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Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration
Author’s Accepted Manuscript Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration Yilin Zhang, Ji Qi, Yuhan Wang, Yu Tian, Junkai Zhang, Tingjing Hu, Maobin Wei, Yanqing Liu, Jinghai Yang www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)32936-X https://doi.org/10.1016/j.ceramint.2017.12.230 CERI17110
To appear in: Ceramics International Received date: 15 November 2017 Revised date: 24 December 2017 Accepted date: 29 December 2017 Cite this article as: Yilin Zhang, Ji Qi, Yuhan Wang, Yu Tian, Junkai Zhang, Tingjing Hu, Maobin Wei, Yanqing Liu and Jinghai Yang, Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.230 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration Yilin Zhanga, Ji Qia, Yuhan Wangb, Yu Tiana, Junkai Zhanga, Tingjing Hua, Maobin Weia, Yanqing Liua,*, Jinghai Yanga,** a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, PR China
b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China
Corresponding author: Jilin Normal University, Changchun 130103, China Corresponding author name: Yanqing Liu, Jinghai Yang E-mail addresses: [email protected] (Yanqing Liu), [email protected] (Jinghai Yang) Abstract Mn-doped BiFeO3 (BiFe1–xMnxO3, x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) polycrystalline multiferroic thin films were successfully synthesized using the facile sol-gel spin-coating method. The crystal structures, surface features, elements valences, and magnetic properties of as-prepared samples were systematically explored. X-ray diffraction and Raman spectroscopy studies revealed the substitutions of Mn into the Fe site and a rhombohedral-to-orthorhombic phase transition. The Field Emission Scanning Electron Microscopy showed a decrease in the average particle sizes and an improvement of surface morphology with increasing the concentration of the substitutes. Energy-dispersive X-ray spectroscopy confirmed the
doping concentration of Mn2+ in the samples. X-ray photoelectron spectroscopy indicated the co-existence of Mn2+/Mn3+ ions in the doped films. The remnant magnetization value of BiFe0.90Mn0.10O3 thin film was found to be approximately six times than that of pure BiFeO3 thin film under a magnetic field of 10 kOe. The enhanced magnetic property of BiFe0.90Mn0.10O3 thin film was mainly ascribed to the structural distortion of spin cycloid and the enhancement of super-exchange interaction between the Fe3+ (Mn2+) and O2- ions. Keywords BiFeO3; Sol-gel technology; Structural phase transition; Magnetic material. 1. Introduction Since multiferroic materials for multifunctional devices have latent applications, such as transducers, spintronic memories, magnetic sensors and information storage, they have aroused an increasing proportion of attention in recent years [1, 2]. BiFeO3 (BFO), one of the most representative multiferroic materials, has intensively attracted tremendous interests due to the co-existence of ferroelectric (TC~1103 K) property and a G-type antiferromagnetic (TN~640 K) property above room temperature [3, 4]. The ferroelectricity of BFO originates from the structural distortion of 6s2 lone pair electrons of Bi3+ ions. The antiferromagnetism derives from the Fe-O-Fe super-exchange interaction [5, 6]. By electric or magnetic fields control the basic physical behaviors and agility, i.e., make BFO a significant material [7]. The recent boom in researches of highly strained BFO imply a new lead-free probe-based system with latency applications in data storage and actuators [8-10]. In addition, rising
research interests in the photovoltaic activity of BFO turn it into a potential candidate for multifunctional devices [11]. However, the drawback of high leakage current of BFO is known as a severe problem, limiting its better applications. On the other hand, BFO possesses a repetitive range cycloid spiral structure and a spin period of 64 nm along [110] direction [12, 13]. As a result, this structure counteracts the macroscopic magnetization and influences the survey of the linear magnetoelectric effect. Resulting from the reason known to specifically chosen doping agent is as usually applied and a valid way to modulate the structure of rhombohedrally distorted perovskite [14-16]. A specifically chosen doping agent will cause the distortion of FeO6 octahedra, which improves the density of surface charge and reduce the concentration of oxygen vacancies [5, 6, 17, 18]. The enhancement of ferromagnetism is due to uncompensated surface spin and the release of potential magnetization locked in the cycloid [7, 14, 19]. For the purpose of overcoming this kind of shortcomings, many research groups have studied the improvement of magnetization of BFO nanoparticles, bulks and films by either doping transition metal ions on Fe-sites, or doping by bivalent and trivalent rare earth ions on Bi-sites. Though several investigations have performed on the rare-earth ( La3+ [20], Er3+ [21], or Cr3+ [22] e.g.) substituted BFO thin films, there are few reports on Mn-doped BFO thin film. Compared with other elements, Mn-doped BFO thin film has the advantage lies in that Mn possesses magnetically activity and the multivalent states
enables the
crystal to be compensated for the charge. Even with a small concentration of doping,
its coupling with Fe ions also exhibits a large magnetization [23, 24]. Chen et al. [25] observed that the magnetization of BFO substituted by various concentrations of Mn and extremely pronounced hysteresis loops appeared in the neighborhood of magnetic field 80 kOe. But it is observed that the second phase (Bi2Fe4O9) appeared for the pure BFO. According to Chen et al. [26] reported that the most substantial increase in remnant magnetization (Mr = 5.1emu/cm3) could be viewed on to BFO doped with Mn of 0.1 concentration in the temperature of 10K. As mentioned above, a series of studies have been done on Mn-doped BFO thin films with different concentrations. However, it is rare to systematic study the effect of Mn-doped on the local electronic structure and magnetic properties of BFO at room temperature. As stated above, it can be expected that Mn doping can improve both remnant and saturation magnetization. In this paper, we utilized an ethylene glycol based sol-gel method to prepare BiFe1-xMnxO3 (BFMOx, x = 0, 0.03, 0.05, 0.10, 0.15, 0.20) polycrystalline thin films, The valence of Mn and state of oxidation in BFMOx thin films were analyzed by the mean of X-ray photoelectron spectroscopy measurements. In addition, the effects of Mn on the crystal structure, surface topography, magnetic properties of BFO thin films at room temperature were studied in detail. 2. Experimental procedure Single-phase polycrystalline Mn doped BFO thin films with different doping concentrations were fabricated using sol-gel technique. The contents of doped Mn of 0, 0.03, 0.05, 0.10, 0.15 and 0.20 had been used to synthesize thin films. By appropriate amounts of high-purity bismuth nitrate [Bi(NO3)3•5H2O], ferric nitrate
[Fe(NO3)3•9H2O] as well as manganese nitrate [H8Mn2O10] were used as the raw materials and were dissolved in ethylene glycol solvent at room temperature using Magnetic Stirrers for several hours. The BFMOx thin films were coated on cleaned silicon substrates via spin coating technology at 1000 rpm for 3s and 4000 rpm for 20s with the uniform precursor solution. Then they were baked at 350 °C for 6 min to remove volatile materials. Both spin coating and baking process were repeated for several times. Finally, all the thin films were crystallized at 500 °C for 1h and were cooled to room temperature in an air atmosphere. Fig. 1 shows the experimental procedure sketch. The structural characteristics of the thin films were done by X-ray diffraction (XRD) on a D/max 3C XRD using Cu Kα radiation (40 kV, 200 mA). A Field Emission Scanning Electron Microscopy (FESEM) Model Hitachi, S-570 was utilized to investigate the surface morphology of these samples and thickness of cross-section. The magnetic hysteresis (M-H) loops of the samples were measured by a Lake Shore 7407 vibrating sample magnetometer (VSM) at room temperature. Raman scattering spectra of BFMOx thin films were recorded in backscattering geometry with a Renishaw MicroRaman spectrometer. X-ray photoelectron spectroscopy (XPS) was conducted via Al Ka line by using a Thermo Scientific ESCALAB 250Xi A1440 system. 3. Results and discussion A sketch map of the BFO crystal structure is introduced in Fig. 2a-d. In order to better comprehend the principle of Mn doped BFO thin films and explain the
superexchange interaction. The red (and blue) ball stands for Fe (and Mn) atom and the gray octahedra with the green ball (O) in the corner represents FeO6 octahedra structure. Fig. 2a and b distinctly display that the lean angle of the superexchange interaction as a result of the doping of Mn due to the stability of the high energy Mn-O bond [27]. It is well known that the BFO thin film possesses a perovskite structure with a rhombohedral distortion (space group R3c). Fig. 2c shows the position of the body-center is occupied by a smaller radius of Fe3+ (0.64 Å), the apex angle is occupied by a larger radius of Bi3+ (1.08 Å), O2+ (1.26 Å) is in the face-center position, six oxygen atoms and its center of Fe atoms form a FeO6 octahedral structure [24, 25]. View Fig. 2d in the positive direction, the Mn ion occupies the central position of the oxygen ions tetrahedron [23]. The replacement of Mn in the Fe site results in a change in the Fe-O-Fe superexchange interactions, and affects the magnetic release of BFO, which can be confirmed by VSM in our work[26]. Fig. 3 shows the XRD patterns of the BFMOx (x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) thin films. All reflections are indexed into the XRD patterns of BFO thin film based on the pervoskite structure which is well consistent with the International Centre for Diffraction Data (ICSD card no. 71-2494) [28]. It indicates that the formation of rhombohedrally distorted perovskite structure (space group R3c) without any secondary phases such as Bi2Fe4O9 and Bi25FeO40 [29]. Fig. 3b shows the magnified X-ray patterns of various concentrations of Mn substituted thin films in 30-35 ranges. When Mn is doped with BFO, the (1 0 4) and (1 1 0) peaks are overlapped into a broad peak. Moreover, (104) and (110) peaks of BFMOx thin films
are found to shift towards a higher angle. These changes indicate that the rhombohedral structure is distorted to either monoclinic or tetragonal via Mn substitution [30]. The refined unit cell parameters are tabulated in Table 1. The lattice constants of pure BFO thin film are a = b= 5.5876 Å and c = 13.8862 Å. The value of the calculated lattice constants for BFMO0.01 are a = b = 5.5767 Å and c = 13.8602 Å, which is the smallest compared to the other samples. When the x is less than or equal to 0.10, these constants slightly decreased with the increase of Mn content. In contrast, when the Mn content is more than 0.10 and less than or equal to 0.20, the lattice constant increases. The similar behavior has also been reported in Er-doped BFO [30]. Thus, these results suggest that the rhombohedral structure is distorted by Mn substitution due to the different ionic radius of Mn compared with Fe [30-32]. The Goldschmidt tolerance factor can be calculated to reflect the distortion effect, by the following simple formula:
t ( RA RO ) / 21/ 2 ( RB RO ) where rA is the radii of Bi cation, rB is the average radius of Fe and Mn cation and rO is the radii of O anion in the appropriate coordination respectively. Preferably, the tolerance factor should be less than two or equal one [33, 34]. The tolerance factor of BFMOx (x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) is presented in Table1. It can be observed that the tolerance factor for BFO is 0.8403. The smaller the tolerance factor, the more violently is the distorting between the lattice. This is owing to the Fe-site ions cannot fill the space completely and instead the oxygen octahedra is leaned, pulling back the space [35]. The Fe-O-Fe bond angle plays a key role in the
conduction and magnetic properties because it controls the orbital overlap and magnetic exchange between O and Fe. From the change of the tolerance factor (t) of BFO to BFMO0.20 thin film, the fact that the distortion increase with the increase of the Mn concentration is obtained. In addition, these values are higher than 0.59, which is the standard for the ratio of the atomic (ionic) radius to the formation of interstitial solid solutions [36]. Therefore, it is difficult for Mn ions to enter the gap position of the BFO crystal. Conversely, it is likely that Mn replaces the Bi site. To understand the changes of vibrational properties induced by the Mn doping in BFO thin films, Raman scattering spectra of BFMOx thin films were carried out at room temperature, as showed in Fig. 4. For rhombohedral perovskite structure (R3c) BFO, the following irreducible expression can be used to summarize Raman activity patterns: Γ=4A1+9E [40]. Usually, low frequency modes are comprehended as nearly relation with Bi-O covalent bond [22, 41, 42]. In BFO thin film, it is noted that the locations of two strong scattering intensity peaks are 167.91 and 216.47 cm-1, which is consistent well with other reported results [37, 43]. As the doping concentration increases, the intensities of the two peaks decrease gradually. According to Yuan et al. [40], the decrease in strength of the A1-1 and E-2 modes involved the changing of Bi-O covalent bonds, owing to the fact that the Bi 6s2 lone pair electrons are weak in Stereo chemistry in the BFMOx thin films. In addition, the two peaks are widened and shifted to the high frequency mode. On the other hand, the high frequency mode is related to the Fe-O vibration [42]. The vibration modes in 400-600 cm-1 in RMnO3 have been interpreted as the antisymmetric bending and
stretching modes of MnO6 associated with the Jahn-Teller distortion [43]. The rise mode is about 625 cm-1 regarded as the basis of the MnO6 octahedral symmetrical stretching mode, which is also increased by Jahn-Teller distortion [43]. Fig. 4b shows a magnified chart in 575-675 cm-1. The high angle shift after doping is attributed to the higher electronegativity of Mn-O than Bi-O. Therefore, the intensity of the Raman peaks decrease and broaden in the BFMOx thin films are attributed into the substitution of Mn in the Fe site, which creates a decrease in the stereochemical activity of the films [44]. Mn ion is the major reason for the transformation of the octahedral distortion to the orthorhombic BFO structure. Consequently, it is reasonable to derive structural distortions appeared in BFMOx thin films. Additionally, Raman spectroscopy also displays features of the Si substrates at the position of 525 cm-1 [45, 46]. Thus, the phase transition of BFMOx can be proved by using XRD and Raman spectra. The SEM images of surface and cross-section of the BFMOx thin films are presented in Fig. 5a-f. It can be seen that there are no clear diffusion and apparent segregation between the Si substrates and the films. Furthermore, the cross-section thicknesses of the BFMOx thin films are estimated to be 429 nm, 396 nm, 220 nm, 204 nm and 189 nm, respectively. From the views of the surface morphologies, grains with wide pores between grains are noticed for BFO thin films. Though all the BFMOx thin films are uniform, they still slightly different. However, BFMOx thin films exhibit less small holes and more tense morphology than the pure BFO thin films. The phenomenon reveals that the Mn substituted BFO thin films can conduct
electrical conductivity to reduce the surface heterogeneity [47]. As can be observed from the histogram of the particle size distribution of all samples in Fig. 6, it is obvious that the average grain size decreases with the increase of Mn doping concentration. The decrease in grain size of BFMOx thin films can be interpreted by the suppression of oxygen vacancy concentration, which leads to slower movement of oxygen ions, thus decreasing the grain growth rate [48]. With the doping concentration of Mn increasing from 0.03 to 0.20, the clusters increased and the individuals decreased in size. Excessive Mn may lead to decrease of grain growth efficiency, because of the energy barrier to grain boundary motion and diffusion [49]. The composition of chemical elements in all samples was examined by EDS tech nique, as shown in Fig. 7a-f. The presence of only Bi, Fe, O and Mn four elements in the BFMOx thin films was confirmed by EDS analysis, indicating that Mn was doped into the samples and have no interference from other impurities elements. Besides, the EDS result suggests that the actual concentrations in BFMOx thin films are approximately 0.0359, 0.0520, 0.0950, 0.1534 and 0.2187, respectively. Fig. 8 shows the X-ray photoelectron spectroscopy (XPS) of Bi4f, Fe2p, O1s and Mn2p in BFMOx, corrected by the binding energy of the C1s line (284.8eV) , respectively. [50]. The spectrum of the Bi4f state is consisting of two different peaks of Bi4f7/2 and Bi4f5/2 at 158.71 and 164.01 eV, respectively. The energy difference between the two peaks of Bi4f is 5.3 eV, indicating the presence of trivalent Bi ions and Bi-O bonds in the BFO thin film. [51, 52]. According to the studies by Yoneda et al. [53], the generation of oxygen vacancies can compensate for the loss of Bi in the
experiment and the defect response reads as follows:
BiO3 / 2 Bi1δ O3 / 2( 1δ) δBi δVB''' 3/ 2δO 3/ 2δVO
The presence of oxygen vacancies is unavoidable via sol-gel method. The electronic structures of BFO thin films modified by Bi vacancies and Mn substitution are reflected in their magnetic behavior [51]. The XPS spectrum expanded from 700 to 740 eV (see Fig. 7 b). The binding energy of Fe2p3/2 and Fe2p1/2 located at 710.61 and 724.16 eV, respectively and are attributed to Fe-O bonds. A satellite peak is noticed at a position greater than 8.35 eV of the Fe2p3/2 principal peak. The satellite peak is deemed to be a feature of the oxidation state of Fe [54]. Due to the different d-orbital electron configurations, Fe2+ and Fe3+ will reveal at 6 eV or 8 eV above the 2p3/2 principal peaks and appear as satellite peaks [52]. Note that satellite peaks appear 8 eV above the 2p3/2 principal peak, indicating that Fe is mainly in the +3 valence state in this sample [52]. It can be proved that the substitution of Mn2+ for volatile Bi3+ ions can lead to suppress the valence fluctuation of Fe ions from +3 to +2 states in BFO thin film. Further, a number of studies have shown that less Fe2+ ions indicate less oxygen vacancies. In order to search the effect of doping Mn on the oxygen state, the O1s peak of the film was measured as shown in Fig. 8c. These are decomposed into three vastly different peaks at around 529.53, 531.61 and 532.7 eV and are ascribed to three distinct O species: O defects (OD), O2-ions (OL) playing a part in Bi-O bonds in the structure and O (OA) such as -OH and -CO3 [54]. According to studies by Yoneda et al., one role of doped Mn ions in BFO is as a hole acceptor [53]. In our experiments, since the Mn element is derived from Mn(NO3)2, divalent
Mn ions will seize a hole and become a trivalent Mn ion. The XPS of the Mn 2p3/2 peaks is shown in Fig. 8d. The primary peak has a binding energy of 641.8 eV. In BFMO0.20 samples, a shoulder peak can be observed below this energy, which is derived from a small amount of Mn2+ (642.2eV). Moreover, the intensities of peaks are getting stronger, demonstrating that the doping concentration gradually increased. This result reveals that Mn substitution decreases the oxygen related defects, which is well consistent with in other Mn-doped BFO samples [45, 54, 55]. So lattice distortion, structure transition and these morphological features will have an influence on the magnetic properties for BFO thin film, which are discussed as follows. Fig. 9a shows the magnetization (M) versus magnetic field (H) curves of 0, 0.03, 0.05, 0.10, 0.15 and 0.20 Mn doped thin films at room temperature. The BFO thin film shows lower spontaneous magnetization compared to other films. A weak residual magnetic moment may be attributed to the angle of inclination between Fe3+ ions. However, the presence of ferromagnetism is governed by the spiral spin structure [56]. This is due to the spatial modulation of the spiral spin structure to prevent the view of the net magnetization [57]. When the Mn is substituted, the M-H loop is no longer linear and displays hysteresis characteristics. With the increase of Mn concentration, the change of microstructure will suppress the spin structure of the cycloid, which
has been confirmed by the XRD results. As anticipated, the Mn
substitution results in weak ferromagnetic moments at room temperature. The saturation magnetization (Ms) values of pure, 0.03, 0.05, 0.10, 0.15 and 0.20 Mn-doped BFO thin films are 40.65, 70.03, 135.50, 154.04, 123.90 and 81.09
emu/cm3, respectively. The remnant magnetization (Mr) values of BFMOx thin films are 2.49, 7.53, 4.50, 14.49, 5.53 and 6.81 emu/cm3. More recently, Deng et al. observed the well-saturated M-H hysteresis loops of BFMO0.10 thin film with Ms value of 5.1 emu/cm3 [26]. Yue et al. reported that the Ms of the Bi0.85Pr0.15Fe0.97Mn0.03O3 thin film is three times higher than of pure BFO thin film, but it is still limited to 1.81emu/cm3 [58]. Furthermore, Ren et al. found the Mn and Cu co-doping thin films present the optimum magnetization (Ms ~ 5.25 emu/cm3) by a simple chemical solution deposition technique [57]. In our work, the magnetization of BFO thin film doped with 0.10 concentration is superior to the other results. The enhancement in Ms for 0.10 Mn doped BFO thin film is attributed to either spiral spin modulation destruction by Mn atoms or Jahn-Teller effect driven by structural distortion. The Jahn-Teller effect leads to fractional spin rearrangement by influencing the cationic interaction. High concentrations of Fe3+ ions in the Fe-O-Fe bond are beneficial for magnetic property display [59]. The decrease in magnetization value after doping of 0.10 Mn is due to the weakening of Fe3+-O-Mn2+ super-exchange interaction at high concentration of Mn [60, 61]. Accordingly, the increased magnetization of Mn doped BFO thin film indicates that it has potential value in application. 4. Conclusions In summary, a series of Mn-doped BFO thin films containing different Mn dopant contents were successfully prepared via a facile sol-gel route. Investigation by XRD reveals that BFO and BFMOx thin films are free of impurities. Raman and XRD
studies of these samples indicate that Mn substitute Fe site and an obvious crystal structure transformation with Mn content increasing is observed in BFMOx thin films. SEM images demonstrate that doping agent has a giant influence on enhanced uniformity of particle formation and crystallite size. X-ray photoelectron spectroscopy (XPS) analyse illustrates that Mn substituting Fe is mainly in the +3 valence state in the samples. The saturated magnetization value of the BFMO0.10 thin film is 14.49 emu/cm3, its remnant magnetization value is 154.04 emu/cm3 at an applied magnetic field of 10 kOe, respectively. Furthermore, we summarize that the reasons for the improvement of magnetization in BFMOx thin films depend mainly on the destroying of the spatially inhomogeneous spin-modulated incommensurate structure and the Fe-O-Fe super-exchange strength. Hence, all of these results can confirm that Mn doped BFO thin films are viable materials for applications in magnetic sensors, information storage and spintronic memories at room temperature. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51608226) and the Program for the development of Science and Technology of Jilin province (Item No. 20150204085GX). References [1] P.C. Sati, M. Kumar, S. Chhoker, Phase evolution, magnetic, optical, and dielectric properties of Zr-substituted Bi0.9Gd0.1FeO3 multiferroics, J. Am. Ceram. Soc. 98 (2015) 1884-1890. [2] W. Ye, G.Q. Tan, G.H. Dong, H.J. Ren, A. Xia, Improved multiferroic properties in
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Figure captions Fig. 1. Schematic diagram of the process for preparing thin film via sol-gel technology. Fig. 2. The superexchange interaction between the (a) Fe-O-Fe and (b) Fe-O-Mn, the perovskite crystal structure simulation patterns of (c) BFO and (d) Mn-substitution. Fig. 3. X-ray diffraction (XRD) patterns of BFMOx films on Si (100) substrates (a) XRD patterns of BFMOx and (b) XRD patterns BFMOx in the range of 2θ from 30 to 34. Fig. 4. (a) Different concentration Mn substituted of BFO thin film Raman scattering diagrams. (b) The magnified patterns of peaks around 575-675 cm-1. Fig. 5. SEM surface images and cross-sectional (a-f) of BFMOx films; (g) EDS images of BFMOx films. Fig. 6. Histograms regarding particle size distribution of all samples. Fig. 7. EDX spectrum of (a) BFO (b) BFMO0.03 (c) BFMO0.05 (d) BFMO0.10 (e) BFMO0.15 (f) BFMO0.20. Fig. 8. XPS spectra of the as-annealed BFMO0.10 thin films in the binding energy regions of (a) Bi4f, (b) Fe2p, (c) O1s and different concentration Mn substituted of BFO film XPS spectra of (d) Mn 2p. Fig. 9. Magnetic hysteresis loops measured at room temperature with the field applied in the plane for BFMOx thin films.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Tables: Table1 The structural parameters (a, b, c, V) and the tolerance factor (t) for BFMOx (x=0, 0.03, 0.05, 0.10, 0.15 and 0.20) thin films. Compositions BFO BFMO0.03 BFMO0.05 BFMO0.10 BFMO0.15 BFMO0.20
ahex = bhex (Å)
chex (Å)
V (Å3)
t
5.5876 5.5850 5.5801 5.5767 5.5852 5.5883
13.8862 13.8853 13.8699 13.8602 13.8722 13.8795
378.66 378.15 376.58 374.02 375.95 376.84
0.8403 0.8402 0.8401 0.8393 0.8388 0.8387
PII: DOI: Reference:
S0272-8842(17)32936-X https://doi.org/10.1016/j.ceramint.2017.12.230 CERI17110
To appear in: Ceramics International Received date: 15 November 2017 Revised date: 24 December 2017 Accepted date: 29 December 2017 Cite this article as: Yilin Zhang, Ji Qi, Yuhan Wang, Yu Tian, Junkai Zhang, Tingjing Hu, Maobin Wei, Yanqing Liu and Jinghai Yang, Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.230 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration Yilin Zhanga, Ji Qia, Yuhan Wangb, Yu Tiana, Junkai Zhanga, Tingjing Hua, Maobin Weia, Yanqing Liua,*, Jinghai Yanga,** a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, PR China
b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China
Corresponding author: Jilin Normal University, Changchun 130103, China Corresponding author name: Yanqing Liu, Jinghai Yang E-mail addresses: [email protected] (Yanqing Liu), [email protected] (Jinghai Yang) Abstract Mn-doped BiFeO3 (BiFe1–xMnxO3, x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) polycrystalline multiferroic thin films were successfully synthesized using the facile sol-gel spin-coating method. The crystal structures, surface features, elements valences, and magnetic properties of as-prepared samples were systematically explored. X-ray diffraction and Raman spectroscopy studies revealed the substitutions of Mn into the Fe site and a rhombohedral-to-orthorhombic phase transition. The Field Emission Scanning Electron Microscopy showed a decrease in the average particle sizes and an improvement of surface morphology with increasing the concentration of the substitutes. Energy-dispersive X-ray spectroscopy confirmed the
doping concentration of Mn2+ in the samples. X-ray photoelectron spectroscopy indicated the co-existence of Mn2+/Mn3+ ions in the doped films. The remnant magnetization value of BiFe0.90Mn0.10O3 thin film was found to be approximately six times than that of pure BiFeO3 thin film under a magnetic field of 10 kOe. The enhanced magnetic property of BiFe0.90Mn0.10O3 thin film was mainly ascribed to the structural distortion of spin cycloid and the enhancement of super-exchange interaction between the Fe3+ (Mn2+) and O2- ions. Keywords BiFeO3; Sol-gel technology; Structural phase transition; Magnetic material. 1. Introduction Since multiferroic materials for multifunctional devices have latent applications, such as transducers, spintronic memories, magnetic sensors and information storage, they have aroused an increasing proportion of attention in recent years [1, 2]. BiFeO3 (BFO), one of the most representative multiferroic materials, has intensively attracted tremendous interests due to the co-existence of ferroelectric (TC~1103 K) property and a G-type antiferromagnetic (TN~640 K) property above room temperature [3, 4]. The ferroelectricity of BFO originates from the structural distortion of 6s2 lone pair electrons of Bi3+ ions. The antiferromagnetism derives from the Fe-O-Fe super-exchange interaction [5, 6]. By electric or magnetic fields control the basic physical behaviors and agility, i.e., make BFO a significant material [7]. The recent boom in researches of highly strained BFO imply a new lead-free probe-based system with latency applications in data storage and actuators [8-10]. In addition, rising
research interests in the photovoltaic activity of BFO turn it into a potential candidate for multifunctional devices [11]. However, the drawback of high leakage current of BFO is known as a severe problem, limiting its better applications. On the other hand, BFO possesses a repetitive range cycloid spiral structure and a spin period of 64 nm along [110] direction [12, 13]. As a result, this structure counteracts the macroscopic magnetization and influences the survey of the linear magnetoelectric effect. Resulting from the reason known to specifically chosen doping agent is as usually applied and a valid way to modulate the structure of rhombohedrally distorted perovskite [14-16]. A specifically chosen doping agent will cause the distortion of FeO6 octahedra, which improves the density of surface charge and reduce the concentration of oxygen vacancies [5, 6, 17, 18]. The enhancement of ferromagnetism is due to uncompensated surface spin and the release of potential magnetization locked in the cycloid [7, 14, 19]. For the purpose of overcoming this kind of shortcomings, many research groups have studied the improvement of magnetization of BFO nanoparticles, bulks and films by either doping transition metal ions on Fe-sites, or doping by bivalent and trivalent rare earth ions on Bi-sites. Though several investigations have performed on the rare-earth ( La3+ [20], Er3+ [21], or Cr3+ [22] e.g.) substituted BFO thin films, there are few reports on Mn-doped BFO thin film. Compared with other elements, Mn-doped BFO thin film has the advantage lies in that Mn possesses magnetically activity and the multivalent states
enables the
crystal to be compensated for the charge. Even with a small concentration of doping,
its coupling with Fe ions also exhibits a large magnetization [23, 24]. Chen et al. [25] observed that the magnetization of BFO substituted by various concentrations of Mn and extremely pronounced hysteresis loops appeared in the neighborhood of magnetic field 80 kOe. But it is observed that the second phase (Bi2Fe4O9) appeared for the pure BFO. According to Chen et al. [26] reported that the most substantial increase in remnant magnetization (Mr = 5.1emu/cm3) could be viewed on to BFO doped with Mn of 0.1 concentration in the temperature of 10K. As mentioned above, a series of studies have been done on Mn-doped BFO thin films with different concentrations. However, it is rare to systematic study the effect of Mn-doped on the local electronic structure and magnetic properties of BFO at room temperature. As stated above, it can be expected that Mn doping can improve both remnant and saturation magnetization. In this paper, we utilized an ethylene glycol based sol-gel method to prepare BiFe1-xMnxO3 (BFMOx, x = 0, 0.03, 0.05, 0.10, 0.15, 0.20) polycrystalline thin films, The valence of Mn and state of oxidation in BFMOx thin films were analyzed by the mean of X-ray photoelectron spectroscopy measurements. In addition, the effects of Mn on the crystal structure, surface topography, magnetic properties of BFO thin films at room temperature were studied in detail. 2. Experimental procedure Single-phase polycrystalline Mn doped BFO thin films with different doping concentrations were fabricated using sol-gel technique. The contents of doped Mn of 0, 0.03, 0.05, 0.10, 0.15 and 0.20 had been used to synthesize thin films. By appropriate amounts of high-purity bismuth nitrate [Bi(NO3)3•5H2O], ferric nitrate
[Fe(NO3)3•9H2O] as well as manganese nitrate [H8Mn2O10] were used as the raw materials and were dissolved in ethylene glycol solvent at room temperature using Magnetic Stirrers for several hours. The BFMOx thin films were coated on cleaned silicon substrates via spin coating technology at 1000 rpm for 3s and 4000 rpm for 20s with the uniform precursor solution. Then they were baked at 350 °C for 6 min to remove volatile materials. Both spin coating and baking process were repeated for several times. Finally, all the thin films were crystallized at 500 °C for 1h and were cooled to room temperature in an air atmosphere. Fig. 1 shows the experimental procedure sketch. The structural characteristics of the thin films were done by X-ray diffraction (XRD) on a D/max 3C XRD using Cu Kα radiation (40 kV, 200 mA). A Field Emission Scanning Electron Microscopy (FESEM) Model Hitachi, S-570 was utilized to investigate the surface morphology of these samples and thickness of cross-section. The magnetic hysteresis (M-H) loops of the samples were measured by a Lake Shore 7407 vibrating sample magnetometer (VSM) at room temperature. Raman scattering spectra of BFMOx thin films were recorded in backscattering geometry with a Renishaw MicroRaman spectrometer. X-ray photoelectron spectroscopy (XPS) was conducted via Al Ka line by using a Thermo Scientific ESCALAB 250Xi A1440 system. 3. Results and discussion A sketch map of the BFO crystal structure is introduced in Fig. 2a-d. In order to better comprehend the principle of Mn doped BFO thin films and explain the
superexchange interaction. The red (and blue) ball stands for Fe (and Mn) atom and the gray octahedra with the green ball (O) in the corner represents FeO6 octahedra structure. Fig. 2a and b distinctly display that the lean angle of the superexchange interaction as a result of the doping of Mn due to the stability of the high energy Mn-O bond [27]. It is well known that the BFO thin film possesses a perovskite structure with a rhombohedral distortion (space group R3c). Fig. 2c shows the position of the body-center is occupied by a smaller radius of Fe3+ (0.64 Å), the apex angle is occupied by a larger radius of Bi3+ (1.08 Å), O2+ (1.26 Å) is in the face-center position, six oxygen atoms and its center of Fe atoms form a FeO6 octahedral structure [24, 25]. View Fig. 2d in the positive direction, the Mn ion occupies the central position of the oxygen ions tetrahedron [23]. The replacement of Mn in the Fe site results in a change in the Fe-O-Fe superexchange interactions, and affects the magnetic release of BFO, which can be confirmed by VSM in our work[26]. Fig. 3 shows the XRD patterns of the BFMOx (x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) thin films. All reflections are indexed into the XRD patterns of BFO thin film based on the pervoskite structure which is well consistent with the International Centre for Diffraction Data (ICSD card no. 71-2494) [28]. It indicates that the formation of rhombohedrally distorted perovskite structure (space group R3c) without any secondary phases such as Bi2Fe4O9 and Bi25FeO40 [29]. Fig. 3b shows the magnified X-ray patterns of various concentrations of Mn substituted thin films in 30-35 ranges. When Mn is doped with BFO, the (1 0 4) and (1 1 0) peaks are overlapped into a broad peak. Moreover, (104) and (110) peaks of BFMOx thin films
are found to shift towards a higher angle. These changes indicate that the rhombohedral structure is distorted to either monoclinic or tetragonal via Mn substitution [30]. The refined unit cell parameters are tabulated in Table 1. The lattice constants of pure BFO thin film are a = b= 5.5876 Å and c = 13.8862 Å. The value of the calculated lattice constants for BFMO0.01 are a = b = 5.5767 Å and c = 13.8602 Å, which is the smallest compared to the other samples. When the x is less than or equal to 0.10, these constants slightly decreased with the increase of Mn content. In contrast, when the Mn content is more than 0.10 and less than or equal to 0.20, the lattice constant increases. The similar behavior has also been reported in Er-doped BFO [30]. Thus, these results suggest that the rhombohedral structure is distorted by Mn substitution due to the different ionic radius of Mn compared with Fe [30-32]. The Goldschmidt tolerance factor can be calculated to reflect the distortion effect, by the following simple formula:
t ( RA RO ) / 21/ 2 ( RB RO ) where rA is the radii of Bi cation, rB is the average radius of Fe and Mn cation and rO is the radii of O anion in the appropriate coordination respectively. Preferably, the tolerance factor should be less than two or equal one [33, 34]. The tolerance factor of BFMOx (x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) is presented in Table1. It can be observed that the tolerance factor for BFO is 0.8403. The smaller the tolerance factor, the more violently is the distorting between the lattice. This is owing to the Fe-site ions cannot fill the space completely and instead the oxygen octahedra is leaned, pulling back the space [35]. The Fe-O-Fe bond angle plays a key role in the
conduction and magnetic properties because it controls the orbital overlap and magnetic exchange between O and Fe. From the change of the tolerance factor (t) of BFO to BFMO0.20 thin film, the fact that the distortion increase with the increase of the Mn concentration is obtained. In addition, these values are higher than 0.59, which is the standard for the ratio of the atomic (ionic) radius to the formation of interstitial solid solutions [36]. Therefore, it is difficult for Mn ions to enter the gap position of the BFO crystal. Conversely, it is likely that Mn replaces the Bi site. To understand the changes of vibrational properties induced by the Mn doping in BFO thin films, Raman scattering spectra of BFMOx thin films were carried out at room temperature, as showed in Fig. 4. For rhombohedral perovskite structure (R3c) BFO, the following irreducible expression can be used to summarize Raman activity patterns: Γ=4A1+9E [40]. Usually, low frequency modes are comprehended as nearly relation with Bi-O covalent bond [22, 41, 42]. In BFO thin film, it is noted that the locations of two strong scattering intensity peaks are 167.91 and 216.47 cm-1, which is consistent well with other reported results [37, 43]. As the doping concentration increases, the intensities of the two peaks decrease gradually. According to Yuan et al. [40], the decrease in strength of the A1-1 and E-2 modes involved the changing of Bi-O covalent bonds, owing to the fact that the Bi 6s2 lone pair electrons are weak in Stereo chemistry in the BFMOx thin films. In addition, the two peaks are widened and shifted to the high frequency mode. On the other hand, the high frequency mode is related to the Fe-O vibration [42]. The vibration modes in 400-600 cm-1 in RMnO3 have been interpreted as the antisymmetric bending and
stretching modes of MnO6 associated with the Jahn-Teller distortion [43]. The rise mode is about 625 cm-1 regarded as the basis of the MnO6 octahedral symmetrical stretching mode, which is also increased by Jahn-Teller distortion [43]. Fig. 4b shows a magnified chart in 575-675 cm-1. The high angle shift after doping is attributed to the higher electronegativity of Mn-O than Bi-O. Therefore, the intensity of the Raman peaks decrease and broaden in the BFMOx thin films are attributed into the substitution of Mn in the Fe site, which creates a decrease in the stereochemical activity of the films [44]. Mn ion is the major reason for the transformation of the octahedral distortion to the orthorhombic BFO structure. Consequently, it is reasonable to derive structural distortions appeared in BFMOx thin films. Additionally, Raman spectroscopy also displays features of the Si substrates at the position of 525 cm-1 [45, 46]. Thus, the phase transition of BFMOx can be proved by using XRD and Raman spectra. The SEM images of surface and cross-section of the BFMOx thin films are presented in Fig. 5a-f. It can be seen that there are no clear diffusion and apparent segregation between the Si substrates and the films. Furthermore, the cross-section thicknesses of the BFMOx thin films are estimated to be 429 nm, 396 nm, 220 nm, 204 nm and 189 nm, respectively. From the views of the surface morphologies, grains with wide pores between grains are noticed for BFO thin films. Though all the BFMOx thin films are uniform, they still slightly different. However, BFMOx thin films exhibit less small holes and more tense morphology than the pure BFO thin films. The phenomenon reveals that the Mn substituted BFO thin films can conduct
electrical conductivity to reduce the surface heterogeneity [47]. As can be observed from the histogram of the particle size distribution of all samples in Fig. 6, it is obvious that the average grain size decreases with the increase of Mn doping concentration. The decrease in grain size of BFMOx thin films can be interpreted by the suppression of oxygen vacancy concentration, which leads to slower movement of oxygen ions, thus decreasing the grain growth rate [48]. With the doping concentration of Mn increasing from 0.03 to 0.20, the clusters increased and the individuals decreased in size. Excessive Mn may lead to decrease of grain growth efficiency, because of the energy barrier to grain boundary motion and diffusion [49]. The composition of chemical elements in all samples was examined by EDS tech nique, as shown in Fig. 7a-f. The presence of only Bi, Fe, O and Mn four elements in the BFMOx thin films was confirmed by EDS analysis, indicating that Mn was doped into the samples and have no interference from other impurities elements. Besides, the EDS result suggests that the actual concentrations in BFMOx thin films are approximately 0.0359, 0.0520, 0.0950, 0.1534 and 0.2187, respectively. Fig. 8 shows the X-ray photoelectron spectroscopy (XPS) of Bi4f, Fe2p, O1s and Mn2p in BFMOx, corrected by the binding energy of the C1s line (284.8eV) , respectively. [50]. The spectrum of the Bi4f state is consisting of two different peaks of Bi4f7/2 and Bi4f5/2 at 158.71 and 164.01 eV, respectively. The energy difference between the two peaks of Bi4f is 5.3 eV, indicating the presence of trivalent Bi ions and Bi-O bonds in the BFO thin film. [51, 52]. According to the studies by Yoneda et al. [53], the generation of oxygen vacancies can compensate for the loss of Bi in the
experiment and the defect response reads as follows:
BiO3 / 2 Bi1δ O3 / 2( 1δ) δBi δVB''' 3/ 2δO 3/ 2δVO
The presence of oxygen vacancies is unavoidable via sol-gel method. The electronic structures of BFO thin films modified by Bi vacancies and Mn substitution are reflected in their magnetic behavior [51]. The XPS spectrum expanded from 700 to 740 eV (see Fig. 7 b). The binding energy of Fe2p3/2 and Fe2p1/2 located at 710.61 and 724.16 eV, respectively and are attributed to Fe-O bonds. A satellite peak is noticed at a position greater than 8.35 eV of the Fe2p3/2 principal peak. The satellite peak is deemed to be a feature of the oxidation state of Fe [54]. Due to the different d-orbital electron configurations, Fe2+ and Fe3+ will reveal at 6 eV or 8 eV above the 2p3/2 principal peaks and appear as satellite peaks [52]. Note that satellite peaks appear 8 eV above the 2p3/2 principal peak, indicating that Fe is mainly in the +3 valence state in this sample [52]. It can be proved that the substitution of Mn2+ for volatile Bi3+ ions can lead to suppress the valence fluctuation of Fe ions from +3 to +2 states in BFO thin film. Further, a number of studies have shown that less Fe2+ ions indicate less oxygen vacancies. In order to search the effect of doping Mn on the oxygen state, the O1s peak of the film was measured as shown in Fig. 8c. These are decomposed into three vastly different peaks at around 529.53, 531.61 and 532.7 eV and are ascribed to three distinct O species: O defects (OD), O2-ions (OL) playing a part in Bi-O bonds in the structure and O (OA) such as -OH and -CO3 [54]. According to studies by Yoneda et al., one role of doped Mn ions in BFO is as a hole acceptor [53]. In our experiments, since the Mn element is derived from Mn(NO3)2, divalent
Mn ions will seize a hole and become a trivalent Mn ion. The XPS of the Mn 2p3/2 peaks is shown in Fig. 8d. The primary peak has a binding energy of 641.8 eV. In BFMO0.20 samples, a shoulder peak can be observed below this energy, which is derived from a small amount of Mn2+ (642.2eV). Moreover, the intensities of peaks are getting stronger, demonstrating that the doping concentration gradually increased. This result reveals that Mn substitution decreases the oxygen related defects, which is well consistent with in other Mn-doped BFO samples [45, 54, 55]. So lattice distortion, structure transition and these morphological features will have an influence on the magnetic properties for BFO thin film, which are discussed as follows. Fig. 9a shows the magnetization (M) versus magnetic field (H) curves of 0, 0.03, 0.05, 0.10, 0.15 and 0.20 Mn doped thin films at room temperature. The BFO thin film shows lower spontaneous magnetization compared to other films. A weak residual magnetic moment may be attributed to the angle of inclination between Fe3+ ions. However, the presence of ferromagnetism is governed by the spiral spin structure [56]. This is due to the spatial modulation of the spiral spin structure to prevent the view of the net magnetization [57]. When the Mn is substituted, the M-H loop is no longer linear and displays hysteresis characteristics. With the increase of Mn concentration, the change of microstructure will suppress the spin structure of the cycloid, which
has been confirmed by the XRD results. As anticipated, the Mn
substitution results in weak ferromagnetic moments at room temperature. The saturation magnetization (Ms) values of pure, 0.03, 0.05, 0.10, 0.15 and 0.20 Mn-doped BFO thin films are 40.65, 70.03, 135.50, 154.04, 123.90 and 81.09
emu/cm3, respectively. The remnant magnetization (Mr) values of BFMOx thin films are 2.49, 7.53, 4.50, 14.49, 5.53 and 6.81 emu/cm3. More recently, Deng et al. observed the well-saturated M-H hysteresis loops of BFMO0.10 thin film with Ms value of 5.1 emu/cm3 [26]. Yue et al. reported that the Ms of the Bi0.85Pr0.15Fe0.97Mn0.03O3 thin film is three times higher than of pure BFO thin film, but it is still limited to 1.81emu/cm3 [58]. Furthermore, Ren et al. found the Mn and Cu co-doping thin films present the optimum magnetization (Ms ~ 5.25 emu/cm3) by a simple chemical solution deposition technique [57]. In our work, the magnetization of BFO thin film doped with 0.10 concentration is superior to the other results. The enhancement in Ms for 0.10 Mn doped BFO thin film is attributed to either spiral spin modulation destruction by Mn atoms or Jahn-Teller effect driven by structural distortion. The Jahn-Teller effect leads to fractional spin rearrangement by influencing the cationic interaction. High concentrations of Fe3+ ions in the Fe-O-Fe bond are beneficial for magnetic property display [59]. The decrease in magnetization value after doping of 0.10 Mn is due to the weakening of Fe3+-O-Mn2+ super-exchange interaction at high concentration of Mn [60, 61]. Accordingly, the increased magnetization of Mn doped BFO thin film indicates that it has potential value in application. 4. Conclusions In summary, a series of Mn-doped BFO thin films containing different Mn dopant contents were successfully prepared via a facile sol-gel route. Investigation by XRD reveals that BFO and BFMOx thin films are free of impurities. Raman and XRD
studies of these samples indicate that Mn substitute Fe site and an obvious crystal structure transformation with Mn content increasing is observed in BFMOx thin films. SEM images demonstrate that doping agent has a giant influence on enhanced uniformity of particle formation and crystallite size. X-ray photoelectron spectroscopy (XPS) analyse illustrates that Mn substituting Fe is mainly in the +3 valence state in the samples. The saturated magnetization value of the BFMO0.10 thin film is 14.49 emu/cm3, its remnant magnetization value is 154.04 emu/cm3 at an applied magnetic field of 10 kOe, respectively. Furthermore, we summarize that the reasons for the improvement of magnetization in BFMOx thin films depend mainly on the destroying of the spatially inhomogeneous spin-modulated incommensurate structure and the Fe-O-Fe super-exchange strength. Hence, all of these results can confirm that Mn doped BFO thin films are viable materials for applications in magnetic sensors, information storage and spintronic memories at room temperature. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51608226) and the Program for the development of Science and Technology of Jilin province (Item No. 20150204085GX). References [1] P.C. Sati, M. Kumar, S. Chhoker, Phase evolution, magnetic, optical, and dielectric properties of Zr-substituted Bi0.9Gd0.1FeO3 multiferroics, J. Am. Ceram. Soc. 98 (2015) 1884-1890. [2] W. Ye, G.Q. Tan, G.H. Dong, H.J. Ren, A. Xia, Improved multiferroic properties in
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Figure captions Fig. 1. Schematic diagram of the process for preparing thin film via sol-gel technology. Fig. 2. The superexchange interaction between the (a) Fe-O-Fe and (b) Fe-O-Mn, the perovskite crystal structure simulation patterns of (c) BFO and (d) Mn-substitution. Fig. 3. X-ray diffraction (XRD) patterns of BFMOx films on Si (100) substrates (a) XRD patterns of BFMOx and (b) XRD patterns BFMOx in the range of 2θ from 30 to 34. Fig. 4. (a) Different concentration Mn substituted of BFO thin film Raman scattering diagrams. (b) The magnified patterns of peaks around 575-675 cm-1. Fig. 5. SEM surface images and cross-sectional (a-f) of BFMOx films; (g) EDS images of BFMOx films. Fig. 6. Histograms regarding particle size distribution of all samples. Fig. 7. EDX spectrum of (a) BFO (b) BFMO0.03 (c) BFMO0.05 (d) BFMO0.10 (e) BFMO0.15 (f) BFMO0.20. Fig. 8. XPS spectra of the as-annealed BFMO0.10 thin films in the binding energy regions of (a) Bi4f, (b) Fe2p, (c) O1s and different concentration Mn substituted of BFO film XPS spectra of (d) Mn 2p. Fig. 9. Magnetic hysteresis loops measured at room temperature with the field applied in the plane for BFMOx thin films.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Tables: Table1 The structural parameters (a, b, c, V) and the tolerance factor (t) for BFMOx (x=0, 0.03, 0.05, 0.10, 0.15 and 0.20) thin films. Compositions BFO BFMO0.03 BFMO0.05 BFMO0.10 BFMO0.15 BFMO0.20
ahex = bhex (Å)
chex (Å)
V (Å3)
t
5.5876 5.5850 5.5801 5.5767 5.5852 5.5883
13.8862 13.8853 13.8699 13.8602 13.8722 13.8795
378.66 378.15 376.58 374.02 375.95 376.84
0.8403 0.8402 0.8401 0.8393 0.8388 0.8387