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Microwave absorption properties of multiferroic BiFeO3 nanoparticles
Materials Letters 63 (2009) 1344–1346
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave absorption properties of multiferroic BiFeO3 nanoparticles Yu-Qing Kang a, Mao-Sheng Cao a,⁎, Jie Yuan b, Xiao-Ling Shi a a b
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China School of Information Engineering, Centre University for Nationality, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 19 January 2009 Accepted 6 March 2009 Available online 16 March 2009 Keywords: Multiferroic BiFeO3 Microwave absorption Sol–gel
a b s t r a c t Multiferroic BiFeO3 (BFO) nanoparticles ranging from 60 nm to 120 nm were synthesized successfully by a sol–gel method, and the microwave absorption properties of BFO nanoparticles were investigated in the range of 12.4 GHz to 18 GHz. The reflection loss of BFO nanoparticles is more than 10 dB (or more than 90%) in the 13.1 GHz–18 GHz range and reaches to 26 dB at 16.3 GHz, which indicated that the BFO is a good candidate for microwave absorption application. The excellent microwave absorption properties of BFO nanoparticles could be attributed to the good electromagnetic match as a consequence of the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles, which has been confirmed by electric and magnetic measurement. Moreover, the nanosizeconfinement effect may also have contribution to the high reflection loss of BFO nanoparticles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic BiFeO3 (BFO) has attracted much attention because it is one of the several compounds that exhibit coexistence of ferroelectricity and ferromagnetism at room temperature, due to its high ferroelectric Curie temperature (TC ~ 1103 K) and G-type antiferromagnetic Néel temperature (TN ~ 647 K) [1]. So it is expected to form a new type of magnetoelectric devices by a combination of ferroelectric and ferromagnetic properties [2–4]. In addition to the potential applications as magnetoelectric devices, BFO might find applications as microwave absorption materials due to its magnetoelectric coupling. As the excellent microwave absorption generally results from the good electromagnetic match, i.e., efficient complementarity between the relative permittivity and permeability [5,6]. In recent years, extensive investigations have been carried out to fabricate microwave absorption materials with good electromagnetic match, such as CNTs/Fe, [7] CNTs/ CoFe2O4, [8] and Ni(C) nanocapsules [9]. These core-shell nanocomposites showed better microwave absorption than the pure core or shell materials. However, the complex fabrication process of these core-shell nanocomposites has been challenging for putting such materials into practical applications. Therefore, it is significant to search other approaches to fabricate microwave absorption materials with good electromagnetic match. In this letter, multiferroic BFO nanoparticles were synthesize by a sol–gel method, and the microwave absorption properties were investigated in detail.
metric proportions (1:1 molar ratio) were dissolved in 2-methoxyethanol (C3H8O2). The solution was adjusted to a pH value of about 4 by adding 2-methoxyethanol and nitric acid. Then citric acid in 1:1 molar ratio with respect to the metal nitrates was added to the solution, followed by polyethylene glycol as a dispersant. The mixture was stirred for 30 min at 50 °C to obtain the sol, which was then kept at 80 °C for 48 h to form the dried gel. The dried gel was then grinded into powders and calcined at 300 °C. The calcined powders were sintered at 500 °C for 2 h, and then cooled rapidly to room temperature. The structure and morphology of the BFO nanoparticles were investigated by X-ray diffraction (XRD) (X'Pert PRO, Cu-Ka) and transmission electron microscope (TEM) (JEM-2010). Electric characterization was carried out on a ferroelectric tester (WS 2000).
2. Experimental details Sol–gel method was employed to prepare BFO nanoparticles. Bismuth nitrate (Bi(NO3)3 · 5H2O) and iron nitrate (Fe(NO3)3 · 9H2O) in stoichio⁎ Corresponding author. Tel./fax: +86 10 68914062. E-mail address: [email protected] (M.-S. Cao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.010
Fig. 1. XRD pattern of as-prepared BiFeO3 nanoparticles. Inset shows the rhombohedral cell of BiFeO3 (O = red, Bi = green, and Fe = blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y.-Q. Kang et al. / Materials Letters 63 (2009) 1344–1346
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3. Results and discussion
Fig. 2. TEM image of BiFeO3 nanoparticles.
Magnetic properties were measured using vibrating sample magnetometer (VSM) (LakeShore 7407). For the electromagnetic parameters measurements carried out using a vector network analyzer (ANRIGSU 37269D), the BFO nanoparticles were pressed into pellets with a dimension of 15.20 mm × 7.56 mm × 2.26 mm and then annealed at 500 °C for 30 min.
Fig. 3. (a) Room temperature ferroelectric hysteresis loop of BiFeO3 nanoparticles. (b) M–H hysteresis loop measured at room temperature for BiFeO3 nanoparticles, and inset shows the partially enlarged curve.
Fig. 1 shows the XRD pattern of the as-prepared BFO nanoparticles. It is clear that the BFO nanoparticles are highly crystallized and all diffraction peaks of BFO nanoparticles can be perfectly indexed to BFO with JCPDS card No. 86-1518. This result indicated that the asprepared BFO is rhombohedrally distorted perovskite with space group R3c (No.161), as shown in the inset. Fig. 2 represents the typical transmission electron microscope (TEM) image of the BFO nanoparticles. It can be seen that the BFO mainly consists of roughly spherical with an average size about 60–120 nm. The corresponding energy dispersive spectroscopy (EDS) of a typical particle shows that the atomic ratio of Bi to Fe is approximately 1:1. The ferroelectric nature of BFO nanoparticles measured at room temperature is shown in Fig. 3 (a). The saturation polarization Ps and remnant polarization Pr are found to be 2.1 μC/cm2 and 1.0 μC/cm2 at a maximum applied electric field of 130 kV/cm, respectively. Although these values are smaller than that reported for most of BFO thin film (it is comparable with that of BFO thin film with a Ps value of 2.2 μC/ cm2 and a Pr value of 0.83 μC/cm2 [10]), they are higher than that of bulk materials [11]. The magnetic hysteresis loops have been measured over ±1.5 T at room temperature, as shown in Fig. 3 (b). It is clear that a saturated magnetic hysteresis loop is obtained with saturation magnetization Ms of about 1.5 emu/g. This result is consistent with a previous work [12], which reports that the Ms decreases with increasing particle size of nanoscale BFO. Inset in Fig. 3 (b) gives the partially enlarged M–H curve, which reveals that the coercive field of the BFO nanoparticles is quite small, similar to BFO nanowires [13], nanotubes [14] and BFO thin film [15]. In conclusion, the as-prepared BFO nanoparticles represent weak ferromagnetic order rather than superparamagnetism. Fig. 4 shows the complex permittivity and complex permeability of the BFO nanoparticles as a function of frequencies in the range of 12.4 GHz to 18 GHz. From Fig. 4 (a), the real permittivity (ɛ′) increases from ~ 13 to ~ 19 at 16.7 GHz and then decreases slightly with
Fig. 4. (a) Complex permittivity and (b) complex permeability of BiFeO3 nanoparticles versus frequencies in the range of 12.4 GHz to 18 GHz.
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Y.-Q. Kang et al. / Materials Letters 63 (2009) 1344–1346
nanoparticles could be attributed to the good electromagnetic match resulted from the coexistence of ferroelectric and weak ferromagnetic order. Moreover, the nanosize-confinement effect may also have contribution to the high reflection loss of BFO nanoparticles. 4. Conclusions
Fig. 5. Reflection loss of bulk BiFeO3 and BiFeO3 nanoparticles versus frequencies.
Multiferroic BFO nanoparticles with particles size of 60–120 nm have been successfully prepared by a sol–gel method. Electric and magnetic measurement confirmed the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles. The BFO nanoparticles showed high reflection loss, which could be attributed to the good electromagnetic match, as well as nanosize-confinement effect. In general, the high reflection loss of BFO nanoparticles is favorable to the application as microwave absorption materials. Acknowledgement
increasing frequency, while the imaginary permittivity (ɛ″) nearly increases with increasing frequency until a maximum value of ~ 7.2. From Fig. 4 (b), it can be seen that the real permeability (μ′) of BFO nanoparticles varies from ~ 1.3 to ~0.8 depending on the frequency, and the imaginary permeability (μ″) increases until attaining a maximum value and then decreases with an average value of about 0.1. The reflection loss, which directly determined the microwave absorption properties of materials, can be defined as [7,8]: RðdBÞ = 20 log j ðzin − 1Þ = ðzin + 1Þj be obtained from Where zin is the normalized input impedance pffiffiffiffiffiffiffiffiffiffiffiffiffi could pffiffiffiffiffiffiffiffiffiffiffiffi ffi the following expression: zin = μ r = er tanh jð2π = cÞ μ r = er fd , where c is the velocity of electromagnetic waves in free space, f is the frequency of microwaves, and d is the thickness of the absorber. Fig. 5 shows the reflection loss of BFO nanoparticles ranging from 12.4 GHz to 18 GHz (solid circles). It can be seen that the reflection loss of BFO is more than 10 dB (or more than 90%) in the 13.1 GHz–18 GHz range with 3.5 mm thickness layer. The maximum reflection loss reaches to 26 dB at 16.3 GHz, which is comparable with that of Fe/CNTs (more than 25 dB) [7], and CNTs/CoFe2O4 (18 dB) [8]. As for comparison, the reflection loss of bulk BFO was also investigated, as shown in Fig. 5 (hollow circles). The reflection loss of bulk BFO, with a maximum value of 9.8 dB at 12.6 GHz, is less than that of BFO nanoparticles. The high reflection loss of BFO
The authors thank the National Natural Science Foundation (Grant No. 50872159), the National Defense Funds (Grant No. 51318030302 and A2220061080) and the Scientific Research Foundation of Graduate School of BIT (Grant No. AA200802) for providing the research grant. References [1] Fischer P, Polomska M, Sosnowska I, Szymanksi M. J Phys C 1980;13:1931. [2] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Science 2003;299:1719–22. [3] Hur N, Park S, Sharma PA, Ahn JS, Guha S, Cheong SW. Nature 2004;429:392–5. [4] Sun T, Pan ZX, Dravid VP, Wang ZY, Yu MF, Wang J. Appl Phys Lett 2006;89:163117. [5] Wadhawan A, Garrett D, Perez JM. Appl Phys Lett 2003;83:2683–5. [6] Cao MS, Qin RR, Qiu CJ, Zhu J. Mater Design 2003;24:391–6. [7] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Adv Mater 2004;16:401–5. [8] Che RC, Zhi CY, Liang CY, Zhou XG. Appl Phys Lett 2006;88:033105. [9] Zhang XF, Dong XL, Huang H, Liu YY, Wang WN, Zhu XG, et al. Appl Phys Lett 2006;89:053115. [10] Palkar VR, John J, Pinto R. Appl Phys Lett 2002;80:1628–30. [11] Kumar MM, Palkar VR, Srinivas K, Suryanarayana SV. Appl Phys Lett 2000;76:2764–6. [12] Mazumder R, Devi PS, Bhattacharya D, Choudhury P, Sen A, Raja M. Appl Phys Lett 2007;91:062510. [13] Gao F, Yuan Y, Wang KF, Chen XY, Chen F, Liu JM. Appl Phys Lett 2006;89:102506. [14] Wei J, Xue DS, Xu Y. Scripta Mater 2008;58:45–8. [15] Li YB, Sritharan T, Zhang S, He XD, Liu Y, Chen TP. Appl Phys Lett 2008;92:132908.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave absorption properties of multiferroic BiFeO3 nanoparticles Yu-Qing Kang a, Mao-Sheng Cao a,⁎, Jie Yuan b, Xiao-Ling Shi a a b
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China School of Information Engineering, Centre University for Nationality, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 19 January 2009 Accepted 6 March 2009 Available online 16 March 2009 Keywords: Multiferroic BiFeO3 Microwave absorption Sol–gel
a b s t r a c t Multiferroic BiFeO3 (BFO) nanoparticles ranging from 60 nm to 120 nm were synthesized successfully by a sol–gel method, and the microwave absorption properties of BFO nanoparticles were investigated in the range of 12.4 GHz to 18 GHz. The reflection loss of BFO nanoparticles is more than 10 dB (or more than 90%) in the 13.1 GHz–18 GHz range and reaches to 26 dB at 16.3 GHz, which indicated that the BFO is a good candidate for microwave absorption application. The excellent microwave absorption properties of BFO nanoparticles could be attributed to the good electromagnetic match as a consequence of the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles, which has been confirmed by electric and magnetic measurement. Moreover, the nanosizeconfinement effect may also have contribution to the high reflection loss of BFO nanoparticles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic BiFeO3 (BFO) has attracted much attention because it is one of the several compounds that exhibit coexistence of ferroelectricity and ferromagnetism at room temperature, due to its high ferroelectric Curie temperature (TC ~ 1103 K) and G-type antiferromagnetic Néel temperature (TN ~ 647 K) [1]. So it is expected to form a new type of magnetoelectric devices by a combination of ferroelectric and ferromagnetic properties [2–4]. In addition to the potential applications as magnetoelectric devices, BFO might find applications as microwave absorption materials due to its magnetoelectric coupling. As the excellent microwave absorption generally results from the good electromagnetic match, i.e., efficient complementarity between the relative permittivity and permeability [5,6]. In recent years, extensive investigations have been carried out to fabricate microwave absorption materials with good electromagnetic match, such as CNTs/Fe, [7] CNTs/ CoFe2O4, [8] and Ni(C) nanocapsules [9]. These core-shell nanocomposites showed better microwave absorption than the pure core or shell materials. However, the complex fabrication process of these core-shell nanocomposites has been challenging for putting such materials into practical applications. Therefore, it is significant to search other approaches to fabricate microwave absorption materials with good electromagnetic match. In this letter, multiferroic BFO nanoparticles were synthesize by a sol–gel method, and the microwave absorption properties were investigated in detail.
metric proportions (1:1 molar ratio) were dissolved in 2-methoxyethanol (C3H8O2). The solution was adjusted to a pH value of about 4 by adding 2-methoxyethanol and nitric acid. Then citric acid in 1:1 molar ratio with respect to the metal nitrates was added to the solution, followed by polyethylene glycol as a dispersant. The mixture was stirred for 30 min at 50 °C to obtain the sol, which was then kept at 80 °C for 48 h to form the dried gel. The dried gel was then grinded into powders and calcined at 300 °C. The calcined powders were sintered at 500 °C for 2 h, and then cooled rapidly to room temperature. The structure and morphology of the BFO nanoparticles were investigated by X-ray diffraction (XRD) (X'Pert PRO, Cu-Ka) and transmission electron microscope (TEM) (JEM-2010). Electric characterization was carried out on a ferroelectric tester (WS 2000).
2. Experimental details Sol–gel method was employed to prepare BFO nanoparticles. Bismuth nitrate (Bi(NO3)3 · 5H2O) and iron nitrate (Fe(NO3)3 · 9H2O) in stoichio⁎ Corresponding author. Tel./fax: +86 10 68914062. E-mail address: [email protected] (M.-S. Cao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.010
Fig. 1. XRD pattern of as-prepared BiFeO3 nanoparticles. Inset shows the rhombohedral cell of BiFeO3 (O = red, Bi = green, and Fe = blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y.-Q. Kang et al. / Materials Letters 63 (2009) 1344–1346
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3. Results and discussion
Fig. 2. TEM image of BiFeO3 nanoparticles.
Magnetic properties were measured using vibrating sample magnetometer (VSM) (LakeShore 7407). For the electromagnetic parameters measurements carried out using a vector network analyzer (ANRIGSU 37269D), the BFO nanoparticles were pressed into pellets with a dimension of 15.20 mm × 7.56 mm × 2.26 mm and then annealed at 500 °C for 30 min.
Fig. 3. (a) Room temperature ferroelectric hysteresis loop of BiFeO3 nanoparticles. (b) M–H hysteresis loop measured at room temperature for BiFeO3 nanoparticles, and inset shows the partially enlarged curve.
Fig. 1 shows the XRD pattern of the as-prepared BFO nanoparticles. It is clear that the BFO nanoparticles are highly crystallized and all diffraction peaks of BFO nanoparticles can be perfectly indexed to BFO with JCPDS card No. 86-1518. This result indicated that the asprepared BFO is rhombohedrally distorted perovskite with space group R3c (No.161), as shown in the inset. Fig. 2 represents the typical transmission electron microscope (TEM) image of the BFO nanoparticles. It can be seen that the BFO mainly consists of roughly spherical with an average size about 60–120 nm. The corresponding energy dispersive spectroscopy (EDS) of a typical particle shows that the atomic ratio of Bi to Fe is approximately 1:1. The ferroelectric nature of BFO nanoparticles measured at room temperature is shown in Fig. 3 (a). The saturation polarization Ps and remnant polarization Pr are found to be 2.1 μC/cm2 and 1.0 μC/cm2 at a maximum applied electric field of 130 kV/cm, respectively. Although these values are smaller than that reported for most of BFO thin film (it is comparable with that of BFO thin film with a Ps value of 2.2 μC/ cm2 and a Pr value of 0.83 μC/cm2 [10]), they are higher than that of bulk materials [11]. The magnetic hysteresis loops have been measured over ±1.5 T at room temperature, as shown in Fig. 3 (b). It is clear that a saturated magnetic hysteresis loop is obtained with saturation magnetization Ms of about 1.5 emu/g. This result is consistent with a previous work [12], which reports that the Ms decreases with increasing particle size of nanoscale BFO. Inset in Fig. 3 (b) gives the partially enlarged M–H curve, which reveals that the coercive field of the BFO nanoparticles is quite small, similar to BFO nanowires [13], nanotubes [14] and BFO thin film [15]. In conclusion, the as-prepared BFO nanoparticles represent weak ferromagnetic order rather than superparamagnetism. Fig. 4 shows the complex permittivity and complex permeability of the BFO nanoparticles as a function of frequencies in the range of 12.4 GHz to 18 GHz. From Fig. 4 (a), the real permittivity (ɛ′) increases from ~ 13 to ~ 19 at 16.7 GHz and then decreases slightly with
Fig. 4. (a) Complex permittivity and (b) complex permeability of BiFeO3 nanoparticles versus frequencies in the range of 12.4 GHz to 18 GHz.
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Y.-Q. Kang et al. / Materials Letters 63 (2009) 1344–1346
nanoparticles could be attributed to the good electromagnetic match resulted from the coexistence of ferroelectric and weak ferromagnetic order. Moreover, the nanosize-confinement effect may also have contribution to the high reflection loss of BFO nanoparticles. 4. Conclusions
Fig. 5. Reflection loss of bulk BiFeO3 and BiFeO3 nanoparticles versus frequencies.
Multiferroic BFO nanoparticles with particles size of 60–120 nm have been successfully prepared by a sol–gel method. Electric and magnetic measurement confirmed the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles. The BFO nanoparticles showed high reflection loss, which could be attributed to the good electromagnetic match, as well as nanosize-confinement effect. In general, the high reflection loss of BFO nanoparticles is favorable to the application as microwave absorption materials. Acknowledgement
increasing frequency, while the imaginary permittivity (ɛ″) nearly increases with increasing frequency until a maximum value of ~ 7.2. From Fig. 4 (b), it can be seen that the real permeability (μ′) of BFO nanoparticles varies from ~ 1.3 to ~0.8 depending on the frequency, and the imaginary permeability (μ″) increases until attaining a maximum value and then decreases with an average value of about 0.1. The reflection loss, which directly determined the microwave absorption properties of materials, can be defined as [7,8]: RðdBÞ = 20 log j ðzin − 1Þ = ðzin + 1Þj be obtained from Where zin is the normalized input impedance pffiffiffiffiffiffiffiffiffiffiffiffiffi could pffiffiffiffiffiffiffiffiffiffiffiffi ffi the following expression: zin = μ r = er tanh jð2π = cÞ μ r = er fd , where c is the velocity of electromagnetic waves in free space, f is the frequency of microwaves, and d is the thickness of the absorber. Fig. 5 shows the reflection loss of BFO nanoparticles ranging from 12.4 GHz to 18 GHz (solid circles). It can be seen that the reflection loss of BFO is more than 10 dB (or more than 90%) in the 13.1 GHz–18 GHz range with 3.5 mm thickness layer. The maximum reflection loss reaches to 26 dB at 16.3 GHz, which is comparable with that of Fe/CNTs (more than 25 dB) [7], and CNTs/CoFe2O4 (18 dB) [8]. As for comparison, the reflection loss of bulk BFO was also investigated, as shown in Fig. 5 (hollow circles). The reflection loss of bulk BFO, with a maximum value of 9.8 dB at 12.6 GHz, is less than that of BFO nanoparticles. The high reflection loss of BFO
The authors thank the National Natural Science Foundation (Grant No. 50872159), the National Defense Funds (Grant No. 51318030302 and A2220061080) and the Scientific Research Foundation of Graduate School of BIT (Grant No. AA200802) for providing the research grant. References [1] Fischer P, Polomska M, Sosnowska I, Szymanksi M. J Phys C 1980;13:1931. [2] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Science 2003;299:1719–22. [3] Hur N, Park S, Sharma PA, Ahn JS, Guha S, Cheong SW. Nature 2004;429:392–5. [4] Sun T, Pan ZX, Dravid VP, Wang ZY, Yu MF, Wang J. Appl Phys Lett 2006;89:163117. [5] Wadhawan A, Garrett D, Perez JM. Appl Phys Lett 2003;83:2683–5. [6] Cao MS, Qin RR, Qiu CJ, Zhu J. Mater Design 2003;24:391–6. [7] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Adv Mater 2004;16:401–5. [8] Che RC, Zhi CY, Liang CY, Zhou XG. Appl Phys Lett 2006;88:033105. [9] Zhang XF, Dong XL, Huang H, Liu YY, Wang WN, Zhu XG, et al. Appl Phys Lett 2006;89:053115. [10] Palkar VR, John J, Pinto R. Appl Phys Lett 2002;80:1628–30. [11] Kumar MM, Palkar VR, Srinivas K, Suryanarayana SV. Appl Phys Lett 2000;76:2764–6. [12] Mazumder R, Devi PS, Bhattacharya D, Choudhury P, Sen A, Raja M. Appl Phys Lett 2007;91:062510. [13] Gao F, Yuan Y, Wang KF, Chen XY, Chen F, Liu JM. Appl Phys Lett 2006;89:102506. [14] Wei J, Xue DS, Xu Y. Scripta Mater 2008;58:45–8. [15] Li YB, Sritharan T, Zhang S, He XD, Liu Y, Chen TP. Appl Phys Lett 2008;92:132908.