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The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 thin films
Journal Pre-proofs The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3−0.45BaTiO3 thin films Ming-Yuan Yan, Jian-Min Yan, Zhi-Xue Xu, Hui Wang, Meng Xu, Guan-Yin Gao, Fei-Fei Wang, Ren-Kui Zheng PII: DOI: Reference:
S0167-577X(20)30148-8 https://doi.org/10.1016/j.matlet.2020.127443 MLBLUE 127443
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
17 December 2019 20 January 2020 29 January 2020
Please cite this article as: M-Y. Yan, J-M. Yan, Z-X. Xu, H. Wang, M. Xu, G-Y. Gao, F-F. Wang, R-K. Zheng, The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3−0.45BaTiO3 thin films, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet. 2020.127443
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 thin films Ming-Yuan Yana,b, Jian-Min Yanb, Zhi-Xue Xub, Hui Wangc, Meng Xub, Guan-Yin Gaod, Fei-Fei Wanga,*, Ren-Kui Zhengb,c,* a Key
Laboratory of Optoelectronic Material and Device, Department of Physics, Shanghai Normal University, Shanghai 200234, China
b
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China c
School of Materials Science and Engineering, Nanchang University, and Jiangxi Engineering Laboratory for Advanced Functional Thin Films, Nanchang 330031, China
d
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
Abstract: 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMOBTO) thin films have been epitaxially grown on (001)-oriented Nb-doped SrTiO3 (NSTO) single-crystal substrates by the pulsed laser deposition. X-ray diffraction, SAED, magnetic hysteresis loop, and piezoresponse force microscopy results show that the structural, and magnetic properties of the BFGMOBTO epitaxial films can be progressively tuned by the oxygen pressure (𝑃O2) during film deposition. For the films prepared at 𝑃O2=5 Pa phase separated into BFGMO and BTO phase, however, with increasing 𝑃O2 to 20 Pa the film evolves from two phases to single phase with the best crystalline quality, largest saturation magnetization, lowest coercive field and relatively easy-switchable ferroelectric domains. Keywords: epitaxial growth, thin films, multiferroic materials, pulsed laser deposition Electronic mail: [email protected] (R.K. Zheng); [email protected] (F.F. Wang) 1
1. Introduction Multiferroic materials have attracted an increasing interest due to their potential applications in novel types of electronic memory devices [1-3]. However, there are very few single-phase multiferroic materials whose magnetoelectric (ME) coupling effects is, in fact, extremely weak at room temperature or only appears at low temperatures [4, 5], which strongly limits their applications. In order to obtain multiferroic materials with strong ME coupling at room temperature, researchers have paid more attention to multiferroic composite materials which can realize ME coupling through lattice strain effects [6]. Since the report of strong ME coupling in BaTiO3CoFe2O4 (BTOCFO) composite thin films [7], the 1-3 type self-assembled nanocomposite films have been actively investigated due to their strong ME coupling effect. As of now, many kinds of perovskite-spinel composite films with the self-assembled nanostructures such as CoFe2O4PbTiO3 [8], BiFeO3CoFe2O4 [9], have been investigated. It is also worth noting that the (1-x)BiFeO3–xBaTiO3 ceramics have attracted much attention in recent years due to their enhanced multiferroic properties [10-12]. However, there is few reports about the (1-x)BiFeO3–xBaTiO3 thin films. In this letter, the 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMOBTO) films were first deposited on (001)-oriented Nb-doped SrTiO3 single-crystal substrates by the pulsed laser deposition. The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties were discussed. 2. Materials and methods The 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 target was synthesized by the rapid sintering technique. High purity Bi2O3 (99.9%), Fe2O3 (99.5%), Ga2O3 (99.99%), MnO2
2
(99.99%), BaCO3 (99.8%), TiO2 (99.9%) were thoroughly mixed and ball milled for 12 h, dried and rapidly heated to 1025 oC in 5 min and calcinated for 10 min and then cooled to room temperature in 5 min. 223 mm pellet obtained by pressing calcinated powders was rapidly heated to 1050 oC again in 5 min and sintered for 10 min and cooled to room temperature in 5 min. BFGMOBTO films were grown on one-side polished (001)-oriented NSTO single-crystal substrates by the pulsed laser deposition (PLD) using a KrF excimer laser (λ=248 nm). During film deposition the substrate temperature, laser energy density, and pulse repetition rate were kept at 660 oC, 2 J/cm2, and 3 Hz, respectively. The films were grown at different fixed oxygen pressures ranging from 5 to 25 Pa. The distance between the target and the substrates is 5 cm. The as-grown films were in situ annealed in 1 atom oxygen for 1 h before cooling down to room temperature. The crystal structure of films was characterized using a PANalytical X’Pert PRO X-ray diffractometer with CuKα1 radiation (λ=1.5406 Å). The atomic force microscopy (AFM) and piezoresponse force microscopy (PFM) images were obtained using a MFP-3D Infinity atomic force microscope (Oxford Instruments Asylum Research Inc.) at room
temperature.
The
magnetic
hysteresis
loops
were
measured
using
a
superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS-XL5). The selected area electron diffraction (SAED) is measured using a Tecnai G2F20 S-Twin transmission electron microscope. 3. Results and discussions Fig. 1(a)
shows
the
X-ray
diffraction
(XRD)
θ-2θ
patterns
of
the
0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMO-BTO) ceramic target. All diffraction peaks can be indexed to the standard pattern (PDF#47-1962) of BaTiO3, indicating that 3
the target is single phase. Fig. 1(b) shows the XRD θ-2θ scan patterns of BFGMOBTO films prepared at different oxygen pressures. The pattern of the BFGMOBTO film grown at 5 Pa oxygen pressure shows two sets of clearly separated diffraction peaks corresponding to the BFGMO (001) and BTO (001) and the BFGMO (002) and BTO (002) diffraction peaks, respectively, which indicates that the BTO and BFGMO films grew hetero-epitaxially on the NSTO substrate. The scans taken on the BFGMO (101), BTO (101), and NSTO (101) of the film prepared at 5 Pa oxygen pressure were shown in Fig. 1(e), where one can find that both the BFGMO and the BTO films had four-fold symmetries which implies the cube-on-cube epitaxial growth of the film on the NSTO (001) substrate. The SAED of BFGMO-BTO film grown at 5 Pa is shown in Fig. 1(f). Three appreciable sets of diffraction patterns can be identified, which further demonstrates the epitaxial relationship between BTO, BFGMO films and NSTO substrate. With increasing the oxygen pressure to 20 Pa, the two splitting peaks gradually merge into one diffraction peak. For the film prepared at 20 Pa, there are only the BFGMO-BTO (001) and (002) diffraction peaks, which corresponds to the SAED results shown in Fig. 1(g). Fig. 1(c) shows the XRD scan rocking curves taken on the BFGMO (002) diffraction peak for the films prepared at different pressures. It can be seen that the intensity of the diffraction peak for the 20-Pa film is much higher than that of films prepared at other pressures. As shown in the Fig. 1(d), the full width at half maximum (FWHM) is only 0.12o for the 20-Pa film, which is the smallest FWHM among these five thin-film samples and implies that its crystalline quality is superior to that of other sample. Cross-sectional SEM image shows that the thickness of the BFGMOBTO film prepared at 20 Pa is 700 nm. Fig. 2(a) and 2(b) show the magnetic hysteresis (M-H) loops of the BFGMO-BTO 4
films prepared at different oxygen pressures. All films display typical feature of the ferromagnetic hysteresis loops at T=10 K and 300 K, indicating that the BFGMOBTO films show weak ferromagnetism. The observed remanent magnetization is presumably due to the canted antiferromagnetic order of Fe–O–Fe spin chains [13]. The change in the bond angle of Fe–O–Fe may arise from the partial substitution of Ba2+ for Bi3+ and Ti4+/Ga3+/Mn4+ for Fe3+, which would break the cycloidal spin structure of the BiFeO3 matrix, thus releasing the locked magnetization and enhancing the ferromagnetism [13]. The maximum saturation magnetization Ms of 2.06 emu/cc and the minimum coercive field of 103.5 Gs was obtained in the film prepared at 20 Pa oxygen pressure. Fig.3 (a)-(i) show the composition and electric-field-induced out-of-plane (OP) PFM phase images of three different thicknesses of the 20-Pa BFGMO-BTO films. Fig. 3(j), 3(k), and 3(m) show the topography of the 175 nm (RMS=1.5 nm), 350 nm (RMS=0.28 nm) and 700 nm (RMS=5.2 nm) BFGMO-BTO films, respectively. As shown in Fig. 3(a)-3(f), the 175 nm and 350 nm films were poled by 10 V, 15 V in 11 and 33 m2 areas. The positive and negative voltage were applied to check the domain reversibility. For these two films, a low voltage of 10 V was able to pole the films where clear contrast between the inner and outer can be observed [Fig. 3(b) and 3(e)]. By applying a larger voltage (15 V) to the tip of the piezoresponse force microscope, the polarization could be switched more completely and the domain structure became more stable [Fig. 3(c) and 3(f)]. With increasing the film thickness to 700 nm, larger voltages were needed to pole the films. It can be seen from the Fig. 3(h), when 15 V voltages were applied to the 700 nm film, only apportion of ferroelectric domain could be poled. With the poling voltages further increased to 25 V, more domains could be switched, indicating thicker BFGMOBTO films require a higher electric field to 5
realize full domain switching.
4. Conclusions In summary, we have prepared BFGMOBTO epitaxial films with good crystalline quality on NSTO (001) substrates using the pulsed laser deposition. XRD and SAED results show that the films prepared at a low oxygen pressure (𝑃O2=5 Pa) phase separated into BFGMO and BTO phases and gradually grew into a single phase BFGMOBTO epitaxial films with increasing oxygen pressure to 20 Pa. Particularly, the films prepared at 20 Pa oxygen pressure shows the best crystallinity, largest saturation magnetization, lowest coercive field, and relatively easy-switchable ferroelectric domains. Our results may help researchers better understand the effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of BiFeO3BaTiO3 based solid solution films. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11974155, 11574214, 11974250). The Science and Technology Commission of Shanghai Municipality (Grant No. 19070502800). References [1] N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, S-W. Cheong, Nature 429 (2004) 392. [2] P. C. Rout, V. Srinivasan, Phys. Rev. Lett. 123 (2019) 107201. [3] N. A. Spaldin, R. Ramesh, Nat. Mater. 18 (2019) 203. [4] G. T. Rado, V. J. Folen, Phys. Rev. Lett. 7 (1961) 31. [5] I. Kornev, M. Bichurin, J. P. Rivera, S. Gentil, H. Schmid, A. G. M.Jansen, P. Wyder, Phys. Rev. B 62 (2000) 12247. [6] J. Ma, J. M. Hu, Z. Li, C.-W. Nan, Adv. Mater. 23 (2011) 1062. [7] H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. M. Ardabili, T. Zhao, L. S. Riba, S. R. 6
Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [8] J. H. Li, I. Levin, J. Slutsker, V. Provenzano, P. K. Schenck, R. Ramesh, J. Ouyang, A. L. Roytburd, Appl. Phys. Lett. 87 (2005) 072909. [9] H. M. Zheng, F. Straub, Q. Zhan, P. L. Yang, W. K. Hsieh, F. Zavaliche, Y. H. Chu, U. Dahmen, R. Ramesh. Adv. Mater. 18 (2006) 2747. [10] T.-H. Wang, Y. Ding, C.-S. Tu, Y.-D. Yao, K.-T. Wu, T.-C. Lin, H.-H. Yu, C.-S. Ku, H.-Y. Lee, Appl. Phys. Lett. 109 (2011) 07D907. [11] S. C. Yang, A. Kumar. V. Petkov, S. Priya, J. Appl. Phys. 113 (2013) 144101. [12] A. S. Priya, I. B. Shameem Banu, S. Anwar, Mater. Lett. 142 (2015) 42. [13] T. J. Park, G. C. Papaefthymiou, A. J. Viescas, Y. Lee, H. Zhou, S. S. Wong, Phys. Rev. B 82 (2010) 024431.
Fig. 1 (a) The XRD patterns of BFGMO-BTO target. (b) The XRD θ-2θ scan pattern of the BFGMO-BTO/NSTO structure under various working oxygen pressure; Inset (b): Cross-sectional SEM image for BFGMO-BTO film prepared under 20 Pa oxygen pressure. (c) XRD rocking curve taken on the (002) diffraction peak of the films under various working pressure. (d) The FWHM value of (002) diffraction peak versus oxygen pressure. (e) The ϕ -scan patterns taken on the (101) diffraction patterns of the BTO (I), BFGMO (II), BFGMO-BTO (III), and NSTO (IV), respectively. SAED pattern taken at the interface for the BFGMO-BTO /NSTO structure prepared at 5 Pa (f) and 20 Pa (g).
7
Fig. 2 Ferromagnetic hysteresis loops of the BFGMO-BTO films prepared at different oxygen pressures, as measured at T=10 (a) and 300 K (b).
Fig. 3. The electric-field-induced domain evolution (a-i) and topography (j-m) of the 20-Pa BFGMOBTO films with thicknesses of 175, 350, and 700 nm.
Credit Author Statement Ming-Yuan Yan and Jian-Min Yan: Prepared thin films and performed ferroelectric and ferromagnetic measurements. Zhi-Xue Xu: Carried out EDS and XPS measurements. Hui Wang and Meng Xu: Participated in the manuscript revision 8
Guan-Yin Gao: Performed XRD and TEM measurements. Fei-Fei Wang: Carried out AFM and PFM measurements. Ren-Kui Zheng: Designed and supervised the research. Ming-Yuan Yan, Jian-Min Yan and Ren-Kui Zheng wrote the paper.
Highlights:
The BFGMO-BTO film is successfully prepared by pulsed laser deposition method.
The structure and magnetic properties highly depend on the oxygen pressure.
The electric field used to switch ferroelectric domains increases with the film thickness.
The work contributes to finding a route to achieve more excellent room-temperature multiferroic films.
9
S0167-577X(20)30148-8 https://doi.org/10.1016/j.matlet.2020.127443 MLBLUE 127443
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
17 December 2019 20 January 2020 29 January 2020
Please cite this article as: M-Y. Yan, J-M. Yan, Z-X. Xu, H. Wang, M. Xu, G-Y. Gao, F-F. Wang, R-K. Zheng, The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3−0.45BaTiO3 thin films, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet. 2020.127443
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 thin films Ming-Yuan Yana,b, Jian-Min Yanb, Zhi-Xue Xub, Hui Wangc, Meng Xub, Guan-Yin Gaod, Fei-Fei Wanga,*, Ren-Kui Zhengb,c,* a Key
Laboratory of Optoelectronic Material and Device, Department of Physics, Shanghai Normal University, Shanghai 200234, China
b
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China c
School of Materials Science and Engineering, Nanchang University, and Jiangxi Engineering Laboratory for Advanced Functional Thin Films, Nanchang 330031, China
d
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
Abstract: 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMOBTO) thin films have been epitaxially grown on (001)-oriented Nb-doped SrTiO3 (NSTO) single-crystal substrates by the pulsed laser deposition. X-ray diffraction, SAED, magnetic hysteresis loop, and piezoresponse force microscopy results show that the structural, and magnetic properties of the BFGMOBTO epitaxial films can be progressively tuned by the oxygen pressure (𝑃O2) during film deposition. For the films prepared at 𝑃O2=5 Pa phase separated into BFGMO and BTO phase, however, with increasing 𝑃O2 to 20 Pa the film evolves from two phases to single phase with the best crystalline quality, largest saturation magnetization, lowest coercive field and relatively easy-switchable ferroelectric domains. Keywords: epitaxial growth, thin films, multiferroic materials, pulsed laser deposition Electronic mail: [email protected] (R.K. Zheng); [email protected] (F.F. Wang) 1
1. Introduction Multiferroic materials have attracted an increasing interest due to their potential applications in novel types of electronic memory devices [1-3]. However, there are very few single-phase multiferroic materials whose magnetoelectric (ME) coupling effects is, in fact, extremely weak at room temperature or only appears at low temperatures [4, 5], which strongly limits their applications. In order to obtain multiferroic materials with strong ME coupling at room temperature, researchers have paid more attention to multiferroic composite materials which can realize ME coupling through lattice strain effects [6]. Since the report of strong ME coupling in BaTiO3CoFe2O4 (BTOCFO) composite thin films [7], the 1-3 type self-assembled nanocomposite films have been actively investigated due to their strong ME coupling effect. As of now, many kinds of perovskite-spinel composite films with the self-assembled nanostructures such as CoFe2O4PbTiO3 [8], BiFeO3CoFe2O4 [9], have been investigated. It is also worth noting that the (1-x)BiFeO3–xBaTiO3 ceramics have attracted much attention in recent years due to their enhanced multiferroic properties [10-12]. However, there is few reports about the (1-x)BiFeO3–xBaTiO3 thin films. In this letter, the 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMOBTO) films were first deposited on (001)-oriented Nb-doped SrTiO3 single-crystal substrates by the pulsed laser deposition. The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties were discussed. 2. Materials and methods The 0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 target was synthesized by the rapid sintering technique. High purity Bi2O3 (99.9%), Fe2O3 (99.5%), Ga2O3 (99.99%), MnO2
2
(99.99%), BaCO3 (99.8%), TiO2 (99.9%) were thoroughly mixed and ball milled for 12 h, dried and rapidly heated to 1025 oC in 5 min and calcinated for 10 min and then cooled to room temperature in 5 min. 223 mm pellet obtained by pressing calcinated powders was rapidly heated to 1050 oC again in 5 min and sintered for 10 min and cooled to room temperature in 5 min. BFGMOBTO films were grown on one-side polished (001)-oriented NSTO single-crystal substrates by the pulsed laser deposition (PLD) using a KrF excimer laser (λ=248 nm). During film deposition the substrate temperature, laser energy density, and pulse repetition rate were kept at 660 oC, 2 J/cm2, and 3 Hz, respectively. The films were grown at different fixed oxygen pressures ranging from 5 to 25 Pa. The distance between the target and the substrates is 5 cm. The as-grown films were in situ annealed in 1 atom oxygen for 1 h before cooling down to room temperature. The crystal structure of films was characterized using a PANalytical X’Pert PRO X-ray diffractometer with CuKα1 radiation (λ=1.5406 Å). The atomic force microscopy (AFM) and piezoresponse force microscopy (PFM) images were obtained using a MFP-3D Infinity atomic force microscope (Oxford Instruments Asylum Research Inc.) at room
temperature.
The
magnetic
hysteresis
loops
were
measured
using
a
superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS-XL5). The selected area electron diffraction (SAED) is measured using a Tecnai G2F20 S-Twin transmission electron microscope. 3. Results and discussions Fig. 1(a)
shows
the
X-ray
diffraction
(XRD)
θ-2θ
patterns
of
the
0.55BiFe0.95Ga0.03Mn0.02O30.45BaTiO3 (BFGMO-BTO) ceramic target. All diffraction peaks can be indexed to the standard pattern (PDF#47-1962) of BaTiO3, indicating that 3
the target is single phase. Fig. 1(b) shows the XRD θ-2θ scan patterns of BFGMOBTO films prepared at different oxygen pressures. The pattern of the BFGMOBTO film grown at 5 Pa oxygen pressure shows two sets of clearly separated diffraction peaks corresponding to the BFGMO (001) and BTO (001) and the BFGMO (002) and BTO (002) diffraction peaks, respectively, which indicates that the BTO and BFGMO films grew hetero-epitaxially on the NSTO substrate. The scans taken on the BFGMO (101), BTO (101), and NSTO (101) of the film prepared at 5 Pa oxygen pressure were shown in Fig. 1(e), where one can find that both the BFGMO and the BTO films had four-fold symmetries which implies the cube-on-cube epitaxial growth of the film on the NSTO (001) substrate. The SAED of BFGMO-BTO film grown at 5 Pa is shown in Fig. 1(f). Three appreciable sets of diffraction patterns can be identified, which further demonstrates the epitaxial relationship between BTO, BFGMO films and NSTO substrate. With increasing the oxygen pressure to 20 Pa, the two splitting peaks gradually merge into one diffraction peak. For the film prepared at 20 Pa, there are only the BFGMO-BTO (001) and (002) diffraction peaks, which corresponds to the SAED results shown in Fig. 1(g). Fig. 1(c) shows the XRD scan rocking curves taken on the BFGMO (002) diffraction peak for the films prepared at different pressures. It can be seen that the intensity of the diffraction peak for the 20-Pa film is much higher than that of films prepared at other pressures. As shown in the Fig. 1(d), the full width at half maximum (FWHM) is only 0.12o for the 20-Pa film, which is the smallest FWHM among these five thin-film samples and implies that its crystalline quality is superior to that of other sample. Cross-sectional SEM image shows that the thickness of the BFGMOBTO film prepared at 20 Pa is 700 nm. Fig. 2(a) and 2(b) show the magnetic hysteresis (M-H) loops of the BFGMO-BTO 4
films prepared at different oxygen pressures. All films display typical feature of the ferromagnetic hysteresis loops at T=10 K and 300 K, indicating that the BFGMOBTO films show weak ferromagnetism. The observed remanent magnetization is presumably due to the canted antiferromagnetic order of Fe–O–Fe spin chains [13]. The change in the bond angle of Fe–O–Fe may arise from the partial substitution of Ba2+ for Bi3+ and Ti4+/Ga3+/Mn4+ for Fe3+, which would break the cycloidal spin structure of the BiFeO3 matrix, thus releasing the locked magnetization and enhancing the ferromagnetism [13]. The maximum saturation magnetization Ms of 2.06 emu/cc and the minimum coercive field of 103.5 Gs was obtained in the film prepared at 20 Pa oxygen pressure. Fig.3 (a)-(i) show the composition and electric-field-induced out-of-plane (OP) PFM phase images of three different thicknesses of the 20-Pa BFGMO-BTO films. Fig. 3(j), 3(k), and 3(m) show the topography of the 175 nm (RMS=1.5 nm), 350 nm (RMS=0.28 nm) and 700 nm (RMS=5.2 nm) BFGMO-BTO films, respectively. As shown in Fig. 3(a)-3(f), the 175 nm and 350 nm films were poled by 10 V, 15 V in 11 and 33 m2 areas. The positive and negative voltage were applied to check the domain reversibility. For these two films, a low voltage of 10 V was able to pole the films where clear contrast between the inner and outer can be observed [Fig. 3(b) and 3(e)]. By applying a larger voltage (15 V) to the tip of the piezoresponse force microscope, the polarization could be switched more completely and the domain structure became more stable [Fig. 3(c) and 3(f)]. With increasing the film thickness to 700 nm, larger voltages were needed to pole the films. It can be seen from the Fig. 3(h), when 15 V voltages were applied to the 700 nm film, only apportion of ferroelectric domain could be poled. With the poling voltages further increased to 25 V, more domains could be switched, indicating thicker BFGMOBTO films require a higher electric field to 5
realize full domain switching.
4. Conclusions In summary, we have prepared BFGMOBTO epitaxial films with good crystalline quality on NSTO (001) substrates using the pulsed laser deposition. XRD and SAED results show that the films prepared at a low oxygen pressure (𝑃O2=5 Pa) phase separated into BFGMO and BTO phases and gradually grew into a single phase BFGMOBTO epitaxial films with increasing oxygen pressure to 20 Pa. Particularly, the films prepared at 20 Pa oxygen pressure shows the best crystallinity, largest saturation magnetization, lowest coercive field, and relatively easy-switchable ferroelectric domains. Our results may help researchers better understand the effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of BiFeO3BaTiO3 based solid solution films. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11974155, 11574214, 11974250). The Science and Technology Commission of Shanghai Municipality (Grant No. 19070502800). References [1] N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, S-W. Cheong, Nature 429 (2004) 392. [2] P. C. Rout, V. Srinivasan, Phys. Rev. Lett. 123 (2019) 107201. [3] N. A. Spaldin, R. Ramesh, Nat. Mater. 18 (2019) 203. [4] G. T. Rado, V. J. Folen, Phys. Rev. Lett. 7 (1961) 31. [5] I. Kornev, M. Bichurin, J. P. Rivera, S. Gentil, H. Schmid, A. G. M.Jansen, P. Wyder, Phys. Rev. B 62 (2000) 12247. [6] J. Ma, J. M. Hu, Z. Li, C.-W. Nan, Adv. Mater. 23 (2011) 1062. [7] H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. M. Ardabili, T. Zhao, L. S. Riba, S. R. 6
Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [8] J. H. Li, I. Levin, J. Slutsker, V. Provenzano, P. K. Schenck, R. Ramesh, J. Ouyang, A. L. Roytburd, Appl. Phys. Lett. 87 (2005) 072909. [9] H. M. Zheng, F. Straub, Q. Zhan, P. L. Yang, W. K. Hsieh, F. Zavaliche, Y. H. Chu, U. Dahmen, R. Ramesh. Adv. Mater. 18 (2006) 2747. [10] T.-H. Wang, Y. Ding, C.-S. Tu, Y.-D. Yao, K.-T. Wu, T.-C. Lin, H.-H. Yu, C.-S. Ku, H.-Y. Lee, Appl. Phys. Lett. 109 (2011) 07D907. [11] S. C. Yang, A. Kumar. V. Petkov, S. Priya, J. Appl. Phys. 113 (2013) 144101. [12] A. S. Priya, I. B. Shameem Banu, S. Anwar, Mater. Lett. 142 (2015) 42. [13] T. J. Park, G. C. Papaefthymiou, A. J. Viescas, Y. Lee, H. Zhou, S. S. Wong, Phys. Rev. B 82 (2010) 024431.
Fig. 1 (a) The XRD patterns of BFGMO-BTO target. (b) The XRD θ-2θ scan pattern of the BFGMO-BTO/NSTO structure under various working oxygen pressure; Inset (b): Cross-sectional SEM image for BFGMO-BTO film prepared under 20 Pa oxygen pressure. (c) XRD rocking curve taken on the (002) diffraction peak of the films under various working pressure. (d) The FWHM value of (002) diffraction peak versus oxygen pressure. (e) The ϕ -scan patterns taken on the (101) diffraction patterns of the BTO (I), BFGMO (II), BFGMO-BTO (III), and NSTO (IV), respectively. SAED pattern taken at the interface for the BFGMO-BTO /NSTO structure prepared at 5 Pa (f) and 20 Pa (g).
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Fig. 2 Ferromagnetic hysteresis loops of the BFGMO-BTO films prepared at different oxygen pressures, as measured at T=10 (a) and 300 K (b).
Fig. 3. The electric-field-induced domain evolution (a-i) and topography (j-m) of the 20-Pa BFGMOBTO films with thicknesses of 175, 350, and 700 nm.
Credit Author Statement Ming-Yuan Yan and Jian-Min Yan: Prepared thin films and performed ferroelectric and ferromagnetic measurements. Zhi-Xue Xu: Carried out EDS and XPS measurements. Hui Wang and Meng Xu: Participated in the manuscript revision 8
Guan-Yin Gao: Performed XRD and TEM measurements. Fei-Fei Wang: Carried out AFM and PFM measurements. Ren-Kui Zheng: Designed and supervised the research. Ming-Yuan Yan, Jian-Min Yan and Ren-Kui Zheng wrote the paper.
Highlights:
The BFGMO-BTO film is successfully prepared by pulsed laser deposition method.
The structure and magnetic properties highly depend on the oxygen pressure.
The electric field used to switch ferroelectric domains increases with the film thickness.
The work contributes to finding a route to achieve more excellent room-temperature multiferroic films.
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