Multifield Control of Domains in a Room-Temperature Multiferroic 0.85

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Multifield Control of Domains in a Room-Temperature Multiferroic 0.85BiTi0.1Fe0.8Mg0.1O3−0.15CaTiO3 Thin Film Tingting Jia,†,‡ Ziran Fan,†,∥ Junxiang Yao,† Cong Liu,† Yuhao Li,† Junxi Yu,† Bi Fu,† Hongyang Zhao,∥ Minoru Osada,‡ Ehsan Nasr Esfahani,⊥ Yaodong Yang,# Yuanxu Wang,¶ Jiang-Yu Li,†,⊥ Hideo Kimura,*,‡ and Zhenxiang Cheng*,†,§,¶ †

Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, Guangdong 518055, China ‡ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan § Institute for Superconducting & Electronic Materials, University of Wollongong, Innovation Campus, North Wollongong, NSW 2500, Australia ∥ Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, Department of Materials Science and Engineering, Wuhan Institute of Technology, 206 Guanggu first Road, Wuhan 430205, China ⊥ Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195, United States # Electronic Materials Research Laboratory, Key Laboratory of The Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China ¶ Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China S Supporting Information *

ABSTRACT: Single-phase materials that combine electric polarization and magnetization are promising for applications in multifunctional sensors, information storage, spintronic devices, etc. Following the idea of a percolating network of magnetic ions (e.g., Fe) with strong superexchange interactions within a structural scaffold with a polar lattice, a solid solution thin film with perovskite structure at a morphotropic phase boundary with a high level of Fe atoms on the B site of perovskite structure is deposited to combine both ferroelectric and ferromagnetic ordering at room temperature with magnetoelectric coupling. In this work, a 0.85BiTi0.1Fe0.8Mg0.1O3−0.15CaTiO3 thin film has been deposited by pulsed laser deposition (PLD). Both the ferroelectricity and the magnetism were characterized at room temperature. Large polarization and a large piezoelectric effective coefficient d33 were obtained. Multifield coupling of the thin film has been characterized by scanning force microscopy. Ferroelectric domains and magnetic domains could be switched by magnetic field (H), electric field (E), mechanical force (F), and, indicating that complex cross-coupling exists among the electric polarization, magnetic ordering and elastic deformation in 0.85BiTi0.1Fe0.8Mg0.1O3− 0.15CaTiO3 thin film at room temperature. This work also shows the possibility of writing information with electric field, magnetic field, and mechanical force and then reading data by magnetic field. We expect that this work will benefit information applications. KEYWORDS: ferroelectricity, magnetism, domain switching, morphotropic phase boundary, thin film

1. INTRODUCTION The ability to write magnetic bits electrically has long been sought after for the next generation of data storage that needs to consume minimum power and possess high information density.1,2 While some successes have been achieved in spincurrent injection into nanosized magnetic cells,3,4 accomplishing electric writing in the absence of current, which often involves multiferroic materials, remains elusive. Multiferroics possess electric and magnetic orders simultaneously and are often magnetoelectrically coupled,5−7 but single-phase multi© XXXX American Chemical Society

ferroic materials are rare, especially at room temperature, due to the mutually exclusive conditions on ferroelectricity and ferromagnetism.2,8 In fact, despite almost 20 years of extensive research, electric writing of magnetic bits has yet to be realized in a single-phase multiferroic at room temperature, despite the demonstration of such capability in multiferroic heterostrucReceived: April 3, 2018 Accepted: June 1, 2018 Published: June 1, 2018 A

DOI: 10.1021/acsami.8b05289 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the perovskite subcell of BTFM-CTO. (b) XRD patterns of the ceramic target and thin film. The peaks marked with * are due to the diffraction from the orthorhombic phase (Pna21). (c) STEM cross-sectional view and compositional EDS mapping. (d) SAED diffraction pattern for BTFM-CTO thin film taken along the [110] zone axis. (e) HRTEM image of the as-deposited thin film.

ferromagnetic superexchange interaction, resulting in the development of polar Bi2Mn4/3Ni2/3O6 with large magnetization at low temperature.12 Using this strategy and taking advantage of the morphotropic phase boundary (MPB) in a solid solution,13 the (1 − x)BiTi3/8Fe2/8Mg3/8O3−xCaTiO3 system with superior ferroelectricity was developed,14 and long-range magnetic order was realized by increasing the concentration of Fe ions beyond percolation, leading to roomtemperature ferromagnetism in the bulk ceramic.15 In this work, we apply this strategy to deposit multiferroic 0.85BiTi0.1Fe0.8Mg0.1O3−0.15CaTiO3 thin film using pulsed laser deposition (PLD) and, with it, successfully realize electric voltage and mechanical writing of both ferroelectric and magnetic bits at room temperature. Guided by the previous studies, we have focused on the system with composition of 0.85BiTi 0.1 Fe 0.8 Mg 0.1 O 3 − 0.15CaTiO 3 (BTFM-CTO), which was shown to be

tures consisting of ferroelectric and ferromagnetic constituents with different configurations. The example closest to success so far is a recently reported superlattice of LuFe2O4 and LuFeO3 that was atomically engineered,9,10 although the magnetism was rather weak, and the electric control of magnetism was only realized at 200 K. In order to make electrically written and magnetically read data storage viable, we need to be able to switch strong magnetization electrically at room temperature, with low voltage, and, ideally, in a single-phase multiferroic film integrated on silicon. Among the several routes known toward multiferroicity, the lone-pair mechanism is most promising,5 as exhibited by BiFeO3, the only room temperature multiferroic that has been very intensively studied, although it is antiferromagnetic despite having strong polarization.11 It was realized that multiple cation occupation of the octahedral B sites in the ABO3 perovskite structure can generate ferromagnetism by exploiting the B


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Figure 2. Ferroelectric behavior of BTFM-CTO thin film: (a) polarization−electric field (P−E) loops and corresponding (b) current−electric field (I−E) loops of BTFM-CTO thin film measured at room temperature and 250 Hz. (c) Positive-up−negative-down (PUND) measurements show the switching and nonswitching polarization as a function of voltage. Inset is the pulse waveform with a pulse width of 2.5 ms and a rise time of 1 ms. In the PUND measurement, five sequential pulses are applied on the capacitors. At the beginning, a negative writing pulse V0 is used to reset the polarization state. After that, a positive pulse V1 is applied to record the switching polarization. Then, another positive pulse V2 is applied to measure the nonswitching polarization which contains only the nonremanent polarization. This also applies to the “negative-down” process. V3 pulse switches the ferroelectric domains to the opposite direction, and V4 just measures the nonremanent polarization. P0−P4 are the corresponding polarization with V0−V4. (d) Butterfly loop in black shows displacement resulting from the piezoresponse to applied voltage and the calculated gradient in the form of d33, at the maximum reaching the d33 value of 127 pm/V. electrodes on the surface of the sample through a shadow mask (100 μm in diameter). The ferroelectric properties of the as-obatained capacitor were measured at RT using an aixACCT TF-1000 ferroelectric tester. The so-called positive-up−negative-down (PUND) measurements were also carried out. A triangle voltage waveform was used; the PUND amplitude was ±25 V; the frequency was 250 Hz: the write pulse rise-time was 1 ms; and the read pulse delay was 2.5 ms. Magnetic properties of the as-deposited BTFMCTO film were measured using a superconducting quantum interference device (SQUID) (Quantum Design MPMS magnetometer). The diamagnetic moment of the substrate was calculated from measurements on the bare substrate and subtracted from the raw data. 2.3. Scanning Force Microscopy Measurement. Local piezoelectric/ferroelectric and magnetic properties were measured using a NanoCute SPI 3800 (Hitachi HiTech Science) scanning force microscopy (SFM) system which enables both piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements. Detail could be found in our previous work.16 The reason for not using the MFM tip to do force scanning was to avoid the magnetization effect from the weak magnetic field of the MFM tip. The mechanical constant was 15 N/m, and the resonance frequency was 139 kHz. The BTFM-CTO thin film was electrically poled by +15 V, −15 V, and +15 V in an area of 6 × 6 μm2, 4 × 4 μm2, and 2 × 2 μm2, successively, to obtain a uniformly arranged box-in-box pattern. For the electric-mechanic-electric lithography, an electric voltage of +10 V was initially applied in an area of 6 × 6 μm2; then, 500 nN was applied in an inner square of 4 × 4 μm2; and finally, an electric voltage of +10 V was applied in a 2 × 2 μm2 area inside the squares again, so the box-in-box pattern completed done at last. The experiment on applying magnetic field on the thin film was carried out on a MFP-3D Infinity AFM (Asylum Research).

ferromagnetic at room temperature with large polarization in the bulk ceramic.14 The unit cell structure is schematically shown in Figure 1a, wherein the A-site ion is displaced along both the [111]p and the [001]p axes, leading to polarization, and there is a coherent magnetic B-site sublattice connected by B−O−B superexchange pathways, resulting in long-range magnetic order when the Fe concentration is beyond percolation limit.15 Importantly, the spin is oriented perpendicular to the polar [111]p direction, making it possible to switch the magnetization electrically.

2. EXPERIMENTAL SECTION 2.1. Film Deposition and Structural Characterization. 0.85BiTi0.1Fe0.8Mg0.1O3−0.15CaTiO3 (BTFM-CTO) thin film was deposited on Pt/TiO2/SiO2/Si substrates by a pulsed laser deposition (PLD) system with the laser source at 355 nm and a repetition rate of 10 Hz. The ceramic target for BTFM-CTO deposition was prepared using a conventional solid-state reaction process. The BTFM-CTO thin film was deposited at around ∼500 °C for 30 min, followed by an in situ thermal annealing process for 10 min, and then cooled down to room temperature. The phases of the target and films were determined by X-ray diffraction (XRD) using Cu Kα radiation using a Rigaku diffractometer. Scanning transmission electron microscopy (STEM) cross-sectional view and compositional Energy-dispersive spectroscopy (EDS) mapping were carried out using a Topcon 002BF system. A high-resolution transmission electron microscope (HRTEM) image of the as-deposited thin film was measured by Tecnai G2 F20 S-Twin. 2.2. Macroscale Electrical and Magnetic Measurements. A capacitor of Pt/BTFM-CTO/Pt was fabricated by depositing Pt top C


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Figure 3. Ferromagnetic behavior of BTFM-CTO thin film: (a) M−H curve measured at room temperature (300 K). The inset is the M−H curve measured at 5 K. (b) M−T curves (ZFC−FC curves) of the BTFM-CTO ceramic target. (c) M−H curve at 5 K, with the inset showing the M−H curve at RT. (d) M−T curves of BTFM-CTO thin film.

is a polar orthorhombic structure (Pna21). For the thin film, it can be observed that the perovskite structure is maintained over the composition range 0.0 < x ≤ 0.2. The sample with the composition of x = 0.15, y = 0.8 was chosen for detailed property studies because it showed the best performance in terms of ferroelectric and magnetic properties. Figure 1c shows a state-of-the-art aberration corrected STEM high-angle annular dark-field (HAADF) image and the corresponding EDS maps of Bi, Fe, Mg, Ti, Ca, and O atoms in the BTFM-CTO thin film. It is obvious that the elements are uniformly distributed in the film, indicating its high crystalline quality. According to the STEM image, the film thickness is ∼330 nm. The selected area electron diffraction pattern (SAED) shown in Figure 1d reveals a splitting of the reflections. The splitting originates from the crystal structure near/in the morphotropic phase boundary region which possesses an orthorhombic structure (O, Pna21) with an in-phase oxygen octahedral tilt. It is suggested that the tetragonal polar order is partly induced from the rhombohedral structure (R, R3c) at the MPB. We also can observe 1/2 weak reflections, indicating the coexistence of a mixture of R and O phases.17 Figure 1e is a cross-sectional HRTEM image of the as-deposited thin film, which shows a continuous and sharp interface between the BTFM-CTO film and the substrate. There are fingerprints of its origin from a domain boundary with different in-plane orientations, as described above.18 3.2. Large Ferroelectric Polarization. Figure 2a shows the ferroelectric polarization (P) as a function of applied electric field (E) for the BTFM-CTO thin film at room temperature (RT) and 100 Hz. The ferroelectric polarization increases with the applied voltage and reaches a maximum remnant polarization with the applied electric field of 777 kV/ cm. The hysteresis loop of the BTFM-CTO thin film shows a

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of BTFM-CTO Thin Film. A pseudobinary solid-solution ceramic target was sintered by the conventional solid-state method. The thin film was deposited by pulsed laser deposition (PLD). Figure 1a illustrates the crystal structure of the ceramic target and thin film, which is a solid solution of the rhombohedral R3c phase and orthorhombic Pna21 phase, having the chemical composition of x = 0.15, y = 0.25, which is the MPB of (1 − x)BiTi(1 − y)/2FeyMg(1 − y)/2O3−xCaTiO3 in the solid solution,15,16 with Ca substituting for some Bi atoms on A sites and Ti and/ or Mg occupying B sites.14,15 The unique structure will cause the two perovskite symmetries to come close to each other in the phase diagram and enhances the piezoelectric response with a high occupancy of Bi ions (up to 92%) on the A sites of perovskite. Furthermore, since the occupancy of Fe ions on B sites is 68%, a percolating network of Fe−O−Fe paths is proposed to provide long-range magnetic ordering in this material.17,18 The X-ray diffraction (XRD) pattern of the ceramic target with the composition of 0.85BiTi0.1Fe0.8Mg0.1O3−0.15CaTiO3 (BTFM-CTO) presents a typical solid solution structure of rhombohedral phase (R) and orthorhombic phase (O) (Figure 1b), which is similar to what was previously reported.14 A series of compositions with x = 0.05, 0.15, 0.2 and y = 0.25, 0.6, 0.8 has been prepared to investigate the evolution of the structural and physical properties around the MPB (see Supporting Information Figures S1−S3). According to the evolution of the peak in angular range of 30−35°, when x = 0.05, the sample is rhombohedral structure (R3c); as x increases to 0.15, it possesses a mixed-phase structure of rhombohedral and orthorhombic (R3c and Pna21); when x increases to 0.2, its D


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ACS Applied Materials & Interfaces large ferroelectric remanent polarization of 2Pr = 191.6 μC/ cm 2 , although we could not completely exclude the contribution from the large leakage current at high voltage. The coercive field (Ec) of the as-deposited BTFM-CTO thin film is large, with 2Ec = 681.8 kV/cm at 300 K. Figure 2b presents the I−E curves measured simultaneously during the P−E hysteresis measurement. The round shape indicates the leakage contribution to the P−E loop. To verify the polarization, a positive-up−negative-down (PUND) measurement19,20 was performed with a triangle pulse, in which the rise time was 2.5 ms and the read pulse delay was 1 ms. The change in polarization when the sample was switched from the negative state of the relaxed remanent polarization into positive saturation indicated switched polarization in the sample, while the change in polarization when the sample was driven to positive saturation from the positive state of the relaxed remanent polarization represented nonswitching polarization. ΔP= Psw = 136.3 μC/cm2 is the switching polarization, which was obtained as shown in Figure 2c. The piezoelectric response of the BTFM-CTO thin film was tested using a piezoresponse force microscope (PFM) system. The butterfly loop shown in Figure 2d is asymmetric, while the piezoelectric effective coefficient d33 could be estimated according to the equation d33 = Δl/ΔV, where Δl is the displacement along the direction of polarization and ΔV is the change in voltage. At an applied voltage of 10 V, the sample reached a maximum effective d33 of about 127 pm/V, which is a higher value than for the ceramic with the same composition (52 pC/N14). The magnetoelectric (ME) effect has also been investigated, as shown in Figure S4. The polarization dependence of the electric field was measured under magnetic field of 0 mT and 44 mT, and the remnant polarization of the thin film was suppressed by applying an inplane magnetic field, indicating that the ME coupling exists in the thin film. 3.3. Magnetic Characterization. Figure 3a and 3c presents the magnetic response as a function of magnetic field for the ceramic target and the as-deposited thin film measured at 5 K and RT. The magnetic moment is 6.49 emu/ cm3 at 500 Oe and 6.34 emu/g for the bulk sample, although the field dependence of the magnetization (M−H) loop shows behavior similar to the ceramic target. It was reported that a weak ferrimagnetism arising from the ferromagnetic canting of a predominantly antiferromagnetic magnetic structure is generally found for BTFM-CTO perovskite ferrits with the composition of x = 0.15, y = 0.8 at the MPB due to the Dzyaloshinsky−Moriya antisymmetric exchange interaction.14 Fe-rich occupancy at the B site of the perovskite structure forms a percolating network of Fe−O−Fe superexchange paths, resulting in the coexistence of magnetic and electric long-range orders.21 Field-cooling (FC) and zero-field-cooling (ZFC) magnetization versus temperature (M−T) curves of the thin film are presented in Figure 3b and 3d. The M−T curves confirm the weak ferromagnetism of the film below 330 K, indicating longrange magnetic ordering in the film which possibly involves magnetoelectric (ME) coupling. Due to the weak magnetization of the thin film, the M−T curves become rough when the temperature is higher than 300 K. Several features can be observed through comparison: (i) similar to reported thin films and the ceramic target, the film shows highly anisotropic behavior in terms of magnetic properties, and the magnetic moments are almost coincident; (ii) the splitting between the ZFC and FC curves appears around 300 K in both cases; and

(iii) the values of the magnetic moment decrease as the temperature increases from low temperature to room temperature. Since the film thickness is only ∼330 nm, the magnetic signal is noisy, although we still could observe a magnetic hysteresis loop at room temperature. We have also investigated the magnetic properties of films with different compositions, as shown in Figure S3. The remanent magnetic moment increases with increasing Fe composition y up to 0.8 at 5 K. A recent report also presents another idea of the origin of the BTFMCTO system:22 oxygen vacancy related exchanges can also induce ferromagnetism by F center exchange, such as with Tin+Vö-Tin+ and Fe3+-Vö-Tin+. Moreover, introducing the CaTiO3 into BiFeO3 could destruct the spin cycloid of BiFeO3, so that the ferromagnetic property of BTFM-CTO would be enhanced. 3.4. Multifield Controlled Domain Switching. Scanning force microscopy (SFM), which is originated from atomic force microscopy (AFM), has already been developed as a powerful facility that integrates advanced techniques (e.g., PFM, magnetic force microscopy (MFM), electrostatic force microscopy (EFM), etc.) to characterize and tune the physical properties of functional materials. In this work, we used the PFM and MFM modes in a SFM system to characterize the magnetoelectric effect in BTFM-CTO thin film. Figure 4a and

Figure 4. Electric lithography and mechanical writing of domains and corresponding magnetic response: (a) topographical image; (b) PFM image; (c) MFM image just after ±15 V writing; (d) topographical image; (e) PFM; and (f) MFM image after +10 V, 500 nN, and +10 V writing.

4d show the topography of the thin film, demonstrating that the film surface is smooth and flat. To investigate the polarization dynamics, Figure 4b shows the results of a writing experiment in the BTFM-CTO thin film. The film was electrically poled by electric bias of +15 V, −15 V, and +15 V in 6 × 6 μm2, 4 × 4 μm2, and 2 × 2 μm2 squares, successively, to obtain a uniformly arranged box-in-box pattern. The dark and bright regions correspond to ferroelectric domain states with +P and −P, respectively, indicating that the as-deposited thin film can be reversibly switched between the two ferroelectric states. Figure 4c displays the corresponding MFM phase image in the same region after electric writing. A sharp contrast in the magnetic domains corresponding to the ferroelectric domains was observed, indicating that magnetic domains in the BTFM-CTO thin film were switched by electric field (E). The contrast between positive polarization and negative polarization is stable, and clear contrast could be observed by a tip short circuit and even maintained after 3 days, E


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Figure 5. SFM characterization of BTFM-CTO thin film. (a) Topographical image. (b) PFM image scanned just after electric writing by 10 V. (c) Corresponding MFM image acquired without an external magnetic field. MFM images of (c) scanned with applied in-plane magnetic field of (d) −2000 Oe, (e) +2000 Oe, and (f) +4000 Oe. The signs “+” and “−” for the magnetic field represent the opposite in-plane magnetic field direction. Note the tilt of the images (d) and (e) is caused by the relocation of the tip after the tip was shifted by magnetic field.

indicating the film’s intrinsic ferroelectric property. In this work, we find a multifield-induced domain switching behavior in as-deposited BTFM-CTO thin film. As shown in Figure 4e and 4f, a similar pattern to that with electric poling was observed by electrical−mechanical−electrical lithography. Accordingly, both ferroelectric and magnetic domains are switched by F, implying that not only local ferro−electro− elastic polarization switching can be induced by the tip stress but also ferroelastic−magnetic domain switching.23 Comparing the topographic image with the unpoled area in the PFM image, it is found that the domain structure does not directly follow the grain structure of the thin film. The corresponding magnetic force microscopy (MFM) image shows bright and dark contrast in the mechanically poled region, indicating that the magnetic domains in BTFM-CTO thin film are also switched by F. We have to note that without an initial positive dc voltage poling the polarization reversal could not be observed when applying F.23 In addition, no obvious contrast of domain switching was observed when the thin film was initially poled by negative voltage. It is because the tip-induced stress can only be applied on the film surface in one direction, i.e., normal to film surface, so that the polarization can only be reversed along one direction by the force applied on the film surface. Therefore, only the polarization initially poled by positive voltage can be reversed by mechanical force. As is wellknown, all the multiaxial ferroelectrics are ferroelastics as well. Therefore, a large enough external strain (uniform or nonuniform) should always lead to reorientation of ferroelectric/ferroelastic domains. When a load is applied onto the film antiparallel to the polarization direction, charge separation occurs in the local region, and thus, a current transient is generated. Increasing the load reverses the current to a nonzero value. Further increasing the loading force results in an increase in the absolute value of current transients up to a local negative maximum, and at the same time, the polarization is reversed. In single-phase multiferroics with both ferroelectric and magnetic properties, the external force and electric field could also induce magnetic spin flipping due to the cross-coupling between electric−elastic−magnetic effects. The magnetoelectric (ME) coupling can be investigated by considering the effect of the magnetic field (H) on the

ferroelectric polarization and magnetization. Further investigation of the effects of magnetic field on the domain switching in BTFM-CTO thin film was carried out in an MFP-3D Infinity AFM system. Figure 5a and 5b is the topographical image and the PFM image after electric lithography. We can also observe magnetic domain switching simultaneously after the electric lithography, as shown in Figure 5c, which is the MFM image of an as-deposited BTFM-CTO thin film. A clear switching pattern was observed, even though the magnetic domains were not completely switched, indicating that the ME coupling in the film is stable at RT. Subsequently, an alternating in-plane magnetic field (H) was applied on the thin film, at −2000 Oe, +2000 Oe, and +4000 Oe, successively. The MFM images were affected differently depending on the field applied (Figure 5d− f). With field of ±2000 Oe, not much contrast change on the magnetic domains was observed, while the +4000 Oe in-plane magnetic field completely switched all the domains and wiped out the letters written by the electric field. This observation gives strong evidence that magnetic domains can be switched by not only electric field but also magnetic field and mechanical force; furthermore, the MFM image in Figure 5c represents the real magnetic domain.

4. CONCLUSIONS In conclusion, we have demonstrated that both ferroelectric and magnetic domains in our 0.85BiTi3/8Fe2/8Mg3/8O3− 0.15CaTiO3 thin film on Pt/TiO2/SiO2/Si substrate could be switched by applying E and loading with F. We have proved that the strain-induced ferroelasticity when applying an external loading force turns out to be a practical means to switch both ferroelectric and magnetic domains in BTFM-CTO thin film. This result gives us a new route to understand the mechanism of ferroelectric−magnetic−elastic coupling in multiferroic materials. It will also enrich our understanding of the strain effects, which will open up new technical opportunities for multifunctional device designs, such as sensors, detectors, highdensity data storage via mechanical means, etc. F


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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tingting Jia: 0000-0003-4352-8211 Ehsan Nasr Esfahani: 0000-0002-5197-8039 Jiang-Yu Li: 0000-0003-0533-1397 Hideo Kimura: 0000-0002-0941-909X Author Contributions

Tingting Jia, Zhenxiang Cheng, and Hideo Kimura proposed the research project. Tingting Jia prepared the thin films. Ziran Fan, Junxiang Yao, Cong Liu, Yuhao Li, and Junxi Yu carried out the characterizations. All authors discussed the results and contributed to the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Key Research and Development Program of China (2016YFA0201001), the Leading Talents Program of Guangdong Province (2016LJ06C372), the National Natural Science Foundation of China (51702351, 51402327, 51571083, 11627801), and the Shenzhen Science and Technology Innovation Committee (JCYJ20170413152832151, JCYJ20170307165829951, JCYJ20170818163902553, KQJSCX20170331162214306). Authors thank Dr. Xujun Su and Prof. Ke Xu in the Suzhou Institute of Nano-Tech and Nano-Bionics for TEM measurements and discussion. The authors also thank Prof. Xinshen Gao and his group in South China Normal University for their help in PFM and MFM measurements.

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ABBREVIATIONS ME, magnetoelectric; SPM, scanning probe microscopy; AFM, atomic force microscope; BTFM-CTO, 0.85BiTi3/8Fe2/8Mg3/8O3−0.15CaTiO3; RT, room temperature; PLD, pulsed laser deposition; XRD, X-ray diffraction; TEM, transmission electron microscope; STEM, scanning transmission electron microscope; SEM, scanning electron microscope; PUND, positive-up−negative-down; PFM, piezoresponse force microscope; MFM, magnetic force microscope; TC, Curie point; TN, Néel transition temperature; H, magnetic field; E, electric field; SQUID, superconducting quantum interference device; FC, field cooling; ZFC, zero field cooling; M−T, magnetization versus temperature

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REFERENCES

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