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CoFe2-xLaxO4 bilayer thin films
Ceramics International 45 (2019) 19504–19512
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Regulation of defect complexes at the interface for improving multiferroicity of Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 bilayer thin films
T
Mintao Xuea, Guoqiang Tana,∗, Ao Xiaa, Zhengjun Chaia, Long Lvb, Huijun Renc, Xixi Rena, Jincheng Lia a
Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an, Shaanxi, 710021, China b College of Cryptography Engineering, Engineering University of PAP, Xi'an, 710086, China c School of Arts and Sciences, Shaanxi University of Science and Technology, Xi'an, Shaanxi, 710021, China
ARTICLE INFO
ABSTRACT
Keywords: BiFeO3/CoFe2O4 Defect complexes Pinning effect Multiferroicity
This work presents the Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 (BLSFMC/CFLxO) multiferroic bilayer films with Au top electrode on the fluorine doped tin oxide (FTO) by sol-gel method. And the change in structure and properties of BLSFMC/CFLxO films are investigated. The results show that the upper layer of BLSFMC is induced structural distortion by doping La3+ in the underlying CFO and the R3m: R structural phase content increases. La3+ blocks the diffusion of Fe2+ ions into the BLSFMC layers, reduces the oxygen vacancies concentration, and weakens the valence fluctuations of Fe, Mn, and Co ions in the upper BLSFMC layer, decreasing the number of defect complexes and improving the insulation properties of the BLSFMC/CFLxO bilayer. The pinned ferroelectric domain at the BSLFMC/CFL0.03O interface is reduced, and the ferroelectricity of the BLSFMC layer is released. The residual polarization of BLSFMC/CFL0.03O Pr = 136.82 μC/cm2, the squareness ratio Rsq = 1.20 and the saturation magnetization Ms = 90 emu/cm3 compared with the single-phase BLSFMC thin films, BLSFMC/CFL0.03O bilayer thin films exhibits excellent ferroelectricity and has a large amount of enhanced ferromagnetism. The structure and multiferroicity properties of the bilayer films can be adjusted by the underlying La3+ dopant.
1. Introduction As a typical multiferroicity material, BiFeO3 exhibits ferroelectricity and G-type antiferromagnetic coexistence at room temperature, with a high ferroelectric Curie temperature (1100 K) and Néel temperature (653 K) [1–4]. Moreover, its ferromagnetic surface and ferroelectric polarization direction (⟨111⟩) are perpendicular to each other, its Gtype antiferromagnetic and ferroelectricity are coupled, and BiFeO3 has been widely used in applications such as spintronics, sensors, and tunable microwave devices [5], making it the compound of choice for many researchers [6–8]. The Bi3+ and Fe2+ ions in BiFeO3 lead to poor insulation in films, which generates a high leakage current density, affecting the practical application of ferroelectric polarization. Researchers commonly dope BiFeO3 films with rare earth elements such as La, Er, Gd, and Pr in the A site of BiFeO3 [9–12] to reduce the oxygen vacancies concentration and the leakage current density, improving the films’ insulation properties and hence their ferroelectricity. Transition metal elements such as Ti, Mn, Ni, and Co are doped in the B site to
∗
allowing the super-exchange of inhibit the valence state of Fe3+ , Fe–O–Fe bonds to improve the antiferromagnetic properties of BiFeO3 [13–16]. Rajput et al. [14] prepared BiFeO3 films doped with Ni and La, demonstrating that an optimal content of La and Ni reduced the oxygen vacancies concentration and significantly reduced the leakage current density (J ~ 10−4 A/cm2). Which displays a well-shaped hysteresis loop, showing enhanced remanent polarization values (Pr ~ 66 μC/ cm2) and a lower coercive field (Ec ~ 0.3 MV/cm). Yue et al. found the structural phase transition of BiFeO3 with Pr, Dy, and Mn as dopants, and the relationship between defects and grain boundary resistance and its effect on leakage conductance were studied. The incorporation of Dy3+ can improve the antiferromagnetic properties of BiFeO3 (Ms ~ 2.39 emu/cm3) [17]. Guo et al. studied the influence of the codoping of Ho, Sr, Mn, and Ni BiFeO3: the defects and oxygen vacancy concentration in the films were reduced, the BiFeO3 films exhibited enhanced ferroelectricity (Pr ~ 193 μC/cm2) under the Fowler-Nordheim (FN) tunneling mechanism, and the presence of Mn4+ released the Fe2+ concentration. The super-exchange of Fe bonds enhances the
Corresponding author. E-mail address: [email protected] (G. Tan).
https://doi.org/10.1016/j.ceramint.2019.07.038 Received 26 April 2019; Received in revised form 2 July 2019; Accepted 3 July 2019 Available online 03 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19504–19512
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antiferromagnetic properties of BiFeO3 (Ms ~ 5.78 emu/cm3) [18]. Ion doping of BiFeO3 mainly results in changes in its structural phase, reducing film defects and leakage current density and improving the ferroelectricity of BiFeO3 to allow the super-exchange of Fe–O–Fe bonds, thus improving film ferromagnetism. However, in BiFeO3 the Fe–O–Fe bond super-exchange effect gives rise to weak ferromagnetism which can be problematic in applications. The researchers further constructed a ferroelectric/ferromagnetic (FE/FM) multiferroicity bilayer film. Sone et al. [19] prepared multiferroicity bilayer films of BiFeO3/CoFe2O4. After compounding the ferromagnetic phase of CoFe2O4, the ferromagnetism of the films was significantly enhanced, and its saturation magnetization was as high as 118 emu/cm3, but the ferroelectricity of the BiFeO3/CoFe2O4 multiferroic heterojunction was weak. C.M. Raghavan et al. find Structural change in (Bi0.95La0.05) (Fe0.97Cr0.03)O3/CoFe2O4 by X-ray diffraction and Raman scattering and low order of leakage current density and enhanced magnetic performance were observed in double layered thin films [20]. Based on results of the multi-doping of BiFeO3, researchers have proposed combining multi-doped BiFeO3 films [21] with the CoFe2O4 ferromagnetic phase to construct Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe23+ xLaxO4 multiferroicity bilayer films, and through the incorporation of La in the underlying CoFe2O4, to regulate the oxygen vacancies concentration defects and built-in electric. These steps improve the films’ ferroelectricity and ferromagnetic properties, making BiFeO3 (ferroelectric layer) and CoFe2O4 (ferromagnetic layer) more compatible and optimizing the multiferroic properties of multiferroicity bilayer films. 2. Experimental Using the sol-gel method to prepare Bi0.79La0.18Sr0.03 Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 (BLSFMC/CFLxO, x ~ 0.00–0.15), multiferroicity bilayer films were placed on an F-doped SnO2 conductive glass substrate. Firstly, Co(NO3)3·6H2O, La(NO3)3, and Fe (NO3)3·9H2O were dissolved in a precursor solution of 2-methoxyethanol and acetic anhydride (3:1 vol ratio) to obtain a 0.2 mol/L precursor solution of CFLxO, and Bi(NO3)3·5H2O (5% excess), La(NO3)3, Sr(NO3)3, Fe(NO3)3·9H2O, C6H9MnO6·2H2O, and Co(NO3)3·6H2O were dissolved in the precursor solution of 2-methoxyethanol and acetic anhydride (3:1 vol ratio) to obtain a 0.2 mol/L precursor solution of BLSFMC. The precursor solution of CFLxO was spin-coated on the glass substrate for 13 s at 3700 r/min, and baked at 185 °C for 9 min, then annealed at 590 °C for 20 min. The above steps were repeated six times to obtain CFLxO films. Then, the precursor solution of BLSFMC was
spin-coated on the CFLxO films at 3700 r/min for 13 s, baked at 185 °C for 9 min, and annealed at 545 °C for 10 min, a cycle repeated 13 times to obtain BLSFMC/CFLxO multiferroicity bilayer films. Au was sputtered on an area of 0.03 mm2 on the top of the bilayer films as the top electrode and annealed at 285 °C for 15 min. X-ray diffraction (XRD) using a Rigaku D/MAX-2200 was employed to characterize the films’ crystal structures and MAUD Rietveld software was used to analyze the XRD data to further determine crystal structure changes. Raman spectroscopy was performed using a A Horiba JYHR800 Raman test system equipped with a 532 nm Ar-ion laser. Monitoring of the leakage current density of the bilayer films was performed using an Agilent B2901A source/measure unit, and the hysteresis loops of the samples were obtained by an aixACCT TFAnalyzer 2000. SQUID-MPMS-XL magnetic measurement characterized the magnetic properties of the bilayer films. 3. Results and discussion Fig. 1 shows the XRD pattern of the BLSFMC/CFLxO multiferroicity bilayer films. Because the BLSFMC ferroelectric layer masks the information of the CFLxO layer, the XRD pattern mainly shows the diffraction peak of BLSFMC, corresponding to the standard card JSPDS No.72–2112 belonging to the perovskite structure. None of the XRD patterns showed a hetero peak, indicating that the bilayer films did not have a miscellaneous phase. Fig. 1(b) is a partially enlarged view of the BLSFMC/CFLxO bilayer films with 2θ ~ 31.5–33°. The (110) and (1–10) double peaks of the BLSFMC layer become gradually weakened and shift to a higher angle with increased La3+ doping in the CFLxO layer. This is because La3+ influences the upper BLSFMC structure after the incorporation of CFLxO, and the mismatched lattice constant causes structural distortion of the BLSFMC layer. In order to further study the structural changes of BLSFMC/CFLxO bilayer films, Rietveld was used to fit the XRD data with MAUD. The results are shown in Fig. 1(c) and Table 1. The BLSFMC in BLSFMC/CFO bilayer films consists mainly of two phases: R3c:H(84%) and R3m:R (16%). With the incorporation of La3+ in the underlying CFO, the R3c:H content in the upper BLSFMC decreases gradually, and the content of R3m:R increases, which indicates that the La3+ not only changes the intensity and growth orientation of the upper BLSFMC diffraction peak but also its structural phase; this structural change affects the multiferroicity properties of bilayer films [22]. Fig. 2(a) shows the XRD pattern of CFLxO films, corresponding to the standard card JSPDS No. 22–1086, which belongs to the cubic
Fig. 1. (a) and (b) X-ray pattern of BLSFMC/CFLxO bilayer films. (c) BLSFMC/CFLxO bilayer films Rietveld diagram.
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Table 1 BLSFMC/CFLxO bilayer films Rietveld data. Samples
Crystal structure
BLSFMC/CFO
Trigonal
BLSFMC/CFL0.03O
Trigonal
BLSFMC/CFL0.06O
Trigonal
BLSFMC/CFL0.09O
Trigonal
BLSFMC/CFL0.12O
Trigonal
BLSFMC/CFL0.15O
Trigonal
Space group
Lattice parameters
R3c:H(84%) R3m:R(16%) R3c:H(67%) R3m:R(33%) R3c:H(57%) R3m:R(43%) R3c:H(42%) R3m:R(58%) R3c:H(24%) R3m:R(76%) R3c:H(9%) R3m:R(91%)
R(%)
a
c
5.60 3.86 5.54 3.91 5.59 3.98 5.58 3.95 5.58 3.98 5.57 3.96
13.74
12.32%
14.09
13.21%
14.21
9.84%
13.96
10.67%
14.08
11.87%
13.99
9.68%
Fig. 2. (a) XRD pattern of CFLxO films. (b) Raman spectrum of CFLxO films.
inverse spinel structure; no impurity phase was found in the XRD pattern. The intensity of the (311) characteristic peak of CFLxO is gradually weakened and broadened as La3+ is incorporated into, the (400) characteristic peak at 43° and the (511) characteristic at 57° also appear reduced in intensity after La3+ doping in CFO. The above is mainly due to the different ionic radii of La3+ and Fe3+ after La3+ doping into CFO it causes lattice distortion of CFO films. Fig. 2(b) shows the Raman spectrum of the CFLO films. The CFO has five Raman active modes (Eg +2A1g+2T2g). As shown in Figure 2b, 306 and 473, 572, 616, and 694 cm−1 correspond to Eg, T2g (2), T2g (1), A1g (2), and A1g (1), respectively. As La3+ is doped into the CFO films, the A1g (1) activemodes peak at 694 cm−1 shows a significant decrease in intensity. The active mode of A1g is related to the Fe–O bond, indicating that the position of Fe3+ is replaced by that of La3+, which causes the tensile active mode strength of the Fe–O bond to weaken. Moreover, the Eg active mode in the low-frequency region is also weakened in strength. The change in the Eg active mode is mainly due to the change in the Fe–O octahedral structure, which causes the Co–O bond to change in the CFO. The above findings further confirm that the incorporation of La3+ causes lattice distortion of the CFO films. Fig. 3 is an SEM image of BLSFMC/CFLxO bilayer films, and the inset shows the change in grain size. The inset of Fig. 3(a) is a cross-sectional SEM image of BLSFMC/CFO. It can be seen that the thickness of the bilayer films is about 520 nm, and the average grain size of the BLSFMC/CFLxO (x ~ 0.00–0.15) bilayer films is 93 nm, 144 nm, 138 nm, 158 nm, 88 nm, and 122 nm, respectively. The upper grain size changes after La3+ is doped into the underlying CFO. Compared with the case without La3+ doping, the BLSFMC/CFLxO films have a broader surface grain size distribution. The BLSFMC/CFL0.03O
bilayer films exhibit a larger average grain size because, after the underlying CFO is doped with La3+, less Fe2+ diffuses into the upper layer to change the structure of the BLSFMC layer. The BLSFMC/ CFL0.12O bilayer films of 0.12 have a small average grain size because excess La3+ incorporated into the underlying CFO will diffuse into the upper layer, causing a change in the BLSFMC layer structure. In addition, the dot elemental analysis of the cross-section was performed by energy dispersive spectroscopy (EDS), as shown in Fig. 3(g and h) and the inset table is the concentration of the elements it was detected that La, Sr, Mn, Co existed in films. The molar ratio of the upper film is Bi0.93La0.22Sr0.05Fe1.35Mn0.06Co0.22O2.17(x = 0.00) and Bi0.94La0.22Sr0.04Fe1.33Mn0.06Co0.23O2.18(x = 0.03), it can be seen the contents of Sr, Mn is close to our expectation. However, because of ion diffusion from underlying CFLxO, the contents of Bi, Co, Fe, La of upper BLSFMC is quite different from our expectation. To further analyze the changes in oxygen vacancies and defects in the BLSFMC/CFLxO bilayer films, XPS analysis was performed, and the data were fitted and analyzed using Avantage software, as shown in Fig. 4. Fig. 4(a) shows the XPS spectra of the BLSFMC/CFLxO bilayer films, and the peak position of La3+ clearly confirms the presence of La3+. Fig. 4(b) shows the XPS peak-fitting result of Fe 2p in the bilayer films. The fitting of Fe 2p is divided into two parts, representing Fe2+ and Fe3+. The concentration ratio of Fe3+/Fe2+ in the bilayer films is 39:61, 65:35, 61:39, 56:44, 46:54, and 35:65, and the formation of Fe2+ defects is inhibited by the incorporation of La3+. The bilayer films have the lowest Fe2+ concentration when the doping amount is 0.03; Fe2+ concentration affects the valence fluctuation of Mn and Co. It can be represented by the chemical formulas (1-1) and (1–2) [23]:
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Fig. 3. SEM image of BLSFMC/CFLxO bilayer films(a-f), the dot elemental analysis of BLSFMC/CFLxO bilayer films by EDS spectra(g-f) and the inset table show the concentration of the elements.
Fe3 + + Mn2 +
Fe 2 + + Mn3 +
(1-1)
Fe3 + + Co2 +
Fe 2 + + Co3 +
(1–2)
Fig. 4(c) and (d) show the Mn-2p and Co-2p peaks in the BLSFMC/ CFLxO bilayer films with the fitted peaks: Mn3+/Mn2+ are 33:67, 44:56, 35:65, 42:58, 41:59, and 32:68. Co3+/Co2+ are 37:63, 46:54, 34:66, 42:58, 43:57, and 35:65 respectively. The decrease in Fe2+ concentration can affect Mn2+ and Co2+. The bilayer films have a small concentration of Mn2+ and Co2+ at a doping amount of 0.03, and the oxygen vacancy concentration depends mainly on the unstable chemical valence of Fe element in BFO. The formation of O vacancies can be expressed by the following chemical formula (1–3) [24]:
4FeFe + 2OO
2FeFe + 2Vo¨ + O2
(1–3)
Fig. 4(e) shows the results of fitting the peak of O1s of the BLSFMC/ CFLxO bilayer films. The ratios of oxygen ion/oxygen vacancies are 91.32:8.68, 97.58:2.42, 95.47: 4.53, 94.33:5.67, 95.87:4.13, and 93.68:6.32. It can be seen that with La3+ incorporated into the bilayer films, the oxygen vacancies concentration is reduced; the bilayer films have the lowest oxygen vacancies concentration when the doping amount is 0.03, which is consistent with the above XPS analysis results. The incorporation of La3+ affects the valence fluctuations in the bilayer films, which further reduces the concentration of the medium defect
2 + −V ··], [Co 2 + −V ··] complex in bilayer films, such as [V Bi −V o··], [MnFe 3+ o o Fe3 + et al.; this will increase the insulation of the films and affect their ferroelectric properties. Fig. 5(a) shows the room temperature leakage current density of bilayer films, and the inset is a schematic diagram of the test model. The leakage current density of the BLSFMC/CFO bilayer films is higher because the lower Fe2+ ions diffuse into the BLSFMC layer. From the above analysis, it can be deduced that Fe2+ increases the formation of oxygen vacancies, affecting the Mn and Co valence states, as well as increasing the number of defect complexes at the interface of BLSFMC/ CFLxO [25–27]. These defect complexes form a conductive path inside the films, creating a higher leakage current density. The Fe3+ concentration is reduced after the underlying is doped with La3+, and a lower Fe2+ concentration reduces the oxygen vacancy concentration; this inhibits the fluctuations of the valence states of Mn and Co and reduces the number of defect complexes at the interface of the bilayer films. The insulation of the films is enhanced, and the thin films leakage current will decrease. Fig. 5(b) shows the leakage current density for different doping concentrations and an applied electric field of 200 kV/ cm. The leakage current densities of the films are 0.026 A/cm2, 0.005 A/cm2, 0.012 A/cm2, 0.018 A/cm2, 0.014 A/cm2, and 0.011 A/cm2. There is minimal leakage current density in the BLSFMC/CFL0.03O films, mainly because they have a larger average grain size, the lowest
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Fig. 4. (a) XPS spectra of BLSFMC/CFLxO bilayer films. (b) Fe 2p analysis. (c) Mn 2p3/2 analysis. (d) Co 2p3/2 analysis. (e) O1s analysis.
oxygen vacancy concentration, and fewer defective complexes. Fig. 6 shows the room temperature hysteresis loops and switching current curves of the BLSFMC/CFLxO bilayer films. The BLSFMC/CFO films exhibit a typical hysteresis loop and switching current curves. The performance parameters are shown in Table 2. The residual polarization of BLSFMC/CFO is Pr = 96 μC/cm2; the coercive field, Ec = 404 kV/cm; and the switching current peak Is = 0.59 mA.
Compared with our previous study, the residual polarization of BLSFMC films is Pr = 188 μC/cm2; the coercive field, Ec = 272.5 kV/cm; and the switching current peak Is = 0.89 mA [21] is lower. This is because after composite the CFO the Fe2+ of CFO diffusion into the upper BLSMFC, generating growth defects such as oxygen vacancy concentration in the BLSFMC, forming a conductive path inside the films and resulting in a large leakage current density. Moreover, some defect complexes were
Fig. 5. Leakage current density diagram of BLSFMC/CFLxO bilayer films.
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Fig. 6. Hysteresis loops and switching current curves of BLSFMC/CFLxO bilayer films.
Rsq = Pr Ps + P1.1Ec Pr
Table 2 Ferroelectric performance parameters of BLSMFC/CFLxO bilayer films. Sample
Pr(μC/cm2)
Ps(μC/cm2)
Ec(kV/cm)
Is(mA)
Rsq
x = 0.00 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15
95.54 136.82 93.16 121.11 104.58 83.18
105.06 145.86 100.99 129.38 112.59 89.53
400.57 425.51 402.12 424.52 422.54 444.46
0.59 0.69 0.54 0.72 0.62 0.46
1.17 1.20 1.16 1.20 1.19 1.18
formed at the BLSFMC/CFO interface, which would produce pinned ferroelectric domains. When the ferroelectric is polarization, it is not easy to switch. The above interaction limits the ferroelectric properties of the bilayer films [28]. When the La3+ content is 0.03, the BLSFMC/ CFL0.03O bilayer films have the largest residual polarization, Pr = 136 μC/cm2, and an enhanced switching current peak, Is = 0.69 mA, while maintaining a low coercive field, Ec = 408 kV/cm. The squareness of the hysteresis loop is an important feature of ferroelectricity. The calculation formula is as follows [29]:
The P1.1Ec is the polarization value corresponding to the 1.1Ec. The squareness of the bilayer films is shown in Table 2. When the La3+ doping amount is 0.03, the bilayer films have the highest squareness of 1.20; this is because after the Fe3+ is doped in the CFO. Corresponds to XPS analysis a lower Fe2+ concentration will reduce the oxygen vacancies concentration and the number of defect complexes at the interface of the bilayer films, as well as improving intrinsic film insulation. The reduction in the number of defect complexes and oxygen vacancies will reduce the pinning effect at the BLSMFC/CFLxO interface, allowing ferroelectric domains to switch easily during polarization, enhancing the ferroelectric properties of BLSFMC/CFL0.03O bilayer films. Fig. 7 is a graph showing the room temperature capacitance-voltage behavior of the BLSFMC/CFLxO films, to further characterize their ferroelectricity. All the bilayer films exhibit the typical butterfly curve of ferroelectrics, and the nonlinear capacitance-voltage relationship indicates the role of inversion of the ferroelectric domains during polarization. The voltage at the peak of the capacitor corresponds to the coercive voltage during ferroelectric polarization. The coercive voltages
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Fig. 7. Capacitance-voltage curves of BLSFMC/CFLxO bilayer films.
Fig. 8. Schematic of ferroelectric enhancement of BLSFMC/CFLxO bilayer films.
of BLSFMC/CFLxO are 2V, 2V, 4V, 2V, 4V, and 4V, (x = 0.00, 0.03, 0.06, 0.09, 0.12, and 0.15) respectively, and the bilayer films have a higher capacitance peak after the underlying is doped with La3+, indicating ferroelectric enhancement. Compared with other bilayer films the BLSFMC/CFL0.03O bilayer films have the highest capacitance peak, maintaining the coercive voltage of 2V and demonstrating an enhanced ferroelectricity. The BLSFMC/CFO bilayer films have a wider capacitance peak (black circle area in Fig. 7(a)) under the forwarding bias due to a higher oxygen vacancy concentration, and defects limit the iron during BLSFMC/CFO inversion. The inversion of the domain indicates hysteresis, and BLSFMC/CFL0.03O has a narrower capacitance peak (the red circle in Fig. 7(a)), indicating that the bilayer films have the lowest oxygen vacancy concentration. This is consistent with the results of the
ferroelectric analysis. Fig. 8 is a schematic of the room temperature ferroelectric performance enhancement of the BLSFMC/CFLxO bilayer films (the purple area is the CFO layer, the yellow area is the BLSFMC layer, the pink area is the Au electrode area, and the other colors are labeled as shown). After compounding the CFO films, under the action of the applied electric field P (red arrow in Fig. 8(a)), there are many Fe2+ ions (red dots) diffusing into the BLSFMC that increase the oxygen vacancies concentration and help to form many defect complexes such as 2 + −V ··], and [Co 2 + −V ··] (the barbed black area in [V Bi −V o··], [MnFe 3+ o o Fe3 + Fig. 8(a)). The defect complexes form a conductive path inside the films, causing a relatively large leakage current density, and due to the diffusion of Fe2+ and oxygen vacancies, a pinned ferroelectric domain
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4. Conclusions Bi0.79La0.18Sr0.03Fe0.94Mn0.02Co0.02O3/CoFe2-xLaxO4 multiferroicity bilayer films were prepared on FTO substrates using a sol-gel method. The underlying CFO layer, doped with La3+, induced structural distortion and a structural phase change in the upper layer of BLSMFC, increasing the average grain size of the bilayer films, and broadening the films' grain size distribution; this broadening reduced the influence of ion diffusion on BLSFMC in the underlying CFO. Adjusting the concentration of Fe2+, Mn2+, and Co2+ in a bilayer film, reduced the number of defect complexes, as well as the pinning effect at the film's interface. The insulation reduced the pinning effect at the BLSFMC/ CFLO interface, making the ferroelectric domain there easier to switch and enhancing film ferroelectricity. BLSMFC/CFL0.03O was found to have enhanced ferroelectricity: Pr = 136.82 μC/cm2, Ps = 145.86 μC/ cm2, Ec = 425.51 kV/cm, Is = 0.69 mA, and Rsq = 1.20, while maintaining strong ferromagnetic properties: Ms = 90 emu/cm3, Mr = 62 emu/cm3, and Hc = 970 Oe.
Fig. 9. Magnetic hysteresis loops of BLSFMC/CFLxO. Table 3 Magnetic performance parameters of BLSMFC/CFLxO bilayer films. 3
3
Acknowledgments
Sample
Mr(emu/cm )
Ms(emu/cm )
Hc(Oe)
x = 0.00 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15
84 62 57 51 36 11
121 90 87 87 63 23
1000 970 868 742 617 424
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), and the Graduate Innovation Fund of Shaanxi University of Science and Technology (SUST-A04).
is produced at the interface of the BLSFMC/CFO bilayer films (red column in Fig. 8(a)); this ferroelectric domain is difficult to switch during polarization [30]. In Fig. 8(c) a wider capacitance peak appears in the C–V curve (the purple circle area in Fig. 8(c)), confirming the pinning effect of the defect complex and the limit of the ferroelectricity of the BLSFMC/CFO bilayer films. The residual polarization of the BLSFMC/CFO bilayer films is only Pr = 96 μC/cm2; the coercive field, Ec = 404 kV/cm; and the switching current is Is = 0.59 mA (Fig. 8(c)). Fig. 8(b) shows that after the CFO is doped with La3+, the charge balance reduces the formation of Fe2+ ions (the red dots in Fig. 8(b)) and reduces Fe2+ diffusion into the upper BLSFMC. This leads to a reduction in the number of defect complexes (the barbed black area in Fig. 8(d)) and an improvement in the intrinsic film insulation. The reduction in the diffusion of Fe2+ and oxygen vacancies also reduces the pinning effect at the BLSFMC/CFLO interface (the number of red columns in Fig. 8(b) is reduced) that make the ferroelectric domains easy to switch during polarization. In Fig. 8(d), a narrow capacitance peak appears in the C–V curve (the yellow circle area in Fig. 8(d)). The narrowing of the capacitor peak indicates that the underlying CFO, doped with La3+, reduces the number of defect complexes, as well as the pinning effect, enhancing the ferroelectricity of the films. The residual polarization of the BLSFMC/CFL0.03O bilayer films increased to Pr = 136 μC/cm2; the switch current peak increased to Is = 0.69 mA; while the coercive field remained low, Ec = 408 kV/cm (Fig. 8(d)). Fig. 9 shows the room temperature hysteresis loops of the BLSFMC/ CFLxO bilayer films and the performance parameters are shown in Table 3. The ferromagnetic properties of the bilayer films become less pronounced after doping with La3+ because the distortion of the structure caused by incorporating La3+ into the CFO layer weakens its ferromagnetism. However, the ferromagnetism of the BLSFMC/CFLxO bilayer films is still stronger than that of the single-phase BLSMFC [21], and the saturation magnetization of the BLSFMC/CFL0.03O bilayer films is 90 emu/cm3, which is 25% lower than that of the BLSFMC/CFO bilayer films but stronger than that in other doped bilayer films. In addition, BLSFMC/CFL0.03O bilayer films exhibit enhanced ferroelectric properties.
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Regulation of defect complexes at the interface for improving multiferroicity of Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 bilayer thin films
T
Mintao Xuea, Guoqiang Tana,∗, Ao Xiaa, Zhengjun Chaia, Long Lvb, Huijun Renc, Xixi Rena, Jincheng Lia a
Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an, Shaanxi, 710021, China b College of Cryptography Engineering, Engineering University of PAP, Xi'an, 710086, China c School of Arts and Sciences, Shaanxi University of Science and Technology, Xi'an, Shaanxi, 710021, China
ARTICLE INFO
ABSTRACT
Keywords: BiFeO3/CoFe2O4 Defect complexes Pinning effect Multiferroicity
This work presents the Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 (BLSFMC/CFLxO) multiferroic bilayer films with Au top electrode on the fluorine doped tin oxide (FTO) by sol-gel method. And the change in structure and properties of BLSFMC/CFLxO films are investigated. The results show that the upper layer of BLSFMC is induced structural distortion by doping La3+ in the underlying CFO and the R3m: R structural phase content increases. La3+ blocks the diffusion of Fe2+ ions into the BLSFMC layers, reduces the oxygen vacancies concentration, and weakens the valence fluctuations of Fe, Mn, and Co ions in the upper BLSFMC layer, decreasing the number of defect complexes and improving the insulation properties of the BLSFMC/CFLxO bilayer. The pinned ferroelectric domain at the BSLFMC/CFL0.03O interface is reduced, and the ferroelectricity of the BLSFMC layer is released. The residual polarization of BLSFMC/CFL0.03O Pr = 136.82 μC/cm2, the squareness ratio Rsq = 1.20 and the saturation magnetization Ms = 90 emu/cm3 compared with the single-phase BLSFMC thin films, BLSFMC/CFL0.03O bilayer thin films exhibits excellent ferroelectricity and has a large amount of enhanced ferromagnetism. The structure and multiferroicity properties of the bilayer films can be adjusted by the underlying La3+ dopant.
1. Introduction As a typical multiferroicity material, BiFeO3 exhibits ferroelectricity and G-type antiferromagnetic coexistence at room temperature, with a high ferroelectric Curie temperature (1100 K) and Néel temperature (653 K) [1–4]. Moreover, its ferromagnetic surface and ferroelectric polarization direction (⟨111⟩) are perpendicular to each other, its Gtype antiferromagnetic and ferroelectricity are coupled, and BiFeO3 has been widely used in applications such as spintronics, sensors, and tunable microwave devices [5], making it the compound of choice for many researchers [6–8]. The Bi3+ and Fe2+ ions in BiFeO3 lead to poor insulation in films, which generates a high leakage current density, affecting the practical application of ferroelectric polarization. Researchers commonly dope BiFeO3 films with rare earth elements such as La, Er, Gd, and Pr in the A site of BiFeO3 [9–12] to reduce the oxygen vacancies concentration and the leakage current density, improving the films’ insulation properties and hence their ferroelectricity. Transition metal elements such as Ti, Mn, Ni, and Co are doped in the B site to
∗
allowing the super-exchange of inhibit the valence state of Fe3+ , Fe–O–Fe bonds to improve the antiferromagnetic properties of BiFeO3 [13–16]. Rajput et al. [14] prepared BiFeO3 films doped with Ni and La, demonstrating that an optimal content of La and Ni reduced the oxygen vacancies concentration and significantly reduced the leakage current density (J ~ 10−4 A/cm2). Which displays a well-shaped hysteresis loop, showing enhanced remanent polarization values (Pr ~ 66 μC/ cm2) and a lower coercive field (Ec ~ 0.3 MV/cm). Yue et al. found the structural phase transition of BiFeO3 with Pr, Dy, and Mn as dopants, and the relationship between defects and grain boundary resistance and its effect on leakage conductance were studied. The incorporation of Dy3+ can improve the antiferromagnetic properties of BiFeO3 (Ms ~ 2.39 emu/cm3) [17]. Guo et al. studied the influence of the codoping of Ho, Sr, Mn, and Ni BiFeO3: the defects and oxygen vacancy concentration in the films were reduced, the BiFeO3 films exhibited enhanced ferroelectricity (Pr ~ 193 μC/cm2) under the Fowler-Nordheim (FN) tunneling mechanism, and the presence of Mn4+ released the Fe2+ concentration. The super-exchange of Fe bonds enhances the
Corresponding author. E-mail address: [email protected] (G. Tan).
https://doi.org/10.1016/j.ceramint.2019.07.038 Received 26 April 2019; Received in revised form 2 July 2019; Accepted 3 July 2019 Available online 03 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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antiferromagnetic properties of BiFeO3 (Ms ~ 5.78 emu/cm3) [18]. Ion doping of BiFeO3 mainly results in changes in its structural phase, reducing film defects and leakage current density and improving the ferroelectricity of BiFeO3 to allow the super-exchange of Fe–O–Fe bonds, thus improving film ferromagnetism. However, in BiFeO3 the Fe–O–Fe bond super-exchange effect gives rise to weak ferromagnetism which can be problematic in applications. The researchers further constructed a ferroelectric/ferromagnetic (FE/FM) multiferroicity bilayer film. Sone et al. [19] prepared multiferroicity bilayer films of BiFeO3/CoFe2O4. After compounding the ferromagnetic phase of CoFe2O4, the ferromagnetism of the films was significantly enhanced, and its saturation magnetization was as high as 118 emu/cm3, but the ferroelectricity of the BiFeO3/CoFe2O4 multiferroic heterojunction was weak. C.M. Raghavan et al. find Structural change in (Bi0.95La0.05) (Fe0.97Cr0.03)O3/CoFe2O4 by X-ray diffraction and Raman scattering and low order of leakage current density and enhanced magnetic performance were observed in double layered thin films [20]. Based on results of the multi-doping of BiFeO3, researchers have proposed combining multi-doped BiFeO3 films [21] with the CoFe2O4 ferromagnetic phase to construct Bi0.79La0.18Sr0.03Fe0.94Mn0.04Co0.02O3/CoFe23+ xLaxO4 multiferroicity bilayer films, and through the incorporation of La in the underlying CoFe2O4, to regulate the oxygen vacancies concentration defects and built-in electric. These steps improve the films’ ferroelectricity and ferromagnetic properties, making BiFeO3 (ferroelectric layer) and CoFe2O4 (ferromagnetic layer) more compatible and optimizing the multiferroic properties of multiferroicity bilayer films. 2. Experimental Using the sol-gel method to prepare Bi0.79La0.18Sr0.03 Fe0.94Mn0.04Co0.02O3/CoFe2-xLaxO4 (BLSFMC/CFLxO, x ~ 0.00–0.15), multiferroicity bilayer films were placed on an F-doped SnO2 conductive glass substrate. Firstly, Co(NO3)3·6H2O, La(NO3)3, and Fe (NO3)3·9H2O were dissolved in a precursor solution of 2-methoxyethanol and acetic anhydride (3:1 vol ratio) to obtain a 0.2 mol/L precursor solution of CFLxO, and Bi(NO3)3·5H2O (5% excess), La(NO3)3, Sr(NO3)3, Fe(NO3)3·9H2O, C6H9MnO6·2H2O, and Co(NO3)3·6H2O were dissolved in the precursor solution of 2-methoxyethanol and acetic anhydride (3:1 vol ratio) to obtain a 0.2 mol/L precursor solution of BLSFMC. The precursor solution of CFLxO was spin-coated on the glass substrate for 13 s at 3700 r/min, and baked at 185 °C for 9 min, then annealed at 590 °C for 20 min. The above steps were repeated six times to obtain CFLxO films. Then, the precursor solution of BLSFMC was
spin-coated on the CFLxO films at 3700 r/min for 13 s, baked at 185 °C for 9 min, and annealed at 545 °C for 10 min, a cycle repeated 13 times to obtain BLSFMC/CFLxO multiferroicity bilayer films. Au was sputtered on an area of 0.03 mm2 on the top of the bilayer films as the top electrode and annealed at 285 °C for 15 min. X-ray diffraction (XRD) using a Rigaku D/MAX-2200 was employed to characterize the films’ crystal structures and MAUD Rietveld software was used to analyze the XRD data to further determine crystal structure changes. Raman spectroscopy was performed using a A Horiba JYHR800 Raman test system equipped with a 532 nm Ar-ion laser. Monitoring of the leakage current density of the bilayer films was performed using an Agilent B2901A source/measure unit, and the hysteresis loops of the samples were obtained by an aixACCT TFAnalyzer 2000. SQUID-MPMS-XL magnetic measurement characterized the magnetic properties of the bilayer films. 3. Results and discussion Fig. 1 shows the XRD pattern of the BLSFMC/CFLxO multiferroicity bilayer films. Because the BLSFMC ferroelectric layer masks the information of the CFLxO layer, the XRD pattern mainly shows the diffraction peak of BLSFMC, corresponding to the standard card JSPDS No.72–2112 belonging to the perovskite structure. None of the XRD patterns showed a hetero peak, indicating that the bilayer films did not have a miscellaneous phase. Fig. 1(b) is a partially enlarged view of the BLSFMC/CFLxO bilayer films with 2θ ~ 31.5–33°. The (110) and (1–10) double peaks of the BLSFMC layer become gradually weakened and shift to a higher angle with increased La3+ doping in the CFLxO layer. This is because La3+ influences the upper BLSFMC structure after the incorporation of CFLxO, and the mismatched lattice constant causes structural distortion of the BLSFMC layer. In order to further study the structural changes of BLSFMC/CFLxO bilayer films, Rietveld was used to fit the XRD data with MAUD. The results are shown in Fig. 1(c) and Table 1. The BLSFMC in BLSFMC/CFO bilayer films consists mainly of two phases: R3c:H(84%) and R3m:R (16%). With the incorporation of La3+ in the underlying CFO, the R3c:H content in the upper BLSFMC decreases gradually, and the content of R3m:R increases, which indicates that the La3+ not only changes the intensity and growth orientation of the upper BLSFMC diffraction peak but also its structural phase; this structural change affects the multiferroicity properties of bilayer films [22]. Fig. 2(a) shows the XRD pattern of CFLxO films, corresponding to the standard card JSPDS No. 22–1086, which belongs to the cubic
Fig. 1. (a) and (b) X-ray pattern of BLSFMC/CFLxO bilayer films. (c) BLSFMC/CFLxO bilayer films Rietveld diagram.
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Table 1 BLSFMC/CFLxO bilayer films Rietveld data. Samples
Crystal structure
BLSFMC/CFO
Trigonal
BLSFMC/CFL0.03O
Trigonal
BLSFMC/CFL0.06O
Trigonal
BLSFMC/CFL0.09O
Trigonal
BLSFMC/CFL0.12O
Trigonal
BLSFMC/CFL0.15O
Trigonal
Space group
Lattice parameters
R3c:H(84%) R3m:R(16%) R3c:H(67%) R3m:R(33%) R3c:H(57%) R3m:R(43%) R3c:H(42%) R3m:R(58%) R3c:H(24%) R3m:R(76%) R3c:H(9%) R3m:R(91%)
R(%)
a
c
5.60 3.86 5.54 3.91 5.59 3.98 5.58 3.95 5.58 3.98 5.57 3.96
13.74
12.32%
14.09
13.21%
14.21
9.84%
13.96
10.67%
14.08
11.87%
13.99
9.68%
Fig. 2. (a) XRD pattern of CFLxO films. (b) Raman spectrum of CFLxO films.
inverse spinel structure; no impurity phase was found in the XRD pattern. The intensity of the (311) characteristic peak of CFLxO is gradually weakened and broadened as La3+ is incorporated into, the (400) characteristic peak at 43° and the (511) characteristic at 57° also appear reduced in intensity after La3+ doping in CFO. The above is mainly due to the different ionic radii of La3+ and Fe3+ after La3+ doping into CFO it causes lattice distortion of CFO films. Fig. 2(b) shows the Raman spectrum of the CFLO films. The CFO has five Raman active modes (Eg +2A1g+2T2g). As shown in Figure 2b, 306 and 473, 572, 616, and 694 cm−1 correspond to Eg, T2g (2), T2g (1), A1g (2), and A1g (1), respectively. As La3+ is doped into the CFO films, the A1g (1) activemodes peak at 694 cm−1 shows a significant decrease in intensity. The active mode of A1g is related to the Fe–O bond, indicating that the position of Fe3+ is replaced by that of La3+, which causes the tensile active mode strength of the Fe–O bond to weaken. Moreover, the Eg active mode in the low-frequency region is also weakened in strength. The change in the Eg active mode is mainly due to the change in the Fe–O octahedral structure, which causes the Co–O bond to change in the CFO. The above findings further confirm that the incorporation of La3+ causes lattice distortion of the CFO films. Fig. 3 is an SEM image of BLSFMC/CFLxO bilayer films, and the inset shows the change in grain size. The inset of Fig. 3(a) is a cross-sectional SEM image of BLSFMC/CFO. It can be seen that the thickness of the bilayer films is about 520 nm, and the average grain size of the BLSFMC/CFLxO (x ~ 0.00–0.15) bilayer films is 93 nm, 144 nm, 138 nm, 158 nm, 88 nm, and 122 nm, respectively. The upper grain size changes after La3+ is doped into the underlying CFO. Compared with the case without La3+ doping, the BLSFMC/CFLxO films have a broader surface grain size distribution. The BLSFMC/CFL0.03O
bilayer films exhibit a larger average grain size because, after the underlying CFO is doped with La3+, less Fe2+ diffuses into the upper layer to change the structure of the BLSFMC layer. The BLSFMC/ CFL0.12O bilayer films of 0.12 have a small average grain size because excess La3+ incorporated into the underlying CFO will diffuse into the upper layer, causing a change in the BLSFMC layer structure. In addition, the dot elemental analysis of the cross-section was performed by energy dispersive spectroscopy (EDS), as shown in Fig. 3(g and h) and the inset table is the concentration of the elements it was detected that La, Sr, Mn, Co existed in films. The molar ratio of the upper film is Bi0.93La0.22Sr0.05Fe1.35Mn0.06Co0.22O2.17(x = 0.00) and Bi0.94La0.22Sr0.04Fe1.33Mn0.06Co0.23O2.18(x = 0.03), it can be seen the contents of Sr, Mn is close to our expectation. However, because of ion diffusion from underlying CFLxO, the contents of Bi, Co, Fe, La of upper BLSFMC is quite different from our expectation. To further analyze the changes in oxygen vacancies and defects in the BLSFMC/CFLxO bilayer films, XPS analysis was performed, and the data were fitted and analyzed using Avantage software, as shown in Fig. 4. Fig. 4(a) shows the XPS spectra of the BLSFMC/CFLxO bilayer films, and the peak position of La3+ clearly confirms the presence of La3+. Fig. 4(b) shows the XPS peak-fitting result of Fe 2p in the bilayer films. The fitting of Fe 2p is divided into two parts, representing Fe2+ and Fe3+. The concentration ratio of Fe3+/Fe2+ in the bilayer films is 39:61, 65:35, 61:39, 56:44, 46:54, and 35:65, and the formation of Fe2+ defects is inhibited by the incorporation of La3+. The bilayer films have the lowest Fe2+ concentration when the doping amount is 0.03; Fe2+ concentration affects the valence fluctuation of Mn and Co. It can be represented by the chemical formulas (1-1) and (1–2) [23]:
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Fig. 3. SEM image of BLSFMC/CFLxO bilayer films(a-f), the dot elemental analysis of BLSFMC/CFLxO bilayer films by EDS spectra(g-f) and the inset table show the concentration of the elements.
Fe3 + + Mn2 +
Fe 2 + + Mn3 +
(1-1)
Fe3 + + Co2 +
Fe 2 + + Co3 +
(1–2)
Fig. 4(c) and (d) show the Mn-2p and Co-2p peaks in the BLSFMC/ CFLxO bilayer films with the fitted peaks: Mn3+/Mn2+ are 33:67, 44:56, 35:65, 42:58, 41:59, and 32:68. Co3+/Co2+ are 37:63, 46:54, 34:66, 42:58, 43:57, and 35:65 respectively. The decrease in Fe2+ concentration can affect Mn2+ and Co2+. The bilayer films have a small concentration of Mn2+ and Co2+ at a doping amount of 0.03, and the oxygen vacancy concentration depends mainly on the unstable chemical valence of Fe element in BFO. The formation of O vacancies can be expressed by the following chemical formula (1–3) [24]:
4FeFe + 2OO
2FeFe + 2Vo¨ + O2
(1–3)
Fig. 4(e) shows the results of fitting the peak of O1s of the BLSFMC/ CFLxO bilayer films. The ratios of oxygen ion/oxygen vacancies are 91.32:8.68, 97.58:2.42, 95.47: 4.53, 94.33:5.67, 95.87:4.13, and 93.68:6.32. It can be seen that with La3+ incorporated into the bilayer films, the oxygen vacancies concentration is reduced; the bilayer films have the lowest oxygen vacancies concentration when the doping amount is 0.03, which is consistent with the above XPS analysis results. The incorporation of La3+ affects the valence fluctuations in the bilayer films, which further reduces the concentration of the medium defect
2 + −V ··], [Co 2 + −V ··] complex in bilayer films, such as [V Bi −V o··], [MnFe 3+ o o Fe3 + et al.; this will increase the insulation of the films and affect their ferroelectric properties. Fig. 5(a) shows the room temperature leakage current density of bilayer films, and the inset is a schematic diagram of the test model. The leakage current density of the BLSFMC/CFO bilayer films is higher because the lower Fe2+ ions diffuse into the BLSFMC layer. From the above analysis, it can be deduced that Fe2+ increases the formation of oxygen vacancies, affecting the Mn and Co valence states, as well as increasing the number of defect complexes at the interface of BLSFMC/ CFLxO [25–27]. These defect complexes form a conductive path inside the films, creating a higher leakage current density. The Fe3+ concentration is reduced after the underlying is doped with La3+, and a lower Fe2+ concentration reduces the oxygen vacancy concentration; this inhibits the fluctuations of the valence states of Mn and Co and reduces the number of defect complexes at the interface of the bilayer films. The insulation of the films is enhanced, and the thin films leakage current will decrease. Fig. 5(b) shows the leakage current density for different doping concentrations and an applied electric field of 200 kV/ cm. The leakage current densities of the films are 0.026 A/cm2, 0.005 A/cm2, 0.012 A/cm2, 0.018 A/cm2, 0.014 A/cm2, and 0.011 A/cm2. There is minimal leakage current density in the BLSFMC/CFL0.03O films, mainly because they have a larger average grain size, the lowest
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Fig. 4. (a) XPS spectra of BLSFMC/CFLxO bilayer films. (b) Fe 2p analysis. (c) Mn 2p3/2 analysis. (d) Co 2p3/2 analysis. (e) O1s analysis.
oxygen vacancy concentration, and fewer defective complexes. Fig. 6 shows the room temperature hysteresis loops and switching current curves of the BLSFMC/CFLxO bilayer films. The BLSFMC/CFO films exhibit a typical hysteresis loop and switching current curves. The performance parameters are shown in Table 2. The residual polarization of BLSFMC/CFO is Pr = 96 μC/cm2; the coercive field, Ec = 404 kV/cm; and the switching current peak Is = 0.59 mA.
Compared with our previous study, the residual polarization of BLSFMC films is Pr = 188 μC/cm2; the coercive field, Ec = 272.5 kV/cm; and the switching current peak Is = 0.89 mA [21] is lower. This is because after composite the CFO the Fe2+ of CFO diffusion into the upper BLSMFC, generating growth defects such as oxygen vacancy concentration in the BLSFMC, forming a conductive path inside the films and resulting in a large leakage current density. Moreover, some defect complexes were
Fig. 5. Leakage current density diagram of BLSFMC/CFLxO bilayer films.
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Fig. 6. Hysteresis loops and switching current curves of BLSFMC/CFLxO bilayer films.
Rsq = Pr Ps + P1.1Ec Pr
Table 2 Ferroelectric performance parameters of BLSMFC/CFLxO bilayer films. Sample
Pr(μC/cm2)
Ps(μC/cm2)
Ec(kV/cm)
Is(mA)
Rsq
x = 0.00 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15
95.54 136.82 93.16 121.11 104.58 83.18
105.06 145.86 100.99 129.38 112.59 89.53
400.57 425.51 402.12 424.52 422.54 444.46
0.59 0.69 0.54 0.72 0.62 0.46
1.17 1.20 1.16 1.20 1.19 1.18
formed at the BLSFMC/CFO interface, which would produce pinned ferroelectric domains. When the ferroelectric is polarization, it is not easy to switch. The above interaction limits the ferroelectric properties of the bilayer films [28]. When the La3+ content is 0.03, the BLSFMC/ CFL0.03O bilayer films have the largest residual polarization, Pr = 136 μC/cm2, and an enhanced switching current peak, Is = 0.69 mA, while maintaining a low coercive field, Ec = 408 kV/cm. The squareness of the hysteresis loop is an important feature of ferroelectricity. The calculation formula is as follows [29]:
The P1.1Ec is the polarization value corresponding to the 1.1Ec. The squareness of the bilayer films is shown in Table 2. When the La3+ doping amount is 0.03, the bilayer films have the highest squareness of 1.20; this is because after the Fe3+ is doped in the CFO. Corresponds to XPS analysis a lower Fe2+ concentration will reduce the oxygen vacancies concentration and the number of defect complexes at the interface of the bilayer films, as well as improving intrinsic film insulation. The reduction in the number of defect complexes and oxygen vacancies will reduce the pinning effect at the BLSMFC/CFLxO interface, allowing ferroelectric domains to switch easily during polarization, enhancing the ferroelectric properties of BLSFMC/CFL0.03O bilayer films. Fig. 7 is a graph showing the room temperature capacitance-voltage behavior of the BLSFMC/CFLxO films, to further characterize their ferroelectricity. All the bilayer films exhibit the typical butterfly curve of ferroelectrics, and the nonlinear capacitance-voltage relationship indicates the role of inversion of the ferroelectric domains during polarization. The voltage at the peak of the capacitor corresponds to the coercive voltage during ferroelectric polarization. The coercive voltages
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Fig. 7. Capacitance-voltage curves of BLSFMC/CFLxO bilayer films.
Fig. 8. Schematic of ferroelectric enhancement of BLSFMC/CFLxO bilayer films.
of BLSFMC/CFLxO are 2V, 2V, 4V, 2V, 4V, and 4V, (x = 0.00, 0.03, 0.06, 0.09, 0.12, and 0.15) respectively, and the bilayer films have a higher capacitance peak after the underlying is doped with La3+, indicating ferroelectric enhancement. Compared with other bilayer films the BLSFMC/CFL0.03O bilayer films have the highest capacitance peak, maintaining the coercive voltage of 2V and demonstrating an enhanced ferroelectricity. The BLSFMC/CFO bilayer films have a wider capacitance peak (black circle area in Fig. 7(a)) under the forwarding bias due to a higher oxygen vacancy concentration, and defects limit the iron during BLSFMC/CFO inversion. The inversion of the domain indicates hysteresis, and BLSFMC/CFL0.03O has a narrower capacitance peak (the red circle in Fig. 7(a)), indicating that the bilayer films have the lowest oxygen vacancy concentration. This is consistent with the results of the
ferroelectric analysis. Fig. 8 is a schematic of the room temperature ferroelectric performance enhancement of the BLSFMC/CFLxO bilayer films (the purple area is the CFO layer, the yellow area is the BLSFMC layer, the pink area is the Au electrode area, and the other colors are labeled as shown). After compounding the CFO films, under the action of the applied electric field P (red arrow in Fig. 8(a)), there are many Fe2+ ions (red dots) diffusing into the BLSFMC that increase the oxygen vacancies concentration and help to form many defect complexes such as 2 + −V ··], and [Co 2 + −V ··] (the barbed black area in [V Bi −V o··], [MnFe 3+ o o Fe3 + Fig. 8(a)). The defect complexes form a conductive path inside the films, causing a relatively large leakage current density, and due to the diffusion of Fe2+ and oxygen vacancies, a pinned ferroelectric domain
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4. Conclusions Bi0.79La0.18Sr0.03Fe0.94Mn0.02Co0.02O3/CoFe2-xLaxO4 multiferroicity bilayer films were prepared on FTO substrates using a sol-gel method. The underlying CFO layer, doped with La3+, induced structural distortion and a structural phase change in the upper layer of BLSMFC, increasing the average grain size of the bilayer films, and broadening the films' grain size distribution; this broadening reduced the influence of ion diffusion on BLSFMC in the underlying CFO. Adjusting the concentration of Fe2+, Mn2+, and Co2+ in a bilayer film, reduced the number of defect complexes, as well as the pinning effect at the film's interface. The insulation reduced the pinning effect at the BLSFMC/ CFLO interface, making the ferroelectric domain there easier to switch and enhancing film ferroelectricity. BLSMFC/CFL0.03O was found to have enhanced ferroelectricity: Pr = 136.82 μC/cm2, Ps = 145.86 μC/ cm2, Ec = 425.51 kV/cm, Is = 0.69 mA, and Rsq = 1.20, while maintaining strong ferromagnetic properties: Ms = 90 emu/cm3, Mr = 62 emu/cm3, and Hc = 970 Oe.
Fig. 9. Magnetic hysteresis loops of BLSFMC/CFLxO. Table 3 Magnetic performance parameters of BLSMFC/CFLxO bilayer films. 3
3
Acknowledgments
Sample
Mr(emu/cm )
Ms(emu/cm )
Hc(Oe)
x = 0.00 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15
84 62 57 51 36 11
121 90 87 87 63 23
1000 970 868 742 617 424
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), and the Graduate Innovation Fund of Shaanxi University of Science and Technology (SUST-A04).
is produced at the interface of the BLSFMC/CFO bilayer films (red column in Fig. 8(a)); this ferroelectric domain is difficult to switch during polarization [30]. In Fig. 8(c) a wider capacitance peak appears in the C–V curve (the purple circle area in Fig. 8(c)), confirming the pinning effect of the defect complex and the limit of the ferroelectricity of the BLSFMC/CFO bilayer films. The residual polarization of the BLSFMC/CFO bilayer films is only Pr = 96 μC/cm2; the coercive field, Ec = 404 kV/cm; and the switching current is Is = 0.59 mA (Fig. 8(c)). Fig. 8(b) shows that after the CFO is doped with La3+, the charge balance reduces the formation of Fe2+ ions (the red dots in Fig. 8(b)) and reduces Fe2+ diffusion into the upper BLSFMC. This leads to a reduction in the number of defect complexes (the barbed black area in Fig. 8(d)) and an improvement in the intrinsic film insulation. The reduction in the diffusion of Fe2+ and oxygen vacancies also reduces the pinning effect at the BLSFMC/CFLO interface (the number of red columns in Fig. 8(b) is reduced) that make the ferroelectric domains easy to switch during polarization. In Fig. 8(d), a narrow capacitance peak appears in the C–V curve (the yellow circle area in Fig. 8(d)). The narrowing of the capacitor peak indicates that the underlying CFO, doped with La3+, reduces the number of defect complexes, as well as the pinning effect, enhancing the ferroelectricity of the films. The residual polarization of the BLSFMC/CFL0.03O bilayer films increased to Pr = 136 μC/cm2; the switch current peak increased to Is = 0.69 mA; while the coercive field remained low, Ec = 408 kV/cm (Fig. 8(d)). Fig. 9 shows the room temperature hysteresis loops of the BLSFMC/ CFLxO bilayer films and the performance parameters are shown in Table 3. The ferromagnetic properties of the bilayer films become less pronounced after doping with La3+ because the distortion of the structure caused by incorporating La3+ into the CFO layer weakens its ferromagnetism. However, the ferromagnetism of the BLSFMC/CFLxO bilayer films is still stronger than that of the single-phase BLSMFC [21], and the saturation magnetization of the BLSFMC/CFL0.03O bilayer films is 90 emu/cm3, which is 25% lower than that of the BLSFMC/CFO bilayer films but stronger than that in other doped bilayer films. In addition, BLSFMC/CFL0.03O bilayer films exhibit enhanced ferroelectric properties.
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