Advances in Materials Science and Engineering
Volume 2019 (2019), Article ID 8976385, 8 pages
https://doi.org/10.1155/2019/8976385

Research Article

Influence of Crystallization Temperature on Structural, Ferroelectric, and Ferromagnetic Properties of Lead-Free Bi0.5(Na0.8K0.2)0.5TiO₃ Multiferroic Films

Ngo Duc Quan,1,2 Tran Quoc Toan,³ Vu Ngoc Hung,² and Minh-Duc Nguyen⁴

¹School of Engineering Physics, Ha Noi University of Science and Technology, 1 Dai Co Viet Road, 100000 Hanoi, Vietnam
²International Training Institute for Materials Science, Hanoi University of Science and Technology, 1 Dai Co Viet Road, 100000 Hanoi, Vietnam
³Institute of Natural Products Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, 100000 Hanoi, Vietnam
⁴MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands

Correspondence should be addressed to Ngo Duc Quan; quan.ngoduc@hust.edu.vn

Received 4 August 2018; Revised 30 September 2018; Accepted 16 December 2018; Published 9 January 2019

Academic Editor: Alessandro Martucci

Copyright © 2019 Ngo Duc Quan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Lead-free Bi0.5(Na0.8K0.2)0.5TiO₃ (abbreviated as BNKT) films have been synthesized via a sol-gel technique on Pt/Ti/SiO₂/Si substrates, and the dependence of the physical properties in BNKT films were investigated as a function of the crystallization temperature. The BNKT films were annealed at different temperatures (600, 650, 700, and 750°C) for 60 min in the air. The results of this study showed that the optimal crystallization temperature is 700°C. At this, the BNKT films exhibited a single perovskite phase structure and high-dense surface. Besides, the remanent (Pr) and maximum (Pm) polarization reached their highest values of 9.2 µC/cm² and 30.6 µC/cm², respectively. All the films showed a weak ferromagnetic behavior with the maximum saturated magnetization (Ms) of 2.1 emu/cm³. These values are equivalent to the highest Pr and Pm values in previous reports on lead-free films.

1. Introduction

Multiferroics are new generation materials which possess simultaneously two or more ferroic orders such as ferro/antiferromagnetism, ferroelectricity, and ferroelasticity. Because of the strong coupling between ferroic orders, multiferroic materials can be added new functionalities. For example, thanks to the magnetoelectricity, we can switch electric polarization by an applied magnetic field or control magnetization by an applied electric field. Additionally, the trends of modern technology are focused on increasing storage density of memories, the miniaturization of devices, higher efficient processes of storing and retrieving data, and so on. Therefore, researches on multiferroic materials have been motivated to illumine the fundamental mechanism as well as technological limits of multiferroics [1]. In the multiferroic materials, BiFeO₃ (BFO) reveals magnetic and ferroelectric properties at room temperature (TC = 830°C and TN = 370°C). While the antiferromagnetic order is formed from the strong exchange between neighboring ions Fe and O, and ferroelectric polarization is originated in the highly polarizable Bi lone pairs [2]. Another multiferroic material, BiMnO₃ (BMO), exhibits magnetic and ferroelectric ground states at TFM = 105 K and TFE = 770 K with a spontaneous polarization (Ps∼16 µC/cm²) and magnetization (Ms∼3.6 µB/Mn), respectively [3, 4]. Besides, when partially substituting Bi³⁺ with isovalent La³⁺, Gajek et al. obtained the materials whose magnetism is more stable than that of pure BMO materials, while remaining multiferroic properties [5]. Murakami et al. reported that BiCrO₃ (BCO) reveals multiferroic behavior with large A-site Bi³⁺. It possesses piezoelectricity and antiferroelectricity at room temperature, and weak ferromagnetism was demonstrated with the Curie temperature of 120 K [6]. The Bi-based multiferroic materials BiMO₃ (where M is transition metals as Mn, Cr, Fe, etc.), having double perovskite structures, are also promising multiferroic candidates. For instant, BiFe0.5Cr0.5O₃ film has the weak magnetism but exhibits a large polarization of around 60 µC/cm² [7, 8]. The hexagonal RMnO₃ materials (R=Y, Ho, Yb, In, Er, Sc, etc.) were studied previously and evidenced the existence of multiferroic properties [9]. In which, YMnO₃ (YMO) materials, being a triangular antiferromagnets, arises antiferromagnetic order at TN = 70 K. And YMO bulk materials exhibit a Pr value of about 5.5 µC/cm² [10]. It is proved that TmMnO₃ and HoMnO₃ also have good multiferroic properties. Tm has the antiferromagnetic order at 75 K and 4.6 K for Ho ion. TmMnO₃ reveals the spontaneous polarization of 0.45 µC/cm², while that of HoMnO₃ has a higher value of 5.6 µC/cm² [1, 11].

Recently, lead-free BNT-BKT (BNKT) ferroelectric materials have been widely studied because Bi³⁺ is highly polarizable. Retaining simultaneously rhombohedral (R3c) and tetragonal (P4mm) phases at the morphotropic phase boundary (MPB) [12, 13], BNKT materials exhibit a large piezoelectric coefficient d₃₃ of 167 pC/N, electromechanical coupling coefficient k₃₃ of 0.56, and high remnant polarization Pr of 38 µC/cm² [14]. Especially, BNKT-based materials exposed the room temperature ferromagnetic behavior [15, 16]. With substituting Mn for Ti⁴⁺, Thanh et al. enhanced the room temperature ferromagnetism in Bi0.5Na0.5TiO₃ materials, which is stemmed from oxygen vacancies [17]. Bi0.5Na0.5TiO₃ materials were proved to reveal weak ferromagnetic property at room temperature when being doped by Cr at the Ti-site, with saturation magnetization of 0.08 µB/Cr at 5 K [18]. However, the observed magnetic and ferroelectric properties in BNKT materials were quite weak and their origin was still unclear.

In the present study, we fabricated lead-free Bi0.5(Na0.8K0.2)0.5TiO₃ (abbreviated as BNKT) films via a sol-gel method on Pt/Ti/SiO₂/Si substrates and investigated the physical properties of BNKT films annealed at different temperatures (600, 650, 700, and 750°C) for 60 min in the air. We found that the optimal crystallization temperature is 700°C. At this, the remanent (Pr) and maximum (Pm) polarization reached their highest values of 9.2 µC/cm² and 30.6 µC/cm², respectively. The saturated magnetization (Ms) was 2.1 emu/cm³.

2. Experimental

The multilayered Bi0.5(Na0.80,K0.20)0.5TiO₃ (BNKT) thin films were formed on Pt/Ti/SiO₂/Si substrates using solutions prepared by a sol-gel technique. Here, the BNKT precursor solution is derived from sodium nitrate (NaNO₃, ≥99%, Sigma-Aldrich), potassium nitrate (KNO₃, ≥99%, Sigma-Aldrich), bismuth nitrate (Bi(NO₃)₃·5H₂O, ≥98%, Sigma-Aldrich), and titanium isopropoxide (Ti[i-OPr]₄, 99%, Sigma-Aldrich) [19]. Acetic acid and 2-methoxyethanol were chosen as co-solvent, and acetylacetone was chosen as ligands. During the process, titanium isopropoxide was first dissolved in acetylacetone to prevent its hydrolysis. In order to compensate the possible Na and K loss during high-temperature annealing, we added their excess amounts of 30% and 20%, respectively. The mixture was stirred at 70°C for 6 hours constantly to form the final solution until a 0.4 M transparent, and stable yellow precursor solution was obtained. Each layer of the BNKT films was formed by spin coating precursor on Pt/Ti/SiO₂/Si substrates at 4000 rpm for 30 s, followed by pyrolysis at 400°C for 10 min. The process was repeated until the BNKT thin films with the required coating layers were obtained. Finally, thermal annealing was carried out at different temperatures of: 600°C, 650°C, 700°C, and 750°C for 60 min to obtain the ferroelectric phase in the BNKT thin films (named as: S600, S650, S700, and S750, respectively). The heating rate is 5°C/min.

Characteristics of the films which are the cross-sectional and surface morphologies were detected by a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and atomic force microscopy (AFM, Bruker Dimension ICON). The crystal structures of BNKT thin films were determined by a Bruker D5005 diffractometer using a Cu-Kα cathode (λ = 1.5406 Å). P–E hysteresis loops were measured under the applied voltages ranging from −25 V to 25 V, frequency of 1000 Hz by using a TF Analyzer 2000 ferroelectric tester (aixACCT Systems GmbH, Germany).

3. Results and Discussion

Figure 1 demonstrates 2D-3D AFM images of BNKT films at different crystallization temperatures: (a) S600, (b) S650, (c) S700, and (d) S750; (e) FE-SEM micrographs of sample S700; and (f) cross-sectional SEM image of sample S700. All films exposed well-defined grain morphologies, and a uniform distribution of grains on the entire surface. For instant, S700 films show a dense microstructure without any traces of cracks detected (Figure 1(e)). The good surface quality of films is confirmed one time again by AFM images with the scanning area 40 μm × 40 μm, as shown in Figures 1(a)–1(d). The AFM images showed relatively smooth and continuous surfaces with small root mean square roughness (RQ) fluctuating from 3.4 nm to 4.8 nm (Table 1). And this value decreased continuously corresponding to an increase in annealing temperature. The RQ has such a small value, and it is confirmed that BNKT films exhibited good surface quality. The well-distributed grains and good surface quality in films will be reliable bases to improve ferroelectricity. The thicknesses of films were determined by cross-sectional FE-SEM images, and Figure 1(f) shows the thickness of the S700 film of approximately 300 nm.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 1: 2D-3D AFM images of BNKT films at different crystallization temperatures: (a) S600, (b) S650, (c) S700, and (d) S750; (e) FE-SEM micrographs of sample S700; (f) cross-sectional SEM image of sample S700.

Table 1: The grain size and root mean square roughness as a function of the annealing temperature.

Annealing temperature (°C)
600
650
700
750

Grain size, D (nm)
45.3
46.5
48.9
49.0

Root mean square roughness, RQ (nm)
4.8
4.6
4.5
3.4

The crystalline structures of BNKT films are deduced from X-ray diffraction patterns in the 2θ ranges of 25°–75° shown in Figure 2(a). All films are well crystallized corresponding to the single-phase perovskite structure, and no other impurity phases are detected [19, 20]. BNKT films in this study possess the optimal composition near the MPB evidenced by the coexisting of rhombohedral and tetragonal phases [21]. With the highest intensity, the (111) orientation is the mixture of orientations as a consequence of the Pt-coated substrate. The (200) peaks, being dominant for all the films, are the preferred orientations. Figure 2(b) illustrated the X-ray diffraction patterns in the 2θ ranges of 46–48°. This result shows that the (200) preferred orientations in the films revealed different intensity values. The lowest value was observed in the BNKT thin film S600. This proved that the BNKT thin film S600 still remains the intermediate pyrochlore phase [22]. When the crystallization temperature increased, the intensity value of (200) orientation significantly raised up and reached the peak at 700°C, before a decrease at 750°C. It is believed that BNKT materials were well crystallized at the annealing temperature of 700°C because the intermediate pyrochlore phase completely transformed into the perovskite phase [22, 23]. Chen et al. reported that BNT-BT films annealed at temperature below 600°C still exhibit a secondary pyrochlore Bi₂Ti₂O₇ phase. And it is completely removed at the annealing temperature of 700°C [23, 24], causing the apparent improvement in crystallinity of the sample. However, the sample annealed at 750°C showed a poorer crystallinity. This may be stemmed from the evaporation of metal ions Bi³⁺, Na⁺, and K⁺ during annealing at high temperature, creating the oxygen vacancies and therefore causing an nonstoichiometry at the surfaces of the BNKT films [25, 26].

(a)

(b)

Figure 2: X-ray diffraction patterns of BNKT films in the 2θ ranges of (a) 25°–75° and (b) 46–48°.

Another reason, the contamination from the substrate may contribute the decrease in crystallinity of the sample annealed at 750°C. Habouti et al. produced the evidence of an interfacial intermetallic layer between Pt and Pb when PZT thin films were annealed at 500°C [27]. By using XPS technique, Bretos et al. [28, 29] also showed the existence of a transient intermetallic PtxPb between PZT and Pt and determined its composition and thickness in films. The intermetallic layer plays the role as a dead layer causing a weak crystallinity in the films. The contamination from the substrate may contribute the decrease in crystallinity of the sample annealed at 750°C.

Additionally, the result also indicates that the particle size enlarged with the increase of the annealing temperature. Won et al. obtained the similar result was when investigating the effect of annealing temperature on the properties of Bi0.5(Na0.85K0.15)0.5TiO₃ thin films [25]. To affirm, we calculated the grain size of BNKT films by using Scherrer equation [30]:

where is the grain size, is the constant related to the crystallite shape (normally taken as 0.9), is the wavelength, is the FWHM, and is the Bragg angle. Table 1 illustrates the grain size of BNKT film as a function of the annealing temperature. Obviously, the value increased significantly from 45.3 nm to 49.0 nm when the annealing temperature was raised from 600°C to 750°C.

Figure 3(a) illustrates the Raman spectra of BNKT films annealed at different temperatures with the wavenumber from 100 cm⁻¹ to 1000 cm⁻¹. All the films revealed the same Raman spectra form, characterizing the perovskite structure. Because of the disorder at the A site and the overlap of Raman modes, the Raman peaks are broad and asymmetry [31]. Using the Lorentzian fitting technique, the peaks correspond to major modes such as A₁ (TO1), A₁ (TO2), B₁ (TO1), A₁ (TO3), and E (LO), deconvoluted and shown in Figure 3(b). The A₁ (TO1) mode at the wavenumber of ∼108 cm⁻¹ is induced by the A-site vibrations. The A₁ (TO2) mode at the wavenumber of 227 cm⁻¹ is resulted from the Ti-O stretching vibrations. Reference [32] reported that the B₁ (TO1) mode (∼281 cm⁻¹), induced by O-Ti-O bending motion, is observed in both tetragonal space groups P4mm and P4bm. Being located at the higher wavenumbers, the A₁ (TO3) and E (LO) modes are contributed by vibrations of TiO₆ oxygen octahedra [31, 33]. However, the Raman data show a little disparity in comparison with the XRD pattern. In detail, with the films annealed at 750°C, the Raman spectra are consistent with the temperature, while the XRD data exhibit the inconsistency with the temperature. This disparity may be stemmed from the combination of the following factors. The first one is the different fields of view of the techniques, where XRD scans a much larger area and hence is more representative. The other cause may be contributed by the different sensitivities of the two techniques. It is well known that the sensitivity of Raman is greater than that of XRD. This is due to the absorption coefficients of the different elements strong influence on the sensitivity of XRD, but this is not the case with Raman. Hence, Raman data showed better results than the former.

(a)

(b)

Figure 3: (a) Raman spectra of BNKT films in the wavenumber ranges of 100–1000 cm⁻¹ and (b) fit curve of S700 films.

Figure 4(a) shows the polarization (P–E) hysteresis loops for the BNKT films annealed at different temperatures. Generally, all the films possessed the same form of P–E hysteresis loops, characterizing for ferroelectric materials. Figure 4 compares the values of Pm, Pr, Pm−Pr, and Ec between the films annealed at different temperatures. In the S600 films, while Pm and Pr have the relatively low values of around 13.7 µC/cm² and 6.9 µC/cm², respectively, Ec possesses the highest value of around 115 kV/cm. The difference between the values of Pm and Pr is 6.9 µC/cm². Pm, Pr, and Pm−Pr are significantly enhanced when the crystallization temperature increases. These parameters reached their highest values of Pm = 30.6 µC/cm², Pr = 9.2 µC/cm², and Pm−Pr = 21.4 µC/cm² at the crystallization temperature of 700°C. However, the opposite was true of Ec. This value continuously reduced from 115 kV/cm to 78 kV/cm with an increase in the crystallization temperature from 600°C to 750°C. This great improvement in ferroelectric properties is due to the intermediate pyrochlore phase completely transformed into the perovskite phase, as shown in Figure 3. Moreover, ferroelectric properties significantly depend on the film’s grain size. So, films possess different grain sizes, and they also exhibit different ferroelectric behaviors. As indicated above, the grain size of BNKT films enlarged continuously with the rise of the annealing temperature. The grain size rises, leading to the repulsive force between neighboring domain walls declines; hence, the ferroelectric films need a lower activation energy for the reorientation of the domains [25, 34]. This promoted to enhance the ferroelectric properties of films. However, when the annealing temperature was increased to 750°C, Pr and Pm values decreased remarkably. This is related to the evaporation of metal ions from films during annealing at high temperature. After being formed, oxygen vacancies were trapped at grain boundaries, causing the current leakage and the polarization degradation [25, 26]. Additionally, the contamination from the substrate may also contribute the decrease of ferroelectric properties of films. The intermetallic layer between BNKT film and Pt substrate annealed at 750°C plays the role as a dead layer causing polarization degradation [27–29].

(a)

(b)

Figure 4: (a) P–E ferroelectric hysteresis loops of BNKT films at the same applied electric field of 300 kV/cm and (b) the maximum polarization (Pm), remnant polarization (Pr), difference between Pm and Pr (Pm−Pr), and coercive field Ec for BNKT films.

Figure 5 illustrates room-temperature magnetic hysteresis (M–H) loops of BNKT films annealed at different temperatures with applied magnetic field of 5 kOe. All the BNKT films, measured at the same magnetic field of 5 kOe show a weak ferromagnetic property, with saturated magnetizations (Ms) of 1.5 emu/cm³, 2.1 emu/cm³, 1.9 emu/cm³, and 1.6 emu/cm³ for the S600, S650, S700, and S750 films, respectively. Especially, the BNKT films S650 and S700 surpass the others in Ms values. It is believed that the evaporation of metal ions Bi³⁺, Na⁺, and K⁺ during annealing at high temperature created the oxygen vacancies on the surfaces of the BNKT films, forming the magnetic moments [35]. The theoretical studies have also proved that magnetic moments may be formed by point defects as anion or cation vacancies [36]. And these magnetic moments are the origin of ferromagnetism in the BNKT films. For the film annealed at higher temperature S750, the magnetism decreases to 1.6 emu/cm³. This may be stemmed from the enlarge in the particle size induced by annealing at higher temperature [37]. As the above analysis, the ferromagnetism is expected to arise from the oxygen vacancies on the surfaces of the grain boundaries, while the ferroelectricity from the grain core [35]. When the grain size increases, the volume fraction of the grain boundary decreases, resulting in a decrease in the ferromagnetism.

(a)

(b)

(c)

(d)

Figure 5: Room-temperature magnetic hysteresis (M‐H) loops of BNKT films annealed at different temperatures with the applied magnetic field of 5 kOe.

4. Conclusion

Lead-free Bi0.5(Na0.8K0.2)0.5TiO₃ (BNKT) films have been successfully fabricated on Pt/Ti/SiO₂/Si substrates via a spin coating-assisted sol-gel method. The influence of crystallization temperature on the microstructures and multiferroic properties of the prepared films was explored in detail. All samples exposed well-defined grain morphologies and a uniform distribution of grains on the entire surface. The investigations showed that the optimal crystallization temperature is 700°C. At this, the BNKT films exhibited significant enhancement of ferroelectric properties with the highest remanent (Pr) and maximum (Pm) polarization of 9.2 µC/cm² and 30.6 µC/cm², respectively. This great improvement in ferroelectric properties is due to the fact that the intermediate pyrochlore phase completely transformed into the perovskite phase. We also revealed a weak ferromagnetic behavior in all the films with the maximum saturated magnetization (Ms) of 2.1 emu/cm³. Magnetic moments which are formed by the oxygen vacancies are the origin of ferromagnetism in the BNKT films.

Data Availability

All the data used to support the findings of this study are included within this manuscript.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was supported by Hanoi University of Science and Technology (HUST) under grant number T2017-LN-08.

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