Room Temperature Exchange Bias in Structure-Modulated Single

Article Cite This: Chem. Mater. 2018, 30, 6156−6163

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Room Temperature Exchange Bias in Structure-Modulated SinglePhase Multiferroic Materials Guopeng Wang,†,● Zezhi Chen,†,● Hongchuan He,† Dechao Meng,‡,∇ He Yang,† Xiangyu Mao,# Qi Pan,† Baojin Chu,† Ming Zuo,‡ Zhihu Sun,∥ Ranran Peng,*,† Zhengping Fu,† Xiaofang Zhai,‡ and Yalin Lu*,‡,†,§,∥,⊥

Chem. Mater. 2018.30:6156-6163. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/09/18. For personal use only.

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CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China ‡ Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China § Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, P. R. China ⊥ Laser Optics Research Center, U.S. Air Force Academy, U.S. Air Force Academy, Colorado 80840, United States # College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China ∇ Microsystem and Terahertz Research Center & Institute of Electronic Engineering, CAEP, Chendu 610200, P. R. China S Supporting Information *

ABSTRACT: Single-phase materials with room temperature (RT) exchange bias (EB) are very important for future applications in spintronic devices, magnetic storage, and sensors. However, such materials are rare because EB normally occurs with ferromagnetic (FM)/antiferromagnetic (AFM) boundaries. In this work, RT EB was realized by breaking the homogeneous structure of long-period Aurivillius oxides through a simple Fe/Ti content modulation. Fantastic intergrowth structures, with different neighboring layer numbers larger than 1, were first observed in this material system with optimized compositions. The intergrowth structure introduces a much larger lattice distortion and probability of Fe−O−Fe interaction, which can largely enhance the EB effect and the EB temperature. Most importantly, RT EB was finally realized in Bi10Fe6+yTi3−yO30+δ (x = 0.2, 0.4) samples with the largest HE of ∼38 Oe measured at 300 K. Significantly, the concurring room temperature multiferroic behavior in such materials may add a new regulating factor for the EB through the application of an electrical field, which is meaningful for future device operations. The findings in this work alleviate the limitation of fabricating Aurivillius oxides with large layer numbers and shed new light on realizing room temperature EB in single-phase materials, with the potential for building future magnetically or electrically controlled spintronic devices.

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applications as devices.12,13 Additionally, complicated and energy-consuming techniques or processes are required to fabricate such artificial materials to combine together components with different lattice parameters and different suitable preparation temperatures.14−18 Consequently, the availability of a single-phase EB material with promising room temperature operation would be more convenient for the realization of potential devices, either in terms of fabrication or device operation. So far, raw single-phase oxides including

INTRODUCTION Exchange bias (EB) induced by interfacial exchange coupling of a ferromagnetic (FM) material with an adjacent antiferromagnetic (AFM) material has been extensively studied for its scientific interest and potential applications in magnetic storage, magnetic tunnel junctions, and spin-electronic devices.1−4 Although a comprehensive understanding of EB is still frusrating researchers, a variety of artificial systems have been developed in the past decade, for example, AFM/FM film,4−7 core−shell nanoparticles,8,9 and other heterogeneous materials consisting of FM and AFM structures.10,11 Unfortunately, EB for most materials exists at temperatures that are far below room temperature, which greatly hinders their potential © 2018 American Chemical Society

Received: July 3, 2018 Revised: August 16, 2018 Published: August 19, 2018 6156

DOI: 10.1021/acs.chemmater.8b02798 Chem. Mater. 2018, 30, 6156−6163

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Chemistry of Materials

Figure 1. (a) XRD patterns for the as-prepared Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4) and Bi10Fe6+xTi3−xO30+δ (Bi10-Fey, y = 0, 0.2, 0.4) samples. (b) The magnified section in the 2θ range ∼29.5−34.3° and (c) the corresponding peak spacing between peak 1 and peak 2 in parts a and b.

a potential strategy for realizing EB materials that function at room temperature.

SmFeO3 single crystals, Bi4.2K0.8Fe2O9+δ nanobelts, and BiFeO3 nanochains have been reported to show RT EB phenomena together with different function mechanisms.14,16,19−22 However, the fabrication of single crystals or nanobelts for device applications remains a big challenge, which usually inflicts a high cost and low output. Therefore, exploring effective RT EB single-phase oxides that can be easily fabricated would be extremely important not only for fundamental physics but also for future device application in spintronics and so forth. Layer-structure Aurivillius oxides with a general formula of Bin+1Fen−3Ti3O3n+3 have drawn increasing attention because of the coexistence of ferroelectric and magnetism in a single material.23−28 Recently, we found that long-period Aurivillius oxides (n ≥ 7) show good EB behavior resulting from the coupling between AFM and spin glass,27−29 which has brought new interest into such materials. Moreover, potential coupling between ferroelectric and ferromagnetism in these oxides may add a new regulating factor for EB through the application of an electrical field, which would become extremely important for future device operation via electrical control.30,31 Unfortunately, the detected EB temperatures in our previous work,27−29 usually 10−50 K below the freezing temperatures (Tf) of the spin glass transition, are all below room temperature (Figure S1). For example, the EB effect in Bi10Fe6Ti3O30 (n = 9), which has a Tf of approximately 280 K, occurs when the temperature is below 250 K.28 Importantly, although the EB temperatures of these oxides increase with their layer number, which makes the anticipated EB temperature for Bi11Fe7Ti3O33 (n = 10) close to RT, it is very difficult to synthesize Aurivillius oxides with n ≥ 10 because of their almost identical formation energies and more fragile stacked structures.32,33 To avoid this inconvenience, instead of merely increasing the layer numbers, alternative strategies would be a big plus. In this work, a facile strategy to modify both exchange bias and multiferroic behavior was proposed by modulating the Fe/ Ti mole ratio in Bi9Fe5Ti3O27 and Bi10Fe6Ti3O30 Aurivillius oxides. With increasing Fe/Ti mole ratio in perovskite slabs, fantastic intergrowth structures with different layer numbers in neighboring layers were obtained. Such a new modulated structure was demonstrated to contribute greatly and intrinsically to the improvement of exchange bias behavior. RT EB behavior was successfully realized in Bi10Fe6+yTi3−yO30+δ (y = 0.4) with the HE of approximately 38 Oe. Significantly, the distinct room temperature multiferroic behavior of these materials may add a new regulating factor to the EB through application of an electrical field. These results strongly indicate

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RESULTS AND DISCUSSION Structure Characterization. Room temperature X-ray diffraction (RT-XRD) patterns for Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4) and Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4) samples are shown in Figure 1a. All the samples present a typical Aurivillius structure without secondary oxides detected, suggesting the high quality of the obtained samples. From the magnified section in the 2θ range ∼29.5−34.3° (Figure 1b), the distance between peaks 1 and 2, noted as the peak spacing, is obviously different for varying x/y values in samples. As shown in Figure 1c, the peak spacing reduced with both x and y in Bi9-Fex and Bi10-Fey, respectively, indicating an increased average lattice parameter c, and thus the increased average values of layer numbers (Figure S2).28 That is to say, when iron (Fe) ions partially substitute titanium (Ti) ions, the homogeneous eight/nine layer structure in Bi9-Fe0/ Bi10-Fe0 is broken, leading to the average layer numbers increasing with x/y. Interestingly, the peak spacing of Bi9-Fe0.4 is slightly lower than that of Bi10-Fe0, which seems to suggest that the average value of the layer numbers in Bi9-Fe0.4 is above 9 (Bi10-Fe0). EDS mappings and the element compositions of the asprepared samples are shown in Figure S3 and Table S1. In all three samples, a homogeneous distribution of Bi, Fe, and Ti elements can be observed, indicating that no impurities are formed. This observation is consistent with the XRD results, in which only peaks corresponding to the Aurivillius structures are detected. To investigate the elemental compositions, the mole ratios of the Ti elements are set as the individual benchmark for each sample (for example, 2.6 in Bi9Fe5.4Ti2.6O27+δ). For all three samples, the calculated mole ratio values of Fe element are almost identical to their desired values, while those of Bi element are excessive. Separation of Bi from the sample under 200 kV electron beam (e-beam) focused irradiation may account for such an excess of Bi element.20 High-angle annular dark-field (HAADF) images of Bi9-Fe0, Bi9-Fe0.4, and Bi10-Fe0 are shown in Figure 2 and Figure S4, which provide more information for the atomic structures of the samples. According to their atomic weight, large bright spots here should correspond to Bi atoms, and small dark spots indicate Fe or Ti atoms, respectively. In the Bi9-Fe0 (100) crystal plane (Figure 2a), orderly perovskite slabs containing eight perovskite layers are sandwiched by two fluorite-like (Bi2O2)2+ layers. Similarly, a homogeneous nine-layer structure 6157


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such special intergrowth structure with a typical structure, we denote it as mixed-layer structure in later discussion. Importantly, unlike Co substituting for Fe, which gradually reduces the layer number from 6 down to 5.5, 5, 4.5, and finally 4,23 the Fe substitution leads to an increase in layer number. The perovskite slabs with larger layer numbers may increase the coupling possibility of Fe−O−Fe, and thus enhance the AFM properties of the samples.28 Meanwhile, the different a/b lattice parameters in the neighboring perovskite slabs, which increase obviously with layer number m,25 may introduce a large internal force at their interface, especially for those samples with a layer number difference >1, and thus bring forth severe lattice distortion. As shown in Figure S5, Fe or Ti atoms (small dark spots) clearly shift off the center of the four adjacent Bi atoms in the (001) plane of Bi9-Fe0.4, suggesting a distorted octahedral, which is similar to previous reports.24,36 Both the increase in layer numbers and the enlarged lattice distortion may benefit the magnetic properties of our modulation samples. Magnetic Characterization. Zero field cooled (ZFC) and field cooled (FC) magnetization of samples is shown in Figure 3a−d and Figure S6. The large divergence between the ZFC and FC plots along with the increased intensity in the FC curves below the bifurcation temperature indicates the onset of weak FM behaior.25 Importantly, a broad peak is observed in the ZFC plot indicating a typical spin-glass-like behavior, which can be further confirmed by the ac susceptibility measurements (Figure S7a−c) and ZFC curves with different applied fields (Figure S7d−f). The Néel temperatures TN are simulated from the turning point of the dM/dT vs T plots, as shown in the inset of Figure 3. For homogeneous Bi9-Fe0 and Bi10-Fe0, TN is below 290 K, while it is approximately 320 and 360 K for Bi9-Fe0.4 and Bi10-Fe0.4, respectively, suggesting a significantly enhanced TN following substitution of the Fe. As

Figure 2. HAADF images of (a) Bi9Fe5Ti3O27 (Bi9-Fe0), (b) Bi10Fe6Ti3O30 (Bi10-Fe0), and (c) Bi9Fe5.4Ti2.6O27+δ (Bi9-Fe0.4). (d− g) Magnified images of the green rectangles in part c.

can be observed in the Bi10-Fe0 (110) plane, as shown in Figure 2b, while for Bi9-Fe0.4 an intergrowth structure can be clearly observed (Figure 2c and Figure S4). Unlike typical intergrowth structures, in which m and (m + 1) perovskite slabs alternately stack between (Bi2O2)2+ layers,34,35 the layer numbers of the perovskite slabs in Bi9-Fe0.4 cover a large span (9−14 layers). To the best of our knowledge, this is the first time such special structures with a large difference (>1) in neighboring layer numbers have been reported. To distinguish

Figure 3. Magnetic characterization of (a) Bi9Fe5Fe3O27 (Bi9-Fe0), (b) Bi9Fe5.4Fe2.6O27+δ (Bi9-Fe0.4), (c) Bi10Fe6Fe3O30 (Bi10-Fe0), and (d) Bi9Fe6.4Fe2.6O30+δ (Bi10-Fe0.4) samples measured at FC (H = 500 Oe) and ZFC modes. The inset shows the plot of dM/dT vs T simulated from the FC data. 6158


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Figure 4. (a) Néel temperature (TN) and remanent magnetization (Mr) at 300 K for Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4) and Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4), with the digits above the TN curve corresponding to the Fe/(Fe + Ti) mole ratio for each sample. (b) TN as a function of Fe/(Fe + Ti) mole ratio for Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4), Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4), and Bi8Fe4+zTi3−zO24+δ (Bi8-Fez, z = 0, 0.2, 0.3, 0.45) samples. (c) Schematic diagram of the enhanced magnetic properties for the novel intergrowth structure compared with the homogeneous structure. Blue rectangles represent the coupling of Fe−O−Fe, and the black ellipses indicate the distortion of the Fe−O octahedron.

the reaction system, we fix the Fe/(Fe + Ti) ratio and slightly increase the Bi content in the Bi10-Fe0 samples. As shown in Figure S11, a slight excess of Bi seems to have a negligible effect on their magnetic properties, implying that a simple nonstoichiometric ratio cannot result in a large difference in performance. Meanwhile, the magnetic properties of Bi9-Fe0.4 and Bi10-Fe0.4 are repeatable, as shown in Figure S12. That is to say, once their raw material ratios are set, the macroscopic performance of the samples is determined. XPS and X-ray absorption near-edge spectroscopy (XANES) analyses of samples are shown in Figure S13a,b. Only Fe3+ ions are detected in our samples, further indicating that the improved magnetic properties of Fe-substituted samples do not originate from the Fe ions with higher valence.39 On the basis of these results, we proposed that the special mixed-layer structures, which can introduce larger lattice distortion and more Fe−O− Fe interaction, should account for the magnetic enhancement in magnetism of Bi8-Fez, Bi9-Fex, and Bi10-Fey (x, y, z ≠ 0) samples, as shown in Figure 4c. A detailed interpretation can be given as follows: (1) The observed AFM roots in the Fe3+− O−Fe3+ interaction in the perovskite slabs, and the weak FM, can be ascribed to the canted AFM structure arising from an antisymmetric spin coupling, namely, the Dzyaloshinskii− Moriya (DM) interaction.40 (2) The increase of layer number in the perovskite slab and the enhanced lattice distortion resulting from the special intergrowth structures improve the

clearly shown in Figure 4a, TN increases almost linearly with increasing Fe substitution content in the separate Bi9-Fex (or Bi10-Fey) system. Interestingly, the Fe-substituted samples show a much higher TN compared to samples with homogeneous structures, even though they have a similar Fe/(Fe + Ti) mole ratio. For example, Bi9-Fe0.3 has a slightly smaller Fe/(Fe + Ti) ratio (0.6625) compared to that of Bi10Fe0 (0.6667); however, the TN of the former (307 K) is much higher than the latter (290 K). Moreover, as shown in Figure 4b, the slopes of the green, blue, and red lines are much sharper than that of the black line, indicating an obvious contribution of the mixed-layer structure to the enhancement of TN. This may also be regarded as evidence for the existence of lattice distortion, which brings forth an intensive impact on TN. M−H curves of Bi9-Fe0, Bi9-Fe0.3, Bi10-Fe0, and Bi10-Fe0.4 measured at 200 and 250 K are shown in Figure S10a,b. Hysteresis loops accompanying unsaturated magnetization even at 30 kOe imply the superposition of both AFM and FM components, consistent with the above analyses of ZFC/ FC curves.37,38 The remanent magnetization for all the samples is shown in Figure 4a and Figure S9c, which shows almost the same trend for TN with Fe substituting content, indicating a similar physical mechanism. These results clearly demonstrate the existence of other mechanisms except for layer number functions.28 To ensure that the structural disorder in the obtained materials is not caused by the nonstoichiometry in 6159


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Figure 5. Typical M−H curves of (a) Bi9Fe5Ti3O27 (Bi9-Fe0) and (b) Bi10Fe6.4Ti3O30+δ (Bi10-Fe0.4) samples measured at 250 K when cooling in ZFC and FC (30 kOe) modes. (c) Fe/(Fe + Ti) mole ratio dependence of Tf of Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4), Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4), and Bi8Fe4+zTi3−zO24+δ (Bi8-Fez, z = 0, 0.2, 0.3, 0.45) samples. (d) Exchange bias field HE at 250 and 300 K of Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4) and Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4) samples.

Figure 6. (a) Remanant polarizations ΔP obtained from the positive-up negative-down (PUND) measurements based on Bi9Fe5+xTi3−xO27+δ (Bi9Fex, x = 0, 0.2, 0.4) and Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.4) with part b showing the corresponding ΔP at 100 kV/cm. (c) Temperature dependence of the dielectric constant ε′ of Bi9-Fe0.4 at varying frequencies. (d) ME coefficient as a function of applied field of Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.4) and Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.4) at 300 K.

phenomenon usually disappears slightly below the peak temperatures (Tf) of the spin glass. Tf values of Bi8-Fez, Bi9Fex, and Bi10-Fey are shown in Figure 5c and are approximately 5−30 K lower than TN. Additionally, Tf shows a similar dependence on the Fe/(Fe + Ti) ratio with TN and Mr,

FM/AFM properties, and thus may have a significant effect on the exchange bias behavior. Exchange Bias. Spin glass detected in ZFC curves and formed via disordered magnetic spins is the major source for exchange bias in Aurivillius oxides,27−29 in which the EB 6160


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Oe cm at 6 kOe, which is much lower than those of Bi9-Fe0 and Bi9-Fe0.4. Such a small ME property should be ascribed to its poor ferroelectric property. Bi9-Fe0.4 exhibits a good ME coefficient ∼280 μV/Oe cm at 6 kOe, which is approximately 4 times larger than that of BiFeO3 (64 μV/Oe cm) at RT.43 Such a good ME seems to suggest that it is possible to further modulate the EB effect via electrical field. Unfortunately, limited by our instruments, we failed to realize such modulation, and future work on device upgradation should be done to accomplish this target.

implying that the special intergrowth structure can intrinsically and significantly benefit the exchange bias. Tf values for Bi9Fe0, Bi9-Fe0.4, Bi10-Fe0, and Bi10-Fe0.4 are 262, 298, 286, and 335 K, respectively. The high value of Tf for Bi10-Fe0.4 implies that its EB behavior may show up even above room temperature. The exchange bias field HE, usually used to characterize the magnitude of EB effects, can be defined as the following eq 1: HE = |H1 + H2|/2

(1)

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where H1 and H2 are the left and the right coercive fields, respectively.36,37 The exchange bias field HE values of all the samples measured at 250 and 300 K are shown in Figure 5d. It can be clearly seen that for Bi9-Fe0.4 HE is approximately 55 Oe at 250 K and 0 Oe at 300 K. Upon enhancement of the measured temperature, AFM order is depressed due to the thermal fluctuations, which results in a weakened interfacial interaction between AFM and FM glass, and thus the lowered EB effect.28 It should also be emphasized that RT exchange bias has been realized in our samples, such as Bi10-Fe0.2 and Bi10-Fe0.4, with RT HE up to ∼38 Oe. These results clearly announce the fulfillment of our task to explore a RT EB material in Aurivillius oxides based on the special Fe/Ti modulation method, which arise from special intergrowth structures. Direct ME Coupling. Coexistence of FM and FE is one of the fascinating properties of Aurivillious oxides, and these may be utilized to control each other.41 Typical ferroelectric hysteresis loops and ferroelectric domains, as shown in Figure S14a−d, suggest unsaturated behavior in these samples, implying intrinsic ferroelectric characterization with the existence of leakage current. To exclude the polarization contribution from the leakage current and to confirm the intrinsic nature of the measured polarization, positive-up negative-down (PUND) measurements are conducted as shown in Figure 6a, with the remanant polarization ΔP obtained from PUND measurements at E = 100 kV/cm summarized in Figure 6b. Unlike remanent magnetization (Mr), ΔP increases slightly with increasing x in Bi9-Fex (x = 0, 0.2, 0.4) samples, but reduces abruptly when changing to Bi10Fey (y = 0, 0.4) samples. It has been reported that FE properties in Aurivillius oxides reduce with an increase in layer number.42 Therefore, a contrary effect on FE behavior may arise due to the competition between increasing layer number and the distorted lattice in these intergrowth structure induced by the Fe substitution: the former reduces FE, while the latter increases it. In Bi9-Fex (x = 0, 0.2, 0.4) samples, the lattice distortion impacts the FE more than the increase in layer number and thus leads to FE increasing with Fe substitution, while when changing to Bi10-Fey, the positive effect from the special lattice distortion vanishes, thus leading to an abrupt reduction in FE properties. The temperature dependence of the dielectric constant (ε′) of Bi9-Fe0, Bi9-Fe0.4, and Bi10-Fe0 is shown in Figure 6c and Figure S15a-b. When increasing the testing temperature, a peak related to the ferroelectric phase transition above 700 °C is observed, suggesting a high Curie temperature in these samples. Magnetoelectric (ME) measurements at RT were carried out to study the coupling of the ferroelectric and magnetic order, as shown in Figure 6d. All the samples show almost linear magnetoelectric coupling versus the applied magnetic field. Although Bi10-Fe0.4 shows the largest magnetic property among all the samples, its ME coefficient is 135 μV/

CONCLUSION In summary, we highlight an effective strategy to realize room temperature exchange bias and multiferroic behavior by varying the Fe/Ti mole ratio in Bi 9 Fe 5 Ti 3 O 27 and Bi10Fe6Ti3O30. Through carefully selecting the Fe/Ti mole ratio, we obtained a novel mixed-structure with layer number difference larger than one. The intrinsic contribution of the mixed-layer structure along with increased Fe/Ti ratio lead to improved exchange bias and multiferroic behavior and a large room temperature HE of ∼38 Oe for Bi10-Fe0.4 and an ME coefficient of ∼280 μV/Oe cm for Bi9-Fe0.4 among all the samples. This finding opens up new insight for the realization of room temperature exchange bias in single-phase multiferroic materials and future possible commercial application in spin valves.

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EXPERIMENTAL SECTION

Different ratios of Fe/Ti were realized through altering x in Bi9Fe5+xTi3−xO27+δ (Bi9-Fex, x = 0, 0.1, 0.2, 0.3, 0.4) samples, with the detailed synthesis steps for the Bi9-Fex powders described as follows: Stoichiometric amounts of Bi(NO3)3·5H2O, Fe(NO3)3· 9H2O, and tetrabutyltitanate (TBT) were dissolved in dilute HNO3 as precursors. Citric acid (CA) was added to the solution with the molar ratio of CA to metal ions set as 2:1. The formed solutions were dried at 140 °C for 12 h, and subsequently calcined at 750 °C for 2 h to remove organic residues and form powders with good crystallization. Finally, the obtained yellow powders were cold-pressed into pellets and then sintered at 930−950 °C for 3 h to obtain dense samples. For the preparation of Bi10Fe6+yTi3−yO30+δ (Bi10-Fey, y = 0, 0.2, 0.4) and Bi8Fe4+zTi3−zO24+δ (Bi8-Fez, z = 0, 0.2, 0.3, 0.45) samples, similar fabrication processes were applied. Crystal structures of the samples were investigated by a D/Max-gA diffractometer (Japan) with Cu Kα radiation. Selected area electron diffraction (SEAD) and high-angle annular dark-field (HAADF) images were obtained using a scanning transmission electron microscope (STEM) (JEM-ARM200F, JEOL) equipped with a spherical aberration corrector. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Thermo ESCALAB 250 equipped with an Al Kα source. Direct current magnetic measurements were made using a Quantum Design physical property measurement system equipped with a vibrating sample magnetometer (PPMS-VSM). Exchange biases were characterized by measuring the magnetic hysteresis loops after cooling the samples from 350 K down to the testing temperatures in ZFC or FC modes. Alternating current susceptibilities were measured using a commercial superconducting quantum interference device. K-edge XANES spectra of Fe were measured on the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) utilizing the fluorescent mode with a 19element Ge solid-state detector. Ferroelectric hysteresis loops with pulsed polarization positive-up negative-down (PUND) were conducted using a Precision LC FE analyzer (Precision LC II Radiant Technology). Dielectric properties were obtained using a dielectric spectrometer (Novocontrol Technologies). Room temperature magnetoelectric (ME) coupling measurements were obtained using a Super ME system (Quantum Design) with ac magnetic fields 6161


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Chemistry of Materials of approximately 2 Oe and dc bias fields up to 6 kOe. Surface morphology and electromechanical properties were measured using piezoresponse force microscopy (PFM) (Bruker DI MultiMode V).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02798.

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Additional XRD, HAADF, EDS, magnetic experiments, XPS, XANES, ferroelectric experiments, PFM, and dielectric results (PDF)

AUTHOR INFORMATION

ORCID

Zhihu Sun: 0000-0002-3898-969X Ranran Peng: 0000-0003-4558-6757 Zhengping Fu: 0000-0002-7896-1333 Author Contributions ●

G.W. and Z.C. contributed equally to this work and should be considered co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2016YFA0401004), the Natural Science Foundation of China (51472228), the External Cooperation Program of BIC, Chinese Academy of Sciences (211134KYSB20130017), Key Research Program of Chinese Academy of Sciences (KGZD-EW-T06) and Hefei Science Center CAS (2016HSCIU004).

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