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Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films
Applied Surface Science 254 (2008) 6762–6765
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films Yi Zhang, Ling-Hua Pang, Ming-Hui Lu, Zheng-Bin Gu, Shan-Tao Zhang *, Chang-Sheng Yuan, Yan-Feng Chen National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 February 2008 Received in revised form 18 April 2008 Accepted 18 April 2008 Available online 1 May 2008
La-substituted BiFeO3, Bi0.8La0.2FeO3, thin films were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscope were used to analyze the structures of the films. The results show the films fabricated under optimized growth condition are (0 1 2) textured. X-ray photoemission spectroscopy results indicate that the oxidation state of Fe ion is Fe3+ in the films without detectable Fe2+. The films show low leakage current and excellent dielectric characters. Multiferroic properties with a remnant ferroelectric polarization of 5.2 mC/cm2 and a remanent magnetization of 0.02 mB/Fe were established. These results have some implications for further research. ß 2008 Elsevier B.V. All rights reserved.
Keywords: BiFeO3 Microstructure Multiferroic La-substituted
1. Introduction Multiferroic materials, which show simultaneously spontaneous electric and magnetic ordering in one phase, have attracted much attention in these years because of its fascinating potential application in such as ferroelectric memory and multi-functional devices [1–4]. The typical single-phase multiferroic perovskite oxides are BiFeO3 (BFO), YMnO3, BiMnO3, etc. Among these multiferroic materials, BFO is well studied, because of its high Curie temperature (TC 1103 K) and Neel temperature (TN 643 K) [1,5]. BFO is a rhombohedrally distorted ferroelectric perovskite with the space group R3c (ar = 3.96 A˚, ar = 0.68) and G-type antiferromagnetic [2,3]. In thin film form, it has large saturated polarization (80 mC/cm2) at room temperature. But comparing with its excellent ferroelectric properties, its magnetic properties are too weak to reach practical standard [6]. How to enhance the magnetic properties of BFO is becoming much more interesting. Recently, much work has been done to enhance the magnetic properties of BFO films. Two main methods were reported. One is chemical doping, the other is to fabricate different epitaxial heterostructure films [7,8]. The former method (such as Lasubstitution for A-site Bi ions) was considered as an effective way to suppress inhomogeneous magnetic spin structure. The inho-
* Corresponding author. E-mail address: [email protected] (S.-T. Zhang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.082
mogeneous magnetic spin structure, which formed by the rotating oxygen octahedral along [1 1 1] axis with a wavelength of about 620 A˚, will cancel the macroscopic magnetization [3]. In fact, how to suppress the spiral spin structure and get ferromagnetism is one of the most important issues [3,6,7]. Our recent work shows that proper La3+-substitution for Bi3+ (Bi0.8La0.2FeO3) can improve magnetic and ferroelectric properties of BFO ceramics simultaneously [9]. In this article, we report the microstructures and multiferroic properties of (0 1 2) textured La-substituted BFO (Bi0.8La0.2FeO3, BLFO) films. The relationship between microstructures and properties is discussed. 2. Experimental The film was fabricated by pulsed laser deposition (PLD) on Pt/Ti/ SiO2/Si substrates. The BLFO ceramic was used as PLD target. The ceramic was prepared by solid-state reaction with the starting materials of Bi2O3, Fe2O3 and La2O3, Bi-excess by 20% to compensate its loss during the process [10]. A Brilliant Nd:YAG laser (Quantel) with 355 nm wavelength was used in the experiment. During each deposition, the deposition rate was 10 Hz, the pulse energy was 200 mJ, the base pressure was 3 105 Pa, and the distance between substrate and target was 5.5 cm. To optimize growth condition of BLFO films, different substrate temperatures and flowing oxygen pressures were applied. After deposition, Pt top electrodes with diameter of 100 mm were sputtered at room
Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
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temperature with mask. Then these capacitors were post-annealed at 500 8C with flowing oxygen. The structure of the BLFO films were characterized by X-ray diffraction (XRD) (Rigaku diffractometer with nickel filtered Cu Ka radiation source) and TECNAI F20 high-resolution transmission electron microscope (HRTEM). The ESCALab MK-II X-ray photoelectron spectroscopy (XPS) with Mg Ka X-ray source (hy = 1253.6 eV) was used to analyze the binding state of Fe 2p electron in the films. The ferroelectric and dielectric measurements were carried out by RT66A test system (Radiant Technologies, USA), a pA meter (HP4140B) and a impedance/phase analyzer (HP4294A), while a superconducting quantum interference device (SQUID) (MPMS XL-7) magnetometer was applied for magnetic tests. 3. Results and discussion 3.1. Structure analysis and cross-sectional studies Fig. 1(a) and (b) shows oxygen pressure-dependent and temperature-dependent XRD patterns of BLFO films on Pt-coated Si substrates, respectively. As shown in Fig. 1(a), the (0 1 2) and (0 2 4) peaks can be clearly observed in the films fabricated at 530 8C under 6 Pa, while they diminish rapidly under 0.1 Pa and 50 Pa. The weak (1 1 0) peak can be observed under 6 Pa and 50 Pa. In Fig. 1(b), the strong (0 1 2) and (0 2 4) peaks are detectable in the films fabricated at 470 8C, 500 8C and 530 8C, while the peaks become weak when substrate temperature is
Fig. 2. (a) Bright field cross-sectional TEM image of BLFO film. (b) HRTEM image of the grain boundary.
higher than 530 8C. It is noted the (1 1 0) peak has the same tendency with temperature. No other peaks are observed in these XRD patterns, showing that these films are single phase. Based on these XRD patterns and by further considering ferroelectric property and leakage current, which will be shown below, the optimized substrate temperature and flowing oxygen pressure for fabricating BLFO films are determined to be 530 8C and 6 Pa, respectively. Hereafter, our report will focus on the BLFO films fabricated under optimized growth condition. It should be noted that the films fabricated under the optimized conditions are (0 1 2) textured. According to a previous report [11], the film was formed from nucleation, and different nucleation with different orientation may have different growth rate. So, if the (0 1 2) oriented BLFO nucleation grows faster than other oriented nucleation, the (0 1 2) orientation will control the growth direction. But, if the nucleates grows heterogeneously, the film will have no preferred orientation. The faster growing rate of (0 1 2) nucleates might explain why the texture can be observed below 530 8C. The bright field cross-sectional TEM image (Fig. 2(a)) shows the thickness of BLFO polycrystalline film is about 130 nm. According to the image, the grain size in the film is about 100 nm in-plane and the arrows in the figure point to the grain boundaries. The pattern shows the film is grown column like. The variant contrast among different grains shows different orientation. More HRTEM observation was taken on the position where the circle marked. Fig. 2(b) shows the HRTEM image of the grain boundary. It can be calculated that the interplanar distances of the grains are 0.393 nm and 0.279 nm corresponding to BLFO (0 1 2) and BLFO (1 1 0), respectively. At the interface of the film and substrate there is an amorphous intermediate layer about 3 nm to relax the strain, which is induced by the misfit between film and substrate. 3.2. XPS studies and dielectric properties
Fig. 1. (a) Pressure-dependent and (b) temperature-dependent XRD patterns of BLFO film.
The Fe oxidation state in textured BLFO films fabricated under the optimized conditions is determined by XPS. The scan of the Fe 2p line is shown in Fig. 3. The peaks at 711.2 eV and 724.6 eV indicate the films are single phase and the oxidation state of Fe ion should be Fe3+ without detectable Fe2+. The existence of oxygen
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Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
Fig. 5. Frequency-dependent dielectric constant and loss tangent of the BLFO film. Fig. 3. X-ray photoelectron spectrums pattern of the Fe 2p line of the BLFO film.
3.3. Ferroelectric and antiferromagnetic properties vacancies and Fe2+, which mainly formed by oxygen deficiency, is considered as the reason for low resistivity of BFO film [9,12,13]. So, amended oxygen stoichiometry and forming phase with single Fe3+ phase can improve the dielectric properties remarkably. Fig. 4 is the leakage current density–applied voltage (J–V) characteristics of the BLFO film prepared under the optimized condition. The leakage currents were recorded with both polarities of the applied voltages from 5 V to 5 V. It is obvious that the leakage current is distinct different when the bias was reversed. Yun et al. [14] and Li et al. [15] observed the same phenomena, respectively. The leakage current density under the 5 V and 5 V applied voltages were 1.2 106 and 1.4 107, respectively. It shows the optimized growth condition can effectively suppress oxygen vacancies, so leakage current and dielectric conductivity can be reduced remarkably [13,16,17]. Fig. 5 shows the frequency-dependent dielectric constant and loss tangent of the BLFO polycrystalline film. With the frequency increase, the dielectric constants decrease gradually without any sudden changes. The dielectric constants decrease a little quicker when the frequency is below 104 Hz and decrease a little slower when beyond 104 Hz, but the total decreasing trend is nearly linear. The loss tangent has almost a constant value until a break at 105 Hz. Beyond 105 Hz, the curve increases more sharply. It is deduced that the film has excellent dielectric character. The La doping could increase the dielectric constant and decrease the loss tangent, which similar with other reports [7,18].
Typical P–V hysteresis loop of the textured films prepared under optimized condition is shown in Fig. 6(a). The loop is not so well saturated but without any breakdown. It is one character of Fe3+ single-phase BFO film, while the concomitant Fe2+ and Fe3+ phase film might obtain an unsaturated lossy loop [12]. It is obtained that the remnant polarization (Pr) is about 5.2 mC/cm2. The magnetic property is shown in Fig. 6(b). The curve is measured with an in-plane magnetic field. The hysteresis loop shows weak ferromagnetic property of the BLFO film, with the saturation magnetization (Ms) and remanent magnetization (Mr) about 0.1 mB/Fe and 0.02 mB/Fe, respectively. These values are larger than other reported values of BFO film deposit on Pt/TiO2/ SiO2/Si by PLD [14]. The result of the M–H curve indicates that Lasubstituted BFO can enhance the ferromagnetic property, compared with pure BFO film [7,8]. As mentioned before, the spiral spin
Fig. 4. J–V characteristic of the BLFO film.
Fig. 6. (a) P–V hysteresis loop and (b) M–H hysteresis loop of the BLFO film.
Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
structure will cancel the macroscopic magnetization and exhibit weak ferromagnetism [3]. La doping can suppress the spiral spin structure and improve the magnetic property [9]. It was reported different oriented epitaxial BiFeO3 films have different ferroelectric properties and magnetic properties. The remanent polarization values of (1 0 1) films and (1 1 1) films are pffiffiffi pffiffiffi approximately 2 and 3 times the value for (0 0 1) films. It is mainly because the spontaneous polarization of BFO film is oriented to (1 1 1), and the Pr of these films were projected onto the (1 1 1) axis [2]. It confirms that the structures change and the orientation of the films have effects on ferroelectric and magnetic properties. This can explain the properties difference between that shown here and other reports. 4. Conclusions In conclusion, La-substituted BFO film was fabricated on Pt/Ti/ SiO2/Si substrates by PLD. The film is (0 1 2) textured grown under optimized growth condition. The single phase of Fe3+ oxidation state was measured by XPS. A typical P–V hysteresis loop with the remnant polarization Pr = 5.2 mC/cm2 is established. The weak ferromagnetic property is also shown in M–H loop with the remanent magnetization Mr = 0.02 mB/Fe. The results show Lasubstitution can improve multiferroic properties of BFO. Acknowledgements This work was supported by the National Nature Science Foundation of China (Contracts 10404012), the State Key Program
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for Basic Research of China (2007CB613202), and the Nature Science Foundation of Jiangsu province (BK2006514). The authors acknowledge support from the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [2] J. Li, J. Wang, M. Wuttig, R. Ramesh, B. Naigang Wang, A.P. Ruette, A.K. Pyatakov, D. Zvezdin, Viehland, Appl. Phys. Lett. 84 (2004) 5261. [3] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401. [4] J. Wang, H. Zheng, Z. Ma, S. Prasertchoung, M. Wuttig, R. Droopad, J. Yu, K. Eisenbeiser, R. Ramesh, Appl. Phys. Lett. 85 (2004) 02574. [5] S.U. Lee, S.S. Kim, M.H. Park, J.W. Kim, H.K. Jo, W.J. Kim, Appl. Surf. Sci. 254 (2007) 1493. [6] F. Bai, J. Wang, M. Wuttig, J. Li, N. Wang, A.P. Pyatakov, A.K. Zvezdin, L.E. Cross, D. Viehland, Appl. Phys. Lett. 86 (2005) 032511. [7] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [8] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, S.G. Lee, Appl. Phys. Lett. 86 (2005) 222903. [9] S.T. Zhang, Y. Zhang, M.H. Lu, C.L. Du, Y.F. Chen, Z.G. Liu, Y.Y. Zhu, N.B. Ming, Appl. Phys. Lett. 88 (2006) 162901. [10] W. Eerenstein, F.D. Morrison, J. Dho, M.G. Blamire, J.F. Scott, N.D. Mathur, Science 307 (2005) 1203a. [11] F. Tyholdt, S. Jorgensen, H. Fjellvag, A.E. Gunnaes, J. Mater. Res. 20 (2005) 2127. [12] V.R. Palkar, J. John, R. Pinto, Appl. Phys. Lett. 80 (2002) 1628. [13] X. Qi, J. Dho, R. Tomov, M.G. Blamire, J.L. MacManus-Driscoll, Appl. Phys. Lett. 86 (2005) 062903. [14] K.Y. Yun, M. Noda, M. Okuyama, H. Saeki, H. Tabata, K. Saito, J. Appl. Phys. 96 (2004) 3399. [15] Y.W. Li, J.L. Sun, J. Chen, X.J. Meng, J.H. Chu, J. Cryst. Growth 285 (2005) 595. [16] J.K. Kim, S.S. Kim, W.J. Kim, A.S. Bhalla, . Guo, Appl. Phys. Lett. 88 (2006) 132901. [17] K.Y. Yun, M. Noda, M. Okuyama, Appl. Phys. Lett. 83 (2003) 3981. [18] H. Uchida, R. Ueno, H. Nakaki, H. Funakubo, S. Koda, Jpn. J. Appl. Phys., Part 2 44 (2005) L561.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films Yi Zhang, Ling-Hua Pang, Ming-Hui Lu, Zheng-Bin Gu, Shan-Tao Zhang *, Chang-Sheng Yuan, Yan-Feng Chen National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 February 2008 Received in revised form 18 April 2008 Accepted 18 April 2008 Available online 1 May 2008
La-substituted BiFeO3, Bi0.8La0.2FeO3, thin films were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscope were used to analyze the structures of the films. The results show the films fabricated under optimized growth condition are (0 1 2) textured. X-ray photoemission spectroscopy results indicate that the oxidation state of Fe ion is Fe3+ in the films without detectable Fe2+. The films show low leakage current and excellent dielectric characters. Multiferroic properties with a remnant ferroelectric polarization of 5.2 mC/cm2 and a remanent magnetization of 0.02 mB/Fe were established. These results have some implications for further research. ß 2008 Elsevier B.V. All rights reserved.
Keywords: BiFeO3 Microstructure Multiferroic La-substituted
1. Introduction Multiferroic materials, which show simultaneously spontaneous electric and magnetic ordering in one phase, have attracted much attention in these years because of its fascinating potential application in such as ferroelectric memory and multi-functional devices [1–4]. The typical single-phase multiferroic perovskite oxides are BiFeO3 (BFO), YMnO3, BiMnO3, etc. Among these multiferroic materials, BFO is well studied, because of its high Curie temperature (TC 1103 K) and Neel temperature (TN 643 K) [1,5]. BFO is a rhombohedrally distorted ferroelectric perovskite with the space group R3c (ar = 3.96 A˚, ar = 0.68) and G-type antiferromagnetic [2,3]. In thin film form, it has large saturated polarization (80 mC/cm2) at room temperature. But comparing with its excellent ferroelectric properties, its magnetic properties are too weak to reach practical standard [6]. How to enhance the magnetic properties of BFO is becoming much more interesting. Recently, much work has been done to enhance the magnetic properties of BFO films. Two main methods were reported. One is chemical doping, the other is to fabricate different epitaxial heterostructure films [7,8]. The former method (such as Lasubstitution for A-site Bi ions) was considered as an effective way to suppress inhomogeneous magnetic spin structure. The inho-
* Corresponding author. E-mail address: [email protected] (S.-T. Zhang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.082
mogeneous magnetic spin structure, which formed by the rotating oxygen octahedral along [1 1 1] axis with a wavelength of about 620 A˚, will cancel the macroscopic magnetization [3]. In fact, how to suppress the spiral spin structure and get ferromagnetism is one of the most important issues [3,6,7]. Our recent work shows that proper La3+-substitution for Bi3+ (Bi0.8La0.2FeO3) can improve magnetic and ferroelectric properties of BFO ceramics simultaneously [9]. In this article, we report the microstructures and multiferroic properties of (0 1 2) textured La-substituted BFO (Bi0.8La0.2FeO3, BLFO) films. The relationship between microstructures and properties is discussed. 2. Experimental The film was fabricated by pulsed laser deposition (PLD) on Pt/Ti/ SiO2/Si substrates. The BLFO ceramic was used as PLD target. The ceramic was prepared by solid-state reaction with the starting materials of Bi2O3, Fe2O3 and La2O3, Bi-excess by 20% to compensate its loss during the process [10]. A Brilliant Nd:YAG laser (Quantel) with 355 nm wavelength was used in the experiment. During each deposition, the deposition rate was 10 Hz, the pulse energy was 200 mJ, the base pressure was 3 105 Pa, and the distance between substrate and target was 5.5 cm. To optimize growth condition of BLFO films, different substrate temperatures and flowing oxygen pressures were applied. After deposition, Pt top electrodes with diameter of 100 mm were sputtered at room
Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
6763
temperature with mask. Then these capacitors were post-annealed at 500 8C with flowing oxygen. The structure of the BLFO films were characterized by X-ray diffraction (XRD) (Rigaku diffractometer with nickel filtered Cu Ka radiation source) and TECNAI F20 high-resolution transmission electron microscope (HRTEM). The ESCALab MK-II X-ray photoelectron spectroscopy (XPS) with Mg Ka X-ray source (hy = 1253.6 eV) was used to analyze the binding state of Fe 2p electron in the films. The ferroelectric and dielectric measurements were carried out by RT66A test system (Radiant Technologies, USA), a pA meter (HP4140B) and a impedance/phase analyzer (HP4294A), while a superconducting quantum interference device (SQUID) (MPMS XL-7) magnetometer was applied for magnetic tests. 3. Results and discussion 3.1. Structure analysis and cross-sectional studies Fig. 1(a) and (b) shows oxygen pressure-dependent and temperature-dependent XRD patterns of BLFO films on Pt-coated Si substrates, respectively. As shown in Fig. 1(a), the (0 1 2) and (0 2 4) peaks can be clearly observed in the films fabricated at 530 8C under 6 Pa, while they diminish rapidly under 0.1 Pa and 50 Pa. The weak (1 1 0) peak can be observed under 6 Pa and 50 Pa. In Fig. 1(b), the strong (0 1 2) and (0 2 4) peaks are detectable in the films fabricated at 470 8C, 500 8C and 530 8C, while the peaks become weak when substrate temperature is
Fig. 2. (a) Bright field cross-sectional TEM image of BLFO film. (b) HRTEM image of the grain boundary.
higher than 530 8C. It is noted the (1 1 0) peak has the same tendency with temperature. No other peaks are observed in these XRD patterns, showing that these films are single phase. Based on these XRD patterns and by further considering ferroelectric property and leakage current, which will be shown below, the optimized substrate temperature and flowing oxygen pressure for fabricating BLFO films are determined to be 530 8C and 6 Pa, respectively. Hereafter, our report will focus on the BLFO films fabricated under optimized growth condition. It should be noted that the films fabricated under the optimized conditions are (0 1 2) textured. According to a previous report [11], the film was formed from nucleation, and different nucleation with different orientation may have different growth rate. So, if the (0 1 2) oriented BLFO nucleation grows faster than other oriented nucleation, the (0 1 2) orientation will control the growth direction. But, if the nucleates grows heterogeneously, the film will have no preferred orientation. The faster growing rate of (0 1 2) nucleates might explain why the texture can be observed below 530 8C. The bright field cross-sectional TEM image (Fig. 2(a)) shows the thickness of BLFO polycrystalline film is about 130 nm. According to the image, the grain size in the film is about 100 nm in-plane and the arrows in the figure point to the grain boundaries. The pattern shows the film is grown column like. The variant contrast among different grains shows different orientation. More HRTEM observation was taken on the position where the circle marked. Fig. 2(b) shows the HRTEM image of the grain boundary. It can be calculated that the interplanar distances of the grains are 0.393 nm and 0.279 nm corresponding to BLFO (0 1 2) and BLFO (1 1 0), respectively. At the interface of the film and substrate there is an amorphous intermediate layer about 3 nm to relax the strain, which is induced by the misfit between film and substrate. 3.2. XPS studies and dielectric properties
Fig. 1. (a) Pressure-dependent and (b) temperature-dependent XRD patterns of BLFO film.
The Fe oxidation state in textured BLFO films fabricated under the optimized conditions is determined by XPS. The scan of the Fe 2p line is shown in Fig. 3. The peaks at 711.2 eV and 724.6 eV indicate the films are single phase and the oxidation state of Fe ion should be Fe3+ without detectable Fe2+. The existence of oxygen
6764
Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
Fig. 5. Frequency-dependent dielectric constant and loss tangent of the BLFO film. Fig. 3. X-ray photoelectron spectrums pattern of the Fe 2p line of the BLFO film.
3.3. Ferroelectric and antiferromagnetic properties vacancies and Fe2+, which mainly formed by oxygen deficiency, is considered as the reason for low resistivity of BFO film [9,12,13]. So, amended oxygen stoichiometry and forming phase with single Fe3+ phase can improve the dielectric properties remarkably. Fig. 4 is the leakage current density–applied voltage (J–V) characteristics of the BLFO film prepared under the optimized condition. The leakage currents were recorded with both polarities of the applied voltages from 5 V to 5 V. It is obvious that the leakage current is distinct different when the bias was reversed. Yun et al. [14] and Li et al. [15] observed the same phenomena, respectively. The leakage current density under the 5 V and 5 V applied voltages were 1.2 106 and 1.4 107, respectively. It shows the optimized growth condition can effectively suppress oxygen vacancies, so leakage current and dielectric conductivity can be reduced remarkably [13,16,17]. Fig. 5 shows the frequency-dependent dielectric constant and loss tangent of the BLFO polycrystalline film. With the frequency increase, the dielectric constants decrease gradually without any sudden changes. The dielectric constants decrease a little quicker when the frequency is below 104 Hz and decrease a little slower when beyond 104 Hz, but the total decreasing trend is nearly linear. The loss tangent has almost a constant value until a break at 105 Hz. Beyond 105 Hz, the curve increases more sharply. It is deduced that the film has excellent dielectric character. The La doping could increase the dielectric constant and decrease the loss tangent, which similar with other reports [7,18].
Typical P–V hysteresis loop of the textured films prepared under optimized condition is shown in Fig. 6(a). The loop is not so well saturated but without any breakdown. It is one character of Fe3+ single-phase BFO film, while the concomitant Fe2+ and Fe3+ phase film might obtain an unsaturated lossy loop [12]. It is obtained that the remnant polarization (Pr) is about 5.2 mC/cm2. The magnetic property is shown in Fig. 6(b). The curve is measured with an in-plane magnetic field. The hysteresis loop shows weak ferromagnetic property of the BLFO film, with the saturation magnetization (Ms) and remanent magnetization (Mr) about 0.1 mB/Fe and 0.02 mB/Fe, respectively. These values are larger than other reported values of BFO film deposit on Pt/TiO2/ SiO2/Si by PLD [14]. The result of the M–H curve indicates that Lasubstituted BFO can enhance the ferromagnetic property, compared with pure BFO film [7,8]. As mentioned before, the spiral spin
Fig. 4. J–V characteristic of the BLFO film.
Fig. 6. (a) P–V hysteresis loop and (b) M–H hysteresis loop of the BLFO film.
Y. Zhang et al. / Applied Surface Science 254 (2008) 6762–6765
structure will cancel the macroscopic magnetization and exhibit weak ferromagnetism [3]. La doping can suppress the spiral spin structure and improve the magnetic property [9]. It was reported different oriented epitaxial BiFeO3 films have different ferroelectric properties and magnetic properties. The remanent polarization values of (1 0 1) films and (1 1 1) films are pffiffiffi pffiffiffi approximately 2 and 3 times the value for (0 0 1) films. It is mainly because the spontaneous polarization of BFO film is oriented to (1 1 1), and the Pr of these films were projected onto the (1 1 1) axis [2]. It confirms that the structures change and the orientation of the films have effects on ferroelectric and magnetic properties. This can explain the properties difference between that shown here and other reports. 4. Conclusions In conclusion, La-substituted BFO film was fabricated on Pt/Ti/ SiO2/Si substrates by PLD. The film is (0 1 2) textured grown under optimized growth condition. The single phase of Fe3+ oxidation state was measured by XPS. A typical P–V hysteresis loop with the remnant polarization Pr = 5.2 mC/cm2 is established. The weak ferromagnetic property is also shown in M–H loop with the remanent magnetization Mr = 0.02 mB/Fe. The results show Lasubstitution can improve multiferroic properties of BFO. Acknowledgements This work was supported by the National Nature Science Foundation of China (Contracts 10404012), the State Key Program
6765
for Basic Research of China (2007CB613202), and the Nature Science Foundation of Jiangsu province (BK2006514). The authors acknowledge support from the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [2] J. Li, J. Wang, M. Wuttig, R. Ramesh, B. Naigang Wang, A.P. Ruette, A.K. Pyatakov, D. Zvezdin, Viehland, Appl. Phys. Lett. 84 (2004) 5261. [3] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401. [4] J. Wang, H. Zheng, Z. Ma, S. Prasertchoung, M. Wuttig, R. Droopad, J. Yu, K. Eisenbeiser, R. Ramesh, Appl. Phys. Lett. 85 (2004) 02574. [5] S.U. Lee, S.S. Kim, M.H. Park, J.W. Kim, H.K. Jo, W.J. Kim, Appl. Surf. Sci. 254 (2007) 1493. [6] F. Bai, J. Wang, M. Wuttig, J. Li, N. Wang, A.P. Pyatakov, A.K. Zvezdin, L.E. Cross, D. Viehland, Appl. Phys. Lett. 86 (2005) 032511. [7] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [8] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, S.G. Lee, Appl. Phys. Lett. 86 (2005) 222903. [9] S.T. Zhang, Y. Zhang, M.H. Lu, C.L. Du, Y.F. Chen, Z.G. Liu, Y.Y. Zhu, N.B. Ming, Appl. Phys. Lett. 88 (2006) 162901. [10] W. Eerenstein, F.D. Morrison, J. Dho, M.G. Blamire, J.F. Scott, N.D. Mathur, Science 307 (2005) 1203a. [11] F. Tyholdt, S. Jorgensen, H. Fjellvag, A.E. Gunnaes, J. Mater. Res. 20 (2005) 2127. [12] V.R. Palkar, J. John, R. Pinto, Appl. Phys. Lett. 80 (2002) 1628. [13] X. Qi, J. Dho, R. Tomov, M.G. Blamire, J.L. MacManus-Driscoll, Appl. Phys. Lett. 86 (2005) 062903. [14] K.Y. Yun, M. Noda, M. Okuyama, H. Saeki, H. Tabata, K. Saito, J. Appl. Phys. 96 (2004) 3399. [15] Y.W. Li, J.L. Sun, J. Chen, X.J. Meng, J.H. Chu, J. Cryst. Growth 285 (2005) 595. [16] J.K. Kim, S.S. Kim, W.J. Kim, A.S. Bhalla, . Guo, Appl. Phys. Lett. 88 (2006) 132901. [17] K.Y. Yun, M. Noda, M. Okuyama, Appl. Phys. Lett. 83 (2003) 3981. [18] H. Uchida, R. Ueno, H. Nakaki, H. Funakubo, S. Koda, Jpn. J. Appl. Phys., Part 2 44 (2005) L561.