Influence of Crystallization Temperature on Structural , Ferroelectric , and Ferromagnetic Properties of Lead-Free Bi 0 . 5 ( Na 0 . 8 K 0 . 2 ) 0 . 5 TiO 3 Multiferroic Films

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


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.erefore, researches on multiferroic materials have been motivated to illumine the fundamental mechanism as well as technological limits of multiferroics [1].In the multiferroic materials, BiFeO 3 (BFO) reveals magnetic and ferroelectric properties at room temperature (T C � 830 °C and T N � 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, BiMnO 3 (BMO), exhibits magnetic and ferroelectric ground states at T FM � 105 K and T FE � 770 K with a spontaneous polarization (P s ∼16 µC/cm 2 ) and magnetization (M s ∼3.6 µB/Mn), respectively [3,4].Besides, when partially substituting Bi 3+ with isovalent La 3+ , 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 BiCrO 3 (BCO) reveals multiferroic behavior with large A-site Bi 3+ .It possesses piezoelectricity and antiferroelectricity at room temperature, and weak ferromagnetism was demonstrated with the Curie temperature of 120 K [6].
e Bi-based multiferroic materials BiMO 3 (where M is transition metals as Mn, Cr, Fe, etc.), having double perovskite structures, are also promising multiferroic candidates.For instant, BiFe 0.5 Cr 0.5 O 3 film has the weak magnetism but exhibits a large polarization of around 60 µC/cm 2 [7,8].e hexagonal RMnO 3 materials (R�Y, Ho, Yb, In, Er, Sc, etc.) were studied previously and evidenced the existence of multiferroic properties [9].In which, YMnO 3 (YMO) materials, being a triangular antiferromagnets, arises antiferromagnetic order at T N � 70 K.And YMO bulk materials exhibit a P r value of about 5.5 µC/cm 2 [10].It is proved that TmMnO 3 and HoMnO 3 also have good multiferroic properties.Tm has the antiferromagnetic order at 75 K and 4.6 K for Ho ion.TmMnO 3 reveals the spontaneous polarization of 0.45 µC/cm 2 , while that of HoMnO 3 has a higher value of 5.6 µC/cm 2 [1,11].
Recently, lead-free BNT-BKT (BNKT) ferroelectric materials have been widely studied because Bi 3+ 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 d 33 of 167 pC/N, electromechanical coupling coefficient k 33 of 0.56, and high remnant polarization P r of 38 µC/cm 2 [14].Especially, BNKT-based materials exposed the room temperature ferromagnetic behavior [15,16].With substituting Mn for Ti 4+ , anh et al. enhanced the room temperature ferromagnetism in Bi 0.5 Na 0.5 TiO 3 materials, which is stemmed from oxygen vacancies [17].Bi 0.5 Na 0.5 TiO 3 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 Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 (abbreviated as BNKT) films via a sol-gel method on Pt/Ti/SiO 2 /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 (P r ) and maximum (P m ) polarization reached their highest values of 9.2 µC/cm 2 and 30.6 µC/cm 2 , respectively.e saturated magnetization (M s ) was 2.1 emu/cm 3 .

Experimental
e multilayered Bi 0.5 (Na 0.80 ,K 0.20 ) 0.5 TiO 3 (BNKT) thin films were formed on Pt/Ti/SiO 2 /Si substrates using solutions prepared by a sol-gel technique.Here, the BNKT precursor solution is derived from sodium nitrate (NaNO 3 , ≥99%, Sigma-Aldrich), potassium nitrate (KNO 3 , ≥99%, Sigma-Aldrich), bismuth nitrate (Bi(NO 3 ) 3 •5H 2 O, ≥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 hightemperature annealing, we added their excess amounts of 30% and 20%, respectively.e 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/SiO 2 /Si substrates at 4000 rpm for 30 s, followed by pyrolysis at 400 °C for 10 min.e 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).e 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).
e 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).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)).e 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).

Results and Discussion
e 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.e RQ has such a small value, and it is confirmed that BNKT films exhibited good surface quality.
e well-distributed grains and good surface quality in films will be reliable bases to improve ferroelectricity.e 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.
e 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 2 Advances in Materials Science and Engineering phases [21].With the highest intensity, the (111) orientation is the mixture of orientations as a consequence of the Ptcoated substrate.e (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 °. is result shows that the (200) preferred orientations in the films revealed different intensity values.e lowest value was observed in the BNKT thin film S600. is 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 Bi 2 Ti 2 O 7 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. is may be stemmed from the evaporation of metal ions Bi 3+ , 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].
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 Pt x Pb between PZT and Pt and determined its composition and thickness in films.
e intermetallic layer plays the role as a dead layer causing a weak crystallinity in the films.
e 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 Bi 0.5 (Na 0.85 K 0.15 ) 0.5 TiO 3 thin films [25].To affirm, we calculated the grain size of BNKT films by using Scherrer equation [30]: 5 µm   where D is the grain size, K is the constant related to the crystallite shape (normally taken as 0.9), λ is the wavelength, β is the FWHM, and θ is the Bragg angle.Table 1 illustrates the grain size of BNKT film as a function of the annealing temperature.Obviously, the D 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 A 1 (TO1), A 1 (TO2), B 1 (TO1), A 1 (TO3), and E (LO), deconvoluted and shown in Figure 3(b).e A 1 (TO1) mode at the wavenumber of ∼108 cm −1 is induced by the A-site vibrations.e A 1 (TO2) mode at the wavenumber of 227 cm −1 is resulted from the Ti-O stretching vibrations.Reference [32] reported that the B 1 (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 A 1 (TO3) and E (LO) modes are contributed by vibrations of TiO 6 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.is disparity may be stemmed from the combination of the following factors.e first one is the different fields of view of the techniques, where XRD scans a much larger area and hence is more representative.
e 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.
is 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.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 P m , P r , P m −P r , and E c between the films annealed at different temperatures.In the S600 films, while P m and P r have the relatively low values of around 13.7 µC/cm 2 and 6.9 µC/cm 2 , respectively, E c possesses the highest value of around 115 kV/cm.e difference between the values of P m and P r is 6.9 µC/cm 2 .P m , P r , and P m −P r are significantly enhanced when the crystallization temperature increases.
ese parameters reached their highest values of P m � 30.6 µC/cm 2 , P r � 9.2 µC/cm 2 , and P m −P r � 21.4 µC/cm 2 at the crystallization temperature of 700 °C.However, the opposite was true of E c . is value continuously reduced from 115 kV/cm to 78 kV/cm with an increase in the crystallization temperature from 600 °C to 750 °C. is 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.e 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]. is promoted to enhance the ferroelectric properties of films.However, when the annealing temperature was increased to 750 °C, P r and P m values decreased remarkably.
is is related to the evaporation of metal ions from films during annealing at high temperature.4 Advances in Materials Science and Engineering 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.e intermetallic layer between BNKT film and Pt substrate annealed at 750 °C plays the role as a dead layer causing polarization degradation [27][28][29].
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 (M s ) of 1.5 emu/cm 3 , 2.1 emu/cm 3 , 1.9 emu/ cm 3 , and 1.6 emu/cm 3 for the S600, S650, S700, and S750 films, respectively.Especially, the BNKT films S650 and S700 surpass the others in M s values.It is believed that the evaporation of metal ions Bi 3+ , Na + , and K + during annealing at high temperature created the oxygen vacancies on the surfaces of the BNKT films, forming the magnetic moments [35].e 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/cm 3 .
is may be stemmed from the Advances in Materials Science and Engineering 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.

Conclusion
Lead-free Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 (BNKT) films have been successfully fabricated on Pt/Ti/SiO 2 /Si substrates via a spin coating-assisted sol-gel method.e 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.e 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 (P r ) and maximum (P m ) polarization of 9.2 µC/cm 2 and 30.6 µC/cm 2 , respectively.is 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 (M s ) of 2.1 emu/cm 3 .Magnetic moments which are formed by the oxygen vacancies are the origin of ferromagnetism in the BNKT films.Advances in Materials Science and Engineering

Figure 1
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 (Figure1(e)).e good surface quality of films is confirmed one time again by AFM images with the scanning area 40 μm × 40 μm, as shown in Figures1(a)-1(d).eAFM images showed relatively smooth and continuous surfaces with small root mean square roughness (RQ) fluctuating from 3.4 nm to 4.8 nm (Table1).And this value decreased continuously corresponding to an increase in annealing temperature.e RQ has such a small value, and it is confirmed that BNKT films exhibited good surface quality.ewell-distributed grains and good surface quality in films will be reliable bases to improve ferroelectricity.e thicknesses of films were determined by cross-sectional FE-SEM images, and Figure1(f ) shows the thickness of the S700 film of approximately 300 nm.e crystalline structures of BNKT films are deduced from X-ray diffraction patterns in the 2θ ranges of 25 °-75

Table 1 :
e 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 Advances in Materials Science and Engineering

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