of Epitaxially Strained
Sr1−xBaxMnO3 Thin Films.
Eric Langenberg, Laura Maurel, Noelia Marcano, Roger Guzmán, Pavel Štrichovanec, Thomas
Prokscha, César Magén, Pedro A. Algarabel and José A. Pardo.
Advanced Materials Interfaces 4, 1601040 (2017).

Laboratorio de Microscopías Avanzadas

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