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Enhanced multiferroic properties and domain structure of La-doped BiFeO3 thin films
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Scripta Materialia 63 (2010) 780–783 www.elsevier.com/locate/scriptamat
Enhanced multiferroic properties and domain structure of La-doped BiFeO3 thin films F. Yan,a T.J. Zhu,b M.O. Laia and L. Lua,* a
Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 13 May 2010; revised 6 June 2010; accepted 6 June 2010 Available online 10 June 2010
BiFeO3 (BFO) and La-doped BFO (BLFO) thin films were grown on Pt/TiO2/SiO2/Si substrate using pulsed laser deposition. The domain structures of the BFO and BLFO were investigated via piezoresponse force microscopy. Highly enhanced ferroelectric properties with great remanent polarization (Pr) of 102 lC cm2 and decreased leakage current density were obtained via La doping. The magnetic property was also increased by the La doping, ascribed to spatial homogenization of the spin arrangement. The mechanisms for the enhancement of ferroelectric and ferromagnetic characteristics are discussed. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Multiferroic thin film; Dielectric properties, Ferroelectricity; Piezoresponse force microscopy
Multiferroics represent a class of materials that covers several ferroic orders [1–3]. Due to their special phenomena and potential applications in multifunctional devices, multiferroic materials have attracted a lot of interest in recent years [4,5]. One of the most widely studied group of multiferroic materials is the orthorhombic RMnO3 (R = Tb, Yb, Dy) manganites, where ferroelectricity is induced by magnetic long-range ordering instead of structural distortions, charge ordering or stereochemical activity [6,7]. Another well-known material is lead-free BiFeO3 (BFO), which at room temperature comprises a single phase with a rhombohedrally distorted perovskite (space group R3c), a ferroelectric Curie temperature (TC 1103 K) and an antiferromagnetic Ne´el (TN 647 K) [8]. BFO has large remanent polarization in the single crystal and thin film states [9–11]. The spontaneous polarization could be as much as 91.5 lC cm2 in theory because 6s2 lone pairs of electrons at the Bi site distort the geometry of the FeO6 anion [12]. The characteristics of good ferroelectricity, high TC and high TN enable the use of multiferroic BFO in real applications [13]. However, applications of BFO need to be overcome several challenges, such as serious electrical leakage [14], ferroelectric reliability [15] and an inhomogeneous magnetic spin structure [16]. Much effort have been de-
* Corresponding author. Tel.: +65 65162356; fax: +65 67791459; email: [email protected]
voted to improving the multiferroic properties of BFO, utilizing the site-engineering concept to introduce chemical pressure into this system [17]. Among many dopants, Sm, Tb, Gd and Nd, partially substituted at the Bi site, have shown improved ferroelectricity and an enhanced homogenization of spin arrangement [18–22]. Lanthanum (La) has also been used to alter the electric and magnetic properties via various techniques. However, ferroelectric and ferromagnetic characterization have fluctuated distinctly across many research reports. For instance, some have reported increased saturated polarization (Ps) and remanent polarization (Pr) of about 2–5 lC cm2, while others have reported values as large as about 50–90 lC cm2 based on different substrates [23–25]. As for magnetization, the inhomogeneous magnetic spin structure of BFO films can also be suppressed by La substitution in order to improve the weak ferromagnetic ordering [26]. Moreover, the switching behavior of domains and domain walls is of crucial importance for the ferroelectric and magnetic properties. Thus, La doping is expected to have an effect on the switching of domains and domain walls. In this study, we report improved polarization and a reduced electric coercive field in La-doped BFO thin films. The influence of isovalent La doping on the crystalline structure, surface topography, domain structures, dielectric properties and magnetic property of BFO film are investigated. Furthermore, the possible causes of the greatly enhanced ferroelectric and ferromagnetic characteristics are discussed.
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.06.013
F. Yan et al. / Scripta Materialia 63 (2010) 780–783
Polycrystalline pure and 5 mol.% La-doped BiFeO3 thin films were grown on (1 1 1) Pt/TiO2/SiO2/Si substrates using pulsed laser deposition. The targets, having compositions of BiFeO3 and Bi0.95La0.05FeO3, with 10% excess bismuth to compensate for the Bi volatilization during annealing and deposition, were synthesized by a conventional solid-state reaction using high-purity Bi2O3, Fe2O3 and La2O3. The structural properties were determined by X-ray diffraction (XRD) with Cu Ka radiation. The characterization of surface morphologies and piezoresponse were carried out using a piezoresponse force microscope (PFM; Asylum Research MFP3De). Pt top electrodes of 100 lm diameter were sputtered onto the surface of the films. The thickness of the films was examined using a Hitachi S4300 field emission scanning electron microscope (FE-SEM). The dielectric properties were measured using a precision impedance analyzer at zero bias voltage. The ferroelectric and magnetic properties were characterized using a Radiant Precision Workstation and a Lakeshore 736 vibrating sample magnetometer (VSM), respectively. Figure 1a shows the XRD spectra of the as-deposited BFO and BLFO thin films. The film exhibits a singlephase perovskite structure, and no secondary phase is observed. The XRD spectra also suggest that the films are polycrystalline with rhombohedral symmetry [17]. The diffraction pattern of the BLFO shows a relatively broad single peak at 2h = 32.8° and no (1 0 4) reflection appears, indicating that the rhombohedral distortion is reduced toward the orthorhombic or tetragonal structure with La doping. The lattice parameter after La doping is larger than pure BFO because the ionic radius of La3+ (0.1032 nm) is slightly larger than that of Bi3+ (0.1030 nm) [23]. The out-of-plane lattice parameters obtained from the XRD spectra are aBFO = 0.3971 nm and aBLFO = 0.3972 nm, respectively. Note that the BLFO film is under greater tensile strain (0.312%) than the BFO film (0.262%) comparison with the bulk BFO (0.396 nm). The tensile strain and the film–substrate misfit may be the cause for the increased tetragonality of the BLFO film. Moreover, the strain could vary the domain behavior and further change the ferroic behavior and magnetic interaction [6]. Figure 1b shows a typical FE-SEM cross-sectional microstructure of the BFO thin film on Pt/TiO2/SiO2/Si substrate, suggesting that the BFO film has dense columnar grains and a thickness of about 350 nm. Figure 2a and d shows the atomic force microscopy (AFM) morphologies of the as-received BFO and BLFO thin film, respectively. The average grain size of the
Figure 1. (a) X-ray diffraction spectra for BFO and BLFO thin film. (b) A typical FE-SEM cross-sectional image of the films.
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BLFO film is approximately 200 nm larger than that of the BFO. The presence of La dopant is effective in controlling the volatility of Bi atoms in order to suppress oxygen vacancy concentration since the energy of the La–O bond (789.6 ± 8 kJ mol1) is stronger than that of Bi–O bond (337.2 ± 12 kJ mol1) [27–29]. The root mean square roughness values are 10.88 and 5.97 nm for BFO and BLFO, respectively, indicating that La doping can also smooth the film surface morphology. The out-of-plane PFM (OP-PFM) amplitude and phase images of the BFO and BLFO film are shown in Figure 2b, c and e, f, respectively. The ferroelectric domain structures of the BFO exhibit a fractal growth habit, a domain size similar to the grain size and upward or downward polarization within each domain (Fig. 2c). The domain structures of the BLFO show more homogeneous domains and the domains are pinned at the ground boundaries (Fig. 2f). The density of the domain walls in the BLFO is less than in the BFO; therefore, it is expected that there is a lower leakage of current in the BLFO since certain domain walls in the BFO are much more conductive than the domains themselves [30]. There exist three types of domain walls, namely those that separate domains 71°, 109° and 180° different in polarization [31]. Domain patterns can develop with either a {1 0 0}-type plane for 109° walls or a {1 0 1}type plane for 71° walls, respectively [32]. As shown from the XRD spectra, the polycrystalline films should be a mixture of all three types of domain wall. The domain size in the BLFO film is larger than that in the BFO film, which could originate from the tensile strain enhanced by the La doping, the bigger grain size or even the misfit dislocation between the film and the substrate [6]. The domain structure and the density of the domain walls of the BLFO also suggests that it can undergo a higher electric bias and is easier to polarizee than the BFO. The leakage current density (J) vs. electric field (E) of the films is shown in Figure 3a. The BFO film shows a low leakage current only when the electric field is below 200 kV cm1, while the leakage of the BLFO film is nearly two orders of magnitude lower than that of the BFO film. The decrease in the leakage current of the BLFO film is believed to be attributed to the single phase, the smoother surface, the reduction in V O and the decreased density of the domain walls. Figure 3b shows the ferroelectric polarization vs. electric field (P–E) curves of the BFO and BLFO thin films measured at 1 kHz, where remnant polarizations Pr of 66 and 102 lC cm2, respectively, are obtained. The values of Pr are similar to those reported previously for other BFO thin films [23]. The lower coercive field of 380 kV cm1 for the BLFO compared with 700 kV cm1 for the BFO could arise from the change in domain switching dynamics because the presence of La ions might break the symmetry between orientations and extend to other low symmetry states [33]. An alternative possible mechanism for the low coercive field could be associated with the domain wall density of the BLFO film [24]. Furthermore, the 6s2 lone-pair orbital of Bi3+ is stereochemically active and responsible for the ferroelectric distortion. The distortions induced by La doping are therefore likely to be caused by the turning
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F. Yan et al. / Scripta Materialia 63 (2010) 780–783
Figure 2. (a and d) AFM images, (b and e) out-of-plane PFM amplitude and (c, f) phase images for BFO and BLFO thin film (1 1 lm2), respectively. The height data show the topography of the image, and the phase data (piezoresponse) are mapped on as color. Bright and dark indicate the upward and downward domain orientation in (c) and (f).
Figure 3. Measurement of electric properties of the Pt/BFO/Pt and Pt/BLFO/Pt capacitors at room temperature: (a) leakage current density; (b) ferroelectric hysteresis loops; (c) electric field dependencies of remanent polarization and the coercive field; and (d) dielectric properties.
off of the lone-pair activity [30]. Figure 3c illustrates the good saturation characteristics of Pr and Ec of BLFO as a function of the electric field. Figure 3d illustrates the dielectric constant, e, and the dielectric loss, tand, as functions of frequency. It is evident that the e and tand values of the films vary slightly with frequency (102–106 Hz). The dielectric constant varies ranging from 320 to 260 for the BLFO and from 220 to 150 for the BFO. It is obvious that the La dopant can significantly enhance the dielectric constant from 190 to 290 at 1 kHz, whereas it has only a small influence on tand, which may be due to the bigger grain size of the BLFO and the low charged defects or defect
000 dipole complexes of the BLFO, such as the V O , V Bi or 0 V Þ . When the frequency is higher than ðV 000 Bi O 105 Hz, a slight increase in tand is observed, implying that the charge carriers in the films cannot follow the external electric field and the maximum electrical energy is transferred to the oscillation ions [34]. Meanwhile, there is a noticeable increase in tand at low frequency, which might be due to space charges in the interface of the films [35]. The magnetization characteristics of both thin films measured at a maximum magnetic field of 5000 Oe applied parallel to the substrate surface at 300 K are shown in Figure 4. The magnetic hysteresis loops show weak ferro-
F. Yan et al. / Scripta Materialia 63 (2010) 780–783
Figure 4. Magnetic properties of BFO and BLFO thin films measured at room temperature.
magnetic ordering in nature with the average saturated magnetization (Ms) ranging from Ms 8 emu cm3 (corresponding to a magnetization of about 0.055 lB Fe1) for the BFO to 15 emu cm3 (0.102 lB Fe1) for the BLFO thin film. There are several possible reasons for this enhancement in magnetization: (i) the spatial homogenization of the spin arrangement [15]; (ii) the distorted spin cycloid structure when La ions are positioned at the Bisite [16]; (iii) the formation of partial Fe2+; (iv) the varied canting angle of the Fe–O–Fe bond [36]; or (v) the increased tensile strain changing the balance between the antiferromagnetic and ferromagnetic interactions [6]. In conclusion, we have successfully grown pure and La-doped BFO thin films on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition. The domain growth habits were investigated via PFM, which indicated that the domain structure has changed and the domain wall density was decreased by La doping. The leakage current density of the BLFO decreased by two orders of magnitude in comparison with that of the BFO. The BLFO thin film shows a greatly enhanced ferroelectric property together with greater remanent polarization and ferromagnetic orderings in comparison with BFO film. The enhanced ferroelectric properties of La-doped BFO were analyzed by domain engineering. A high dielectric constant and low dielectric loss of the BLFO ensure good ferroelectric and piezoelectric behavior. This work was performed under the financial support from the National University of Singapore. [1] M. Bibes, A. Barthe´le´my, Nat. Mater. 7 (2008) 425. [2] W. Eerenstein, N.D. Mathur, J.F. Scott, Nat. Mater. 442 (2006) 759. [3] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. [4] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [5] Y. Yamasaki, S. Miyasaka, Y. Kaneko, J.P. He, T. Arima, Y. Tokura, Phys. Rev. Lett. 96 (2006) 207204. [6] S. Venkatesan, C. Daumont, B.J. Kooi, B. Noheda, J.T.M.D. Hosson, Phys. Rev. B. 80 (2009) 214111. [7] D. Rubi, S. Venkatesan, B.J. Kooi, J.T.M.D. Hosson, T.T.M. Palstra, B. Noheda, Phys. Rev. B. 78 (2008) 020408(R).
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Scripta Materialia 63 (2010) 780–783 www.elsevier.com/locate/scriptamat
Enhanced multiferroic properties and domain structure of La-doped BiFeO3 thin films F. Yan,a T.J. Zhu,b M.O. Laia and L. Lua,* a
Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 13 May 2010; revised 6 June 2010; accepted 6 June 2010 Available online 10 June 2010
BiFeO3 (BFO) and La-doped BFO (BLFO) thin films were grown on Pt/TiO2/SiO2/Si substrate using pulsed laser deposition. The domain structures of the BFO and BLFO were investigated via piezoresponse force microscopy. Highly enhanced ferroelectric properties with great remanent polarization (Pr) of 102 lC cm2 and decreased leakage current density were obtained via La doping. The magnetic property was also increased by the La doping, ascribed to spatial homogenization of the spin arrangement. The mechanisms for the enhancement of ferroelectric and ferromagnetic characteristics are discussed. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Multiferroic thin film; Dielectric properties, Ferroelectricity; Piezoresponse force microscopy
Multiferroics represent a class of materials that covers several ferroic orders [1–3]. Due to their special phenomena and potential applications in multifunctional devices, multiferroic materials have attracted a lot of interest in recent years [4,5]. One of the most widely studied group of multiferroic materials is the orthorhombic RMnO3 (R = Tb, Yb, Dy) manganites, where ferroelectricity is induced by magnetic long-range ordering instead of structural distortions, charge ordering or stereochemical activity [6,7]. Another well-known material is lead-free BiFeO3 (BFO), which at room temperature comprises a single phase with a rhombohedrally distorted perovskite (space group R3c), a ferroelectric Curie temperature (TC 1103 K) and an antiferromagnetic Ne´el (TN 647 K) [8]. BFO has large remanent polarization in the single crystal and thin film states [9–11]. The spontaneous polarization could be as much as 91.5 lC cm2 in theory because 6s2 lone pairs of electrons at the Bi site distort the geometry of the FeO6 anion [12]. The characteristics of good ferroelectricity, high TC and high TN enable the use of multiferroic BFO in real applications [13]. However, applications of BFO need to be overcome several challenges, such as serious electrical leakage [14], ferroelectric reliability [15] and an inhomogeneous magnetic spin structure [16]. Much effort have been de-
* Corresponding author. Tel.: +65 65162356; fax: +65 67791459; email: [email protected]
voted to improving the multiferroic properties of BFO, utilizing the site-engineering concept to introduce chemical pressure into this system [17]. Among many dopants, Sm, Tb, Gd and Nd, partially substituted at the Bi site, have shown improved ferroelectricity and an enhanced homogenization of spin arrangement [18–22]. Lanthanum (La) has also been used to alter the electric and magnetic properties via various techniques. However, ferroelectric and ferromagnetic characterization have fluctuated distinctly across many research reports. For instance, some have reported increased saturated polarization (Ps) and remanent polarization (Pr) of about 2–5 lC cm2, while others have reported values as large as about 50–90 lC cm2 based on different substrates [23–25]. As for magnetization, the inhomogeneous magnetic spin structure of BFO films can also be suppressed by La substitution in order to improve the weak ferromagnetic ordering [26]. Moreover, the switching behavior of domains and domain walls is of crucial importance for the ferroelectric and magnetic properties. Thus, La doping is expected to have an effect on the switching of domains and domain walls. In this study, we report improved polarization and a reduced electric coercive field in La-doped BFO thin films. The influence of isovalent La doping on the crystalline structure, surface topography, domain structures, dielectric properties and magnetic property of BFO film are investigated. Furthermore, the possible causes of the greatly enhanced ferroelectric and ferromagnetic characteristics are discussed.
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.06.013
F. Yan et al. / Scripta Materialia 63 (2010) 780–783
Polycrystalline pure and 5 mol.% La-doped BiFeO3 thin films were grown on (1 1 1) Pt/TiO2/SiO2/Si substrates using pulsed laser deposition. The targets, having compositions of BiFeO3 and Bi0.95La0.05FeO3, with 10% excess bismuth to compensate for the Bi volatilization during annealing and deposition, were synthesized by a conventional solid-state reaction using high-purity Bi2O3, Fe2O3 and La2O3. The structural properties were determined by X-ray diffraction (XRD) with Cu Ka radiation. The characterization of surface morphologies and piezoresponse were carried out using a piezoresponse force microscope (PFM; Asylum Research MFP3De). Pt top electrodes of 100 lm diameter were sputtered onto the surface of the films. The thickness of the films was examined using a Hitachi S4300 field emission scanning electron microscope (FE-SEM). The dielectric properties were measured using a precision impedance analyzer at zero bias voltage. The ferroelectric and magnetic properties were characterized using a Radiant Precision Workstation and a Lakeshore 736 vibrating sample magnetometer (VSM), respectively. Figure 1a shows the XRD spectra of the as-deposited BFO and BLFO thin films. The film exhibits a singlephase perovskite structure, and no secondary phase is observed. The XRD spectra also suggest that the films are polycrystalline with rhombohedral symmetry [17]. The diffraction pattern of the BLFO shows a relatively broad single peak at 2h = 32.8° and no (1 0 4) reflection appears, indicating that the rhombohedral distortion is reduced toward the orthorhombic or tetragonal structure with La doping. The lattice parameter after La doping is larger than pure BFO because the ionic radius of La3+ (0.1032 nm) is slightly larger than that of Bi3+ (0.1030 nm) [23]. The out-of-plane lattice parameters obtained from the XRD spectra are aBFO = 0.3971 nm and aBLFO = 0.3972 nm, respectively. Note that the BLFO film is under greater tensile strain (0.312%) than the BFO film (0.262%) comparison with the bulk BFO (0.396 nm). The tensile strain and the film–substrate misfit may be the cause for the increased tetragonality of the BLFO film. Moreover, the strain could vary the domain behavior and further change the ferroic behavior and magnetic interaction [6]. Figure 1b shows a typical FE-SEM cross-sectional microstructure of the BFO thin film on Pt/TiO2/SiO2/Si substrate, suggesting that the BFO film has dense columnar grains and a thickness of about 350 nm. Figure 2a and d shows the atomic force microscopy (AFM) morphologies of the as-received BFO and BLFO thin film, respectively. The average grain size of the
Figure 1. (a) X-ray diffraction spectra for BFO and BLFO thin film. (b) A typical FE-SEM cross-sectional image of the films.
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BLFO film is approximately 200 nm larger than that of the BFO. The presence of La dopant is effective in controlling the volatility of Bi atoms in order to suppress oxygen vacancy concentration since the energy of the La–O bond (789.6 ± 8 kJ mol1) is stronger than that of Bi–O bond (337.2 ± 12 kJ mol1) [27–29]. The root mean square roughness values are 10.88 and 5.97 nm for BFO and BLFO, respectively, indicating that La doping can also smooth the film surface morphology. The out-of-plane PFM (OP-PFM) amplitude and phase images of the BFO and BLFO film are shown in Figure 2b, c and e, f, respectively. The ferroelectric domain structures of the BFO exhibit a fractal growth habit, a domain size similar to the grain size and upward or downward polarization within each domain (Fig. 2c). The domain structures of the BLFO show more homogeneous domains and the domains are pinned at the ground boundaries (Fig. 2f). The density of the domain walls in the BLFO is less than in the BFO; therefore, it is expected that there is a lower leakage of current in the BLFO since certain domain walls in the BFO are much more conductive than the domains themselves [30]. There exist three types of domain walls, namely those that separate domains 71°, 109° and 180° different in polarization [31]. Domain patterns can develop with either a {1 0 0}-type plane for 109° walls or a {1 0 1}type plane for 71° walls, respectively [32]. As shown from the XRD spectra, the polycrystalline films should be a mixture of all three types of domain wall. The domain size in the BLFO film is larger than that in the BFO film, which could originate from the tensile strain enhanced by the La doping, the bigger grain size or even the misfit dislocation between the film and the substrate [6]. The domain structure and the density of the domain walls of the BLFO also suggests that it can undergo a higher electric bias and is easier to polarizee than the BFO. The leakage current density (J) vs. electric field (E) of the films is shown in Figure 3a. The BFO film shows a low leakage current only when the electric field is below 200 kV cm1, while the leakage of the BLFO film is nearly two orders of magnitude lower than that of the BFO film. The decrease in the leakage current of the BLFO film is believed to be attributed to the single phase, the smoother surface, the reduction in V O and the decreased density of the domain walls. Figure 3b shows the ferroelectric polarization vs. electric field (P–E) curves of the BFO and BLFO thin films measured at 1 kHz, where remnant polarizations Pr of 66 and 102 lC cm2, respectively, are obtained. The values of Pr are similar to those reported previously for other BFO thin films [23]. The lower coercive field of 380 kV cm1 for the BLFO compared with 700 kV cm1 for the BFO could arise from the change in domain switching dynamics because the presence of La ions might break the symmetry between orientations and extend to other low symmetry states [33]. An alternative possible mechanism for the low coercive field could be associated with the domain wall density of the BLFO film [24]. Furthermore, the 6s2 lone-pair orbital of Bi3+ is stereochemically active and responsible for the ferroelectric distortion. The distortions induced by La doping are therefore likely to be caused by the turning
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Figure 2. (a and d) AFM images, (b and e) out-of-plane PFM amplitude and (c, f) phase images for BFO and BLFO thin film (1 1 lm2), respectively. The height data show the topography of the image, and the phase data (piezoresponse) are mapped on as color. Bright and dark indicate the upward and downward domain orientation in (c) and (f).
Figure 3. Measurement of electric properties of the Pt/BFO/Pt and Pt/BLFO/Pt capacitors at room temperature: (a) leakage current density; (b) ferroelectric hysteresis loops; (c) electric field dependencies of remanent polarization and the coercive field; and (d) dielectric properties.
off of the lone-pair activity [30]. Figure 3c illustrates the good saturation characteristics of Pr and Ec of BLFO as a function of the electric field. Figure 3d illustrates the dielectric constant, e, and the dielectric loss, tand, as functions of frequency. It is evident that the e and tand values of the films vary slightly with frequency (102–106 Hz). The dielectric constant varies ranging from 320 to 260 for the BLFO and from 220 to 150 for the BFO. It is obvious that the La dopant can significantly enhance the dielectric constant from 190 to 290 at 1 kHz, whereas it has only a small influence on tand, which may be due to the bigger grain size of the BLFO and the low charged defects or defect
000 dipole complexes of the BLFO, such as the V O , V Bi or 0 V Þ . When the frequency is higher than ðV 000 Bi O 105 Hz, a slight increase in tand is observed, implying that the charge carriers in the films cannot follow the external electric field and the maximum electrical energy is transferred to the oscillation ions [34]. Meanwhile, there is a noticeable increase in tand at low frequency, which might be due to space charges in the interface of the films [35]. The magnetization characteristics of both thin films measured at a maximum magnetic field of 5000 Oe applied parallel to the substrate surface at 300 K are shown in Figure 4. The magnetic hysteresis loops show weak ferro-
F. Yan et al. / Scripta Materialia 63 (2010) 780–783
Figure 4. Magnetic properties of BFO and BLFO thin films measured at room temperature.
magnetic ordering in nature with the average saturated magnetization (Ms) ranging from Ms 8 emu cm3 (corresponding to a magnetization of about 0.055 lB Fe1) for the BFO to 15 emu cm3 (0.102 lB Fe1) for the BLFO thin film. There are several possible reasons for this enhancement in magnetization: (i) the spatial homogenization of the spin arrangement [15]; (ii) the distorted spin cycloid structure when La ions are positioned at the Bisite [16]; (iii) the formation of partial Fe2+; (iv) the varied canting angle of the Fe–O–Fe bond [36]; or (v) the increased tensile strain changing the balance between the antiferromagnetic and ferromagnetic interactions [6]. In conclusion, we have successfully grown pure and La-doped BFO thin films on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition. The domain growth habits were investigated via PFM, which indicated that the domain structure has changed and the domain wall density was decreased by La doping. The leakage current density of the BLFO decreased by two orders of magnitude in comparison with that of the BFO. The BLFO thin film shows a greatly enhanced ferroelectric property together with greater remanent polarization and ferromagnetic orderings in comparison with BFO film. The enhanced ferroelectric properties of La-doped BFO were analyzed by domain engineering. A high dielectric constant and low dielectric loss of the BLFO ensure good ferroelectric and piezoelectric behavior. This work was performed under the financial support from the National University of Singapore. [1] M. Bibes, A. Barthe´le´my, Nat. Mater. 7 (2008) 425. [2] W. Eerenstein, N.D. Mathur, J.F. Scott, Nat. Mater. 442 (2006) 759. [3] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. [4] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [5] Y. Yamasaki, S. Miyasaka, Y. Kaneko, J.P. He, T. Arima, Y. Tokura, Phys. Rev. Lett. 96 (2006) 207204. [6] S. Venkatesan, C. Daumont, B.J. Kooi, B. Noheda, J.T.M.D. Hosson, Phys. Rev. B. 80 (2009) 214111. [7] D. Rubi, S. Venkatesan, B.J. Kooi, J.T.M.D. Hosson, T.T.M. Palstra, B. Noheda, Phys. Rev. B. 78 (2008) 020408(R).
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