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Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics
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Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar, Sandeep Chhoker
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S0022-3697(13)00313-2 http://dx.doi.org/10.1016/j.jpcs.2013.09.003 PCS7167
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Journal of Physics and Chemistry of Solids
Received date: 27 February 2013 Revised date: 9 August 2013 Accepted date: 5 September 2013 Cite this article as: Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar, Sandeep Chhoker, Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2013.09.003 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.
Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar* and Sandeep Chhoker* Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, Noida- 201307, India ABSTRACT Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics were synthesized by solid state reaction method. Effects of Dy substitution on structural distortion, magnetic and optical properties of BiFeO3 were examined by X-diffraction, Raman and UV-Visible spectroscopy. The samples were found to crystallize in rhombohedral structure of BiFeO3 with R3c space group. The reduction in lattice parameters and unit cell volume indicate the distortion in FeO6 octahedra of the rhombohedral structure without any signature of phase transformation up to x=0.12. The predictable weak ferromagnetic hysteresis loops can be observed in the Dy doped samples with maximum remnant magnetization of 0.2103 emu/g for x=0.12. The weak ferromagnetism is ascribed to the suppressed spiral spin structure and magnetically active characteristic of Dy3+ ions together with ferromagnetic coupling between Dy3+ and Fe3+ ions. With optical band gap in visible region, Dy doped BiFeO3 ceramics are potential material for optoelectronic device and solar cell applications. Keywords: Multiferroics, Raman Spectroscopy, Magnetic properties, Optical properties. Corresponding author Email: [email protected] Phone: +91 120 2594360; Fax: +91 120 2400986
1
1. Introduction Multiferroic materials showing both ferromagnetic and ferroelectric ordering in a single phase have attracted great attention due to its potential in advanced device applications and fascinating basic physics [1-2]. Amongst all types of ABO3 multiferroic materials, BiFeO3 (BFO) is the most promising material for a given application due to its high ferroelectric Curie (TC~1103 K) and antiferromagnetic Neel (TN~643 K) temperatures [3]. The G-type antiferromagnetic nature and modulated spiral spin structure with long periodicity of 62 nm restrict the observation of macroscopic magnetization in bulk BFO, which prohibits multiferroic properties [4]. Recently, weak ferromagnetism is reported in thin films and nanoparticles owning to the size effect [5-6]. On the other hand, improved magnetic properties have been observed by substituting rare earth (Pr3+, Gd3+, Ho3+ and Dy3+) ions at A-site [7-10] and transition metal (Ti4+ and Mn4+) ions at B-site [1112] . The substitution of magnetically active Dy3+ ions at A-site has shown stabilization of perovskite structure with an enhancement in the ferroelectric and magnetic properties of BFO [13]. However, there are rarely any reports on structural analysis by Raman spectroscopy as well as optical properties of Dy doped BFO bulk ceramics. This triggered us to synthesize Bi1-xDyxFeO3 multiferroic ceramics and study its structural, magnetic and optical properties. 2. Experimental Bi1-xDyxFeO3 ceramics with x = 0.00, 0.03, 0.05, 0.07, 0.10 and 0.12 (named BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 respectively) were prepared by solid state reaction method using high purity (99.99%, Sigma Aldrich) Bi2O3, Fe2O3 and Dy2O3 powders. First these oxides were weighed in stoichiometric proportion and then mixed and ground in mortar pestle in acetone medium for 4 hrs to make homogenous mixture of oxides and calcined at 7000C for 2hrs. These calcined powders are pressed into pellets of 10 mm diameter and thickness of 1 mm. These pellets are sintered at 800°C (BFO, BDF-3 and BDF-5 samples) and 820°C (BDF-7, BDF-10 and BDF-12 samples) for 2 hrs. X-ray diffraction (XRD) patterns were recorded by Bruker D8 Advance diffractometer with
2
CuKα radiation. Raman spectra were recorded by Renishaw Raman spectrophotometer using 514.5 nm Laser beam with spot size 1μm. The power of laser was kept below 5 mW in order to avoid any heating of samples. The room temperature magnetic properties were measured by SQUID magnetometer. Optical properties were determined by UV–Visible diffuse reflectance (Ocean optics UV–Visible 4000). 3. Results and discussion Fig. 1(a) shows the XRD patterns of Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics. All BDF samples, except BDF-3, exhibited single phase perovskite rhombohedral structure of BFO (JCPDS card no. 71-2494). The XRD patterns indicate that the doping stabilizes the rhombohedral structure of BFO by suppressing impurity phases. The shifting of (104) and (110) peaks towards higher 2θ values and reduced separation between these peaks (Fig. 1(b)) indicate the distortion in the rhombohedral structure of BFO without any signature of structural transformation up to x=0.12. This behaviour is unlike Dy doped BFO nanoparticles in which structural transformation has been reported for x=0.10 [14]. However, in Dy doped BFO bulk ceramics, the structural transformation from rhombohedral to orthorhombic is expected for x>0.12. The phase coexistence in broad concentration range has already been reported for other rare earth doped BFO ceramics [8]. The smaller Dy3+ (0.92 Å) ions substitution for the bigger Bi3+ (1.17 Å) ions lead to the distortion in the BFO lattice which enhances the Fe-O-Fe bond angle together with contraction of the unit cell. The lattice parameter, unit cell parameter and Fe-O-Fe bond angle for BDF samples are shown in Table 1. The continuous increase in the Fe-O-Fe bond angle might improve the magnetic properties of BDF samples. Figure 2 shows the room temperature Raman spectra for BDF samples in the wavenumber range 50-700 cm-1. Mode assignment in BiFeO3 is a complex and often confusing issue as discussed by Bielecki et. al. [15]. The problem is the strong angular dispersion and the optical axis in [111] direction. There is no natural surface on a single crystal parallel or perpendicular to the optical axis due to the general direction of optical axis. The situation is even more complex in bulk samples due
3
to random orientation of grains. As a result, mixed kinds of modes are observed with shifted frequencies and modified intensities instead of pure vibrational modes. Raman patterns of BDF samples correspond to the spectral feature of typical Raman spectra of rhombohedral of BiFeO3 as reported by other reports [16-22]. There is no obvious change in Raman spectra of BDF samples with increasing Dy indicating the absence of any structural phase transition unlike Dy doped BFO nanoparticles [14]. Figure 3 shows the field dependent magnetization of BFO and BDF ceramics measured at room temperature. BFO, BDF-3 and BDF-5 samples show antiferromagnetic nature, while BDF-7, BDF10 and BDF-12 samples exhibit weak ferromagnetic nature with higher values of remnant magnetization. The incorporation of Dy3+ ions into the BFO lattice increases the remnant and net magnetization (MH) (inset of Fig. 3(b)) and broadening of loops occur. The values of remnant magnetization (Mr) are found to be 0.009, 0.0233, 0.0314, 0.0544, 0.1400 and 0.2103 emu/g for BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples, respectively. The remnant and net magnetization observed for BDF-12 sample are much higher than the reported values for A-site doped and codoped BFO bulk ceramics [9, 13 and 23-25]. Several reasons contribute to the improvement of magnetism in BDF samples. First, the spiral spin structure of BFO is suppressed by the substitution of Dy3+ ions for Bi3+ ions which leads to the structural distortion giving rise to an enhanced magnetization. The Dy substitution only suppressed and could not destroy the spiral spin structure completely. However, for BDF-12 sample, larger Fe-O-Fe angle suggests highly distorted FeO6 octahedra which are responsible for the strong ferromagnetic interaction [26-27]. The continuous increase in Fe-O-Fe bond angle, which is closely related to magnetic structure, is in good agreement with observed M-H loops. Second, the enhancement of magnetization could be attributed to the formation of Bi-O-Dy chains which became magnetic sub-lattices [24]. The Dy doped BFO system has antiferromagnetic Fe−O−Fe interaction coupled with a weak ferromagnetic component, which comes from the canted Fe sublattice due to Dzyaloshinskii−Moriya interaction, which leads to the linear increase with field in the M−H curve [28]. Besides this we presume that
4
the ferromagnetic coupling between Dy3+ and Fe3+ ions contributes to the enhanced magnetization to some extent. UV-visible absorption spectra of pure and BDF samples (Fig. 4) consist two absorption bands. The strong band in the range 450-600 nm is attributed to metal to metal transition and the weak band around 700 nm is assigned to the crystal field transitions. The absorption cut-off wavelength increases with increasing Dy content and the corresponding band gap is calculated as 2.25, 2.21, 2.20, 2.19, 2.12 and 2.09 eV for BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples, respectively. The obtained values of the band gap for BDF samples are consistent with the reported values for pure and doped BFO [14, 19, 29 and 30] and smaller than BFO thin film [31] and BFO nanoparticles [32]. The decrease in band gap of BDF samples may be attributed to the rearrangement of molecular orbitals and distortion induced in the FeO6 octahedra [33]. The energy band gap in visible region makes BDF samples suitable for photocatalytic and solar cell applications. The room temperature dielectric constant and dielectric loss of Dy doped BFO ceramics are shown in Fig. 5 (a) and (b). The values of dielectric constant decreased with increasing Dy doping. At lower frequencies the relaxation behaviour has been observed for BDF-3 and BDF-5 samples. However, dielectric constant for BDF-7, BDF-10 and BDF-12 samples remains stable in whole frequency range. This indicated dipoles with small effective mass mainly contribute to dielectric constant, instead of charge defects with large effective masses [26]. The dielectric loss also decreased with increasing Dy concentration indicating that Dy-substitution at A-site effectively suppressed the formation of oxygen vacancies which results in decreased conductivity. 4. Conclusions The presence of distorted rhombohedral structural without any signature of phase transformation up to x=0.12 sample has been established from XRD and Raman spectroscopy. The change in lattice parameters and Fe-O-Fe bond angle indicated the distortion in the BFO lattice due to Dy
5
substitution. The enhancement in magnetization is ascribed to the partial suppression of modulated spiral spin structure due to distortion and ferromagnetic coupling between Dy3+ and Fe3+ ions. Also, the decrease in the band gap is ascribed to the rearrangement of molecular orbitals and distortion induced in the FeO6 octahedra with increasing Dy concentration. Thus the Dy doping in BFO ceramics has been found to exhibit interesting optical properties in visible region along with improved magnetic properties. Acknowledgements This work was financially supported by Department of science and Technology (DST), India through grant no. SR/FTP/PS-91/2009. Prakash Chandra Sati is also thankful to DST for providing INSPIRE Fellowship. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21. [3] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu B et al. Science 299 (2003) 1719. [4] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401. [5] J. Liu, M. Li, L. Pei, J. Wang, Z. Hu, X. Wang, X. Zhao, EPL 89 (2010) 57004. [6] M. Kumar, K.L. Yadav, G.D. Verma, Mater. Lett. 62 (2008) 1159. [7] B. Yu, M. Li, Z. Hu, L. Pei, D. Guo, X. Zhao, S. Dong, Appl. Phys. Lett. 93 (2008) 182909. [8] V. A. Khomchenko, V. V. Shvartsman, P. Borisov, W. Kleemann, D. A. Kiselev, I. K. Bdikin, J. M. Vieira, A. L. Kholkin, Acta Materialia 57 (2009) 5137. [9] N. V. Minh, N. G. Quan, J. Alloy. Comp. 509 (2011) 2663. [10] S. Zhang, W. Luo, D. Wang, Y. Ma, Mater. Lett. 63 (2009) 1820. [11] M. Kumar, K. L. Yadav, J. Appl. Phys. 100 (2006) 074111. [12] S. Chauhan, M. Kumar, S. Chhokar, S. C. Katyal, H. Singh, M. Jewariya, K. L. Yadav, Solid
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State. Commun. 152 (2012) 525. [13] P. Uniyal, K. L. Yadav, J. Phys. : Condens. Matter. 21 (2009) 012205. [14] M. Arora, P. C. Sati, S. Chauhan, H. Singh, K. L.Yadav, S. Chhoker, M. Kumar, Mater. Lett. 96 (2013) 71. [15] J. Bielecki, P. Svedlindh, D. T. Tibebu, S. Cai, S. G. Eriksson, L. Borjesson, C. S. Knee, Phys. Rev. B 86 (2012) 184422. [16] H. Fukumura, S. Matsui, H. Harima, T. Takahashi, T. Itoh, K. Kisoda, M. Tamada, Y. Noguchi, M. Miyayama, J. Phys.: Condens. Matter 19 (2007) 365224. [17] A. Jaiswal, R. Das, T. Maity, K. Vivekanand, S. Adyanthaya, Pankaj Poddar, J. Phys. Chem. C 114 (2010) 12432. [18] A. Jaiswal, R. Das, K. Vivekanand, P. M. Abraham, S. Adyanthaya, and P. Poddar, J. Phys. Chem. C 114 (2010) 2108. [19] M. M. Shirolkar, R. Das, T. Maity, P. Poddar, S. K. Kulkarni, J. Phys. Chem. C 116 (2012) 19503. [20] S. Gupta, M. Tomar and V. Gupta, J. Exp. Nanosci. iFirst (2012) 1–6. [21] G. L.Yuan, S. W. Or, H. L. W. Chan, J. Appl. Phys. 101 (2007) 064101. [22] N. Jeon, D. Rout, I. W. Kim, S. J. L. Kang, Appl. Phys. Lett. 98 (2011) 072901. [23] D. Maurya, H. Thota, A. Garg, B. Pandey, P. Chand, H. C. Verma, J. Phys. : Condens. Matter. 21 (2009) 026007. [24] F. Z. Qian, J. S. Jiang, D. M. Jiang, W. G. Zhang, J. H. Liu, J. Phys. D: Appl. Phys. 43 (2010) 025403. [25] P. C. Sati, M. Arora, S. Chauhan, S. Chhoker, M. Kumar, J. Appl. Phys. 112 (2012) 094102. [26] Reetu, A. Agarwal, S. Sanghi, Ashima, J. Appl. Phys. 110 (2011) 073909. [27] C. Lan, Y. Jiang, S. Yang, J. Mater. Sci. 46 (2011) 734. [28] D. P. Dutta, B. P. Mandal, R. Naik, G. Lawes, A. K. Tyagi, J. Phys. Chem. C, 117 (2013) 2382.
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[29] B. Bhushan, A. Basumallick, S. K. Bandopadhyay, N. Y. Vasanthacharya, D. Das, J. Phys. D: Appl. Phys. 42 (2009) 065004. [30] A. Mukherjee, S. M. Hossain, M. Pal, S. Basu, Appl. Nanosci. 2 (2012) 305. [31] P. Chen, N. J. Podraza, X. S. Xu, A. Melville, E. Vlahos, V. Gopalan, R. Ramesh, D. G. Schlom, J. L. Musfeldt, Appl. Phys. Lett. 96 (2010) 131907. [32] P. Chen, X. Xu, C. Koenigsmann, A. C. Santulli, S. S. Wong, J. L. Musfeldt, Nano Lett., 10 (2010) 4526. [33] B. Bhushan, A. Basumallick, N. Y. Vasanthacharya, S. Kumar, D. Das, Solid State Sci., 12 (2010) 1063.
FIGURE CAPTION: Fig. 1. XRD patterns of Bi1-xDyxFeO3 ceramics with x=0, 0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 2. Room temperature Raman spectra of Bi1-xDyxFeO3 ceramics with x=0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 3. Room temperature M-H curves for Bi1-xDyxFeO3 ceramics. The inset shows variation of Mr and MH with increasing Dy content. Fig. 4. UV-Visible diffuse absorption spectra for Bi1-xDyxFeO3 ceramics with x=0, 0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 5. Frequency dependence of dielectric constant and dielectric loss of Bi1-xDyxFeO3 ceramics at room temperature
8
Table 1. Lattice parameters and bond angle of BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples Samples
a (Å)
c (Å)
V (Å)3
Fe-O-Fe (deg.)
BFO
5.5786
13.8667
373.7270
146.2755
BDF-3
5.5735
13.8502
372.6015
153.472
BDF-5
5.57065
13.83374
371.7766
155.2346
BDF-7
5.56778
13.82769
371.2311
156.0725
BDF-10
5.56623
13.81484
370.6797
158.3336
BDF-12
5.57043
13.80784
371.0511
159.3345
Highlights •
Dy doped BiFeO3 ceramics were synthesised by solid state reaction method.
•
XRD and Raman spectra confirm the distorted rhombohedral structure of BDF samples.
•
Enhanced magnetic properties have been observed for BDF samples.
•
The band gap variation in visible region makes BDF samples suitable for solar cell
applications.
9
Figure(s)
Figure 1
(b)
(a) BDF-12
BDF-7
BDF-5
* * Bi Fe O 2 4 9
10
20
( 128)
( 036)
( 220)
&
Fe O 2 39
( 134)
( 300) ( 018)
( 122)
( 116)
24
( 131)
* **
& Bi
( 208)
BFO
( 202)
( 006)
( 110)
( 104)
** *
( 024)
BDF-3
( 012)
Inte nsity (a.u.)
BDF-10
*
30
40
2
50
(degree)
60
70
80
31.5
32.2
Figure(s)
Figure
2
3 BDF-5 BDF-7 BDF-10 BDF-12
Intensity (a.u.)
BDF-
100
200
300
400
500
Raman Shift (cm
1
-
)
600
700
Figure(s)
r
Figu e 3
BFO B
2
B B B
1
B
D D D D D
F-3 F-5 F-7 F-10 F-12
0
-2
2.0
0.15
1.6
0.10
1.2
0.05
0.8
(emu/g)
0.20
MH
(emu/g)
-1
Mr
Ma
gnetiz
a
tion (emu/g)
3
0.00
0.4 0.00
Dy
0.06
ra
concent
0.12
tion (x)
-3 -80
-60
-40
Ma
-20
0
20
40
gnetic field (kOe)
60
80
Figure(s)
Figure 4 1.0
BFO
BDF-5
BDF-3
0.90
0.20
0.8 0.85 0.6 0.15 0.80
Absorbance
0.4
0.75
0.2
0.10
0.0 400
0.70 500
600
700
800
400
500
600
700
800
400
500
600
700
800
0.65
BDF-7
0.15
BDF-12
BDF-10 0.60 0.35 0.55
0.50
0.30 0.10
0.45 0.25 0.40
0.35 0.20 0.30
0.05 400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
400
500
600
700
800
Figure(s)
Figure 5 0.6
600
Dielectric Constant
(a) 500
(b)
BDF-3
BDF-5
BDF-5
BDF-7
BDF-7
BDF-10
BDF-10
BDF-12
BDF-12
400
0.4
300
0.2
200
100
0.0
0
1k
10k
100k
Frequency (Hz)
1M
1k
10k
100k
1M
Frequency (Hz)
Dielectric Loss
BDF-3
Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar, Sandeep Chhoker
www.elsevier.com/locate/jpcs
PII: DOI: Reference:
S0022-3697(13)00313-2 http://dx.doi.org/10.1016/j.jpcs.2013.09.003 PCS7167
To appear in:
Journal of Physics and Chemistry of Solids
Received date: 27 February 2013 Revised date: 9 August 2013 Accepted date: 5 September 2013 Cite this article as: Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar, Sandeep Chhoker, Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2013.09.003 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.
Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics Prakash Chandra Sati, Manisha Arora, Sunil Chauhan, Manoj Kumar* and Sandeep Chhoker* Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, Noida- 201307, India ABSTRACT Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics were synthesized by solid state reaction method. Effects of Dy substitution on structural distortion, magnetic and optical properties of BiFeO3 were examined by X-diffraction, Raman and UV-Visible spectroscopy. The samples were found to crystallize in rhombohedral structure of BiFeO3 with R3c space group. The reduction in lattice parameters and unit cell volume indicate the distortion in FeO6 octahedra of the rhombohedral structure without any signature of phase transformation up to x=0.12. The predictable weak ferromagnetic hysteresis loops can be observed in the Dy doped samples with maximum remnant magnetization of 0.2103 emu/g for x=0.12. The weak ferromagnetism is ascribed to the suppressed spiral spin structure and magnetically active characteristic of Dy3+ ions together with ferromagnetic coupling between Dy3+ and Fe3+ ions. With optical band gap in visible region, Dy doped BiFeO3 ceramics are potential material for optoelectronic device and solar cell applications. Keywords: Multiferroics, Raman Spectroscopy, Magnetic properties, Optical properties. Corresponding author Email: [email protected] Phone: +91 120 2594360; Fax: +91 120 2400986
1
1. Introduction Multiferroic materials showing both ferromagnetic and ferroelectric ordering in a single phase have attracted great attention due to its potential in advanced device applications and fascinating basic physics [1-2]. Amongst all types of ABO3 multiferroic materials, BiFeO3 (BFO) is the most promising material for a given application due to its high ferroelectric Curie (TC~1103 K) and antiferromagnetic Neel (TN~643 K) temperatures [3]. The G-type antiferromagnetic nature and modulated spiral spin structure with long periodicity of 62 nm restrict the observation of macroscopic magnetization in bulk BFO, which prohibits multiferroic properties [4]. Recently, weak ferromagnetism is reported in thin films and nanoparticles owning to the size effect [5-6]. On the other hand, improved magnetic properties have been observed by substituting rare earth (Pr3+, Gd3+, Ho3+ and Dy3+) ions at A-site [7-10] and transition metal (Ti4+ and Mn4+) ions at B-site [1112] . The substitution of magnetically active Dy3+ ions at A-site has shown stabilization of perovskite structure with an enhancement in the ferroelectric and magnetic properties of BFO [13]. However, there are rarely any reports on structural analysis by Raman spectroscopy as well as optical properties of Dy doped BFO bulk ceramics. This triggered us to synthesize Bi1-xDyxFeO3 multiferroic ceramics and study its structural, magnetic and optical properties. 2. Experimental Bi1-xDyxFeO3 ceramics with x = 0.00, 0.03, 0.05, 0.07, 0.10 and 0.12 (named BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 respectively) were prepared by solid state reaction method using high purity (99.99%, Sigma Aldrich) Bi2O3, Fe2O3 and Dy2O3 powders. First these oxides were weighed in stoichiometric proportion and then mixed and ground in mortar pestle in acetone medium for 4 hrs to make homogenous mixture of oxides and calcined at 7000C for 2hrs. These calcined powders are pressed into pellets of 10 mm diameter and thickness of 1 mm. These pellets are sintered at 800°C (BFO, BDF-3 and BDF-5 samples) and 820°C (BDF-7, BDF-10 and BDF-12 samples) for 2 hrs. X-ray diffraction (XRD) patterns were recorded by Bruker D8 Advance diffractometer with
2
CuKα radiation. Raman spectra were recorded by Renishaw Raman spectrophotometer using 514.5 nm Laser beam with spot size 1μm. The power of laser was kept below 5 mW in order to avoid any heating of samples. The room temperature magnetic properties were measured by SQUID magnetometer. Optical properties were determined by UV–Visible diffuse reflectance (Ocean optics UV–Visible 4000). 3. Results and discussion Fig. 1(a) shows the XRD patterns of Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics. All BDF samples, except BDF-3, exhibited single phase perovskite rhombohedral structure of BFO (JCPDS card no. 71-2494). The XRD patterns indicate that the doping stabilizes the rhombohedral structure of BFO by suppressing impurity phases. The shifting of (104) and (110) peaks towards higher 2θ values and reduced separation between these peaks (Fig. 1(b)) indicate the distortion in the rhombohedral structure of BFO without any signature of structural transformation up to x=0.12. This behaviour is unlike Dy doped BFO nanoparticles in which structural transformation has been reported for x=0.10 [14]. However, in Dy doped BFO bulk ceramics, the structural transformation from rhombohedral to orthorhombic is expected for x>0.12. The phase coexistence in broad concentration range has already been reported for other rare earth doped BFO ceramics [8]. The smaller Dy3+ (0.92 Å) ions substitution for the bigger Bi3+ (1.17 Å) ions lead to the distortion in the BFO lattice which enhances the Fe-O-Fe bond angle together with contraction of the unit cell. The lattice parameter, unit cell parameter and Fe-O-Fe bond angle for BDF samples are shown in Table 1. The continuous increase in the Fe-O-Fe bond angle might improve the magnetic properties of BDF samples. Figure 2 shows the room temperature Raman spectra for BDF samples in the wavenumber range 50-700 cm-1. Mode assignment in BiFeO3 is a complex and often confusing issue as discussed by Bielecki et. al. [15]. The problem is the strong angular dispersion and the optical axis in [111] direction. There is no natural surface on a single crystal parallel or perpendicular to the optical axis due to the general direction of optical axis. The situation is even more complex in bulk samples due
3
to random orientation of grains. As a result, mixed kinds of modes are observed with shifted frequencies and modified intensities instead of pure vibrational modes. Raman patterns of BDF samples correspond to the spectral feature of typical Raman spectra of rhombohedral of BiFeO3 as reported by other reports [16-22]. There is no obvious change in Raman spectra of BDF samples with increasing Dy indicating the absence of any structural phase transition unlike Dy doped BFO nanoparticles [14]. Figure 3 shows the field dependent magnetization of BFO and BDF ceramics measured at room temperature. BFO, BDF-3 and BDF-5 samples show antiferromagnetic nature, while BDF-7, BDF10 and BDF-12 samples exhibit weak ferromagnetic nature with higher values of remnant magnetization. The incorporation of Dy3+ ions into the BFO lattice increases the remnant and net magnetization (MH) (inset of Fig. 3(b)) and broadening of loops occur. The values of remnant magnetization (Mr) are found to be 0.009, 0.0233, 0.0314, 0.0544, 0.1400 and 0.2103 emu/g for BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples, respectively. The remnant and net magnetization observed for BDF-12 sample are much higher than the reported values for A-site doped and codoped BFO bulk ceramics [9, 13 and 23-25]. Several reasons contribute to the improvement of magnetism in BDF samples. First, the spiral spin structure of BFO is suppressed by the substitution of Dy3+ ions for Bi3+ ions which leads to the structural distortion giving rise to an enhanced magnetization. The Dy substitution only suppressed and could not destroy the spiral spin structure completely. However, for BDF-12 sample, larger Fe-O-Fe angle suggests highly distorted FeO6 octahedra which are responsible for the strong ferromagnetic interaction [26-27]. The continuous increase in Fe-O-Fe bond angle, which is closely related to magnetic structure, is in good agreement with observed M-H loops. Second, the enhancement of magnetization could be attributed to the formation of Bi-O-Dy chains which became magnetic sub-lattices [24]. The Dy doped BFO system has antiferromagnetic Fe−O−Fe interaction coupled with a weak ferromagnetic component, which comes from the canted Fe sublattice due to Dzyaloshinskii−Moriya interaction, which leads to the linear increase with field in the M−H curve [28]. Besides this we presume that
4
the ferromagnetic coupling between Dy3+ and Fe3+ ions contributes to the enhanced magnetization to some extent. UV-visible absorption spectra of pure and BDF samples (Fig. 4) consist two absorption bands. The strong band in the range 450-600 nm is attributed to metal to metal transition and the weak band around 700 nm is assigned to the crystal field transitions. The absorption cut-off wavelength increases with increasing Dy content and the corresponding band gap is calculated as 2.25, 2.21, 2.20, 2.19, 2.12 and 2.09 eV for BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples, respectively. The obtained values of the band gap for BDF samples are consistent with the reported values for pure and doped BFO [14, 19, 29 and 30] and smaller than BFO thin film [31] and BFO nanoparticles [32]. The decrease in band gap of BDF samples may be attributed to the rearrangement of molecular orbitals and distortion induced in the FeO6 octahedra [33]. The energy band gap in visible region makes BDF samples suitable for photocatalytic and solar cell applications. The room temperature dielectric constant and dielectric loss of Dy doped BFO ceramics are shown in Fig. 5 (a) and (b). The values of dielectric constant decreased with increasing Dy doping. At lower frequencies the relaxation behaviour has been observed for BDF-3 and BDF-5 samples. However, dielectric constant for BDF-7, BDF-10 and BDF-12 samples remains stable in whole frequency range. This indicated dipoles with small effective mass mainly contribute to dielectric constant, instead of charge defects with large effective masses [26]. The dielectric loss also decreased with increasing Dy concentration indicating that Dy-substitution at A-site effectively suppressed the formation of oxygen vacancies which results in decreased conductivity. 4. Conclusions The presence of distorted rhombohedral structural without any signature of phase transformation up to x=0.12 sample has been established from XRD and Raman spectroscopy. The change in lattice parameters and Fe-O-Fe bond angle indicated the distortion in the BFO lattice due to Dy
5
substitution. The enhancement in magnetization is ascribed to the partial suppression of modulated spiral spin structure due to distortion and ferromagnetic coupling between Dy3+ and Fe3+ ions. Also, the decrease in the band gap is ascribed to the rearrangement of molecular orbitals and distortion induced in the FeO6 octahedra with increasing Dy concentration. Thus the Dy doping in BFO ceramics has been found to exhibit interesting optical properties in visible region along with improved magnetic properties. Acknowledgements This work was financially supported by Department of science and Technology (DST), India through grant no. SR/FTP/PS-91/2009. Prakash Chandra Sati is also thankful to DST for providing INSPIRE Fellowship. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21. [3] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu B et al. Science 299 (2003) 1719. [4] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401. [5] J. Liu, M. Li, L. Pei, J. Wang, Z. Hu, X. Wang, X. Zhao, EPL 89 (2010) 57004. [6] M. Kumar, K.L. Yadav, G.D. Verma, Mater. Lett. 62 (2008) 1159. [7] B. Yu, M. Li, Z. Hu, L. Pei, D. Guo, X. Zhao, S. Dong, Appl. Phys. Lett. 93 (2008) 182909. [8] V. A. Khomchenko, V. V. Shvartsman, P. Borisov, W. Kleemann, D. A. Kiselev, I. K. Bdikin, J. M. Vieira, A. L. Kholkin, Acta Materialia 57 (2009) 5137. [9] N. V. Minh, N. G. Quan, J. Alloy. Comp. 509 (2011) 2663. [10] S. Zhang, W. Luo, D. Wang, Y. Ma, Mater. Lett. 63 (2009) 1820. [11] M. Kumar, K. L. Yadav, J. Appl. Phys. 100 (2006) 074111. [12] S. Chauhan, M. Kumar, S. Chhokar, S. C. Katyal, H. Singh, M. Jewariya, K. L. Yadav, Solid
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State. Commun. 152 (2012) 525. [13] P. Uniyal, K. L. Yadav, J. Phys. : Condens. Matter. 21 (2009) 012205. [14] M. Arora, P. C. Sati, S. Chauhan, H. Singh, K. L.Yadav, S. Chhoker, M. Kumar, Mater. Lett. 96 (2013) 71. [15] J. Bielecki, P. Svedlindh, D. T. Tibebu, S. Cai, S. G. Eriksson, L. Borjesson, C. S. Knee, Phys. Rev. B 86 (2012) 184422. [16] H. Fukumura, S. Matsui, H. Harima, T. Takahashi, T. Itoh, K. Kisoda, M. Tamada, Y. Noguchi, M. Miyayama, J. Phys.: Condens. Matter 19 (2007) 365224. [17] A. Jaiswal, R. Das, T. Maity, K. Vivekanand, S. Adyanthaya, Pankaj Poddar, J. Phys. Chem. C 114 (2010) 12432. [18] A. Jaiswal, R. Das, K. Vivekanand, P. M. Abraham, S. Adyanthaya, and P. Poddar, J. Phys. Chem. C 114 (2010) 2108. [19] M. M. Shirolkar, R. Das, T. Maity, P. Poddar, S. K. Kulkarni, J. Phys. Chem. C 116 (2012) 19503. [20] S. Gupta, M. Tomar and V. Gupta, J. Exp. Nanosci. iFirst (2012) 1–6. [21] G. L.Yuan, S. W. Or, H. L. W. Chan, J. Appl. Phys. 101 (2007) 064101. [22] N. Jeon, D. Rout, I. W. Kim, S. J. L. Kang, Appl. Phys. Lett. 98 (2011) 072901. [23] D. Maurya, H. Thota, A. Garg, B. Pandey, P. Chand, H. C. Verma, J. Phys. : Condens. Matter. 21 (2009) 026007. [24] F. Z. Qian, J. S. Jiang, D. M. Jiang, W. G. Zhang, J. H. Liu, J. Phys. D: Appl. Phys. 43 (2010) 025403. [25] P. C. Sati, M. Arora, S. Chauhan, S. Chhoker, M. Kumar, J. Appl. Phys. 112 (2012) 094102. [26] Reetu, A. Agarwal, S. Sanghi, Ashima, J. Appl. Phys. 110 (2011) 073909. [27] C. Lan, Y. Jiang, S. Yang, J. Mater. Sci. 46 (2011) 734. [28] D. P. Dutta, B. P. Mandal, R. Naik, G. Lawes, A. K. Tyagi, J. Phys. Chem. C, 117 (2013) 2382.
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[29] B. Bhushan, A. Basumallick, S. K. Bandopadhyay, N. Y. Vasanthacharya, D. Das, J. Phys. D: Appl. Phys. 42 (2009) 065004. [30] A. Mukherjee, S. M. Hossain, M. Pal, S. Basu, Appl. Nanosci. 2 (2012) 305. [31] P. Chen, N. J. Podraza, X. S. Xu, A. Melville, E. Vlahos, V. Gopalan, R. Ramesh, D. G. Schlom, J. L. Musfeldt, Appl. Phys. Lett. 96 (2010) 131907. [32] P. Chen, X. Xu, C. Koenigsmann, A. C. Santulli, S. S. Wong, J. L. Musfeldt, Nano Lett., 10 (2010) 4526. [33] B. Bhushan, A. Basumallick, N. Y. Vasanthacharya, S. Kumar, D. Das, Solid State Sci., 12 (2010) 1063.
FIGURE CAPTION: Fig. 1. XRD patterns of Bi1-xDyxFeO3 ceramics with x=0, 0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 2. Room temperature Raman spectra of Bi1-xDyxFeO3 ceramics with x=0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 3. Room temperature M-H curves for Bi1-xDyxFeO3 ceramics. The inset shows variation of Mr and MH with increasing Dy content. Fig. 4. UV-Visible diffuse absorption spectra for Bi1-xDyxFeO3 ceramics with x=0, 0.03, 0.05, 0.07, 0.10 and 0.12. Fig. 5. Frequency dependence of dielectric constant and dielectric loss of Bi1-xDyxFeO3 ceramics at room temperature
8
Table 1. Lattice parameters and bond angle of BFO, BDF-3, BDF-5, BDF-7, BDF-10 and BDF-12 samples Samples
a (Å)
c (Å)
V (Å)3
Fe-O-Fe (deg.)
BFO
5.5786
13.8667
373.7270
146.2755
BDF-3
5.5735
13.8502
372.6015
153.472
BDF-5
5.57065
13.83374
371.7766
155.2346
BDF-7
5.56778
13.82769
371.2311
156.0725
BDF-10
5.56623
13.81484
370.6797
158.3336
BDF-12
5.57043
13.80784
371.0511
159.3345
Highlights •
Dy doped BiFeO3 ceramics were synthesised by solid state reaction method.
•
XRD and Raman spectra confirm the distorted rhombohedral structure of BDF samples.
•
Enhanced magnetic properties have been observed for BDF samples.
•
The band gap variation in visible region makes BDF samples suitable for solar cell
applications.
9
Figure(s)
Figure 1
(b)
(a) BDF-12
BDF-7
BDF-5
* * Bi Fe O 2 4 9
10
20
( 128)
( 036)
( 220)
&
Fe O 2 39
( 134)
( 300) ( 018)
( 122)
( 116)
24
( 131)
* **
& Bi
( 208)
BFO
( 202)
( 006)
( 110)
( 104)
** *
( 024)
BDF-3
( 012)
Inte nsity (a.u.)
BDF-10
*
30
40
2
50
(degree)
60
70
80
31.5
32.2
Figure(s)
Figure
2
3 BDF-5 BDF-7 BDF-10 BDF-12
Intensity (a.u.)
BDF-
100
200
300
400
500
Raman Shift (cm
1
-
)
600
700
Figure(s)
r
Figu e 3
BFO B
2
B B B
1
B
D D D D D
F-3 F-5 F-7 F-10 F-12
0
-2
2.0
0.15
1.6
0.10
1.2
0.05
0.8
(emu/g)
0.20
MH
(emu/g)
-1
Mr
Ma
gnetiz
a
tion (emu/g)
3
0.00
0.4 0.00
Dy
0.06
ra
concent
0.12
tion (x)
-3 -80
-60
-40
Ma
-20
0
20
40
gnetic field (kOe)
60
80
Figure(s)
Figure 4 1.0
BFO
BDF-5
BDF-3
0.90
0.20
0.8 0.85 0.6 0.15 0.80
Absorbance
0.4
0.75
0.2
0.10
0.0 400
0.70 500
600
700
800
400
500
600
700
800
400
500
600
700
800
0.65
BDF-7
0.15
BDF-12
BDF-10 0.60 0.35 0.55
0.50
0.30 0.10
0.45 0.25 0.40
0.35 0.20 0.30
0.05 400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
400
500
600
700
800
Figure(s)
Figure 5 0.6
600
Dielectric Constant
(a) 500
(b)
BDF-3
BDF-5
BDF-5
BDF-7
BDF-7
BDF-10
BDF-10
BDF-12
BDF-12
400
0.4
300
0.2
200
100
0.0
0
1k
10k
100k
Frequency (Hz)
1M
1k
10k
100k
1M
Frequency (Hz)
Dielectric Loss
BDF-3