RhombohedralâOrthorhombic Ferroelectric Morphotropic Phase

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Rhombohedral-Orthorhombic Ferroelectric Morphotropic Phase Boundary Associated with Polar Vortex in BiFeO Films 3

Wanrong Geng, Xiangwei Guo, Yinlian Zhu, Yunlong Tang, Yanpeng Feng, Minjie Zou, Yujia Wang, Mengjiao Han, Jinyuan Ma, Bo Wu, Wentao Hu, and Xiuliang Ma ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05449 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Rhombohedral-Orthorhombic Ferroelectric Morphotropic Phase Boundary Associated with Polar Vortex in BiFeO3 Films Wanrong Geng,1,2 Xiangwei Guo,1,2 Yinlian Zhu,1* Yunlong Tang,1 Yanpeng Feng,1,3 Minjie Zou,1,2 Yujia Wang,1 Mengjiao Han,1,3 Jinyuan Ma,1,2,4 Bo Wu,1,2 Wentao Hu,1,2 Xiuliang Ma1,4* 1

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Wenhua Road 72, 110016 Shenyang, China 2

University of Science and Technology of China, Jinzhai Road 96, 230026 Hefei, China

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University of Chinese Academy of Sciences, Yuquan Road 19, 100049 Beijing, China

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State Key Lab of Advanced Processing and Recycling on Non-ferrous Metals, Lanzhou

University of Technology, Langongping Road 287, 730050 Lanzhou, China *Correspondence authors: E-mail address: [email protected] (Y.L. Zhu) or [email protected] (X. L. Ma) KEYWORDS: BiFeO3 ferroelectric films, vortices, morphotropic phase boundary, aberrationcorrected scanning transmission electron microscopy, first-principles calculations

ABSTRACT: Strongly correlated oxides exhibit multi-degrees of freedoms which are potential for mediating exotic phases with exciting physical properties, such as polar vortex recently found in ferroelectric oxides films. Polar vortex is stabilized by competition between charge, lattice and/or orbital degrees of freedom, which displays vortex-ferroelectric phase transitions and emergent

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chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nano-scale vortex arrays in [110]-oriented BiFeO3 films. These vortex arrays are stabilized in ultrathin BiFeO3 layers sandwiched by two coherently grown orthorhombic scandate layers, exhibiting a ferroelectric morphotropic phase boundary constituted by a mixed-phase structure of polar orthorhombic BiFeO3 and rhombohedral BiFeO3. Clear polarization switching and piezoelectric signals were observed in these multilayers as revealed by piezoresponse force microscopy. This work presents a feature of polar vortex in BiFeO3 films showing morphotropic phase boundary character, which offers potential degree of manipulating phase components and properties of ferroelectric topological structures.

Topological states of spin condensations as vortices,1 skyrmions2-4 and bobber phases5 exhibiting exciting properties in ferromagnetics have motivated further conjectures of other kind of order parameter condensation states, such as potential dipole condensation as vortices or topological domain structures in low dimensional ferroelectric materials with perovskite structure.6-10 While the large anisotropy energy embarrasses the emergence of vortex phase in ferroelectrics, recently periodically arrayed flux-closure domains were obtained in PbTiO3 multilayers.11-13 In addition, ferroelectric vortices were subsequently observed in PbTiO3/SrTiO3 superlattices.14-16 Importantly, these polarization topologies are realized to generate a range of physical phenomena in ferroelectric and multiferroic materials. For example, enhanced electrical conductivity has been discovered along 1D vortex/antivortex cores in BiFeO3 (BFO) thin films detected by a combination of scanning probe microscopy technique and phase field simulations.17 Besides, by employing phase field simulations, high electroelastic fields were obtained in vortex


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cores in BFO thin films.18 Using Bragg Coherent Diffraction Imaging, the three-dimensional vortex in BaTiO3 nanoparticle is imaged in operando and the vortex core exhibits the reversible hysteretic transformation path.19 In PbTiO3/SrTiO3 multilayers, a mixed-phase system of a vortex phase and a ferroelectric a1/a2 phase formed a mesoscale, fiber-textured superstructure.15 In particular, emergent chirality were identified for the vortex phases in PbTiO3/SrTiO3 superlattices which is potential for designing information storage and processing devices.20 In this work, we report the finding of rhombohedral (R)-orthorhombic (O) ferroelectric morphotropic phase boundary (MPB) in (110)-oriented pure BFO thin films deposited on orthorhombic substrates using pulsed laser deposition (PLD) technique. Particularly, 2D nanoscale vortex arrays were identified associated with this MPB state, which are stabilized in ultrathin BFO layers sandwiched between two orthorhombic structures. Cs-corrected scanning transmission electron microscopy (STEM) imaging reveals these vortex arrays are constituted by a two-phase coexistence of polar O-BFO and R-BFO, which further form the R-O ferroelectric MPB in pure BFO, where the polar O-BFO is proved to be Pna21 phase by first-principles calculations. Our results provide valuable information for controlling the spontaneous formation of nano-scale vortex arrays and manipulating phase components by polarization-based topologies in ferroelectric thin films. RESULTS AND DISCUSSION BiFeO3 is multiferroic with coupled ferroelectricity and antiferromagnetism at room temperature.21 It has received intensive attention due to its special properties including electromechanical coupling,22 magnetoelectric coupling,23 domain conductance,24 photovoltaic effect,25 and potential applications in nonvolatile memory.26 The structure of BFO (sketched in Figure 1a) is rhombohedral with the space group of R3c.27 Its structure can be described as a simple


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cubic perovskite cell with rhombohedral distortion, having a pseudo-cubic lattice constant of apc = 3.965 Å and a distortion angle of α = 0.6°. For BFO unit cells, the relative displacement directions of Fe ions (denoted as δFe) are in the opposite direction of ferroelectric spontaneous polarization.11, 28, 29 Different from (001)-oriented BFO thin films extensively studied before, we fabricated the BiFeO3/GdScO3/BiFeO3 (BFO/GSO/BFO) multilayers on orthorhombic TbScO3(010)O (TSO) single-crystal substrates (with lattice constants of aO = 5.465 Å, bO = 5.729 Å, cO = 7.917 Å, subscript O denote orthorhombic indices), as shown in Supplementary Figure S1. It is noted that [010] orthorhombic direction corresponds to [110] pseudo-cubic direction. Thus, large anisotropic strains are imposed on BFO layers by scandate substrates with the magnitudes of lattice mismatch along two in-plane directions: -2.578% (compressive, along [100]O) and -0.164% (compressive, along [001]O), respectively. As shown in the schematic of the BFO unit cell projected along [11̅ 0] direction (Figure 1b), (110)-oriented substrates have stronger control over BiFeO3 layers with two B-O-B’ bond connections across hetero-interfaces.30 The detailed BFO structural information is characterized using Transmission Electron Microscopy (TEM) and STEM. As shown in Figure 1c, a dark-field image of BFO(10 nm)/GSO(3 nm)/BFO(10 nm) multilayers grown on TbScO3 (010)O substrates (sample 1, Supplementary Figure S1) is given. Vertical stripe domain structures can be observed in the first BFO layer. Furthermore, obvious lattice distortion of potential 109ºdomains is identified in the first BFO layer (Figure 1d, HAADF-STEM imaging), reflected by satellite spots around the (110) reflection (highlighted with white arrows in the inserted Fast Fourier Transformations (FFT) image (Figure 1d)). Additional insights into the regular arrangement of 109° domains are obtained from the reciprocal space mapping results (Supplementary Figure S2). The domain walls marked by blue and red dotted lines (Figure 1d) are localized according to the in-plane lattice rotation mapping obtaining by Geometric Phase Analysis


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(GPA) using the TSO substrate as a reference (Figure 2a), which shows periodically alternate inplane lattice rotation of 109°domains. To verify the polarization switching of the multilayers, the ferroelectric local switching behaviors are given in Figure 1e and 1f by using piezoresponse force microscopy (PFM). The measurements are repeated three times to improve the signal to noise ratio, among which the loop shapes remain approximately the same for each time. Figure 1e is the local phase hysteresis signal of the multilayers, indicating clear switching behavior. Figure 1f displays the characteristic butterfly loops in out-of-plane PFM amplitude signals for the multilayers, demonstrating the nature of ferroelectric property. Furthermore, the phase hysteresis and amplitude butterfly loops are characterized by some biases under the applied voltage. The asymmetric behavior of the PFM loops may be attributed to the different work functions between the PFM conducting tip and TbScO3 substrate. The differences in work functions will result in a potential difference31 in the BiFeO3 multilayers. In addition, the effect of the internal bias field in the BiFeO3 multilayers cannot be ruled out, which may contribute to the asymmetric behavior of the PFM loops to some extent.


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Figure 1. Periodical 109°domains in BiFeO3(10 nm)/GdScO3(3 nm)/BiFeO3(10 nm) multilayers grown on TbScO3(010)O substrates. (a) Schematic of one BiFeO3 unit cell. (b) The ball-and-stick model of BiFeO3 unit cell projected along [11̅ 0] direction, with out-of-plane direction being [110]. (c) A dark field image taken along [11̅0] zone axis. (d) High resolution HAADF-STEM image of


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the multilayers. The inset is the (110) reflection for FFT corresponding to the first BFO layer. (e) Local piezoresponse phase and (f) amplitude hysteresis loops of the multilayers. To elucidate the detailed structure of 109°domains, in-plane and out-of-plane lattice rotation mappings corresponding to Figure 1d are shown in Figure 2a and 2b, respectively. Bright and dark stripe-like contrasts are observed in Figure 2a, implying the 109° domain walls distribute alternately in the BFO layer. In the meanwhile, obvious periodic triangular areas with clear boundaries are identified in Figure 2b, which locate at the terminations of 109°domain walls near the BFO/GSO and BFO/TSO interfaces. Besides, the boundary between triangular areas and 109° domains exhibit obvious out-of-plane lattice rotation discrepancy, implying structure differences. In particular, the triangular areas at the bottom ends of 109°domain walls have opposite out-ofplane lattice rotation relative to the counterparts at the upper ones, as indicated by white dotted zigzag lines in Figure 2b. To demonstrate the feature of triangular areas and their interaction with 109°domain walls, Figure 2c gives the atomically resolved HAADF-STEM image corresponding to the area outlined by the yellow rectangle in Figure 2b. Figure 2d shows the HAADF-STEM image (Figure 2c) with an overlay of spontaneous polarization direction in BiFeO3. The clockwiseanticlockwise nano-scale vortices can be identified at the terminations of adjacent 109°domain walls, forming 2D polar vortex arrays in Figure 2d. A clockwise-anticlockwise vortex unit is outlined by the white rectangle in Figure 2d, which consists of one clockwise vortex, one anticlockwise vortex and two 109°domain walls. Figure 2e and 2f are magnified images further exhibiting two special areas marked by “3” and “4” in Figure 2d. Continuous polarization rotation curling around the terminals of 109°domain walls can be visible in Figure 2e and Figure 2f. Figure 2g depicts the simplified schematic showing the vortex configuration at the terminations of 109° domain walls based on experimental data in Figure 2d. In-depth analyses of BFO polarization


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information in a single vortex structure reveal that the vortex consists of two parts: the 109°domain walls terminals and the triangular areas adjacent to hetero-interfaces. BFO unit cells in triangular areas mainly display in-plane polarization projected along [11̅0] direction, meaning that BFO in these areas are not R phase. These results above suggest the possible phase transition of BFO in triangular areas. In other words, the nano-scale vortex arrays locating at the termination of 109° domain walls exhibit a feature of two-phase coexistence. Similar vortex nanodomain arrays, consisting of two 109°domain walls and a mirrored pair of inclined 180°domain walls, were observed previously in BiFeO3 thin films.28 However, different from the vortex nanodomains of the single rhombohedral phase, the polar vortex in the present study features a ferroelectric morphotropic phase boundary constituted by a mixed-phase structure of polar orthorhombic BiFeO3 and rhombohedral BiFeO3. We will discuss the issue of two-phase coexistence later.


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Figure 2. Observation of vortex arrays in BiFeO3(10 nm)/GdScO3(3 nm)/BiFeO3(10 nm) multilayers grown on TbScO3(010)O substrates. GPA analyses of STEM data revealing the distributions of (a) in-plane lattice rotation (RX) and (b) out-of-plane lattice rotation (RY) for Figure 1d. The inset gives the definition of RX and RY. (c) The HAADF-STEM image for the yellow rectangle in (b); (d) HAADF-STEM image with an overlay of spontaneous polarization direction of BiFeO3 based on (c); The details of (e) the clockwise vortex and (f) anticlockwise vortex labelled as “3” and “4” in (d), both showing a continuous polarization rotation of BiFeO3. (g) Schematic showing vortex configurations in BiFeO3 based on experimental data in (d).


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As mentioned, vortices are stabilized in BFO/GSO/BFO multilayers grown on TSO(010)O substrates. Thus, the special sandwich construction, in which the BFO ultrathin films are clamped between two orthorhombic structures, may facilitate the formation of vortices. We have further grown simpler systems, GSO(3 nm)/BFO(7 nm) multilayers grown on GSO(010)O substrates (Sample 2, Supplementary Figure S1) and BFO(10 nm)/TSO(10 nm)/BFO(10 nm) multilayers grown on TSO(010)O substrates (Sample 3, Supplementary Figure S1), with symmetric insulating boundary conditions between orthorhombic substrates and buffer layers for the first BFO layers. Periodical 109°domains are obtained in the first BFO layers for both multilayers, as shown in Supplementary Figure S4. Obvious vortices, constituted by mixed-phase structures of polar OBFO and R-BFO, are clearly visible in both Figure 3a-d and Figure 3e-h, no matter whether the orthorhombic structure is GbScO3 or TbScO3. Figure 3d and 3h are the colour contour plots of the in-plane lattice rotation for A-site sublattice based on the extraction of A-site atom positions refined by Gaussian peak fitting in Matlab.32 In summary, the nano-scale vortex arrays can be stabilized when the BFO layers are sandwiched between two orthorhombic structures.


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Figure 3. Identification of vortices in the first BFO layers with symmetric insulating boundary condition. (a) HAADF-STEM image with an overlay of spontaneous polarization direction of BiFeO3 for GdScO3(3 nm)/BiFeO3(7 nm) multilayers grown on GdScO3(010)O substrates. The details of the clockwise vortex (b) and anticlockwise vortex (c) labelled as “1” and “2” in (a). (d) The colour contour plot of RX for A-site sublattice corresponding to (a). (e) HAADF-STEM image with an overlay of spontaneous polarization direction of BiFeO3 for BiFeO3(10 nm)/TbScO3(10 nm)/BiFeO3(10 nm) multilayers grown on TbScO3(010)O substrates. The details of the clockwise vortex (f) and anticlockwise vortex (g) labelled as “3” and “4” in (e). (h) The colour contour plot


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of RX for A-site sublattice corresponding to (e). In both cases, clockwise-anticlockwise vortex pairs are observed in BFO layers. To illuminate the feature of vortices existed in (110)-oriented BFO thin films, the two-phase coexistence happened at the terminations of 109°domain walls is a non-negligible aspect. Figure 4a is the atomic-resolved HAADF-STEM image of the BiFeO3(10 nm)/GdScO3(3 nm)/BiFeO3(10 nm) multilayers grown on TbScO3(010)O substrates (Sample 1, Supplementary Figure S1), in which 109°domain walls are marked with red dotted lines. These 109°domain walls are not ending at interfaces, but several unit cells away from the interfaces and residing in BFO layers. Figure 4bd are the FFT images of the white rectangles labeled “1” to “3” in Figure 4a, respectively. Obvious super-modulation peaks (marked by white boxes in Figure 4b and Figure 4d) can be identified, which are the fingerprints of orthorhombic symmetry, while the 109°domains marked by the white rectangle numbered “2” in Figure 4a still remain rhombohedral symmetry. Namely, the MPB of rhombohedral and orthorhombic ferroelectric phases form at the terminations of 109°domain walls associated with nano-scale vortex phase with electric toroidal order.


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Figure 4. Phase transition in BiFeO3 layers. (a) HAADF-STEM image of BiFeO3(10 nm)/GdScO3(3 nm)/BiFeO3(10 nm) multilayers grown on TbScO3(010)O substrates. (b-d) FFT of white rectangles in (a) labelled as “1” to “3”. (e) The colour contour plot of RX for A-site sublattice corresponding to the yellow rectangle in (a). The colour contour plot of B-site ionic displacement vectors with respect to A-site sublattice along (f) [001] and (g) [110] directions, respectively, corresponding to (e).


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To confirm the two-phase coexistence, Figure 4e gives the 2D colour contour plot of in-plane lattice rotation mapping corresponding to the yellow rectangle in Figure 4a. A considerable modulation of Bi spacing can be visible for the triangular area, with the rotation angle being positive and negative alternately, akin to the in-plane lattice rotation character of TSO substrates. The magnitude of averaged in-plane rotation angles of the triangular area is 7.9°. This special stripe-like ordered distribution is the result of in-plane lattice kink character of O-BFO phase projected along [11̅ 0] direction.33 While for R-BFO, its in-plane lattice rotation mapping shows uniform distribution. Thus, it reconfirms the coexistence of O-BFO phase and R-BFO phase, which is in good consistency with the FFT data above, and constitutes a ferroelectric MPB combined with the matrix BFO. To identify the structure of the O-BFO phase, the colour contour plots of B-site ionic displacement vectors with respect to A-site sublattice along [001] and [110] directions are shown in Figure 4f and Figure 4g, respectively. In Figure 4f, orthorhombic phases in triangular area have obvious polarization with the direction being opposite to the in-plane polarization direction inside 109°domains. The magnitude of averaged Fe ionic displacement in triangular area reaches 0.19 Å, which is larger than the averaged Fe ionic displacement magnitude in R-BFO (about 0.14 Å). However, BFO unit cells in triangular area display no obvious out-of-plane component of polarization, as shown in Figure 4g. Besides, the lattice parameters of the polar O-BFO in triangular areas are estimated by the colour contour plots of in-plane and out-of-plane lattice spacing maps of the HAADF-STEM image (Supplementary Figure S5), as shown in Supplementary Figure S6. The magnitudes of averaged in-plane and out-of-plane lattice constants are 3.87 Å and 5.54 Å, respectively. Thus, both polarization and structural order parameters are modulated for O-BFO in triangular areas.


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Furthermore, we performed first-principles calculations to determine the space group and ferroelectric properties of the polar O-BFO. The technical details can be found in the Methods part and the calculated results are given in Table 1. We take the non-polar O-BFO with the space group of Pbnm as a reference. Based on both lattice constants (Supplementary Figure S6) and averaged Fe ionic displacements (Figure 4f and Figure 4g) extracted from the experiments, the atomic structure of the reference was modified. During the ionic relaxation to obtain the atomic structure of polar O-BFO, the coordinates of Bi and Fe along the [001] direction were fixed to guarantee the stability of the Fe ionic displacements. Lattice parameters

Pna21

a = 5.596 Å b = 5.539 Å c = 7.746 Å α = β = γ = 90º

Atom

Wyc.

x

y

z

Bi

4a

0.549

0.500

0.225

Fe

4a

0.998

0.503

0.500

O

4a

0.286

0.709

0.051

O

4a

0.311

0.691

0.472

O

4a

0.974

0.415

0.253

Polarization

Pz = 0.47 C m-2 Pxy = 0 C m-2

AFD pattern & angles

a-a-c+ Φ = 14.1º θ = 14.2º

Table 1. The atomic coordinates, Wyckoff positions, polarization values and AFD pattern (following the definition of Glazer34) of the Pna21 phase obtained from LSDA+U calculations. The lattice parameters are the experimental values. Pz and Pxy stand for the component along the z axis (corresponding to the [001] direction in the experiment) and the one in the xy plane. The rotation angles Φ and θ of AFD refer to the components rotating around the z axis and tilting around the axis in the xy plane, respectively.


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According to the symmetric analyses, the space group of the polar O-BFO is identified as Pna21. The oxygen octahedral rotation pattern of this phase is a-a-c+, which is the same as that of TSO substrates. The calculated in-plane rotation angles are ±8.15°, which are consistent with the experimental results (Figure 4e). More importantly, the Pna21 phase has an additional ferroelectric polarization (Pz = 0.47 C m-2) along the [001] axis. In fact, the polar O-BFO found by us is analogous to the Pna21-I phase with the polarization of about 0.5 C m-2, as reported by Stengel et al. previously.35 This phase can be stabilized by an electric field along the [001] direction.35 It is expected that the combined effect of the local built-in electric field and interfacial oxygen octahedral coupling may facilitate the BFO in triangular areas adopting a polar Pna21 phase. On the one hand, the formation of the polar Pna21 phase within vortices can be understood through an electrostatic energy consideration (Figure 5). Inhomogeneous local electric fields exist at the terminations of vertical 109°domain walls near uncompensated hetero-interfaces, as depicted in Figure 5a. The local built-in fields facilitate the generation of dipole moment vectors between positive and negative charge centers (Figure 5b-c). As a result, the polar O-BFO forms due to the local electric fields. Namely, the polar O-BFO in triangular areas are induced by the local electric fields, which is supported by the first-principles calculations reported previously.35 To some extent, the in-plane polarization of the polar O-BFO play a significant role in stabilizing 109°domain configurations.36


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Figure 5. Schematics showing the formation of polar O-BFO phase. (a) Stereo schematic showing the detailed 109°domain patterns in BFO layers. The black and red arrows denote the polarization of R-BFO and O-BFO, respectively. (b) Projection of the non-polarized O-BFO unit cell along [11̅ 0] direction. (c) Schematic showing O-BFO polarized under local built-in electric fields. The purple shade area emphasizes Fe ionic displacements relative to the center line of the BFO unit cell along [001] direction (the blue broken line). On the other hand, the stronger interfacial oxygen octahedral coupling and large compressive strains from high index (110)-oriented substrates may also be the decisive factors for the formation of polar O-BFO phase, constituting a ferroelectric MPB combined with R-BFO matrix. The giant electromechanical responses37, 38 discovered at MPB make the ferroelectrics be promising for a variety of applications. It is known that compositional variation or mechanical pressure may both drive the formation of MPB. By varying the composition of complex-structured solid solutions, such as Pb(Zrx, Ti1-x)O3, a rhombohedral-to-tetragonal phase boundary is reported.39 For pure compound ferroelectric oxides, mechanical pressure is applied to induce the formation of MPB. The two-phase boundary of rhombohedral phase and tetragonal phase is stabilized in BiFeO3 through the large compressive strain imposed by substrates.40 Driven by interfacial oxygen octahedral coupling, the coexistence of rhombohedral phase and orthorhombic phase is reported


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in BiFeO3 ultrathin films.33 Besides, PbTiO3 can also display a large MPB under extremely high pressure.41 Thus, the strain and the stronger interfacial oxygen octahedral coupling imparted by underlying substrates can effectively alter the stable crystal structure of BFO thin films. In our case, the large compressive strain (up to -2.578%), exerted by (110)-oriented TSO substrates, makes it possible for the phase transition in BFO thin films,42 which further positions BFO on a MPB constituted by its R and O polymorphs. Besides, as shown in Figure 1b, (110)-oriented substrates have stronger clamping on BiFeO3 thin films30 with two B-O-B’ bond connections along [110] directions across hetero-interface than generally studied (001)-oriented substrates, which only permit one B-O-B’ bond connection. Thus, (110)-oriented substrates enable a great control over FeO6 octahedron tilt patterns through substrate proximity effects.43 The stronger interfacial oxygen octahedral coupling between TSO substrates and BFO thin films forces BFO unit cells adjacent to interfaces to adopt the symmetry of TSO substrates. Thus, the oxygen octahedral rotation symmetry of the polar O-BFO phase is a-a-c+, following the definition of Glazer.34 Furthermore, these vortex arrays are also obtained in BFO/TSO/BFO/TSO(010)O and GSO/BFO/GSO(010)O multilayers, with symmetric insulating boundary conditions for the first BFO layers. The specific sandwich construction of BFO layers clamped between two orthorhombic phases stabilize the nano-scale vortex arrays. The vortex phase, which is characterized by electric toroidal order, exhibits a ferroelectric MPB constituted by a modulated O-BFO and R-BFO, where the continuous polarization rotation between R-BFO and polar O-BFO may suggest good piezoelectric properties. CONCLUSIONS In summary, we have found nano-scale vortex arrays in (110)-oriented BFO multilayers. Analyses of experimental results clearly demonstrate that these vortex arrays are stabilized by


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orthorhombic symmetry, which clamps the BFO layers in sandwich construction multilayers, exhibiting a feature of a ferroelectric MPB. The MPB is constituted by the polar O-BFO and RBFO deduced from atomically resolved order-parameter mappings. It is proposed that the formation of the polar O-BFO phase is the synergetic result, which involves the combination of the stronger interfacial oxygen octahedral coupling across hetero-interfaces and the localized electrostatic energies. Our results illustrate a viable mechanism of vortices formation and a trait of two ferroelectric phase coexistence for polar vortex with electric toroidal order, which will offer potential degree of manipulating phase components and properties of ferroelectric topological structures. METHODS Film Deposition Details. Using pulsed laser deposition (PLD) with a Coherent ComPex PRO 201 F KrF (λ=248 nm) excimer laser, a series of multilayers on scandate substrates were deposited, as labeled below: BFO(10 nm)/GSO(3 nm)/BFO(10 nm)/TSO(010)O, GSO(3 nm)/BFO(7 nm)/GSO(010)O, BFO(10 nm)/TSO(10 nm)/BFO(10 nm)/TSO (010)O. The scandate substrates used here are commercial substrates without extra chemical or heat treatment. Before deposition, the substrates were heated to 850 °C for 15 minutes to clean the substrate surfaces and then cooled down to the film deposition temperature (10 °C min -1). BFO target (1 mol% Bi-enriched) was pre-sputtered for 15 minutes to clean its surface. A repetition rate of 6 Hz, substrate temperature of 800 °C, and oxygen pressure of 90 mTorr were used when growing BFO layers. A repetition rate of 3 Hz, substrate temperature of 700 °C, and oxygen pressure of 75 mTorr were used when growing the GSO and TSO buffer layers. After deposition,


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these films were annealed at growth temperature in an oxygen pressure of 0.5 atm for 15 minutes and then cooled slowly to room temperature at a rate of 5 °C min-1. STEM sample preparation, (S)TEM observation, geometry phase analysis (GPA), and 2D gaussian peaks fitting. The samples for TEM and STEM observation were prepared by traditional process: slicing, gluing, grinding, dimpling and finally ion milling. A Gatan 691 PIPS was used for ion milling. HAADF-STEM images were recorded using aberration-corrected (scanning) transmission electron microscope (Titan Cubed 60-300 kV microscope (FEI) equipped with double Cs corrector from CEOS, and operated at 300 kV). The diffraction contrast images were recorded using a conventional TEM (Tecnai G2 F30 (FEI) working at 300 kV). Strain analyses were based on GPA,44 which were carried out using Gatan Digital Micrograph software. Atom positions were accurately determined using 2D Gaussian peaks fitting in Matlab,29, 32 thus making it possible to acquire the information of the lattice spacing, B-site ionic displacement and A-site ionic in-plane and out-of-plane rotation. It should be noted that a wiener filter of HAADF and a low-pass annular mask restricted to the instrument resolution limit of the images were used in order to reduce the noise in obtained images. An algorithm was used in Matlab to correct nonlinear drift distortions of scanning transmission electron microscopies in HAADF images by using image pairs with orthogonal scan directions.45 First-principles calculations. The density functional theory calculations were employed in the local spin-density approximation (LSDA) scheme, with the Hubbard U (Ueff = 3.8 eV) applied to the 3d orbitals of Fe, as implemented in the software package VASP.46-48 The Bi’s 5d106s26p3, Fe’s 3p63d64s2 and O’s 2s22p4 were treated as the valence states within the projector augmented wave (PAW) method.48 A √2 × √2× 2 (20-atom) BFO cell was built to allow the relevant ferroelectric and antiferrodistortive structural distortions, and the antiferromagnetic order was set as G-type. A


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6 × 6 × 4 Monkhorst-Pack k-point grid was used for integrations within the Brillouin zone.49 The plane wave cut-off energy was chosen as 500 eV. The ionic relaxation was considered as convergent when the force on every atom was less than 1 meV Å-1. Ferroelectric polarization was calculated in the Berry phase method.50, 51 The symmetry analysis was done with the Materials Studio software. Piezoresponse force microscopy (PFM). Polarization switchings of the multilayers were observed using a commercial piezoresponse force microscopy (Cypher, Asylum Research, US) in the dual AC resonance tracking (DART) mode. Conductive Ti/Ir coated silicon cantilevers (Asylum Research, ASYELEC-01-R2, US) were used for PFM hysteresis loop measurements. The typical tip radius is less than 25 nm and the force constant is ≈ 3 N m−1. XRD reciprocal space maps (RSM). XRD reciprocal space mappings were performed using a high-resolution X-ray diffractometer (BRUKER, D8 Advance). ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. The supporting information includes: sample information, reciprocal space mapping results, determination of spontaneous polarization of BFO, dark field images, HAADF, geometry phase analyses, the information of the lattice spacing (PDF)

The authors declare no competing financial interest.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Yinlian Zhu: 0000-0002-0356-3306 Author Contributions X.L.M. and Y.L.Z. conceived the project of interfacial characterization in oxides by using aberration-corrected STEM. W.R.G., Y.L.Z., and X.L.M. designed the experiments. W.R.G. performed the thin films growth and STEM observations. X.W.G. and Y.J.W. carried out Firstprinciples calculations. Y.L.T., Y.P.F., M.J.Z., M.J.H., J.Y.M., B.W. and W.T.H. participated in the thin films growth and STEM imaging. All authors contributed to the discussions and manuscript preparation. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51571197, 51501194, 51401212 and 51671194), National Basic Research Program of China (2014CB921002), and the Key Research Program of Frontier Sciences CAS (QYZDJ-SSWJSC010). Y. L. T. acknowledges the IMR SYNL-T.S. Kê Research Fellowship and the Youth Innovation Promotion Association CAS (No. 2016177). We are grateful to Mr. B. Wu and Mr.


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BRIEFS. A ferroelectric morphotropic phase boundary is associated with polar vortex in (110) BiFeO3 films Table of Contents Figure


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RhombohedralâOrthorhombic Ferroelectric Morphotropic Phase

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