Co multi-doped BiFeO3 thin films

Accepted Manuscript Multiferroic properties of La/Er/Mn/co multi-doped BiFeO3 thin films Yun Liu, Guoqiang Tan, Meiyou Guo, Zhengjun Chai, Long Lv, Mintao Xue, Xixi Ren, Jincheng Li, Huijun Ren, Ao Xia PII:

S0272-8842(19)30592-9

DOI:

https://doi.org/10.1016/j.ceramint.2019.03.053

Reference:

CERI 20992

To appear in:

Ceramics International

Received Date: 15 February 2019 Revised Date:

6 March 2019

Accepted Date: 8 March 2019

Please cite this article as: Y. Liu, G. Tan, M. Guo, Z. Chai, L. Lv, M. Xue, X. Ren, J. Li, H. Ren, A. Xia, Multiferroic properties of La/Er/Mn/co multi-doped BiFeO3 thin films, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.03.053. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Multiferroic Properties of La/Er/Mn/Co Multi-Doped BiFeO3 Thin Films Yun Liu1,*, Guoqiang Tan1, Meiyou Guo1, Zhengjun Chai1, Long Lv2, Mintao Xue1, Xixi Ren1, Jincheng Li1, Huijun Ren3, Ao Xia1 1

Shaanxi Key Laboratory of Green Preparation and Functionalization for

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Inorganic Materials, School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China

College of Cryptography Engineering, Engineering University of PAP, Xi’an

710086, China 3

School of Arts and Sciences, Shaanxi University of Science & Technology,

Xi’an, Shaanxi 710021, China

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*Email address: [email protected] (Y. Liu)

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Abstract: Bi0.9-xLaxEr0.1Fe0.96Co0.02Mn0.02O3 (BLaxEFMCO) thin films were prepared by sol-gel method. The grain size, grain boundary resistance, oxygen vacancies and the amount of Fe2+ of the films were reduced by multi-ion doping to reduce the built-in electric field of the films. An applied voltage was adopted to regulate the

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effects of the directional alignment of the oxygen vacancies, defects, and defect pairs on the ferroelectric domains at the grain boundaries to control the ferroelectric polarization of the films. Meanwhile, the capacitance peak also reveals the effects of

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the ferroelectric domains switching, the migration of oxygen vacancies, and the directional alignment of defect pairs on the ferroelectric properties. In addition, the remnant polarization value of the BLa0.01EFMCO thin film reaches 152 µC/cm2, the

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squareness of the hysteresis loop (Rsq) is calculated to be 1.03, and the maximum switching current is 1.50 mA. The typical butterfly curves under positive and negative electric fields indicate the films with the enhanced ferroelectric properties. Moreover, the BLa0.01EFMCO thin film exhibits the enhanced ferromagnetic properties, and its saturation magnetization (Ms) is 2.32 emu/cm3. Therefore, the ferroelectric properties of the BFO film can be enhanced by the multi-ion doped BFO film to reduce the grain boundary resistance (Rgb), the interface Schottky barrier formed by the asymmetric electrode material at the top and bottom of the film, and the built-in electric field formed by the film internal defect or defect pairs. 1

ACCEPTED MANUSCRIPT Keywords: BiFeO3; Multi-Doping; Built-in electric field; Grain boundary resistance; Ferroelectric properties 1 INTRODUCTION Bismuth ferrite (BiFeO3, BFO) has been intensely studied as one of the most

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promising ferroelectric materials, since it can display ferroelectricity (high Curie temperature (TC) =1103 K) and weak antiferromagnetism (Neel temperature (TN) =643 K) above room temperature [1-4]. The coexistence of ferroelectricity and ferromagnetism is thought to be in favor of generating magnetization in BFO thin

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films under an electric field and inducing ferroelectric polarization under a magnetic field, which provides a new idea for the design of ferroelectric multi-function devices.

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However, the problems existing in the preparation of BFO are that the oxygen vacancies generated by Bi element volatilization and the charge defects produced by the non-metering ratio both cause a large leakage current in the film, which restricts the application in practice. Consequently, reducing the leakage current and improving the electrical properties are the main problems in the application of BFO thin films.

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BFO exhibits a distorted perovskite structure. The rare earth elements, such as Gd, La, Nd and Sm [5-8] and it can replace the A-site (Bi), reducing oxygen vacancies, stabilizing the oxygen octahedral structure and enhancing the uniformity of the spin

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arrangement, which can improve the ferroelectric properties. The replacement of the B-site (Fe) by the transition metal ions, such as Mn, Co and Cr [9-11] can enhance the magnetic spin structure and improve the ferromagnetism of the film [12]. Yan et al.

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studied that the Ru-doped BFO films (BFRO) exhibited a saturated P-E hysteresis loop (the remnant polarization value Pr ~99 µC/cm2), and the improved ferroelectricity was attributed to the decrease of the defects concentrations. The improved magnetic properties (the saturation magnetization value Ms ~16.53 emu/cm3) were derived from the ordered coupling of the ferroelectric and the ferromagnetic [13]. Singh et al. prepared La and Ni co-doped BiFeO3 thin films on Pt/Ti/SiO2/Si (100) substrates by chemical solution method. The grain size was increased with the increase of doping concentration, the leakage current of the films was 10-3A/cm2 lower than that of un-doped 100A/cm2 by about three orders of magnitude, and the 2

ACCEPTED MANUSCRIPT remnant polarization of the films at room temperature was 70µC/cm2, which was attributed to the structural change caused by doping and the decrease of oxygen vacancies concentrations [14]. Hu et al. used Zn and Ti to co-dope BFO films, the leakage current of the films was significantly reduced to 2×10-8 A/cm2 compared with

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un-doped 1×10-5 A/cm2, and the remnant polarization value at room temperature was 84 µC/cm2, which was attributed to the formation of oxygen vacancies and defects in the defective complex that reduced the oxygen vacancies concentrations [15]. Zheng et al. prepared Er, Co and Mn co-doped BiFeO3 thin films by chemical solution

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deposition method. The larger grain size and the dense uniform morphology were favorable for suppressing the leakage current and Bi0.90Er0.10Fe0.96Co0.02Mn0.02O3

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exhibited a high remnant polarization value (Pr ~104.43 µC/cm2) and excellent ferroelectric properties. The result showed the contribution of structural phase transition and reduction of oxygen vacancies caused by ternary doping [16]. Chai et al. prepared Sr, Gd, Mn and Co co-doped BiFeO3 thin films by sol-gel method. It was found that multi-doping could regulate the structural transition and oxygen vacancy

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concentration. The remnant polarization value of the film reached 108 µC/cm2, and the switching current was 1.4 mA. The improvement of ferroelectricity was attributed to the multi-doping to suppress the formation of oxygen vacancies, the reduction of

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the built-in electric field at the interface and the structural phase transition [17]. These reports indicated that multi-doping could reduce the leakage current of BFO films and improve the ferroelectric properties. The improved ferroelectric and ferromagnetic

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properties of the films by multi-co-doping are attributed to the structural phase transitions and the reduction of oxygen vacancies. Most ferroelectric studies were still based on the single remnant polarization value and the increase in the value of the polarization switching current. However, based on capacitance measurement, these researches on the contribution of the ferroelectric domain switching and the defect pairs to the ferroelectric properties of thin films are still less. According to the work of our research group on the doping substitution of BFO thin films [18-22], we prepared Bi0.9-xLaxEr0.1Fe0.96Co0.02Mn0.02O3 (BLaxEFMCO) films multi-co-doped with La, Er, Mn and Co, and the contribution of the refined 3

ACCEPTED MANUSCRIPT grain size, the oxygen vacancy concentration, the defects and the defect pairs and the effects of the build-in electric field formed inside the film on the ferroelectric domain at the grain boundary by multi-ion-doped regulation the ferroelectric polarization process to ferroelectric properties is investigated in detail.

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2 EXPERIMENTAL SECTION 2.1 Preparation

Bi0.9-xLaxEr0.1Fe0.96Co0.02Mn0.02O3 thin films were deposited on the FTO/glass substrates by using sol-gel method. With Bi(NO3)3‧5H2O (Bi excess 5%), Er(NO3)3‧6H2O,

Fe(NO3)3‧9H2O,

C4H6MnO4‧4H2O,

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La(NO3)3‧nH2O,

Co(NO3)2‧6H2O as raw materials, the BLaxEFMCO precursor solution was prepared

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at a molar ratio of 0.9-x:x:0.1:0.96:0.02:0.02, and acetic anhydride and ethylene glycol methyl ether at a volume ratio of 3:1 were used as a solvent to obtain a stable and uniform precursor with the concentration of 0.3 mol/L. The mixed solution was stirred at room temperature for 2 h and then allowed to stand for 12 h to obtain the final sol. The spin-coating speed and the time of the films prepared by the spin

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coating method were 4000 rpm and 15 s while the baking temperature and the time were 200℃ and 8 min. Then the as-prepared films were annealed at 550℃ for 10 min. The precursor was subjected to 13 times repeated the spin coating, baking and

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layer annealing to obtain desired thickness of the film. The circulation of an Au electrode with an area of 0.314 mm2 was sputtered on the film surface by using a

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small ion sputtering apparatus and annealed at 285℃ in order to investigate the electrical behavior.

2.2 Characterization

The structure of the thin film was analyzed by using a D/MAX-2200PC X-ray

diffractometer (Rigaku, Japan) using Cu Kα radiation at =1.5406 Å and the tube current was 40 mA, the tube voltage was 40 kV. The scanning speed was 7°/min, the step size was 0.02°, and the scanning range was 15°-70°. Raman spectroscopy measurement was conducted on a Horiba JYHR800 Raman system equipped with an Ar ion laser for excitation at 532 nm. The surface morphology and the thickness of 4

ACCEPTED MANUSCRIPT thin films were obtained by a field emission scanning electron microscopy (FE-SEM, Hitachi S4800, JEOL, Japan). The 0.314 mm2 Au electrode was prepared on the surface of thin films by sputtering using an SBC-12 ion sputtering instrument. X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos Ltd.) was used to examine the

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chemical bonding states of thin films and Al/Mg double anode X-ray was used. An ac impedance at 0.02 kHz~2 MHz and the capacitance-voltage measurements were performed using an Agilent E4980A concise LCR meter. The leakage current of the thin films was measured using an Agilent B2901A measurement system. The

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ferroelectric measurements were acquired using an aix ACCT TF-Analyzer 2000, the voltage range was 0~100V. The magnetic hysteresis loops were obtained using a

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superconducting quantum interference magnetic measuring system (SQUID MPMS-XL-7), the test temperature range was 2~400 K, the magnetic field was up to 7T, the high resolution was 0.2e, and the sensitivity was about 1×10-7 emu. 3 RESULT and DISCUSSION

Fig. 1(a) shows the XRD patterns of Bi0.9-xLaxEr0.1Fe0.96Co0.02Mn0.02O3

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(BLaxEFMCO) thin films. The diffraction peaks of the BLaxEFMCO thin films, which are consistent with the PDF standard card (JCPDS No. 20-0169), exhibit a twisted rhombohedral perovskite with an R3c space group. No secondary phases were

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detected. Fig. 1(b) shows a partial enlarged XRD patterns in the 2θ ranges of 21.5~24° and 45.5~47.5°. The diffraction peaks intensities of the (101) and (202) are gradually increased for BLaxEFMCO (x=0~0.07) while the thin film crystallinity is increased. It

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can be seen that the (202) diffraction peaks of all the thin films are shifted to a high angle, and the (101) diffraction peaks of BLa0.05EFMCO and BLa0.09EFMCO are obviously shifted to a high angle, too. According to the Bragg equation [23] 2dsinθ=nλ, the smaller ionic radius of La3+(1.06 Å) replace the position of the larger Bi3+(1.08 Å), the interplanar spacing is reduced, and the structure of the films is changed, indicating that the La3+ replaces part of the Bi3+ and enters the lattice of BFO.

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Fig.1. (a) XRD patterns of BLaxEFMCO thin films; (b) partial enlarged XRD patterns in the 2θ

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ranges of 21.5~24°and 45.5~47.5°

To further analyze the structural transitions of BLaxEFMCO thin films, Raman

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scattering spectra are used to investigate the effects of La3+ doping on the film structure. Fig. 2 depicts the Raman scattering spectra of the BLaxEFMCO thin films at room temperature. It can be seen that the nine vibrational peaks at 147, 171, 219, 466, 264, 304, 367, 523 and 611 cm-1 correspond to A1-1, A1-2, A1-3, E-3, E-4, E-6, E-7, E-8 and E-9 modes, respectively. The group theory indicates that the 10 atoms in the

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unit cell of BFO rhombohedral R3c space group give rise to 13 Raman phonon modes (Г=4A1+9E) in the zone center [24]. To obtain the exact peaks positions, the measured spectrum could be properly fitted by decomposing the fitted curves into the individual Lorentzian components [25]. With the increase of the doping concentration of La3+,

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the relative intensities of the vibration peaks of the A-mode are weakened and shifted to the low-frequency, and the A mode in the low frequency range is associated with

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the vibration of the Bi-O bond in the films [26]. It can also be seen that the doping of the Bi3+ contributes the Bi-O bond to change. The variation in the Bi-O bond changes the lattice parameters of the films. However, the intensities of the vibration peaks of E-4 and E-9 modes of BLaxEFMCO thin films are gradually increased and the changes in the E mode are attributed to the stretching and compression of the (Fe, Mn, Co)O6 octahedron, which indicates that the La3+ doping changes the vibration of the Bi-O bond and causes the Fe-O bond synergistic effects, leading to the changes of the structure of the films. These are in accordance with the results of the XRD analyses.

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Fig. 2 Raman scattering spectra of BLaxEFMCO thin films

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Fig. 3 shows the SEM surface morphologies of BLaxEFMCO thin films. The films exhibit the smooth and dense surface morphologies. When the La3+ doping

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concentration is less than 0.05, La3+ acts as a crystal nucleating agent and the nucleation rate is increased during the recrystallization of the films, and the growth rate is decreased to result in the grain refinement. When the doping concentration of La3+ is greater than or equal to 0.05, La3+ acts as a crystal growth agent. The crystal growth rate is larger than the nucleation rate of the crystal nuclei, and abnormally the

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grown grains appear. A large number of the fine grains gather around the grain boundaries of the large grains, and the grain boundaries are increased. The phenomenon is more obvious in BLa0.07EFMCO and BLa0.09EFMCO films. As the

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La3+ doping concentration is increased from 0.00 to 0.09, the average grain sizes (Dgs) are decreased from 93.95 nm to 60.64 nm (the inset of Fig.3(a)-(f)), further

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demonstrating that La3+ doping increases the number of the grain boundaries. The cross-sectional illustration in Fig.3(b) shows that the thickness of the BLa0.01EFMCO film is 550 nm, and the interfaces between the BLaxEFMCO thin films and the SnO2 substrate observed are clear, indicating that the interfaces do not present any remarkable solid phase reaction diffusion.

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Fig.3. BLaxEFMCO thin films: the surface morphology and the grain size distribution of the inset

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(a) x=0.00; (b) x=0.01 (the left illustration of the cross-sectional image); (c) x=0.03; (d) x=0.05; (e) x=0.07; (f) x=0.09;

An ac impedance spectrum of the BLaxEFMCO thin films at room temperature shown in Fig.4 suggests that the relationship between the real part (Z') of the complex impedance and the imaginary part (Z"). It can be seen that all the films have only one

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semicircle, indicating that only one portion (the grain or the grain boundary) in the film dominates the impedance of the sample. The impedance curves of different films are fitted, and the corresponding resistance values are obtained according to the intersection of the arc and the real axis as shown in Tab. 1. Rg and Rgb represent the

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resistance of the grain and the grain boundary, respectively. Cg and Cgb represent the capacitance of the grain and the grain boundary, respectively. It can be concluded that

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the simulated data are very well-combined with the experimental data. It can be clearly seen that a small difference is found to be in capacitance between the grain and the grain boundary in the BLaxEFMCO thin films. Compared with the Rg and Rgb of the BEFMCO film, the Rgs of BLaxEFMCO (x=0.01~0.09) thin films are reduced, the Rgb of the BLa0.01EFMCO thin film is decreased, the Rgbs of the BLaxEFMCO (x=0.03~0.09) thin films are increased, and the Rgbs of the BLa0.07EFMCO and BLa0.09EFMCO thin films are increased conspicuously. It is related to the abnormal growth of BLa0.07EFMCO and BLa0.09EFMCO films in Fig. 3(e) and 3(f). The grain boundaries are increased, the defects are easy to gather, and the leakage channels are 8

ACCEPTED MANUSCRIPT longer. The grain boundaries are mainly in the change of electrical properties. These phenomena indicate that the impedance of this part of the film is mainly derived from the contribution of the dielectric relaxation of the grain boundary, which is beneficial to the dense structure of the film to reduce leakage current. Therefore, La3+ doping

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plays a key role in the grain and the grain boundary resistance of the thin film.

Fig. 4 The AC impedance diagrams of BLaxEFMCO thin films Tab. 1 The AC impedance parameters of BLaxEFMCO thin films R(Ω)

Rg(Ω)

CPE-g(F)

Rgb(Ω)

CPE-gb(F)

0

4.2739×10-7

1.0254×107

2.181×10-10

2.8678×107

4.573×10-10

0.01

8.4705×10-7

8.4056×106

3.055×10-10

2.5179×107

5.841×10-10

0.03

25.36

7.0631×106

2.279×10-10

3.9266×107

3.409×10-10

0.05

38.17

4.4733×106

2.810×10-10

3.6544×107

4.398×10-10

0.07

18.82

5.8806×106

2.694×10-10

4.3825×107

3.786×10-10

19.6

7.8168×106

2.735×10-10

7.7904×107

3.538×10-10

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0.09

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Samples

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The XPS analyses of the BLaxEFMCO films are shown in Fig. 5. The full spectra

of the XPS of the films are shown in Fig. 5(a), and the photoelectron peaks corresponding to Bi, Fe and O, and the peaks corresponding to La 3d, Er 4p, Mn 2p and Co 2p can be clearly observed. Fig. 5(b) shows the chemical states of Fe in the BLaxEFMCO thin films. Two peaks correspond to Fe2+ and Fe3+ by Lorentz-Gauss fitting Fe 2p, respectively, whereas, the valence fluctuation and the change of oxygen vacancy concentration in both Fe2+ and Fe3+ can affect leakage current of the film. The ratios of Fe3+/Fe2+ in the BLaxEFMCO thin films (x=0~0.09) are calculated to be 18:82, 31:69, 23:77, 27:73, 28:72 and 33:67, respectively. These results manifest that 9

ACCEPTED MANUSCRIPT the amount of Fe2+ is decreased by the introduction of La3+. The formation of Fe2+ is accompanied by the appearance of oxygen vacancies to maintain the charge balance. The Equations are shown in (1) and (2), and thus Fe2+ is a sign as the presence of oxygen vacancies. + O ↔ 2Fe

+ V ∙∙ + O

(1)

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2Fe

2Bi + 3O ↔ 2V ′′′ + 3V ∙∙ + Bi O

(2)

Fig. 5(c) shows XPS spectra of Co 2p and Mn 2p orbitals of BLaxEFMCO

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(x=0~0.09) thin films. The valence states of Co2+, Co3+ and Mn2+ of Mn3+ are obtained by the fractional fitting. The proportions of Co2+/Co3+ and Mn2+/Mn3+ in the

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BLaxEFMCO (x=0~0.09) thin films are 58%, 69%, 61%, 64%, 66% and 70%, and 56%, 68%, 63%, 64%, 65%, and 73%, respectively. The valence fluctuations may occur in Co2+ and Mn2+ that can be expressed by Equations (3) and (4). It can be seen that Mn2+ and Co2+ can suppress the formation of Fe2+. Mn

↔ Fe

+ Mn

(3)

+ Fe

↔ Fe

+ Co

(4)

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Co

+ Fe

2MnO ↔ 2Mn

+ V ∙∙ + 2O×

(5) (6)

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2CoO ↔ 2Co

+ V ∙∙ + 2O×

Fig. 5(d) shows the O 1s peak schematic diagrams of BLaxEFMCO thin films. Two peaks of the O 1s orbital can be obtained by the peak-fitting of the O 1s orbital of

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the thin films. The peaks of the BLaxEFMCO (x=0~0.09) thin films are located at 529.45/530.41, 528.54/529.61, 528.81/529.88, 528.58/529.84, 528.98/529.92 and 529.51/530.58, respectively. The peak with the low binding energy corresponds to the lattice position of O2- at the film, and the peak with the high binding energy is correlated to the formation of oxygen vacancies. The peaks at the two binding energies indicate that oxygen vacancies generate in the BLaxEFMCO thin films. Compared with these fitted peak areas, the proportions of oxygen vacancies in the BLaxEFMCO (x=0~0.09) thin films are calculated to be 11%, 5%, 7%, 6%, 6% and 5%, respectively. It is obvious that the oxygen vacancies concentrations in the 10

ACCEPTED MANUSCRIPT BLaxEFMCO thin films are reduced because the substitution of La3+ for Bi3+ can reduce the volatilization of Bi3+ and the generation of oxygen vacancies, which is coincided with the tendency of Fe2+ (shown in Fig. 5(b)). The defects, such as V ′′′

and V ∙∙ are generated owing to the volatility of Bi3+ and the valence fluctuation of and Co

are formed and the multiple

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Fe3+ while the defects, such as Mn

valence ions are doped. The equations are shown in (3-5) and (3-6). To maintain the internal charge balance of the film, V ∙∙ and V ′′′ , and Mn ) -V ∙∙ and (Co

are

) -V ∙∙ defect pairs, further

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combined to form V ′′′ -V ∙∙ , (Mn

and Co

forming the built-in electric field (EIn) inside the film. Therefore, the reduction of

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oxygen vacancies and Fe2+ leads to the decrease in leakage current of the BLaxEFMCO thin films, and the formed defects affect the ferroelectric properties of the films.

Tab. 2 The orbital bonding energy of Fe 2p, Co 2p, Mn 2p and O 1s of the BLaxEFMCO thin films and the corresponding peak area Samples

0.03

529.45

530.41

780.81

786.34

639.53

640.87

Area(%)

83

18

89

11

41

58

56

44

Positions(eV)

708 .49

710.48

528.54

529.61

779.59

784.58

640.69

641.68

Area(%)

69

31

95

5

31

69

68

32

Positions(eV)

709.11

711.2

528.81

529.88

780.69

786.07

639.26

640.41

77

23

93

7

39

61

63

37

Positions(eV)

708.79

710.49

528.58

529.84

780.11

785.36

640.86

641.99

Area(%)

72

28

94

6

36

64

64

36

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0.07 0.09

Mn 2p

711.85

Area(%) 0.05

Co 2p

709.33

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0.01

O 1s

Positions(eV)

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0.00

Fe 2p3/2

Positions(eV)

709.3

711.38

528.98

529.92

780.76

785.90

641.42

642.19

Area(%)

72

28

94

6

34

66

65

35

Positions(eV)

709.7

711.52

529.51

530.58

781.25

786.31

640.8

641.75

Area(%)

67

33

95

5

30

70

73

27

11

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Fig. 5 XPS spectra of the BLaxEFMCO thin films: (a) full spectra; (b) Fe 2p; (c) Co 2p and Mn 2p; (d) O 1s;

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Fig. 6(a) plots the leakage current densities of the BLaxEFMCO thin films, and the inset shows the leakage current density curves of the magnified diagram of the ±175 kV/cm~±364 kV/cm electric field. The scanning voltage direction is +V !"

&

→ 0 → +V

!" 。The

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→ %V

!"

→0

leakage current densities are measured as 2.73×10-3

A/cm2, 9.09×10-4 A/cm2, 9.00×10-4 A/cm2, 9.63×10-4 A/cm2, 8.26×10-4 A/cm2 and

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5.14×10-4 A/cm2 for the BLaxEFMCO thin films at 200 kV/cm in +V

!"

→ 0,

respectively. It is obvious that the leakage current densities of the BLaxEFMCO thin films are gradually decreased and are smaller than that of the BEFMCO thin film as the concentration of La3+ doping is increased, indicating that La3+ can effectively inhibit oxygen vacancy and reduce the amount of Fe2+, and the average grain size is decreased (the illustration in Fig. 3 (a)-(f)), the grain boundary is increased, thereby effectively suppressing the leakage current [27]. Moreover, the poor symmetry of the leakage current density under positive and negative electric fields, and the minimum

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ACCEPTED MANUSCRIPT leakage current density at the non-zero electric field of the BLaxEFMCO (x=0.03, 0.05, 0.07, 0.09) thin films are obtained, which is due to the formation of the asymmetric interface barrier and the formation of the built-in electric field (EIn) caused by the defects inside the BLaxEFMCO films. In addition, the leakage current

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density curves of all the BLaxEFMCO thin films show the resistive switching behavior [28], and Fig. 6(b) shows the high-low resistive switching ratio (HRS/LRS) of all the films under the positive electric field. The resistive switching ratios of the BLaxEFMCO (x=0~0.09) thin films under the applied electric field (~100kV/cm) are

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calculated to be 1.1, 1.01, 1.30, 1.30, 1.12, 1.36, respectively. These ratios indicate that the resistance effects of the film are increased by La3+ doping, but the switching

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ratios are still smaller, which is due to the weak charged defect directional migration. To clearly understand the leakage current mechanisms of the BLaxEFMCO thin films, the bulk-limited ohmic mechanism shown in Equation (7), the space charge limited conduction (SCLC) shown in Equation (8), and the interface-limited Schottky emission shown in Equation (9) are analyzed. The formulas are as follows:

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'( =

*+,- .

(7)

345 4- +. 6

(8)

/

'0121 =

?* @ ⁄&A4- BDE FG H

I

(9)

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'8 = 9: ;<= >

7/

where U is the applied voltage; µ is the free carrier mobility; q and E are the basic charge and the applied electric field strength, respectively; no is the intrinsic carrier

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density due to the intrinsic defect; εr and εo are the static dielectric constant and the vacuum permittivity, respectively; d is the film thickness of the samples; T is the absolute temperature; k is the optical permittivity; Ф is the Schottky barrier height; KB is the optics permittivity; and A is a constant. The LogE~LogJ curves of the BLaxEFMCO thin films are obtained by the piecewise linear fitting processing. Fig. 6 (c) shows the LogE~LogJ curves of the thin films under the positive bias (+V

!"

→ 0). It is observed that all the films have a

linear slope close to 1 at the low electric field, which is the ohmic conduction

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ACCEPTED MANUSCRIPT mechanism. The ohmic conduction and the SCLC mechanisms govern the leakage behavior of the BLa0.01EFMCO and the BLa0.05EFMCO thin films at the high electric fields. However, the only SCLC conduction dominates the leakage behavior of the BLaxEFMCO (x=0.00, 0.03, 0.07 and 0.09) thin films at the high electric field. Fig. !" ).

It

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6(d) plots the LogE~LogJ curves of the thin films under negative bias (0 → %V

is observed that the thin films are ohmic conduction mechanisms at the low electric field; The BEFMCO and the BLa0.09EFMCO thin films are subject to the SCLC behavior at the high electric field, while the BLaxEFMCO (x=0.01, 0.03, 0.05, and

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0.07) thin films are identified to be the ohmic conduction and the SCLC conduction mechanisms at the high electric field. Fig. 6(e) shows the LogE~LogJ curves of the !"

→ 0). The fitting results are consistent with that

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thin films under negative bias (%V

of Fig. 6(c), the BLa0.01EFMCO film also exhibits the ohmic conduction mechanism at the high electric field. Fig. 6(f) shows the LogE~LogJ curves of the thin films under &

positive bias (0 → +V

!" ).

It is seen that the SCLC mechanism dominates the leakage

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behavior of the BEFMCO and the BLa0.09EFMCO thin films at the high electric field, and the ohmic conduction and the SCLC mechanisms are responsible for the

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BLaxEFMCO (x=0.01, 0.03, 0.05 and 0.07) thin films at the high electric field.

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Fig. 6 BLaxEFMCO thin films: (a) Leakage current density (illustrated as the leakage current density curves of the magnified diagram of the ±175kV/cm~±364 kV/cm electric field); (b) The resistive switching ratio under the positive electric field; (c), (d), (e) and (f) LogE~LogJ function diagrams of the BLaxEFMCO films under positive and negative electric fields in different processes (1,2,3 and 4).

The BLa0.01EFMCO and the BLa0.05EFMCO thin films exhibit the bulk conduction mechanisms (ohmic and SCLC) under the positive voltage and the high electric field, indicating that the conduction mechanisms of the BLa0.01EFMCO and the BLa0.05EFMCO thin films are mainly governed by oxygen vacancies and defects. The films exhibit the smaller Rgb and the less grain boundaries, and the lower oxygen 15

ACCEPTED MANUSCRIPT vacancy concentrations of the BLa0.01EFMCO and the BLa0.05EFMCO, 5% and 6%, respectively (as seen in Fig. 5(d)), forming the smaller built-in electric field (EIn) and based on the ohmic and the space charge conduction mechanisms. The BLa0.09EFMCO thin film exhibits the SCLC conduction mechanism under the positive

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voltage and the high electric field. Although the BLa0.09EFMCO film shows the lower oxygen vacancy concentration, 5% (shown in Fig. 5(d)), the Rgb of the thin film is larger, the grain boundaries are more, and the larger built-in electric field (EIn) is easily formed. As the applied electric field is increased, more and more electrons are

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injected into the BLa0.09EFMCO film, the larger built-in electric field (EIn) inside the film captures the free carriers and gradually transfers to the trap-limited space charge

leakage current is reduced.

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limiting current conduction, forming the SCLC conduction mechanism, and the

Fig. 7 shows the P-E loop and the I-E curves of the BLaxEFMCO (x=0.00~0.09) thin films, measured at room temperature and with the frequency of 1 kHz together in the applied electric field of 818 kV/cm. The well-symmetric saturated P-E hysteresis

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loop with a remnant polarization (Pr) of ~152 µC/cm2 and the coercive filed (Ec) of ~302 kV/cm of the BLa0.01EFMCO thin film are obtained. It is of great interest to note that the coercive field of the P-E loop corresponds to the switching current peak of the

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I-E curve. The switching current value of the BLa0.01EFMCO thin film reaches a maximum (1.50 mA), which can be attributed to the decrease in the grain boundary, the decrease in Rgb and the smaller built-in electric field (EIn) formed by the defect

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make the BLa0.01EFMCO film ferroelectric domain easily switched by La3+ replacing Bi3+. The remnant polarization values of the BLaxEFMCO thin films are decreased and the switching current peaks are widened with the further increasing concentration of La3+ doping, which can be well-explained that the excessive doping induced the grain size refinement and the increase in grain boundaries (shown in Fig.3). Vacancies and charged defects tend to accumulate the vicinity of the grain boundaries, and thus oxygen vacancies and defects easily form the defect pairs [29], forming the large built-in electric field (EIn). These defect pairs can pinch the ferroelectric domains, the ferroelectric domains can not follow the change of the applied electric field. The 16

ACCEPTED MANUSCRIPT oxygen vacancies, the defects and the defect pairs form the build-in electric field (EIn) to compensate for the internal charge balance of the film and reduce the polarization of the applied electric field, resulting in the lower ferroelectricity of the BLaxEFMCO (x=0.03~0.09) film than that of the BEFMCO film.

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The ferroelectricity of the BLaxEFMCO thin films can also be described by using the hysteresis loop rectangle (Rsq) [30]. Rsq=(Pr/Ps)+(P1.1Ec/Pr), where Pr is the remnant polarization at the zero electric field, Ps is the saturation polarization, and P1.1Ec is the polarization at the electric field strength of 1.1Ec. For an ideal hysteresis loop, the

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squareness is 2. As summarized in Tab. 3, the Rsq values of BLaxEFMCO are decreased dramatically from ~1.03 to 0.58, indicating that with the further

ferroelectricity is deteriorated.

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introduction of La3+, the ferroelectricity of the film is suppressed and the

Tab. 3 The related ferroelectric parameters of the BLaxEFMCO thin films x=0

x=0.01

x=0.03

x=0.05

x=0.07

x=0.09

Is (mA)

1.30

1.50

0.45

0.42

0.34

0.28

Pr (µC/cm2)

115

152

82.4

84

75

57

Ec (kV/cm)

308.5

302

352.1

281.6

245

214

Ps (µC/cm2)

135.7

181.7

106

119.5

119.7

110.8

Rsq

1.06

1.03

0.98

0.85

0.71

0.58

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Samples

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Fig.7. The ferroelectric hysteresis loop (P-E) and the corresponding polarization current curves (I-E) of the BLaxEFMCO thin films at room temperature and with the frequency of 1 kHz: (a) x=0;

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(b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.07; and (f) x=0.09;

The capacitance-voltage loops (C-V) of the BLaxEFMCO thin films at 1 kHz at

room temperature are shown in Fig. 8. A significant symmetry to a butterfly curve is observed in the BLa0.01EFMCO thin film under positive and negative electric fields and two peaks characterize the spontaneous polarization domain switching. The maximum values of two significant capacitance peaks under positive and negative electric fields correspond to the maximum coercive voltage which indicates the BLa0.01EFMCO film with the remarkable ferroelectricity and the switching completely ferroelectric domain. The butterfly curve with a roundish shape under 18

ACCEPTED MANUSCRIPT positive and negative electric fields and the maximum value of the capacitance peak at the positive applied voltage region of the BEFMCO, the BLa0.05EFMCO together with BLa0.03EFMCO thin films are obtained, indicating that there is an additional capacitance due to oxygen vacancies, defects and defect pairs inside the film [31]. It

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can be interpreted that the switching of the ferroelectric domains are suppressed by the higher oxygen vacancy concentration inside the BEFMCO film, the large grain size and the large domain size. The leakage current is large. However, the grain sizes are refine, the grain boundaries are increased and oxygen vacancy concentrations of

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the BLa0.03EFMCO and the BLa0.05EFMCO films are decreased by La3+ doping (seen in Fig. 5(d)). The oxygen vacancies and the charging defects tend to accumulate

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around the grain boundaries, forming the defect pairs easily, which will pinch more ferroelectric domains to cause the ferroelectric domains to be difficult to turn over, and the film polarization process is hindered. The true ferroelectric properties of the BLa0.01EFMCO film are further explained. Due to more moving ions or charge accumulation at the interface of the film-electrode, the significant asymmetry to the

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butterfly curves of the BLa0.07EFMCO and the BLa0.09EFMCO thin films at positive and negative voltages is observed and the grain size is sharply refined and abnormally grown. The grain boundary is increased, and the pinning behavior of the defect pairs

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gathered around the grain boundary makes the ferroelectric domain switching process more difficult, increasing the applied voltage to cause the polarization switching of the ferroelectric domain to be hindered intensely. This further verifies the introduction

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of excess La3+, the ferroelectricity of the BLaxEFMCO films is suppressed, and the ferroelectricity is deteriorated.

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Fig. 8 The capacitance-voltage (C-V) loop of the BLaxEFMCO thin films at room temperature and

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with the frequency of 1 kHz: (a) x=0; (b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.07 and (f) x=0.09;

The ferroelectric enhancement mechanism diagram of the BLaxEFMCO thin

films is plotted in Fig. 9. It can be seen from the upper part of Fig. 10 that the replacement of Bi3+ by La3+ affects the particle size of the films [32-33]. The grain size of the BLaxEFMCO films is reduced, the growth of the ferroelectric domain is inhibited, the grain boundary is increased, the charge defects are easy to gather around the grain boundary, the grain boundary resistance (Rgb) of BLa0.01EFMCO is decreased, and the grain boundary resistance (Rgb) of BLa0.09EFMCO is increased. Due to the volatilization of Bi3+ and the valence state fluctuation of Fe3+, the large 20

ACCEPTED MANUSCRIPT built-in electric field (EIn) is formed by a large number of oxygen vacancies, defects and defects pairs are generated in the film, and the EIn of BLa0.01EFMCO is smaller than that of BLa0.09EFMCO (the blue double-headed arrow marked in the middle of Fig. 10). The BLa0.01EFMCO thin film exhibits a saturated hysteresis loop and a

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polarization current induced by a large polarization switching and a capacitance peak induced by the switching of the ferroelectric domain (as seen in Fig. 9 with a red asterisk mark), indicating that the voltage of the capacitance switching peak of the C-V coincides with the voltage of the ferroelectric polarization current switching peak

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of I-V (shown in Fig. 9 with the black polarization current curve and the red C-V curve), which shows the BLa0.01EFMCO film with the real and excellent ferroelectric

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properties. The grain size, the remnant polarization and the polarization current of the BLa0.09EFMCO thin film are significantly reduced, and the additional capacitance peaks (marked by the red hearts in Fig. 9) are also observed, and the remnant polarization value of the BLa0.09EFMCO film is 57 µC/cm2 (the green hysteresis loop in Fig. 9). With the increase of La3+ doping concentration, the grain size of the

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BLaxEFMCO films is decreased continuously, the grain boundaries are increased, the defects are accumulated around the grain boundary increase, the pinning effects of ferroelectric domains are increased, and the ferroelectric domain switching is

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suppressed (the C-V butterfly curves at positive and negative voltages exhibit the significant asymmetry in Fig. 8), resulting in the lower remnant polarization values of the BLaxEFMCO films. In addition, since the grain boundary is a paraelectric phase

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[34], the grains in the film indicate the discontinuity of polarization [35], so that the charge caused by the polarization of the BLaxEFMCO films cannot be balanced or compensated, and the built-in electric field (EIn) in the opposite direction of the BLaxEFMCO films is generated [36]. The applied electric field (E) compensates for the internal charge balance of the film, which also reduces the remnant polarization value. As a result, the polarization current peaks of the BLaxEFMCO films are broadened, the instantaneous current peak values caused by the polarization switching are gradually reduced, and the switching of the ferroelectric domain is suppressed. The BLaxEFMCO films exhibit an additional capacitance peak and the capacitance 21

ACCEPTED MANUSCRIPT peak symmetries at positive and negative voltages are decreased. The narrow, inclined and asymmetrical hysteresis loops are observed. Therefore, the multi-ion doped BFO film can make the doped element reduce the grain boundary resistance (Rgb) and lower the interface Schottky barrier formed by the asymmetric electrode material at

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the top and the bottom of the films. The leakage current is subjected to the ohmic and the space charge limited conduction mechanism and the built-in electric field formed by defects or defects in the film is decreased. In this way, the ferroelectric properties of the BFO film can be improved by the multi-doping. Otherwise, only the grain size

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is reduced, and the grain boundary resistance (Rgb) is not considered. The magnitude of the built-in electric field can inhibit the switching of the domain to realize the

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ferroelectric properties-enhancing regulation of the BFO film.

Fig. 9 Ferroelectric enhancement mechanism diagram of the BLaxEFMCO (x=0.01, 0.09) thin film

Fig. 10 depicts the magnetic hysteresis loops (M-H) of the BLaxEFMCO

(x=0.00~0.09) thin films at room temperature. The inset of the top shows the enlarged M-H curves of the BLaxEFMCO thin films at -200Oe~200Oe and the bottom shows the room temperature remnant magnetization value and the saturation magnetization value (Hc~2500 Oe) as a function of La3+ concentration. The weak ferromagnetism of the BLaxEFMCO thin films at room temperature is observed. The remnant 22

ACCEPTED MANUSCRIPT magnetization values and the saturation magnetization values at the magnetic coercive field of 2500 Oe of BLaxEFMCO (x=0.00~0.09) thin films are measured as 0.31, 0.28, 0.29, 0.26, 0.19 and 0.30 emu/cm3 and 2.12, 2.21, 2.15, 1.82, 1.51 and 2.33 emu/cm3, respectively. The BLa0.09EFMCO thin film exhibits the enhanced magnetization

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(Ms~2.33 emu/cm3, Mr~0.30 emu/cm3) which is mainly due to the substitution of La3+ for Bi3+, whereas the doping induced the magnetic spin cycloid structure can inhibit the special spatially modulated helical magnetic structure to a certain extent [37-39],

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thereby releasing the part of the magnetic properties.

Fig.10. The magnetic hysteresis loop of the BLaxEFMCO thin films at room temperature: the top of the illustration showing enlarged magnetic hysteresis loop at -200 Oe~200 Oe, the bottom of

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the inset representing remanent magnetization value and the saturation magnetization value (at the coercive field ~2500 Oe) with the increasing concentration of La3+

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4 CONCLUSION

Bi0.9-xLaxEr0.1Fe0.96Co0.02Mn0.02O3 (BLaxEFMCO, x=0.00~0.09) thin films were

successfully prepared by sol-gel method. With the increase of La3+ doping concentration, the grain size of BLaxEFMCO film is decreased continuously, the grain boundaries are increased, the defects accumulated around the grain boundary are increased, the pinning effects of ferroelectric domains are increased, and the ferroelectric domain switching is suppressed, making the remnant polarization values of the BLaxEFMCO thin films lower. The charges caused by polarization of the BLaxEFMCO films cannot be balanced or compensated, and the built-in electric field 23

ACCEPTED MANUSCRIPT in the opposite direction is generated in the BLaxEFMCO films. The applied electric field (E) compensates the internal charge balance of the film, and also reduces the remnant polarization value of the film. The synergistic effects of the two one lead to the widening of the polarization current peaks of the BLaxEFMCO films, the peak

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value of instantaneous current generated by polarization switching is gradually reduced, and the ferroelectric domain switching is suppressed, showing the additional capacitance peaks and the capacitance peaks with the reduced symmetry at positive and negative voltages are the narrow, tilted and asymmetrical hysteresis loops.

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Therefore, the doped element can reduce the grain boundary resistance (Rgb) by multi-ions doping the BFO film, and the interface Schottky barrier formed by the

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asymmetric electrode material at the top and the bottom of the film. The leakage current flow mechanism is subjected to ohmic and the space charge conduction and the built-in electric field formed by defects or defects in the film can enhance the ferroelectric properties of the BFO films. The remnant polarization value of the BLa0.01EFMCO film with the minimum grain boundary resistance (Rgb) reaches 152

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µC/cm2, the hysteresis loop rectangle (Rsq) is 1.03, and the maximum switching current is 1.50 mA. Meanwhile, a typical butterfly under positive and negative electric fields further indicates that the film exhibits the good ferroelectric properties. Furthermore, the BLa0.01EFMCO thin film shows the enhanced magnetic properties,

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and its saturation magnetization (Ms) is 2.32 emu/cm3. ACKNOWLEDGMENTS

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This work is supported by the Shaanxi Province Key Research and Development

Plan (2018GY-107); the Project of the National Natural Science Foundation of China (Grant No. 51372145); the Major Research Projects of the Ministry of Science and Technology of China (2017YFC0210803); the Graduate Innovation Fund of Shaanxi University of Science &Technology (SUST-A04). REFERENCES [1]

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