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Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain Wenrui Zhang, Mingtao Li, Aiping Chen, Leigang Li, Yuanyuan Zhu, Zhenhai Xia, Ping Lu, Philippe Boullay, Lijun Wu, Yimei Zhu, Judith MacManus Driscoll, Quanxi Jia, Honghui Zhou, Jagdish Narayan, Xinghang Zhang, and Haiyan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03773 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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ACS Applied Materials & Interfaces

Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain Wenrui Zhang,1 Mingtao Li,2,3 Aiping Chen,4,5 Leigang Li,1 Yuanyuan Zhu,1 Zhenhai Xia,2 Ping Lu,6 Philippe Boullay,7 Lijun Wu,8 Yimei Zhu,8 Judith L. MacManus-Driscoll,9 Quanxi Jia,5 Honghui Zhou,10 Jagdish Narayan,10 Xinghang Zhang,11 Haiyan Wang*,1,4 1

Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, USA, 2 Department of Materials Science and Engineering, and Department of Chemistry, University of North Texas, Denton, Texas 76203, USA, 3 International Research Center for Renewable Energy, State key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China, 4 Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, USA, 5 Center for Integrated Nanotechnologies, MS K771, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 6 Sandia National Laboratories, Albuquerque, New Mexico 87185, USA 7 CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Boulevard Maréchal Juin, F-14050 Caen Cedex 4, France, 8 Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA, 9 Department of Materials Science, University of Cambridge, Cambridge, CB2 3QZ, UK, 10 Department of Materials Science and Engineering, NSF Center for Advanced Materials and Smart Structures, North Carolina State University, Raleigh, NC 27695, USA 11 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, USA.

Keywords: Self-assembly, Layered oxides, Strain engineering, Multiferroic, Interface

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ABSTRACT

Layered complex oxides emerge as one of leading topics in fundamental materials science because of their strong interplay of intrinsic charge, spin, orbital and lattice. As a fundamental basis of heteroepitaxial thin film growth, interfacial strain can be used to design materials exhibiting new phenomena beyond their conventional form. Here we report a strain-driven selfassembly of Bismuth-based supercells (SC) with a two-dimensional (2D) layered structure. With integrated experimental tools and first-principles calculations, we investigate the full SC structure and elucidate the fundamental growth mechanism achieved by the strain-enabled selfassembled atomic layer stacking. The unique SC structure exhibits room-temperature ferroelectricity, enhanced magnetic responses and distinct optical bandgap from the conventional cubic structure. This study reveals the important role of interfacial strain modulation and atomic rearrangement in self-assembling a layered singe-phase multiferroic, which opens up a promising avenue in searching and designing novel 2D layered complex oxides with enormous promises.


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1. INTRODUCTION Layered compounds represent an intriguing material family with diverse material systems including graphene,1,2 transition metal dichalcogenides,3 boron nitride,4 X2Te3 (X=Bi, Sb),5 AB2 (A=Mo, Ta, Nb, Ni, W, B=S, Se, Te),6 and oxide-based transition metal compounds.7,8 Nonoxide based 2D layered materials have attracted wide attention in the past decade with most efforts devoted to synthesizing large scale layered materials,9 understanding their basic physical properties,10 and enabling new generation electronic devices.11 Nevertheless, the research scope of layered functional compounds is far beyond the above. Complex oxides have been the focus of condensed matters physics and material science for decades, associated with the discovery of high

temperature

superconductivity,12 colossal

magnetoresistance,13

and

single-phase

multiferroics,14,15 Tuning the functionalities by manipulating the interplay among charge, spin, orbital and lattice through the multilayers, superlattices and vertical architectures has achieved remarkable success.16,17 Room-temperature multiferroics have received intense research interests because of various advanced technological applications including magnetoelectric random access memory and spintronic tunnel junction devices.18 However, it is challenging to integrate pronounced room-temperature ferromagnetism and ferroelectricity in single-phase materials due to the contradictory requirements of electron orbital occupancy.19 Recently, perovskite-related layered materials like Aurivillius and Ruddlesden−Popper phases exhibit great flexibility of structure construction,20 which offers great promises of designing new layered oxide compounds with desired multifunctionalities. One method to grow such a structure is to artificially control the stacking sequence of selected 2D atomic crystal planes based on the molecular beam epitaxy technique.7 In parallel with the artificial process, self-assembly of layer stacking is another 3 ACS Paragon Plus Environment


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promising route for designing new layered structures. Introducing the biaxial strain through the substrate in thin film epitaxy growth has served as a very effective way to manipulate the film microstructure and physical properties.21,22 The Bi-Fe-Mn-O system has been widely studied in the search for room-temperature multiferroics. The growth of this system is complex because of the possible intermixing of Fe and Mn ions and the structure distortion from the Bi 6s2 lone pair effect. Both cubic and layered SC structures were obtained from the same starting Bi2FeMnO6 phase as the substrate parameters and the associated strain and interface were changed.15,23 Under identical growth condition, the conventional pseudo-cubic Bi2FeMnO6 phase is favorably maintained when directly grown on SrTiO3 (STO) substrates because of the small lattice misfit strain (ca. -0.6%), while a novel layered SC phase has been realized on LaAlO3 (LAO) substrates owing to a more pronounced misfit strain (ca. -2.0%). Most previous research efforts have focused on the cubic phase which resembles ordered double perovskites and exhibits obvious ferroelectricity but relatively weak ferromagnetism at room temperature.23-25 However, the understanding of the self-assembled layered stacking in the novel SC structure is limited, and the unique structure distortion may exhibit drastically different physical properties compared to its cubic counterpart. Here using a well-integrated theoretical and experimental approach, we fully determine the SC structure, uncover its fundamental growth mechanism, and demonstrate interesting optical, magnetic and ferroelectric properties at room temperature. 2. RESULTS AND DISCUSSION The Bi3Fe2Mn2O10+δ (BFMO) SC films were grown on LaAlO3 (LAO) (001) substrates using pulsed laser deposition from a Bi2FeMnO6 target (See Supporting information for experimental details). The atomic microstructure was first studied with the aberration-corrected scanning transmission electron microscope (STEM) in high angle annular dark-field (HAADF) 4 ACS Paragon Plus Environment

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mode (Supporting Information Figure S1). A ~5-nm-thick Bi2FeMnO6 interlayer with a tetragonal distortion (c/a=1.15) has been observed between the substrate and the upper film. By examining the cross-sectional STEM images along the [100]p and [010]p zone axis, a misfit layer stacking was identified, where Bi2O2 matches FeMnO4 layers in 1:1 and 4:3 along the [100]p and [010]p axis, respectively. P stands for the perovskite phase, as the simplified pseudo-cubic phase is applied in this work to discuss the zone axis in SC. The HAADF STEM results determine the basic structural information of the layered phase, which includes the cation positions and estimated lattice parameters (a[100]p~3.99 Å, b[010]p~11.97 Å and c[001]p~19.40 Å). The energydispersive X-ray spectroscopy (EDS) analysis reveals the cation ratio of Bi:Fe:Mn in SC is 3:2:2, which is different from the original composition (2:1:1) in the target. Such composition modulation and SC phase formation are only observed in the film grown on LAO substrate, while the film grown on STO substrate shows the cubic perovskite structure with the original target composition. BFMO SC has been investigated by precession electron diffraction tomography (PEDT) in order to obtain crystallographic information from an area corresponding to the whole film thickness, i.e., representative of the BFMO SC structure at a relatively large scale. Figure 1a and 1b present the (0kl) and (1kl) reciprocal space sections reconstructed from the PEDT data and used for a first visual search of systematic absences, which allows us to propose an A-centering of the structure. After data extraction and reduction, the structure solution was performed using the charge-flipping algorithm implemented in Superflip26 assuming an orthorhombic cell with a Amm2 space group and lattice parameters of a~4.0Å, b~11.8Å and c~19.4Å. The obtained structural model was completed using Fourier difference maps and refined against the PEDT data leading to the structure presented in Supporting information (Table S1, Figure S2-S4). 5 ACS Paragon Plus Environment


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The PEDT study allows confirming our first results with information on the localization of oxygen atomic positions and the confirmation that the BFMO SC possesses, at room temperature, a non-centrosymmetric structure. This self-assembled layered structure can be seen as an intergrowth of Bi2O2 and FeMnO4 infinite layers stacked regularly along the growth direction of the film. These Bi2O2 layers are similar to the ones found in Aurivillius compounds where they alternate regularly with perovskite layers. Projected along the stacking direction (Supporting information Figure S4), isolated Bi2O2 layers would define a square lattice with a parameter corresponding approximatively to 3.85Å i.e. a perovskite type lattice. The FeMnO4 layers, made of edge-sharing [Fe0.5Mn0.5]O6 octahedra, are not related to perovskite. This is actually the distinctive feature of the BFMO SC. These FeMnO4 layers can be described as being a slice of two octahedra thick taken along the [110]N direction of a NaCl structure, where N stands for the NaCl phase. Projected along the stacking direction (Supporting information Figure S4), these layers define a rectangular lattice whose dimensions would relate to an hypothetical (Fe,Mn)O structure with a NaCl-type lattice i.e. ~4.4Å along [001]N and ~3.1Å along [110]N aligned, respectively, along [100]P and [010]P. For this last direction, it exists a significant mismatch with the Bi2O2 layers and one needs four units of the FeMnO4 layers to match approximately 3 units of the Bi2O2 layers. Considering this, the composition can be written as 3 Bi2O2 for 4 FeMnO4 leading to the composition Bi3Fe2Mn2O11 where a full occupancy of oxygen atomic positions is assumed. While commensurate here, such a layered structure can be related to misfit layered structures. The in-plane distances given above are rough estimations from crystal chemistry considerations but, from this simple picture, it is expected that this in-plane lattice mismatch between Bi2O2 and FeMnO4 layers has to be accommodated to stabilize the structure.


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Notably, in BFMO SC the Bi2O2 layers are in tension compared to their analog in Aurivillius phases while the FeMnO4 layers are in compression compared to their analog in NaCl structure. To further understand the SC crystal structure and complete the structural information obtained by PEDT, in-depth STEM and EDS characterization were performed. Using a STEM annular bright field (ABF) imaging method, we were able to visualize light elements such as oxygen in SC. Figure 2a and 2b show the high-resolution STEM ABF images of BFMO SC along the [100]p and [010]p axis, respectively. These images were slightly processed by low-pass filter to improve the signal/noise ratio. The insets show the enlarged images along [100]p and [010]p axis for investigating the oxygen positions in SC. Indeed, the oxygen ions away from the cations were observed and marked in yellow. The others close to cations (marked in orange) were difficult to be resolved and thus were analyzed by first principle calculations as discussed in a later section. Figure 2c shows intensity profiles measured in different atomic planes along the [100]p axis. The presence of oxygen vacancy was revealed from the contrast intensity variation, which is believed to lead to mixed valence states of Fe/Mn cations. The XPS spectra of the Fe and Mn 2p3/2 in BFMO cubic and supercell films further confirm the coexistence of a small amount of Fe2+ with a major Fe3+ state in both phases, while the amount of Fe2+ in BFMO SC is slightly higher than that in BFMO cubic structure (Supporting Information Figure S5). A single Mn3+ state was observed in these two phases. In order to determine the existence of any long-range ordering of Fe and Mn cations in SC, atomic-scale EDS mapping was performed along the two primary [100]p and [010]p axis. Figure 2d shows the combined color map of the Bi (L+M, in red), Mn (Kα, in green) and Fe (Kα, in blue) along the [100]p axis (Supporting information Figure S6 includes the selected STEM image and individual element maps). The corresponding X-ray line profiles were extracted and shown in Figure 2e. Compared to the clear 7 ACS Paragon Plus Environment


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periodic arrangement in Bi2O2 layers, no preferential long-range ordering of Fe and Mn could be identified. Similarly, Fe/Mn disordering has also been observed in SC along its [010] axis, as shown in Figure 2f. The noisy distribution of Fe/Mn compared to Bi in Figure 2g may relate to the misfit 4:3 stacking in the projection view. First principle calculations reveal the important role of oxygen vacancy on the structural stability of the BFMO SC phase. With obtained structural and chemical data, ab initio local structural relaxations and total energy calculations were performed. Figure 3a shows the formation energy (Ef) as a function of the number of oxygen vacancy. Four repetitive units of the SC primitive cells were employed for the calculations. As the oxygen vacancy increases from a starting oxygen-stoichiometric composition of Bi24Fe16Mn16O88, Ef reaches a minimum value at the composition of Bi24Fe16Mn16O84. The inset in Figure 3a identifies the possible oxygen vacancy position based on the calculation results. The calculated lattice parameter varies accordingly with the oxygen vacancy, and the ones for Bi24Fe16Mn16O84 are consistent with experimental observations (Supporting information Figure S7 and Figure S8). High-resolution STEM imaging reveals detailed structural evolution across the interlayer region in both [100]p and [010]p directions (Supporting information Figure S9). Figure 3b shows that the SC in-plane lattice parameter along the [100]p axis gradually increases from the interlayer to the layered structure, while an abrupt increase of the lattice parameter along the [010]p axis occurs at their interface. The asymmetric in-plane lattice parameter evolution suggests different strain relaxation states along these two directions,27 which may relate to the asymmetric Poisson’s ratio28 or thermal expansion29 of the BFMO SC. As the substrate-induced strain builds up through the interlayer region, the direction with a larger lattice distortion forms in-plane misfit dislocations and results in the misfit layered stacking in the SC film. Based on the 8 ACS Paragon Plus Environment

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structural calculations and experimental observations, the growth mechanism of BFMO SC phase is proposed and relates to strain relaxation and interface reconstructions. In the interlayer region, asymmetric strain relaxation occurs: a larger compressive strain (ε1) is preserved for direction [010]p, and a smaller compressive strain (ε2) for direction [100]p (Figure 3c). This results in asymmetric lattice distortion and yields different atomic displacements along the two in-plane directions. The strain-driven movement of A and B oxygen ions forms incomplete oxygen octahedra, acting as trapping sites for incoming Bi adatoms. Compared to the 1:1 Bi:(Fe/Mn) matching in direction [100]p, the larger compressive strain in direction [010]p yields a narrower lattice spacing and attracts extra Fe/Mn cations to stack with original Bi ions, leading to a 4:3 Bi:(Fe/Mn) matching. As a result, reconstructed interface occurs and associates with significant strain relaxation, after which the Bi2O2 layer stabilizes and alternates with the growth of FeMnO4 layers (Figure 3d-3g). As the Bi-layered stacking is a relatively stable configuration as seen in Aurivillius phases, the key to growing novel self-assembled SCs requires the formation of the partial octahedral plane similar to the FeMnO4 layers. Using this growth strategy, we have successfully achieved a family of Bi-based self-assembled SC systems by exploring similar [M1-xM’x]O6 edge-sharing octahedral planes (M, M’= Fe, Co, Ni, Mn, Cr) with the Bilayer as the building block. Detailed results of other systems in this family will be reported in separate studies. To probe the physical properties of the BFMO SC, a 50-nm-thick SC film was deposited on 10-nm-thick CeO2-buffered LAO substrate. The CeO2 buffer was selected because it is epitaxially matched with both the LSMO bottom electrode and the upper BFMO SC. The CeO2 layer also removes the interlayer formation and results in the direct growth of the BFMO SC,30 thus providing a unique way to rule out the possible effect from the interlayer region on the 9 ACS Paragon Plus Environment


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physical property measurements of the BFMO SC. XRD and TEM results show high-quality growth of BFMO SC on buffered substrates (Supporting information Figure S10). Figure 4a shows the optical transmittance spectra of the BFMO SC heterostructure and CeO2-buffered LAO substrate. The unique atomic ordering in BFMO SC structure yields a large absorption in the region of 300-600 nm, revealing a direct bandgap of ~3.27 eV as shown in the inset. This is distinct from the results of the cubic Bi2Fe1-xMnxO6 structure, which varies from 1.1 to 2.7 eV depending on the composition of the component phases.31 Magnetic measurements of the BFMO SC show a saturated magnetization (Ms) of 192 emu/cc and a coercivity (Hc) of 198 Oe at 300 K and 4000 Oe, as shown in Figure 4b. The magnetization behavior changes slightly as temperature decreases. At 10 K, Ms (Hc) increases to 212 emu/cc (246 Oe). The inset of Figure 4b shows the temperature-dependent magnetization curves measured without or with an in-plane magnetic field of 1000 Oe. The measured magnetization decreases less than 10 % when temperature increases from 10 K to 350 K, which suggests a high magnetic transition temperature (Tc). The intrinsic spin alignment is induced by a competition between several exchange couplings which are influenced by the bond angle and length, spin orbital occupation and interaction.32,33 A stronger exchange interaction is expected from the two-fold enhanced magnetization (~192 emu/cc) in SC than that (~90 emu/cc) in the BFMO cubic phase at room temperature.23 It is postulated that the magnetic coupling is correlated to the zigzagged alignment of Fe and Mn cations stacked within Bi2O2 layers, which facilitates the Fe/Mn orbital hybridization and reconstructs spin configurations.24,34 To investigate the electrical properties of BFMO SC, bottom (LSMO) and top (Pt) electrodes were grown so that Pt/BFMO SC/CeO2/LSMO thin film capacitors were prepared. Figure 4c shows the results of leakage current density versus electric field (J-E) characteristics of 10 ACS Paragon Plus Environment

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a representative capacitor. The J-E loops of two separate cycles were plotted to demonstrate reproducibility of the test. Compared to the leaky behavior in the ultrathin CeO2 buffer, the SCbased capacitor exhibits a low level of leakage current density (~0.01 A/cm2 at 200 kV/cm) similar to that in pure BFO films.35,36 At E below 220 kV/cm, an Ohmic contact behavior has been observed for both negative and positive biases, as seen from the linear relationship of log(J) vs log(E) with a slope of 1. At higher E, slight asymmetric J-E characteristic was seen. Detailed fitting results suggest that Poole-Frenkel emission and Fowler-Nordheim tunneling mechanisms dominate the J-E loop at negative and positive biases, respectively (Supporting information Figure S11). The ferroelectric properties of the BFMO SC film have been investigated by piezoresponse force microscopy (PFM). As the tip bias sweeps from -5V to 5V to -5V, a characteristic hysteresis behavior has been observed in both phase and amplitude curves (Figure 4d), suggesting room-temperature ferroelectricity for BFMO SC. A coercive field of ~298 kV/cm (1.49V over 50 nm) was obtained for BFMO SC. The [001]p polarization axis in the BFMO SC distinguishes itself from the conventional Aurivillius phase which confines the polarization in the in-plane direction.37 The berry phase method calculates a spontaneous polarization of 6.7 µC/cm2 in the BFMO SC, which is consistent with the experimental result.15 The observed ferroelectricity may be related to the biaxial compressive strain preserved in the SC structure, which likely causes tetragonal distortion of the unit cell along the [001]p axis and induces a spontaneous polarization axis in this direction. Other factors, including the Bi 6s2 long pair effect38 and the electric-field-induced migration of oxygen vacancies within FeMnO4 layers, may also contribute to the ferroelectric effect.39 3. CONCLUSIONS



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In conclusion, the combined experimental (HAADF and ABF-STEM, EDS and PEDT) and first-principles calculations reveal the detailed structure and growth mechanism of the new class of self-assembled Bismuth-based two-dimensional (2D) layered oxide structures, later identified with a lattice space group of Amm2. The substrate-induced biaxial strain promotes asymmetric lattice distortion and interface reconstruction in the initial interlayers, and subsequently facilitates the self-assembled layer stacking of the following atomic crystals. The BFMO SC structure shows an optical bandgap of ~3.27 eV and allows the coexistence of roomtemperature ferromagnetism and ferroelectricity. This work opens up a strain-driven approach to tailor new multifunctional layered oxide films and provides new insights towards novel material designs for high-performance nanoelectronic devices. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. STEM image of the BFMO SC structure; atomic scale EDS maps of single elements, PED tomography and DFT calculation details; STEM images of interfacial area along two directions; STEM, EDS maps, XRD and detailed I-V fitting results of BFMO SC-based heterostructures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Author contributions


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H.W. conceived the project and designed the experiment. W.Z. and L. L. grew the BFMO film. WZ conducted the XRD, ferroelectric and magnetic measurements. M.L. and Z.X. conducted the crystal structure analysis by first-principles calculations. W.Z., A.C., Y.Z., H.Z. and J.N. performed the STEM HAADF measurement and image analysis. L.W. and Y.M. performed the STEM ABF measurement. P.L. investigated the crystal structure with the EDS mapping technique. P.B. performed the PEDT experiment and analysis. All authors discussed the results and commented on the manuscript. ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (ONR, N00014-15-1-2362, for thin film growth) and the U.S. National Science Foundation (NSF, Ceramic Program, DMR-0846504 (for high resolution STEM analysis)). W. Z. and H.W. acknowledge the support from NSF (DMR-1401266). M. L. and Z. X. acknowledge the support partially from AFOSR MURI (FA9550-12-1-0037). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DEAC04-94AL85000. The work at Brookhaven National Laboratory was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering, under Contracts no. DE-AC02-98CH10886. A portion of the electron microscopy experiments were performed at National Center for Electron Microscopy (NCEM), which is supported by the Office of Science, Office of Basic Energy Sciences of the US Department of Energy under Contract No. DE-AC02-05CH11231. W.Z. is grateful to Drs. Peter Ercius, Jim Ciston and Chengyu Song for additional help and fruitful discussions at NCEM. W.Z. thanks Qiyuan Wu for helpful discussion on the XPS analysis. 13 ACS Paragon Plus Environment


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Figures



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Figure 1. (a) 0kl and (b) 1kl reciprocal space sections reconstructed from the PEDT data. Background images: the red grid is a guide to the eyes to highlight the substrate lattice. The lattice mismatch between BFMO and LAO is clear along both [100]* and [010]* directions (see also Figure S2). Insert images: a green grid is used to highlight the BFMO SC lattice where the condition hkl: k+l=2n (A-lattice centering) is observed. Reflections coming from the substrate are encircled in red. Dashed red circles are for reflections due to multiple scattering.


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Figure 2. STEM ABF image of BFMO SC films along (a) [100]p and (b) [010]p zone axes. Insets show corresponding enlarged STEM ABF images of repetitive SC units. (c) Highresolution ABF image of the area including the Bi2O2 and FeMnO4 layers. The intensity scans measure the oxygen position variation and identify the oxygen vacancies. Atomic scale EDS color maps along (d) [100]p and (f) [010]p zone axes. Line profiles for Bi (L+M, red), Mn (Kα, green) and Fe (Kα, blue) identifying the Fe/Mn ordering along (e) [100]p and (g) [010]p zone axes.



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Figure 3. (a) Formation energy of the Bi24Fe16Mn16O88-x SC phase as a function of the number of oxygen vacancies. Inset shows the position of oxygen vacancy for the lowest formation energy. (b) In-plane lattice parameter variation of the SC phase along the [010]p and [100]p axis at the interlayer region. (c) Schematic of partial strain relaxation in different in-plane directions resulting in different movements of A and B oxygen ions, which forms the trapping site of upcoming Bi adatoms. Atomic modeling ((d) and (f)) and STEM images ((e) and (g)) along the [010]p and [100]p directions illustrating the SC growth from the interlayer formation to the alternative layered stacking.


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Figure 4. (a) Optical transmittance spectra as a function of wavelength of CeO2/LAO and BFMO/CeO2/LAO heterostructures. Inset shows the Tauc plot analysis determining a direct bandgap of 3.27 eV for the BFMO SC film. (b) Magnetization hysteresis loops of the BFMO SC film measured at different temperatures. Insets shows the magnetization of the BFMO SC film as a function of temperature under zero field and an in-plane field of 1000 Oe. (c) Leakage current density as a function of electric field of the BFMO/CeO2/LSMO heterostructure measured at room temperature. Two separate curves are measured to demonstrate the reproductivity of the result. (d) Phase and amplitude switching behavior as a function of tip bias of the BFMO/CeO2/LSMO heterostructure.



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Table of Content Figure


Two-Dimensional Layered Oxide Structures ... - ACS Publications

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