Abnormal magnetic property in Fe-Doped BaTiO3 multiferroics

Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 49 ABNORMAL MAGNETIC PROPERTY IN Fe-DOPED BaTiO3 MULTIFERROICS Nguyen Van Dang1*, Nguyen Thi Dung1, Nguyen Van Khien1, To Manh Kien2, Nguyen Khac Hung1 1College of Science - TNU, 2Xuan Mai High School – Hanoi City SUMMARY Samples of multiferroic BaTi1-xFexO3 material (x = 0.0, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.2 and 0.3) were synthesized by using the solid-state reaction method. The influence of Fe substitution for Ti on the crystalline structure and the magnetic property of BaTi1-xFexO3 samples were investigated. The obtained X-ray diffraction patterns showed that the structure of the material sensitively depends on Fe dopant content, x, and transforms gradually from the tetragonal (P4mm) phase to the hexagonal (P63/mmc) one with increasing x. All of the samples exhibit both ferroelectricity and ferromagnetism at room temperature. The result of magnetization measurements showed that the magnetization at a magnetic field as high as 1 T abnormally depends on x, increases with increasing x to a maximum at x=0.1 then decreases monotonously afterward. There are possibilities to account for this anomalous magnetic behavior such as the Fe3+-to-Fe4+ and/or Ti4+-to-Ti3+ change(s) induced by oxygen vacancies. The substitution of Fe into Ti sites also changes the conductivity of the material and impurity (acceptor) levels in the band gap, which can be evident from the absorption spectra, electric impedance, and time- dependent leakage current measured at room temperature. Key words: Multiferroics, XRD, UV - visible spectra, magnetization, model for ferromagnetism INTRODUCTION* BaTiO3 has been well known for its ferroelectricity at room temperature, high permittivity, wide band gap, and numerous dielectric-based applications. It has also attracted great attention of basic research due to its different polymorphs, which exist at various temperature ranges: rhombohedral (T < -90 0C), orthorhombic (-90 0C < T < 5 0C), tetragonal (0 oC < T < 130 oC), cubic (130 oC 1460 0C). Nevertheless, partially doping Fe or Mn with high content, for instance, may stabilize the hexagonal polymorph at room temperature [1- 5]. Moreover, Ren [6] reported a giant electrostrain effect (about 0.75% at 200 V/mm) in Fe-doped BaTiO3 single crystals based on a new mechanism of the symmetry of point defects, in which defect dipole moments yield internal storing forces for recovering the reversibility of domain switching. * Tel: 0983 009975, Email: nvdangsptn@yahoo.com Increasing attention has been also paid to studying the Fe-doped BaTiO3 material because of its interesting magneto-optical properties [7,8]. Most importantly, the Fe- doped BaTiO3 material is currently addressed to investigate as a multiferroic material, in which magnetism and ferroelectricity coexist in a structurally-single phase system at room temperature [1,2,9], which is of importance in the development of multifunctional materials for spintronic devices [8-10]. In this report, we present our experimental results for BaTi1- xFexO3 ceramic samples. The influence of the Fe doping on the crystalline structure is investigated using X-ray diffraction patterns and Raman scattering spectra, and on the band structure via UV absorption spectra and electric impedance and leakage current measurements. Most importantly, we report an abnormal behavior of the magnetization with respect to the doping content of Fe ions. Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 50 EXPERIMENTAL Ceramic BaTi1-xFexO3 samples with various x values in the range of x = 0.0 to x = 0.3 were prepared using conventional solid-state reaction method from BaCO3, TiO2, and Fe2O3 powders (99.99% in purity). The detail of sample preparation was described in our previous report [3, 14-17]. Crystalline phase structure was studied using both the X-ray diffraction carried out on a Siemens D5000 X-ray diffractometer using CuKα (λ = 1.54056 Å) radiation with the 2θ angle in the range of 20-80o and step size of 0.02o, and using Raman scattering spectra measured on a Micro Raman LABRAM 1B system in the range of 100-1000 cm-1. The excitation light was provided by an Ar+ laser, using the 488 nm line. The absorption spectrum was measured on a Jasco 670 UV in the range of 300-1200 nm. The electric impedance was measured on an LCR meter 3550, Tegam. The leakage current was measured on a Precision Premier, Radiant. The magnetization measurements were done on a PPMS 6000 system. All of the characterizations were performed at room temperature. RESULTS AND DISCUSSIONS Figure 1 presents the X-ray diffraction patterns of our BaTi1-xFexO3 samples. It is clearly seen that for x = 0.0 the tetragonal phase of the BaTiO3 material is indicated by single peaks at 2θ = 31.50, 38.80, etc. (lattice constants of a = b = 3.988 Å and c = 4.026 Å). Nevertheless, when Fe dopant content is higher than or equal to 0.07, the hexagonal phase of the BaTiO3 perovskite structure starts to show up in coexistence with the tetragonal phase. This coexistence can be observed clearly from the separation of original tetragonal peaks such as at 2θ = 31.50, 38.80, and 56.20 into twin-like peaks, where the positions of original tetragonal peaks are fixed while the hexagonal peaks locate in their vicinity. The analysis of our X- ray diffraction data reveals a gradual transformation from the tetragonal phase to the hexagonal one with increasing x. This phenomenon is called the stabilization of the hexagonal polymorph in the bulk form of BaTiO3 when doping Ti with Fe or Mn [1-5, 14-17]. Figure 1. X-ray diffraction patterns for samples with 0 ≤ x ≤ 0.12 measured at room temperature. The peaks and the Miller indices for the respective corresponding planes of the tetragonal (circles) and the hexagonal (squares) phases are indicated. Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 51 Notice that there are no peaks on the X-ray patterns responsible for any unexpected impure crystal phase such as iron oxide(s), titanium oxide, barium oxide, etc. shown in all patterns able to be detected by our Siemens D5000 X-ray diffractometer. This means that our samples BaTi1-xFexO3 are of high quality for a further quantitative analysis, which will be published elsewhere. Figure 2. Raman spectra of samples with various Fe dopant contents measured at room temperature. The inset shows the Raman spectrum of undoped BaTiO3 sample. Raman scattering spectroscopy is a useful tool to study the lattice dynamics as well as structural phases. We measured Raman scattering spectra for all samples in order to understand further how Fe dopant content affects on the structural phase transformation. Fig. 2 presents the Raman spectra in the range of 100-1000 cm-1 for all samples. Firstly, the Raman spectrum of our sample with x = 0 is shown in the inset of Fig. 2, in which the typical Raman peaks characteristic of BaTiO3 tetragonal phase are indicated at 180 cm-1 (E mode), 265 cm-1 and 520 cm-1 (A1 mode), 306 cm-1 (B1 mode), and 719 cm-1 (A1 mode). When increasing Fe dopant content x > 0.07, three new peaks appear at 154 cm-1, 218 cm-1, and 636 cm-1, respectively. According to the standard Raman scattering spectra of the hexagonal BaTiO3 single crystal at room temperature revealed by Yamaguchi et al. [13], these new peaks all belong to the hexagonal phase and concretely assigned as follows: peaks at 154 cm-1 and 636 cm-1 are respectively assigned to the one-phonon scattering of the E1g mode and A1g mode, but the peak at 218 cm-1 is assigned to the E1g mode, also called a broad band at 200 cm-1, which may be caused by two-phonon scattering [13]. Secondly, while peaks at 250 cm-1 and 265 cm-1 of the tetragonal phase disappear completely, those at 520 cm-1 and 719 cm-1 of this phase are reduced into shoulders. Nevertheless, the phonon modes responsible for these peaks do not become Raman-inactive, but the peaks are relatively low in height compared to the ones of the hexagonal phase and are suppressed into the spectral background. Eventually, both our Raman scattering and X-ray diffraction measurements are in agreement to illustrate the transformation between tetragonal and hexagonal phases in our samples of BaTi1- xFexO3 ceramics. Figure 3. UV absorption spectra for samples with various Fe dopant content measured at room temperature. The inset shows schematically the band structure in the presence of impurity bands As has been well known that undoped BaTiO3 is a material of wide band gap of about 3.2 eV, this can be seen clearly with an absorption edge in the absorption spectrum in the UV region for the sample x=0.0 shown in Fig. 3. When doping Fe for Ti, the impurity levels of Fe ions can be created inside the gap, which form impurity bands (see the inset of Fig. 3). At low Fe dopant content, the distances between these bands and those between the valence band and the lowest impurity band act like effective band gaps. Nevertheless, with increasing Fe dopant content, the widths of these impurities bands increase and these bands could overlap together. This physical picture of how Fe doping affects on the band gap structure can be applied to interpret the changes of the absorption spectra shown in Fig. 3. The 200 400 600 800 1000 Raman Shift (cm-1) In te n sit y (a. u ) 140 280 420 560 700 719 520 306 265 180 x = 0.0 In te n sit y (a. u ) Raman Shift (cm -1) x = 0.07 x = 0.08 x = 0.09 x = 0.10 x = 0.11 x = 0.12 x = 0.20 x = 0.30 400 600 800 1000 1200 A bs o rb a n ce (a . u . ) Wavelength (nm) x = 0.00 x = 0.07 x = 0.08 x = 0.09 x = 0.10 x = 0.11 x = 0.12 x = 0.20 x = 0.30 Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 52 broadening of the absorption spectra with respect to x implies a tendency of widening and overlapping of impurity bands with increasing Fe dopant content. This physical picture also accounts for the increasing conductivity with respect to x from our electric impedance (Fig. 4) and linkage current (Fig. 5) measurements, which show that Fe-doped BaTiO3 samples behave more metallic with increasing x. More detailed addressing the influence of Fe dopant content on the ferroelectricity and conductivity of the Fe-doped BaTiO3 samples regarding the electric impedance and leakage current measurements in Figs. 4 and 5 will be discussed elsewhere. Magnetization versus magnetic field (M-H) curves measured at room temperature are presented in Fig. 6 for all samples. It is clearly seen in the figure that all of the samples exhibit ferromagnetism at room temperature. However, the coercivity, Hc, and magnetization at 1 Tesla, M1T, do not vary monotonically with respect to x. Instead, they both show clear maxima at x = 0.12 and x = 0.10 for Hc and M1T, respectively (see the inset of Fig. 6). To our knowledge, this abnormal magnetic behavior is for the first time observed by our group. According to the Mossbauer experiments carried by Lin et al. [1] for Fe-doped BaTiO3, when substituting for Ti4+ sites the Fe ions only exist in the Fe3+ valence state, but not in Fe2+ or Fe4+ at all in the cases of high population of Oxygen vacancies. In these cases, Fe3+ ions occupy at pentahedral (penta) and octahedral (octa) sites with a decrease of the ratio of penta sites to octa sites when increasing x. The competition between super-exchange interactions (e.g., between oct Fe3+ ions, between penta Fe3+ ions, between penta Fe3+ and octa Fe3+) could be employed to account for the ferromagnetism in the Fe-doped BaTiO3. Nevertheless, Lin et al [2] also showed that when reducing dramatically the population of Oxygen vacancies by treating the samples in rich-Oxygen atmosphere, for instance, some Fe3+ ion can be oxidized to a higher Fe4+ valence state. The existence of mixed valence state of Fe ions implies the appearance of the double exchange interaction, which favors the ferromagnetism and enhances both magnetization and coercivity. This accounts for the case of our sample with x = 0.12, where the coercivity is highest with a comparably high magnetization. Moreover, the transferring from Ti4+ to Ti3+ valence states and associated super-exchange/double exchange interactions among them could be another possibility. Finally, the oxygen vacancies play the role as effective positive charge sites, which attract electrons to crow around them. As a result, the spins of these electrons may be polarized by magnetic moments of local Fe ion sites, which also contribute to the change of ferromagnetism in our samples. In order to get more insight into the origin of the ferromagnetism, further investigation should be done. Figure 4. Results for the electric impedance measured at room temperature for samples with various Fe dopant content. 0 1.2 2.4 3.6 0 4 8 12 data fit Z' (x107 Ω) Z" (x1 07 Ω ) x = 0.0a) 0 2 4 6 data fit 0 1.2 2.4 3.6 x = 0.07b) Z' (x107 Ω) Z" (x1 07 Ω ) 0 1.4 2.8 4.2 0 0.8 1.6 2.4 x = 0.3 data fit e) 0 1 2 3 4 5 0 1 2 3 4 data fit1 fit2 Z" (x 10 6 Ω ) Z' (x106 Ω) 0 2.4 4.8 7.2 0 0.8 1.6 2.4 data fit1 fit2 Z" (x 10 6 Ω ) Z' (x107 Ω) Z' (x106 Ω) Z" (x1 05 Ω ) 0 0.8 1.6 2.4 0 2 4 6 data fit x = 0.10c) Z' (x107 Ω) Z" (x1 07 Ω ) 0 0.8 1.6 2.4 0 2.5 5 7.5 data fit x = 2.0d) Z" (x1 06 Ω ) Z' (x106 Ω) Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 53 Figure 5. Time dependence of leakage currents measured at room temperature for all samples. The inset shows the result for sample x=0.3 separately plotted due to out-of-scale magnitudes. Figure 6. M-H curves of BaTi1-xFexO3 ceramics with various Fe dopant contents measured at room temperature. The inset shows Hc and M1T versus x. CONCLUSIONS We have reported our experimental investigation of how Fe doping affects on structural and magnetic properties of BaTi1- xFexO3 ceramics synthesized by using the solid-state reaction method. When increasing the Fe dopant content, the crystalline structure transform gradually from the tetragonal phase to hexagonal phase indicated in both X-ray diffraction patterns and Raman spectra. Moreover, increasing x, the impurity bands inside the band gap, which are created by Fe ion levels, broaden and overlap leading to the broadening of the UV absorption spectrum and to more metallic property of the samples in terms of the electric impedance and leakage current measurements. Finally, the abnormal variation of coercivity and magnetization with respect to x could be interpreted due to transferring valence states of Fe and/or Ti ions as a result of the variation of oxygen vacancies population with increasing Fe dopant content. REFERENCES [1]. F. Lin, D. Jiang, X. Ma, and W. Shi, J. Magn. Magn. Mater. 320 (2008) 691. [2]. F. Lin, D. Jiang, X. Ma, and W. Shi, Physica B 403(17) (2008) 2525. [3]. N. V. Dang, T. D. Thanh, L. V. Hong, V. D. Lam, and The-Long Phan, J.Appl.Phys. 110 (2011) 043914-7. [4]. J. Akimoto, Y. Gotoh, and Y. Osawa, Acta Crystallogr., Sect. C: Cryst.Struct. Commun. 50 (1994) 160. [5]. N. Phoosit, T. Tunkasiri, J. Tontrakoon, and S. Phanichphant, J. Miscrosc. Soc. Thailand, 20(1) (2006) 64. [6]. X. Ren, Nature Materials 3 (2004) 91. [7]. A. Mazur, O. F. Schirmer, and S. Mendricks, Appl. Phys. Lett. 70 (1997) 2395. [8]. Rajamani, G. F. Dionne, D. Bono, and C. A. Ross, J. Appl. Phys. 98 (2005) 063907. [9]. R. Maier, J. L. Cohn, J. J. Neumeier, L. A. Bendersky, Appl. Phys. Lett. 78 (2001) 2536. [10]. K. F. Wang, J.-M. Liu, and Z. F. Ren, Ad. in Phys. 58 (2009) 321. [11]. M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) 123 [12]. C. W. Nan, M. I. Bichurin, S. X. Dong, and D. Vieland, J. Appl. Phys. 103 (2008) 031101. [13]. H. Yamaguchi, H. Uwe, T. Sakudo, and E. Sawaguchi, J. Phys. Soc. Jpn. 56 (1987) 589. [14]. N. V. Dang, Ha M. Nguyen, P.-Y. Chuang, T. D. Thanh, V. D. Lam, C.-H. Lee, L. V. Hong, Chinese Journal of Physics 50 (2) (2012) 262-270 [15]. N. V. Dang, Ha M. Nguyen, Pei-Yu Chuang, Jie-Hao Zhang, T. D. Thanh, Chih-Wei Hu, Tsan- Yao Chen, Hung-Duen Yang, V. D. Lam, Chih- Hao Lee and L. V. Hong, Journal of Applied Physics 111 (2012) 07D915-3. [16]. Ha M. Nguyen, N. V. Dang, Pei-Yu Chuang, T. D. Thanh, Chih-Wei Hu, Tsan-Yao Chen, V. D. Lam, Chih-Hao Lee and L. V. Hong, Appl. Phys.Lett. 99 (2011) 202501-3. [17]. N. V. Dang, T. D. Thanh, V. D. Lam, L. V. Hong, and The-Long Phan, Journal of Applied Physics 111 (2012) 113913-9. 5.667 10-8 8.5 10-8 1.133 10-7 1.417 10-7 1.7 10-7 1.983 10-7 0 200 400 600 800 1000 0.0 0.07 0.08 0.09 0.10 0.11 0.12 M ea su re d Cu rr en t ( A m ps ) Time (ms) 0 1.75 10-5 3.5 10-5 0 300 600 900 x = 0.3 M e as u re d Cu rr e n t (A m ps ) Time (ms) -0.1 -0.05 0 0.05 0.1 -1 104 -5000 0 5000 1 104 x = 0.00 x = 0.07 x = 0.08 x = 0.09 x = 0.10 x = 0.11 x = 0.12 x = 0.2 x = 0.3 M (em u /g ) H (Oe) 0 0.035 0.07 0.105 0 0.1 0.2 0.3 M (emu/g) M (em u /g ) Fe concentration (x) H c (O e) 0 500 1000 1500 2000 Hc (Oe) Nguyễn Văn Đăng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 49 - 54 54 TÓM TẮT TÍNH CHẤT TỪ DỊ THƯỜNG CỦA MULTIFERROICS BaTiO3 PHA TẠP Fe Nguyễn Văn Đăng1*, Nguyễn Thị Dung1, Nguyễn Văn Khiển, Tô Mạnh Kiên2, Nguyễn Khắc Hùng1 1Trường Đại học Khoa học - ĐH Thái Nguyên, 2Trường THPT Xuân Mai – Thành phố Hà Nội Vật liệu multiferroic BaTi1-xFexO3 (với x = 0.0, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.2 và 0.3), được tổng hợp bằng phương pháp phản ứng pha rắn. Ảnh hưởng của sự thay thế Fe cho Ti lên cấu trúc tinh thể và tính chất từ của vật liệu BaTi1-xFexO3 đã được khảo sát chi tiết. Kết quả phân tích giản đồ nhiễu xạ tia X và phổ tán xạ Raman cho thấy, cấu trúc của vật liệu chuyển từ cấu trúc tứ giác (P4mm) sang cấu trúc lục giác (P63/mmc) khi nồng pha tạp x tăng và phụ thuộc mạnh vào nồng độ pha tạp x. Tất cả các mẫu pha tạp đều đồng tồn tại cả tính chất sắt điện và tính chất sắt từ ở nhiệt độ phòng. Kết quả đo từ độ phụ thuộc từ trường của các mẫu cho thấy, từ độ của các mẫu tại từ trường 1 Tesla phụ thuộc không tuyến tính vào x: ban đầu khi x tăng, từ độ của các mẫu tăng và cực đại khi x = 0.1, sau đó từ độ giảm khi x tăng. Tính chất từ dị thường trong các mẫu pha tạp có thể liên quan đến sự thay đổi hóa trị từ Fe3+ sang Fe4+/hoặc từ Ti4+ sangTi3+ có nguyên nhân từ sự khuyết thiếu oxi trong mẫu. Ảnh hưởng của sự thay thế Fe cho Ti lên tính chất dẫn và độ rộng vùng cấm của vật liệu cũng được chúng tôi khảo sát thông qua các phép đo phổ hấp thụ, phổ tổng trở và dòng rò ở nhiệt độ phòng. Từ khoá: Multiferroics, nhiễu xạ tia X, phổ hấp thụ, từ độ, mô hình sắt từ. * Tel: 0983 009975, Email: nvdangsptn@yahoo.com