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Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics
Accepted Manuscript Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics N. Zhang, Y.W. Yang, J. Su, D.Z. Guo, X.N. Liu, N. Ma, Z.M. Liu, M. Zhao, X.T. Li, Y.Y. Guo, F.G. Chang, J.-M. Liu PII:
S0925-8388(16)30893-3
DOI:
10.1016/j.jallcom.2016.03.262
Reference:
JALCOM 37150
To appear in:
Journal of Alloys and Compounds
Received Date: 24 January 2016 Revised Date:
29 March 2016
Accepted Date: 30 March 2016
Please cite this article as: N. Zhang, Y.W. Yang, J. Su, D.Z. Guo, X.N. Liu, N. Ma, Z.M. Liu, M. Zhao, X.T. Li, Y.Y. Guo, F.G. Chang, J.-M. Liu, Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.262. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Manuscript submitted to Journal of Alloys and Compounds
Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics
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N. Zhang1,2 a),Y.W. Yang1, J. Su1, D. Z. Guo1, X. N. Liu1, N. Ma1, Z. M. Liu1, M. Zhao1, X. T. Li1, Y. Y. Guo3, F. G. Chang,1b) & J.-M. Liu2 1
Henan Key Laboratory of Photovoltaic Materials, Department of Physics, Henan Normal University, Xinxiang 453007, China
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 3
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2
College of Electronic Science and Engineering, Nanjing University of Posts and
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Telecommunications, Nanjing 210003, China
[Abstract] A series of Bi1−xAxFeO3 (A=Ba2+, Sm3+) compounds have been synthesized in order to reveal the influence of oxygen vacancies on the magnetic behavior of BiFeO3. While the rhombohedral structure is retained for all compositions (0≤x≤0.2), the Ba2+ and Sm3+
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substitution induces different structural disorder, leading to an enhancement in magnetization. Temperature dependence of magnetization between 300K and 900K indicates that the increased oxygen vacancy due to Ba2+ substitution gradually turns the original antiferromagnetic behavior of BiFeO3 into a ferromagnetic one. In contrast, the
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antiferromagnetic nature of BiFeO3 is maintained in the whole Sm3+ substitution range, in which the concentration of oxygen vacancies is preserved. This result reveals unambiguously
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that the oxygen vacancies also play a crucial role in deciding the magnetic behavior of BiFeO3. The intrinsic magnetic property of BiFeO3 is originated from not only the Fe3+/Fe3+ but also the unbalanced Fe3+/Fe2+ antiferromagnetic superexchange interactions.
Keywords: BiFeO3, oxygen vacancies, magnetic property. PACS Numbers: 75.85.+t; 75.50.Pp
a) E-mail:–[email protected] b) E-mail: [email protected]
ACCEPTED MANUSCRIPT 1. Introduction The multiferroic bismuth ferrite (BFO), which belongs to the perovskite structured (ABO3) family, crystallizes into distorted rhombohedral structures with the space group R3c and turns into antiferromagnetic phase at the Néel temperature (TN) of 643 K while maintains
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the ferroelectricity with a ferroelectric transition temperature (TC) of 1103 K.1Due to its large magnetic-electric coupling at room temperature, the multiferroic BFO becomes one of the most promising candidates for potential applications in the fields of multiple-state memories, spintronics, magneto-electric sensor devices and so on,2-5 and has attracted extensive research
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attentions.
However, even though BFO exhibits attractive properties, it also has some sticky issues
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to confront. Due to the volatilization of Bi2O3 in the sample synthesis process, the impurities of Bi2Fe4O9, Bi25FeO40, et.al are commonly existent in BFO.6,7 Additionally, oxygen vacancies arising from the volatilization are always present in BFO and can compel the Fe3+ to change valence state (leading to Fe2+) in order to compensate the charge unbalance in BFO.8 Associated with the impurities and oxygen vacancies, the high leakage current often
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adversely affects the effective measurements of the ferroelectric and dielectric properties of BFO. On the other hand, the magnetic structure of BFO is related to the complex superexchange interaction between Fe3+ ions and forms a cycloidal spiral spin structure with a spin periodicity of 62 nm.1,5 The antiferromagnetic nature of BFO cancels out the net
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macroscopic magnetization in BFO.9-13 Aiming to overcome these difficulties, many efforts have been made and it has been
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demonstrated that the enhanced magnetization, dielectric constant, ferroelectric polarization and magneto-electric properties of BFO can be realized through nanotechnology and partially substituting the Bi-and/or Fe-sites of BFO with various equivalent/heterovalent cations.13-29 Moreover, the optical band gap of BFO can be tuned even into the visible region through substituting the Bi-site with alkaline earth metals.13 Among these works, it is interesting to notice that, apart from the structural distortion-related suppression of spiral spin structure, the present of oxygen vacancies is considered to be one possible reason for the enhancement of magnetization in BFO.20-28 However, the underline physical mechanism of the oxygen vacancies related magnetization enhancement of BFO is still unclear. Additionally, although
ACCEPTED MANUSCRIPT some interesting magnetic phase transitions are observed in pure and alkaline earth substituted BFO systems at high temperatures (500K-900K),21-25 little attention has been paid to the effect of oxygen vacancies on the high temperature magnetic phase transitions. The present paper presents the results of a systemic investigation on the influence of
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oxygen vacancies on the high temperature magnetic behavior of BFO. The concentration of oxygen vacancies can be modulated through Bi-sites element substitution: the Bi-sites partial substitution with divalent ions increases the concentration of oxygen vacancies,17,23 while the replacement of Bi-sites with rare earth elements will not create extra oxygen vacancies.18,19
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For this purpose, Ba and Sm are adopted in this work to substitute the Bi3+ sites to adjust the number of oxygen vacancies, and high temperature magnetic measurements of Bi1−xAxFeO3
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(A=Ba, Sm) compounds have been carried out. It is our intention to clearly reveal the role of oxygen vacancies in deciding the magnetic behavior of BFO through comparing the high temperature magnetic evolutions of Bi1−xAxFeO3 (A=Ba, Sm) compounds, and then to get a comprehensive understanding of the magnetic phase diagram of BFO. Different from the previous studies of Ba and Sm substituted BFO systems,17,18 the main focus of the current
has been largely ignored.
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work is the role of oxygen vacancies in the magnetic transitions at high temperatures, which
2. Experimental details
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The facile sol–gel technique was employed to synthesize a series of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds. The stoichiometric amounts of high purity Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, Ba(NO3)2 and Sm(NO3)3·6H2O were mixed and dissolved in equal amount
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nitric acid with added triple-distilled water. An equal amount of ethylene glycol was added with continuous stirring to the obtained solution. Some glacial acetic acid and citric acid were added into this solution to obtain precursor solution. The solution was heated at 80°C until a spongy gel was formed, and then heated at 350°C to obtain xerogel powders. Then the xerogel powders were thoroughly grinded in the agate mortar and pressed into pellets of 13 mm in diameter and about 1 mm in thickness. At the end of the process, the pellets were calcinated at 830°C for 5min to get high quality Bi1−xAxFeO3 samples. The structural and phase composition analyses of the samples were carried out with a Bruker D8 Advance x-ray diffractometer with Cu Kα radiation in the range of 20° to 60° at
ACCEPTED MANUSCRIPT room temperature. The morphology and particle size distribution were determined using the high-resolution scanning electron microscopic (HRSEM) images. The chemical compositions and valence state of Fe in Bi1−xAxFeO3 samples were determined through X-ray photoelectron spectroscopy (XPS). In order to probe the magnetic evolution, magnetization measurements
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of the samples were performed using vibrating sample magnetometer (VSM). The magnetization loops were obtained at selected temperatures in the field range between -3T≤H≤+3T. Meanwhile, the magnetization versus temperature plots were measured at 5 kOe over the temperature range of 300K 900K. The antiferromagnet Néel temperature TN was
magnetization signal on the heating cycle.
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3. Results and discussion
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revealed in the magnetization measurement and was signed by the anomalies in the
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Fig.1. (Color online) Room temperature powder X-ray diffraction patterns of (a) Bi1–xBaxFeO3 and (b) Bi1–xSmxFeO3 (x=0,0.05,0.1,0.15,0.2). The inset of (a) and (b) shows
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the dominant diffraction peaks for samples with x=0, 0.05, 0.10, 0.15, 0.2 from bottom to up, respectively.
The room temperature X-ray diffraction (XRD) patterns of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) samples are displayed in Fig. 1, which reveal the polycrystalline nature of all the specimen. Different from the previous literatures on the heterovalent substituted BFO samples,15,18,21 no structural phase transition has been observed up to 20% Ba or Sm substituting. All the reflection peaks were indexed to the distorted perovskite structure with rhombohedral lattice type and R3c space group. Some traces of secondary phases Bi25FeO40
ACCEPTED MANUSCRIPT (less than 5%) were also observed in the XRD patterns. Aforementioned, the occurrence of this imprurity phase is due to the Bi2O3 volatilization during heat treatment. Analysis of XRD patterns of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) samples reveals that, due to the ionic radius difference among Bi3+(RBi3+=1.03Å), Ba2+(RBa3+=1.35Å) and Sm3+(RSm3+=0.958Å),30 the
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substitution of Bi3+ with Ba2+ and Sm3+ results in different structural distortion: the volume of the unit cell expands with increase the Ba2+ content while contracts when the Sm3+ substitution increase. This is evidenced by the different shift in the peak positions of the XRD pattern of Bi1−xAxFeO3 compounds, as shown in the inset of Figs. 1(a) and 1(b). Here, no
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matter the Bi3+ is substituted by Ba2+ or Sm3+, the resultant structural distortion would lead to modulations in lattice parameters, Fe–O–Fe bond angles and bond lengths of BFO.13,18,23
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Since the complex superexchange interaction between Fe3+ ions is sensitive to the Fe–O–Fe bond angles of BFO, the Ba2+ and Sm3+ substitution induced structural distortion certainly
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will affect the magnetic behavior of BFO.
nm.
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Fig. 2. The SEM micrographs of Bi0.8A0.2FeO3 (A=Ba, Sm) compounds. The scale bar is 200
The surface morphology and particle distribution of typical Bi0.8A0.2FeO3 (A=Ba, Sm) compounds were investigated using scanning electron microscopy (SEM) and are shown in Figs. 2. The images show that the grains are roughly rectangular for Bi0.8A0.2FeO3 (A=Ba, Sm) samples and are nonuniformly distributed over the entire surface with certain degree of porosity. In order to identify the element, chemical shift, and oxidation state of Fe of the pure and Ba/Sm substituted BFO samples, detailed X-ray Photoelectron Spectroscopy (XPS) analysis
ACCEPTED MANUSCRIPT was performed. The XPS survey spectra of the pure BFO and selected Bi0.8A0.2FeO3 (A=Ba, Sm) compounds are displayed in Fig.3. As one can see that, the Bi, Fe, and O elements are generally present in the three samples, while the substituted Ba (Sm) element is only identified in Bi0.8Ba0.2FeO3 (Bi0.8Sm0.2FeO3) compound. As to the unexpected element C, it is
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well known that, the C 1s peak is taken as a reference for the calibration of the binding energy values. So, the XPS results further confirm the chemical compositions of the pure BFO and
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Bi0.8A0.2FeO3 (A=Ba, Sm) compounds.
Fig. 3. XPS spectra of the BFO and Bi0.8A0.2FeO3 (A=Ba, Sm) compounds. For pure BFO, it is widely considered that the Fe ions are a combination of Fe3+/ Fe2+
ACCEPTED MANUSCRIPT states.8 In order to evaluate the oxidation state of Fe, and to explore whether the substitution of Bi with Ba/Sm has an effect on the oxidation state of Fe, the narrow Fe 2p line in the XPS spectra for Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds are presented in Fig.4(a). Based on the spectra of BFO, the Fe 2p core level splits into two peaks of Fe 2p3/2 and Fe 2p1/2. The
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asymmetric Fe 2p3/2 peak can be further separated into two independent peaks centered at 709.9 eV and 711 eV, as shown in Fig.4(b). Generally, the binding energy values of 709eV and 711eV are considered to correspond to the oxidation states of Fe2+ and Fe3+,31-34 respectively. So, the asymmetric shape of Fe 2p3/2 peak indicates the combination of Fe3+ and
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Fe2+ oxidation states in BFO. By calculating the percentage area under the peak obtained from
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fitting the Fe 2p3/2 spectrum, the concentration of Fe2+ is estimated to be 12% in pure BFO.
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Fig.4. (a) Fe 2p XPS spectra for Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds. (b) Lorentzian dividing of the Fe 2p3/2 core level for pure BFO. The inset of (b) presents the
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Ba/Sm concentration dependent of Fe2+ percentage.
As to the substituted Bi1−xAxFeO3 (A=Ba, Sm) compounds, the substitution of Bi with Ba/Sm leads to a change in the binding energy value of Fe 2p3/2 when compared to pure BFO, as shown in Fig.4(a). By fitting the Fe 2p3/2 spectrum of Bi1−xAxFeO3 (A=Ba, Sm) compounds, it has been found that the percentage of Fe2+ is increased with increasing Ba substitution but remains roughly unchanged in the Bi1−xSmxFeO3 samples, as indicated in the inset of Fig.4(b). Refer to the different valence state between Bi3+, Ba2+ and Sm3+, it is easy to understand that increasing Ba2+ concentration will form more oxygen vacancies, while the Sm3+ substitution has little effect on the modulation of oxygen vacancies.17-19,23 The existence of Fe2+ will bring
ACCEPTED MANUSCRIPT forth the superexchange interactions between Fe2+ and Fe3+ ions through the oxygen anion (Fe3+−O2-−Fe2+), which will coexistent and compete with the general Fe3+−O2-−Fe3+ antiferromagnetic superexchange interaction. So, the fluctuations in Fe2+/Fe3+ ratio is crucial
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for understanding the modulation of the magnetic behavior of BFO.
Fig. 5. (Color online) Measured M-H hysteresis loops at room temperature of Bi1−xAxFeO3
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(A=Ba, Sm) (0≤x≤0.2) samples.
To address the issue of the role of oxygen vacancies in affecting the magnetic properties of BFO, room temperature magnetic hysteresis loops have been measured over ±3 T for all
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the samples. The data for all the compositions i.e. Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) are shown in Fig. 5. For pure BFO, the Fe3+ magnetic moments are coupled ferromagnetically
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within the pseudocubic (111) planes but antiferromagnetically between adjacent planes, which is defined as the G-type antiferromagnetic structure.1 Driving by the Dzyaloshinsky–Moriya (DM) interaction,35 the tilting of spins direction of the (111) planes resulting in a net uncompensated moment. Unfortunately, the net magnetization is usually canceled out due to the modulation of the cycloidal spiral spin structure,1 so a linear magnetization hysteresis (M–H) loop and negligible residual magnetization are observed for BFO. As for the substituted Bi1−xAxFeO3 compounds, thin hysteresis loops are observed with increase in Ba concentration while enhanced remnant magnetization is obtained in the Sm substituted compounds. These loops indicate the appearance of ferromagnetic character in first glance
ACCEPTED MANUSCRIPT and the remnant magnetization of Bi1−xBaxFeO3 compounds can be compared to that of Bi1−xSmxFeO3 compound. We believe the lattice distortion of Bi1−xAxFeO3 should be one important reason for the improvement in the magnetic properties of BFO. By taking ionic sizes of Bi3+ and Ba2+/ Sm3+,30 the substitution of Ba2+/ Sm3+ ions in BFO lattice distorts the
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original rhombohedral structure of BFO. Through modulating the Fe–O–Fe bond angle and Fe–O bond distances, the distorted rhombohedral perovskite structure suppresses the spiral spin structure and then releases the latent magnetization inhabited in BFO, allowing a weak ferromagnetic ordering in Bi1−xAxFeO3 compounds.7,13,36 Moreover, related with the increased
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oxygen vacancies, the enhanced Fe3+−O2-−Fe2+ superexchange interactions are considered to be another important factor for the enhancement of BFO in the Bi1−xBaxFeO3 system.20-23 In
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the Bi1−xSmxFeO3 system, due to the ionic sizes of Sm3+ is smaller than that of Bi3+, the introduction of Sm3+ in BFO will shorten the Sm–O and Fe–O distances, which will enhance the exchange interaction between Sm3+–Sm3+ and Fe3+–Sm3+ and then lead to the increase of
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the magnetization.18,19
Fig. 6. M–T curves of (a) Bi1–xBaxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) and (b) Bi1–xSmxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) compounds measured at 5 kOe.
In order to investigate the high temperature magnetic phase transition and reveal the intrinsic magnetic interaction of BFO, the temperature dependence of magnetization (M-T) was measured in the temperature range 300 K–900 K at a field of 5kOe for all the samples. The results are shown in Fig.6. Since the BFO is considered to be antiferromagnetic below
ACCEPTED MANUSCRIPT TN~643K, the magnetization increases with increasing temperature up to 643K, where a distinct peak appears indicating the antiferromagnetic phase transition. For the Bi1−xAxFeO3 compounds, it is interesting to observe that, even though both Ba2+ and Sm3+ substitution result in the enhancement of magnetization, the original antiferromagnetic behavior of BFO
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gradually turns into ferromagnetic behavior with increasing Ba2+ concentration but maintains antiferromagnetic feature in the whole Sm3+ substitution range, as evidenced from the magnetic-temperature curves of Bi1−xAxFeO3 compounds given in Figs.6(a) and 6(b).
Since the structural distortion exists commonly in both Bi1–xBaxFeO3 and Bi1–xSmxFeO3
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compounds, so the different concentration of oxygen vacancies should be a critical factor responsible for the different high temperature magnetic evolution of these two systems. As
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mentioned above, the Fe2+ commonly exists in BFO. The increased Ba2+ concentration produces more Fe2+ ions which consequently enhance the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction, competing with the initial Fe3+−O2-−Fe3+ antiferromagnetic superexchange interaction and enventually resulting in the ferromagnetic behavior of the high-level Ba2+ substituted compounds. As to the Bi1−xSmxFeO3 samples,
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because Sm3+ is in the same oxidation state as that of Bi3+, the concentration of oxygen vacancies remains essentially unchanged with Sm substitution. Therefore, only relatively small enhancement of magnetization is achieved in the Bi1−xSmxFeO3 (0≤x≤0.2) samples
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below TN ~643K, leaving the antiferromagnetic order Fe3+−O2-−Fe3+ to be persevered as the dominant interaction. And this small magnetization enhancement may be originated from the structural distortion related spin canting and the Sm3+−Sm3+ and Sm3+−Fe3+ exchange
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interaction.18,19,37,38
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Fig. 7. (Color online) M-H loops of Bi0.8Ba0.2FeO3 sample at selected temperatures.
On further analyzing these M-T curves, it has been found that in these Ba2+ substituted samples, the antiferromagnetic phase transition peak becomes smaller when the Ba2+
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concentration is increased, implying the dominant role of Fe3+−O2−Fe3+ is gradually replaced by the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction.
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Furthermore, the Fe3+−O2-−Fe3+ can only be maintained below TN~643K, while the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic order can exist up to a much higher temperature (T~778K), indicating a much stronger superexchange interaction. In order to verify this high temperature ferromagnetic-like phase transition in Bi1−xBaxFeO3 (0.1
ACCEPTED MANUSCRIPT that the ferromagnetic-like transition arises from the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction and the magnetization, which depends on the
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difference between Fe3+ and Fe2+, is relatively small.
Fig.8. (Color online) Schematic plots of the interaction strength and magnetization versus
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Ba2+ concentration for Bi1–xBaxFeO3 (0≤x≤0.2) samples at room temperature. Finally, based on the analysis of high temperature magnetic behaviors of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds, we are able to roughly establish the relationship between the magnetic superexchange interaction and the concentration of oxygen vacancies of BFO, as
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shown in Fig.8. According to this graph, with increasing oxygen vacancies, the dominated magnetic structure of Bi1−xBaxFeO3 changes from cycloidal spiral spin structure into the
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ferrimagnetic order, resulting in the obvious enhancement of magnetization. It is worth nothing to mention that, since the oxygen vacancies always exist in BFO, the formation of uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction is reasonable. Then, we can infer that the general magnetic structure of BFO should be the coexistence of cycloidal spiral spin structure and unballanced antiferromagnetic spin structure. So, even though the present of oxygen vacancies increases the leakage current and makes the measurement of ferroelectric property more difficult, they are important for the magnetic properties of BFO. Proper adjusting the concentration of oxygen vacancies, it is possible to effectively enhance the magnetization of BFO.
ACCEPTED MANUSCRIPT 4. Conclusions In summary, we have investigated the high temperature magnetic behavior of a series Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds in detail. The XRD analysis of Bi1−xAxFeO3 confirms the rhombohedral space group of R3c and the structural distortion with Ba2+/Sm3+
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substitution. Mainly due to the structural distortion and the increased concentration of oxygen vacancies, obvious enhancement of magnetization has been observed in Ba2+ and Sm3+ substituted compounds. Interestingly, the original antiferromagnetic behavior of BFO, which is maintained in all Sm3+substituted samples, alters into a ferromagnetic one with increasing
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Ba2+ concentration. The XPS narrow scan of the Fe 2p spectra reveals that the increased concentration of oxygen vacancies due to the substitution of Ba2+ result in the increased
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percentage of Fe2+, and consequently the ferromagnetic behavior in Bi1−xBaxFeO3 samples. So, we conclude that the oxygen vacancies also play an important role in deciding the magnetic behavior of BFO ceramic.
Acknowledgement
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This work was supported by the Natural Science Foundation of China (U1204111, 11504093, 11304158), the Specialized Research Fund for the Doctoral Program of Higher Education, and the open project of Nanjing National Laboratory of Microstructures (2013M531677 and
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M26002).
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38. M. Al-Haj, Cryst. Res. Technol. 45 (2010) 89–93.
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Magn. Magn. Mater. 322 (2010) 2251–2255.
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1. The Ba/Sm-substitution induces structural distortion in BiFeO3 (BFO). 2. The Ba-substitution increases the concentration of oxygen vacancies. 3. The increased oxygen vacancies alter the magnetic structure of BFO (AFM
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–FM). 4. The oxygen vacancies play a crucial role in deciding the magnetic behavior of BFO.
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5. The general spin structure of BFO should be the coexistence of AFM and FM.
S0925-8388(16)30893-3
DOI:
10.1016/j.jallcom.2016.03.262
Reference:
JALCOM 37150
To appear in:
Journal of Alloys and Compounds
Received Date: 24 January 2016 Revised Date:
29 March 2016
Accepted Date: 30 March 2016
Please cite this article as: N. Zhang, Y.W. Yang, J. Su, D.Z. Guo, X.N. Liu, N. Ma, Z.M. Liu, M. Zhao, X.T. Li, Y.Y. Guo, F.G. Chang, J.-M. Liu, Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.262. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Manuscript submitted to Journal of Alloys and Compounds
Role of oxygen vacancies in deciding the high temperature magnetic properties of Ba and Sm substituted BiFeO3 ceramics
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N. Zhang1,2 a),Y.W. Yang1, J. Su1, D. Z. Guo1, X. N. Liu1, N. Ma1, Z. M. Liu1, M. Zhao1, X. T. Li1, Y. Y. Guo3, F. G. Chang,1b) & J.-M. Liu2 1
Henan Key Laboratory of Photovoltaic Materials, Department of Physics, Henan Normal University, Xinxiang 453007, China
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 3
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2
College of Electronic Science and Engineering, Nanjing University of Posts and
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Telecommunications, Nanjing 210003, China
[Abstract] A series of Bi1−xAxFeO3 (A=Ba2+, Sm3+) compounds have been synthesized in order to reveal the influence of oxygen vacancies on the magnetic behavior of BiFeO3. While the rhombohedral structure is retained for all compositions (0≤x≤0.2), the Ba2+ and Sm3+
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substitution induces different structural disorder, leading to an enhancement in magnetization. Temperature dependence of magnetization between 300K and 900K indicates that the increased oxygen vacancy due to Ba2+ substitution gradually turns the original antiferromagnetic behavior of BiFeO3 into a ferromagnetic one. In contrast, the
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antiferromagnetic nature of BiFeO3 is maintained in the whole Sm3+ substitution range, in which the concentration of oxygen vacancies is preserved. This result reveals unambiguously
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that the oxygen vacancies also play a crucial role in deciding the magnetic behavior of BiFeO3. The intrinsic magnetic property of BiFeO3 is originated from not only the Fe3+/Fe3+ but also the unbalanced Fe3+/Fe2+ antiferromagnetic superexchange interactions.
Keywords: BiFeO3, oxygen vacancies, magnetic property. PACS Numbers: 75.85.+t; 75.50.Pp
a) E-mail:–[email protected] b) E-mail: [email protected]
ACCEPTED MANUSCRIPT 1. Introduction The multiferroic bismuth ferrite (BFO), which belongs to the perovskite structured (ABO3) family, crystallizes into distorted rhombohedral structures with the space group R3c and turns into antiferromagnetic phase at the Néel temperature (TN) of 643 K while maintains
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the ferroelectricity with a ferroelectric transition temperature (TC) of 1103 K.1Due to its large magnetic-electric coupling at room temperature, the multiferroic BFO becomes one of the most promising candidates for potential applications in the fields of multiple-state memories, spintronics, magneto-electric sensor devices and so on,2-5 and has attracted extensive research
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attentions.
However, even though BFO exhibits attractive properties, it also has some sticky issues
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to confront. Due to the volatilization of Bi2O3 in the sample synthesis process, the impurities of Bi2Fe4O9, Bi25FeO40, et.al are commonly existent in BFO.6,7 Additionally, oxygen vacancies arising from the volatilization are always present in BFO and can compel the Fe3+ to change valence state (leading to Fe2+) in order to compensate the charge unbalance in BFO.8 Associated with the impurities and oxygen vacancies, the high leakage current often
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adversely affects the effective measurements of the ferroelectric and dielectric properties of BFO. On the other hand, the magnetic structure of BFO is related to the complex superexchange interaction between Fe3+ ions and forms a cycloidal spiral spin structure with a spin periodicity of 62 nm.1,5 The antiferromagnetic nature of BFO cancels out the net
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macroscopic magnetization in BFO.9-13 Aiming to overcome these difficulties, many efforts have been made and it has been
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demonstrated that the enhanced magnetization, dielectric constant, ferroelectric polarization and magneto-electric properties of BFO can be realized through nanotechnology and partially substituting the Bi-and/or Fe-sites of BFO with various equivalent/heterovalent cations.13-29 Moreover, the optical band gap of BFO can be tuned even into the visible region through substituting the Bi-site with alkaline earth metals.13 Among these works, it is interesting to notice that, apart from the structural distortion-related suppression of spiral spin structure, the present of oxygen vacancies is considered to be one possible reason for the enhancement of magnetization in BFO.20-28 However, the underline physical mechanism of the oxygen vacancies related magnetization enhancement of BFO is still unclear. Additionally, although
ACCEPTED MANUSCRIPT some interesting magnetic phase transitions are observed in pure and alkaline earth substituted BFO systems at high temperatures (500K-900K),21-25 little attention has been paid to the effect of oxygen vacancies on the high temperature magnetic phase transitions. The present paper presents the results of a systemic investigation on the influence of
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oxygen vacancies on the high temperature magnetic behavior of BFO. The concentration of oxygen vacancies can be modulated through Bi-sites element substitution: the Bi-sites partial substitution with divalent ions increases the concentration of oxygen vacancies,17,23 while the replacement of Bi-sites with rare earth elements will not create extra oxygen vacancies.18,19
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For this purpose, Ba and Sm are adopted in this work to substitute the Bi3+ sites to adjust the number of oxygen vacancies, and high temperature magnetic measurements of Bi1−xAxFeO3
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(A=Ba, Sm) compounds have been carried out. It is our intention to clearly reveal the role of oxygen vacancies in deciding the magnetic behavior of BFO through comparing the high temperature magnetic evolutions of Bi1−xAxFeO3 (A=Ba, Sm) compounds, and then to get a comprehensive understanding of the magnetic phase diagram of BFO. Different from the previous studies of Ba and Sm substituted BFO systems,17,18 the main focus of the current
has been largely ignored.
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work is the role of oxygen vacancies in the magnetic transitions at high temperatures, which
2. Experimental details
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The facile sol–gel technique was employed to synthesize a series of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds. The stoichiometric amounts of high purity Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, Ba(NO3)2 and Sm(NO3)3·6H2O were mixed and dissolved in equal amount
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nitric acid with added triple-distilled water. An equal amount of ethylene glycol was added with continuous stirring to the obtained solution. Some glacial acetic acid and citric acid were added into this solution to obtain precursor solution. The solution was heated at 80°C until a spongy gel was formed, and then heated at 350°C to obtain xerogel powders. Then the xerogel powders were thoroughly grinded in the agate mortar and pressed into pellets of 13 mm in diameter and about 1 mm in thickness. At the end of the process, the pellets were calcinated at 830°C for 5min to get high quality Bi1−xAxFeO3 samples. The structural and phase composition analyses of the samples were carried out with a Bruker D8 Advance x-ray diffractometer with Cu Kα radiation in the range of 20° to 60° at
ACCEPTED MANUSCRIPT room temperature. The morphology and particle size distribution were determined using the high-resolution scanning electron microscopic (HRSEM) images. The chemical compositions and valence state of Fe in Bi1−xAxFeO3 samples were determined through X-ray photoelectron spectroscopy (XPS). In order to probe the magnetic evolution, magnetization measurements
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of the samples were performed using vibrating sample magnetometer (VSM). The magnetization loops were obtained at selected temperatures in the field range between -3T≤H≤+3T. Meanwhile, the magnetization versus temperature plots were measured at 5 kOe over the temperature range of 300K 900K. The antiferromagnet Néel temperature TN was
magnetization signal on the heating cycle.
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3. Results and discussion
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revealed in the magnetization measurement and was signed by the anomalies in the
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Fig.1. (Color online) Room temperature powder X-ray diffraction patterns of (a) Bi1–xBaxFeO3 and (b) Bi1–xSmxFeO3 (x=0,0.05,0.1,0.15,0.2). The inset of (a) and (b) shows
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the dominant diffraction peaks for samples with x=0, 0.05, 0.10, 0.15, 0.2 from bottom to up, respectively.
The room temperature X-ray diffraction (XRD) patterns of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) samples are displayed in Fig. 1, which reveal the polycrystalline nature of all the specimen. Different from the previous literatures on the heterovalent substituted BFO samples,15,18,21 no structural phase transition has been observed up to 20% Ba or Sm substituting. All the reflection peaks were indexed to the distorted perovskite structure with rhombohedral lattice type and R3c space group. Some traces of secondary phases Bi25FeO40
ACCEPTED MANUSCRIPT (less than 5%) were also observed in the XRD patterns. Aforementioned, the occurrence of this imprurity phase is due to the Bi2O3 volatilization during heat treatment. Analysis of XRD patterns of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) samples reveals that, due to the ionic radius difference among Bi3+(RBi3+=1.03Å), Ba2+(RBa3+=1.35Å) and Sm3+(RSm3+=0.958Å),30 the
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substitution of Bi3+ with Ba2+ and Sm3+ results in different structural distortion: the volume of the unit cell expands with increase the Ba2+ content while contracts when the Sm3+ substitution increase. This is evidenced by the different shift in the peak positions of the XRD pattern of Bi1−xAxFeO3 compounds, as shown in the inset of Figs. 1(a) and 1(b). Here, no
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matter the Bi3+ is substituted by Ba2+ or Sm3+, the resultant structural distortion would lead to modulations in lattice parameters, Fe–O–Fe bond angles and bond lengths of BFO.13,18,23
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Since the complex superexchange interaction between Fe3+ ions is sensitive to the Fe–O–Fe bond angles of BFO, the Ba2+ and Sm3+ substitution induced structural distortion certainly
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will affect the magnetic behavior of BFO.
nm.
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Fig. 2. The SEM micrographs of Bi0.8A0.2FeO3 (A=Ba, Sm) compounds. The scale bar is 200
The surface morphology and particle distribution of typical Bi0.8A0.2FeO3 (A=Ba, Sm) compounds were investigated using scanning electron microscopy (SEM) and are shown in Figs. 2. The images show that the grains are roughly rectangular for Bi0.8A0.2FeO3 (A=Ba, Sm) samples and are nonuniformly distributed over the entire surface with certain degree of porosity. In order to identify the element, chemical shift, and oxidation state of Fe of the pure and Ba/Sm substituted BFO samples, detailed X-ray Photoelectron Spectroscopy (XPS) analysis
ACCEPTED MANUSCRIPT was performed. The XPS survey spectra of the pure BFO and selected Bi0.8A0.2FeO3 (A=Ba, Sm) compounds are displayed in Fig.3. As one can see that, the Bi, Fe, and O elements are generally present in the three samples, while the substituted Ba (Sm) element is only identified in Bi0.8Ba0.2FeO3 (Bi0.8Sm0.2FeO3) compound. As to the unexpected element C, it is
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well known that, the C 1s peak is taken as a reference for the calibration of the binding energy values. So, the XPS results further confirm the chemical compositions of the pure BFO and
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Bi0.8A0.2FeO3 (A=Ba, Sm) compounds.
Fig. 3. XPS spectra of the BFO and Bi0.8A0.2FeO3 (A=Ba, Sm) compounds. For pure BFO, it is widely considered that the Fe ions are a combination of Fe3+/ Fe2+
ACCEPTED MANUSCRIPT states.8 In order to evaluate the oxidation state of Fe, and to explore whether the substitution of Bi with Ba/Sm has an effect on the oxidation state of Fe, the narrow Fe 2p line in the XPS spectra for Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds are presented in Fig.4(a). Based on the spectra of BFO, the Fe 2p core level splits into two peaks of Fe 2p3/2 and Fe 2p1/2. The
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asymmetric Fe 2p3/2 peak can be further separated into two independent peaks centered at 709.9 eV and 711 eV, as shown in Fig.4(b). Generally, the binding energy values of 709eV and 711eV are considered to correspond to the oxidation states of Fe2+ and Fe3+,31-34 respectively. So, the asymmetric shape of Fe 2p3/2 peak indicates the combination of Fe3+ and
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Fe2+ oxidation states in BFO. By calculating the percentage area under the peak obtained from
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fitting the Fe 2p3/2 spectrum, the concentration of Fe2+ is estimated to be 12% in pure BFO.
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Fig.4. (a) Fe 2p XPS spectra for Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds. (b) Lorentzian dividing of the Fe 2p3/2 core level for pure BFO. The inset of (b) presents the
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Ba/Sm concentration dependent of Fe2+ percentage.
As to the substituted Bi1−xAxFeO3 (A=Ba, Sm) compounds, the substitution of Bi with Ba/Sm leads to a change in the binding energy value of Fe 2p3/2 when compared to pure BFO, as shown in Fig.4(a). By fitting the Fe 2p3/2 spectrum of Bi1−xAxFeO3 (A=Ba, Sm) compounds, it has been found that the percentage of Fe2+ is increased with increasing Ba substitution but remains roughly unchanged in the Bi1−xSmxFeO3 samples, as indicated in the inset of Fig.4(b). Refer to the different valence state between Bi3+, Ba2+ and Sm3+, it is easy to understand that increasing Ba2+ concentration will form more oxygen vacancies, while the Sm3+ substitution has little effect on the modulation of oxygen vacancies.17-19,23 The existence of Fe2+ will bring
ACCEPTED MANUSCRIPT forth the superexchange interactions between Fe2+ and Fe3+ ions through the oxygen anion (Fe3+−O2-−Fe2+), which will coexistent and compete with the general Fe3+−O2-−Fe3+ antiferromagnetic superexchange interaction. So, the fluctuations in Fe2+/Fe3+ ratio is crucial
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for understanding the modulation of the magnetic behavior of BFO.
Fig. 5. (Color online) Measured M-H hysteresis loops at room temperature of Bi1−xAxFeO3
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(A=Ba, Sm) (0≤x≤0.2) samples.
To address the issue of the role of oxygen vacancies in affecting the magnetic properties of BFO, room temperature magnetic hysteresis loops have been measured over ±3 T for all
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the samples. The data for all the compositions i.e. Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) are shown in Fig. 5. For pure BFO, the Fe3+ magnetic moments are coupled ferromagnetically
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within the pseudocubic (111) planes but antiferromagnetically between adjacent planes, which is defined as the G-type antiferromagnetic structure.1 Driving by the Dzyaloshinsky–Moriya (DM) interaction,35 the tilting of spins direction of the (111) planes resulting in a net uncompensated moment. Unfortunately, the net magnetization is usually canceled out due to the modulation of the cycloidal spiral spin structure,1 so a linear magnetization hysteresis (M–H) loop and negligible residual magnetization are observed for BFO. As for the substituted Bi1−xAxFeO3 compounds, thin hysteresis loops are observed with increase in Ba concentration while enhanced remnant magnetization is obtained in the Sm substituted compounds. These loops indicate the appearance of ferromagnetic character in first glance
ACCEPTED MANUSCRIPT and the remnant magnetization of Bi1−xBaxFeO3 compounds can be compared to that of Bi1−xSmxFeO3 compound. We believe the lattice distortion of Bi1−xAxFeO3 should be one important reason for the improvement in the magnetic properties of BFO. By taking ionic sizes of Bi3+ and Ba2+/ Sm3+,30 the substitution of Ba2+/ Sm3+ ions in BFO lattice distorts the
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original rhombohedral structure of BFO. Through modulating the Fe–O–Fe bond angle and Fe–O bond distances, the distorted rhombohedral perovskite structure suppresses the spiral spin structure and then releases the latent magnetization inhabited in BFO, allowing a weak ferromagnetic ordering in Bi1−xAxFeO3 compounds.7,13,36 Moreover, related with the increased
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oxygen vacancies, the enhanced Fe3+−O2-−Fe2+ superexchange interactions are considered to be another important factor for the enhancement of BFO in the Bi1−xBaxFeO3 system.20-23 In
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the Bi1−xSmxFeO3 system, due to the ionic sizes of Sm3+ is smaller than that of Bi3+, the introduction of Sm3+ in BFO will shorten the Sm–O and Fe–O distances, which will enhance the exchange interaction between Sm3+–Sm3+ and Fe3+–Sm3+ and then lead to the increase of
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the magnetization.18,19
Fig. 6. M–T curves of (a) Bi1–xBaxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) and (b) Bi1–xSmxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) compounds measured at 5 kOe.
In order to investigate the high temperature magnetic phase transition and reveal the intrinsic magnetic interaction of BFO, the temperature dependence of magnetization (M-T) was measured in the temperature range 300 K–900 K at a field of 5kOe for all the samples. The results are shown in Fig.6. Since the BFO is considered to be antiferromagnetic below
ACCEPTED MANUSCRIPT TN~643K, the magnetization increases with increasing temperature up to 643K, where a distinct peak appears indicating the antiferromagnetic phase transition. For the Bi1−xAxFeO3 compounds, it is interesting to observe that, even though both Ba2+ and Sm3+ substitution result in the enhancement of magnetization, the original antiferromagnetic behavior of BFO
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gradually turns into ferromagnetic behavior with increasing Ba2+ concentration but maintains antiferromagnetic feature in the whole Sm3+ substitution range, as evidenced from the magnetic-temperature curves of Bi1−xAxFeO3 compounds given in Figs.6(a) and 6(b).
Since the structural distortion exists commonly in both Bi1–xBaxFeO3 and Bi1–xSmxFeO3
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compounds, so the different concentration of oxygen vacancies should be a critical factor responsible for the different high temperature magnetic evolution of these two systems. As
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mentioned above, the Fe2+ commonly exists in BFO. The increased Ba2+ concentration produces more Fe2+ ions which consequently enhance the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction, competing with the initial Fe3+−O2-−Fe3+ antiferromagnetic superexchange interaction and enventually resulting in the ferromagnetic behavior of the high-level Ba2+ substituted compounds. As to the Bi1−xSmxFeO3 samples,
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because Sm3+ is in the same oxidation state as that of Bi3+, the concentration of oxygen vacancies remains essentially unchanged with Sm substitution. Therefore, only relatively small enhancement of magnetization is achieved in the Bi1−xSmxFeO3 (0≤x≤0.2) samples
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below TN ~643K, leaving the antiferromagnetic order Fe3+−O2-−Fe3+ to be persevered as the dominant interaction. And this small magnetization enhancement may be originated from the structural distortion related spin canting and the Sm3+−Sm3+ and Sm3+−Fe3+ exchange
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interaction.18,19,37,38
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Fig. 7. (Color online) M-H loops of Bi0.8Ba0.2FeO3 sample at selected temperatures.
On further analyzing these M-T curves, it has been found that in these Ba2+ substituted samples, the antiferromagnetic phase transition peak becomes smaller when the Ba2+
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concentration is increased, implying the dominant role of Fe3+−O2−Fe3+ is gradually replaced by the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction.
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Furthermore, the Fe3+−O2-−Fe3+ can only be maintained below TN~643K, while the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic order can exist up to a much higher temperature (T~778K), indicating a much stronger superexchange interaction. In order to verify this high temperature ferromagnetic-like phase transition in Bi1−xBaxFeO3 (0.1
ACCEPTED MANUSCRIPT that the ferromagnetic-like transition arises from the uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction and the magnetization, which depends on the
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difference between Fe3+ and Fe2+, is relatively small.
Fig.8. (Color online) Schematic plots of the interaction strength and magnetization versus
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Ba2+ concentration for Bi1–xBaxFeO3 (0≤x≤0.2) samples at room temperature. Finally, based on the analysis of high temperature magnetic behaviors of Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds, we are able to roughly establish the relationship between the magnetic superexchange interaction and the concentration of oxygen vacancies of BFO, as
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shown in Fig.8. According to this graph, with increasing oxygen vacancies, the dominated magnetic structure of Bi1−xBaxFeO3 changes from cycloidal spiral spin structure into the
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ferrimagnetic order, resulting in the obvious enhancement of magnetization. It is worth nothing to mention that, since the oxygen vacancies always exist in BFO, the formation of uncompensated Fe3+−O2-−Fe2+ antiferromagnetic superexchange interaction is reasonable. Then, we can infer that the general magnetic structure of BFO should be the coexistence of cycloidal spiral spin structure and unballanced antiferromagnetic spin structure. So, even though the present of oxygen vacancies increases the leakage current and makes the measurement of ferroelectric property more difficult, they are important for the magnetic properties of BFO. Proper adjusting the concentration of oxygen vacancies, it is possible to effectively enhance the magnetization of BFO.
ACCEPTED MANUSCRIPT 4. Conclusions In summary, we have investigated the high temperature magnetic behavior of a series Bi1−xAxFeO3 (A=Ba, Sm) (0≤x≤0.2) compounds in detail. The XRD analysis of Bi1−xAxFeO3 confirms the rhombohedral space group of R3c and the structural distortion with Ba2+/Sm3+
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substitution. Mainly due to the structural distortion and the increased concentration of oxygen vacancies, obvious enhancement of magnetization has been observed in Ba2+ and Sm3+ substituted compounds. Interestingly, the original antiferromagnetic behavior of BFO, which is maintained in all Sm3+substituted samples, alters into a ferromagnetic one with increasing
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Ba2+ concentration. The XPS narrow scan of the Fe 2p spectra reveals that the increased concentration of oxygen vacancies due to the substitution of Ba2+ result in the increased
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percentage of Fe2+, and consequently the ferromagnetic behavior in Bi1−xBaxFeO3 samples. So, we conclude that the oxygen vacancies also play an important role in deciding the magnetic behavior of BFO ceramic.
Acknowledgement
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This work was supported by the Natural Science Foundation of China (U1204111, 11504093, 11304158), the Specialized Research Fund for the Doctoral Program of Higher Education, and the open project of Nanjing National Laboratory of Microstructures (2013M531677 and
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M26002).
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ACCEPTED MANUSCRIPT 35. I. Dzyaloshinsky, J. Phys. Chem. Solids 4 (1958) 241; T. Moriya, Phys. Rev. 120 (1960) 91. 36. T. Durga Rao, T. Karthik, S. Asthana, J. Rare Earth 31 (2013) 370. 37. Y. J. Zhang, H. G. Zhang , J. H. Yin, H. W. Zhang, J. L. Chen, W. Q. Wang, G. H. Wu, J.
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38. M. Al-Haj, Cryst. Res. Technol. 45 (2010) 89–93.
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Magn. Magn. Mater. 322 (2010) 2251–2255.
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1. The Ba/Sm-substitution induces structural distortion in BiFeO3 (BFO). 2. The Ba-substitution increases the concentration of oxygen vacancies. 3. The increased oxygen vacancies alter the magnetic structure of BFO (AFM
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–FM). 4. The oxygen vacancies play a crucial role in deciding the magnetic behavior of BFO.
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5. The general spin structure of BFO should be the coexistence of AFM and FM.