O A

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Journal of Applied Sciences Research, 9(3): 2432-2438, 2013
ISSN 1819-544X
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES
Synthesis and characterization of Cobalt-doped nano-structure barium strontium
titanate prepared by Sol–gel process
1
M.A. Ahmed, 2M. Kamal, 3F.G. El Desouky, 3E. Girgis, 3I.S.A. Farag and 3I.K. Batttisha
1
Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt
Metal Physics Laboratory, Physics Department, Faculty of Science, Mansoura University, Mansoura 35516,
Egypt.
3
National Research Center (NRC), Solid State Physics Department, Giza, Egypt.
2
ABSTRACT
Multiferroic materials possess at least two ferroic properties which make them particularly suitable for
multifunctional device applications [Chunqing Wang et al.,(2012). The purpose of this work was to get onedimensional multiferroics by doping of Co2+ into ferroelectrics. Multiferroic Ba0.9Sr0.1 Ti0.90Co0.1O3 nanoparticle
was prepared via a sol–gel method. The magnetic hysteresis loop confirmed the expected enhancing of
ferromagnetic properties with a saturation magnetization of 127.58 memu/g, a retentivity of 19.656 memu/g,
and a coercivity of 93.951 Oe measured at room temperature. Structural study revealed the tetragonal BST
phase by using XRD. The crystallite sizes obtained were 52 and 31 nm for Ba0.9 Sr0.1 Ti O3 and Ba0.9Sr0.1
Ti0.90Co0.1O3, respectively. The TEM was used to confirm the presence of the nano-structure phase giving 50
and 30 nm, respectively which is agree with that calculated data from XRD.
Key words: Sol gel, SEM, TEM, Multiferroic, nano-structure, Magnetic hysteresis loop.
Introduction
Perovskite-based multiferroic materials where more than one ferroic order coexists, have been widely
investigated (Spaldin et al.,2005 and Fiebig et al., 2005) due to their significance for fundamental physics and
device applications. This characteristic makes them especially suitable for use in multifunctional devices such as
memory devices, sensors and actuators (Vaz CAF et al.,2010). The coexistence of ferromagnetic and
ferroelectric orders is a challenge to the conventional mechanism of ferroelectricity in perovskite-based
materials with ABO3 structure, because an empty d orbital (such as Ti4+ in BaTiO3) at a B-site should be present
for B-site cation off-centering, which will break up the existing ferromagnetic order (Hill et al., 2000). In recent
years (Martin et. al., 2007 and Wu et al., 2009), considerable efforts were put into the single-phase
multiferroic materials and multiferroic magnetoelectric composites (Zheng et al., 2004 and Ortega et al., 2008).
However, it remains a major challenge to prepare novel multiferroics with excellent properties. Recently, there
has been an increasing interest in the synthesis of multiferroics by doping transition metals into ferroelectric
materials. For example, many studies of this direction had been reported in the materials of Fe-doped BaTiO3
single crystals (Chakraborty et al., 2011), Mn -doped Ba0.93Bi0.07TiO3 ceramics (Wang et al., 2010), and Codoped BaTiO3 thin films (Lin YH et al., 2008). Until quite recently, not only have ferromagnetic properties of
Co-doped BaTiO3 been displayed, high-performance programmable memory devices based on Co-doped
BaTiO3 have also been fabricated (Yan et al., 2011). However, the coupling between ferroelectric and
ferromagnetic orders is highly useful for the new type of multiferroic devices by taking advantage of both
magnetic and electric polarization, such as electrically read-write magnetic memory devices and other
magnetoelectric devices (Prinz et al.,1998 and Zutic et al.,2004). Moreover, multiferroic materials possess
several distinctive electronic and structural properties such as orbital and charge ordering, local moment
formation, and Jahn–Teller distortions (Filippetti et al.,2002 and Barrett et al.,2010).
Barium strontium titanate, abbreviated as BaxSr1_xTiO3 (BST), is the solid solution of BaTiO3 and SrTiO3.
Previous studies mainly focused on BST have been mainly used for dielectric devices such as capacitors for
dynamic random access memory because of its high dielectric constant and low dissipation factor (Giridharan et
al.,2001and Saha, et al., 2000). BST thin films have also been widely investigated for microwave applications
such as phase shifters, tunable filters, tunable resonators, and switches [Su et al. 2000 and Heindl et. al., 2007),
and concerning Fe-doped BaxSr1_xTiO3 samples, the overwhelming majority focus on BSTF thin films (Gong et.
al., 2007 and Lorenz et al.,2003). the Co2+ ions substitute at Ti4+ site are in the Co2+ state, which tends to
Corresponding Author: I.K. Battisha, National Research Center (NRC), Solid State Physics Department, Giza, Egypt
University, Giza, Egypt
E-mail: [email protected], [email protected]
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J. Appl. Sci. Res., 9(3): 2432-2438, 2013
generate oxygen vacancies and internal dipole required by change neutrality. Such oxygen vacancies are
expected to make the cubic structure favorable in energy (Ihrig, et al., 1978 and Hagenmann, et al.,1979).
In this work, we aim to investigate the structural and multiferroic properties of BST samples in powder
form by doping it with 10 mol % Co2+ ions doped BST ceramics exhibit excellent, almost single-phase
structures, which indicate that the preparation of tetragonal BST, Large amount of doping of Co2+ ions which
mainly acts as an acceptors to replace Ti in the B-site, leading to the appearance of lattice defects and vacancies.
The structure and phase identification will be evaluated by XRD.The TEM and SEM of pure barium strontium
titinate (B10ST) and B10ST10CO revealed the presence of the nano-phase in the prepared samples.
Ferromagnetism of BSTCO is simultaneously observed. The magnetic measurements was carried out at room
temperature using lakeshore vibrating sample magnetometer (VSM 7410) model lakeshore 7110.
Materials and methods
2.1. Samples preparation:
Nano-structure pure barium strontium titanate, BaSrTiO3, Ba1-xSrxTiO3, where (x = 0.1), (B10ST) and
doped with Co2+ ions, Ba1-xSrxTi 1-yCoyO3, where (x = 0.1 & y = 0.1) (B10ST10C) in powder forms were
prepared, respectively by a modified sol–gel method.
Our samples have been prepared using barium acetate (Ba(Ac)2) (99%,Sisco Research Laboratories PVT.
LTD, India), Sr(Ac)2 and titanium butoxide (Ti(C4H9O)4), (97%,Sigma–Aldrich,Germany) as the starting
materials; acetylacetone (AcAc,C5H8O2), (98 %, Fluka, Switzerland) acetic acid (HAc)–H2O mixture (96 %,
Adwic, Egypt) were adopted as solvents of (Ti(C4H9O)4, and Ba(Ac)2, respectively. Cobalt nitrate was added to
the final solution with molar ratio 10 mol %. Crystallization of the gel was achieved by calcinating in air for 4h
at 850◦C, in a muffle furnace type (Carbolite CWF 1200).
The phases of the obtained samples are characterized by x-ray diffraction (XRD) (BRUKUR D8
ADVANCED TARGET Cu Kα with Secondary monochromatic KV= 40, mA = 40 Germany) in a wide range of
Bragg angle from10° up to 80° using Cu Ka (1.5406˚A) radiation with a step size of 0.02 at room temperature.
The crystallite size (G) is determined from the Scherrer’s equation;
G = K / D cos
(1)
Where K is the Scherer constant, in the present case K = (0.9),  is the wavelength and D is the full width
(in radians) of the peak at half maximum (FWHM) intensity.
The microstructure and surface morphology of the samples were observed by (TEM) transmission electron
microscope (using JEOL JEM-1230 equipment operating at 120 kV with attached CCD camera). The coarse and
fine microstructures and the morphology of all the BST and BSTC were depicted by using Scanning electron
microscope (SEM) JEOL JSM-63o1F. The SEM gives information on the surface morphology of the sample,
which can help us check whether the growth has taken place or not. The SEM produced 2 D image and reveals
topographic filature of the sample, which allow us to examine the diameter of the nano-particle, shape and
density of the prepared samples. Both techniques (TEM and SEM) were used to determine particle size snd
grain size and uniformity of the sample analysis.
Magnetization hysteresis (M–H) measurements were carried out at room temperature using
lakeshore vibrating sample magnetometer (VSM 7410) model lakeshore 7110.
3. Results:
The XRD patterns of pure, Ba0.9 Sr0.1TiO3, B10ST and doped sample with Co2+ ions Ba0.9 Sr0.1Ti(1-x)CoxO3,
where (x = 0 & 0.1), (BST10CO), respectively calcined at 850°C for 4h are presented in Fig. 1 (a and b), the
samples revealed a polycrystalline nature. Weak line corresponding to the residual carbonates phases, such as
BaCO3, SrCO3 and (Ba, Sr) CO3 were appeared in Fig. 1(b) resulting from the reaction of BaO with atmospheric
CO2 and the burn-out of organic materials or as a result of incomplete calcinations.
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Intensity (a.u)
J. Appl. Sci. Res., 9(3): 2432-2438, 2013
(b)
(101)
(100)
(200)
(002)
(111)
(a)
10
20
30
40
(211)
(202)
(212) (103)
(201)
50
60
70
80
2
Intensity(a.u)
Fig. 1: Room temperature XRD patterns of Ba0.9 Sr0.1Ti1-x CoxO3, where (a) x=0, and (b) x=0.1, respectively.
(b)
Intensity(a.u)
44.6
44.8
45.0
45.2
2
45.4
45.6
45.8
(a)
31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.4
2
Fig. 2: Expansion of the angles 2θ° located between 31° to 32.4° for (a) and 44.8° to 46° for (b).
Figure 3 (a and b) shows the representative TEM of the nano-structure B10ST and doped sample
BS10T10C thermally synthesized in air for 4 h at 850oC. The figure shows some degree of agglomerates.
The surface morphologies obtained through Scanning Electron Microscope (SEM) of the pure B10ST
powder sample calcinated for 4 hours at 850oC are shown in Figure 4.
Fig. 5 (a and b) shows the magnetic property of nano-structure (a) B10ST and (b) B10ST10C measured at
room temperature. Ferromagnetism of BSTCO is simultaneously observed. The magnetic measurement was
carried out at room temperature using lakeshore vibrating sample magnetometer (VSM 7410) model lakeshore
7110.
Discussion:
The XRD patterns of pure, Ba0.9 Sr0.1TiO3, B10ST and doped sample with Co2+ ions Ba0.9 Sr0.1Ti(1-x)CoxO3,
where (x = 0 & 0.1), (BST10CO), respectively calcined at 850°C for 4h are presented in Fig. 1. The samples
revealed a polycrystalline nature. By indexing the XRD patterns it was found that it is consistent with the
tetragonal perovskite structure, which corresponds to ICCD card number [44–0093]. Moreover, a splitting of the
peak in the range between 44.8 and 46o is observed and confirms the presence of the tetragonal phase (Gervais
et al.,2004 and Lin et al.,2010). Weak line corresponding to the residual carbonates phases, such as BaCO3,
SrCO3 and (Ba, Sr) CO3 were appeared in Fig. 1(b) resulting from the reaction of BaO with atmospheric CO2
and the burn-out of organic materials or as a result of incomplete calcinations (Battishaa et. al.,2010), also may
be due to large amount of doping of Co2+ ions which, mainly acts as an acceptors to replace Ti in the B-site,
leading to the appearance of lattice defects and vacancies( Saikat Maitra et al., 2013). The lattice parameters of
the pure and doped samples are carefully determined, as listed in Table 1. It exhibits a slight variation, but the
crystal structures remain almost the same. The values of lattice parameter (a) and (c) is increased by doping with
the Co2+ ions, indicating a lattice expansion of B10ST10C system. The unit cell volume increases from 63.9598
(Ǻ)3 up to 64.1837 (Ǻ)3, which can be attributed to the different sizes between Co2+ and Ti4+ ions (0.65 ˚A for
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J. Appl. Sci. Res., 9(3): 2432-2438, 2013
Co2+ and 0.68 ˚A for Ti4+) (Shannon et. al., 1976). These vacancies may be favorable for stabilizing the
tetragonal structure of BSTC systems.
Fig. 3: The TEM micrograph of (a) pure B10ST and (b) (B10ST10C) powders samples calcinated at 850oC for
4 h.
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Fig. 4: The SEM micrograph of the nano-structure (B10ST) powder sample calcinated at 850oC for 4 h.
0.04
0.15
(a)
(b)
0.10
M(emu/g)
M(emu/g)
0.02
0.00
-0.02
0.05
0.00
-0.05
-0.10
-0.04
-0.15
-6000-4000-2000
0
2000 4000 6000
H Oe
-10000
0
10000
H Oe
Fig. 5: Magnetic hysteresis loop of nano-structure Ba1-xSrxTi 1-y CoyO3, where (x = 0.1& y = 0.1) (a) (B10ST)
and (b) (B10ST10C), measured at room temperature.
Figure 3 (a and b) shows the representative TEM of BS10T and BS10T10C thermally synthesized in air for
4 h at 850o, respectively. Some degree of agglomerates has been found in the clusters consisting of many small
grains especially for the pure B10ST. The calculated average particle size from TEM was about 50 and 33.nm
for B10ST and B10ST10C, respectively, which is nearly equal to the value obtained from XRD for the pure and
doped sample with 10 mol % of Co2+ ions. The TEM was used to confirm the data obtained from XRD patterns
and that the sample is in nano-scale. Images in Figure 3 clearly show that the additive of cobalt oxide has leads
to a grains refining,
Figure 4. Shows Surface morphologies obtained through Scanning Electron Microscope (SEM) of the pure
B10ST and powder calcined for 4 hours at 850oC are shown in Figure 4. (a), respectively. The image shows
comparatively more accumulated grains with higher density showing increase in grain growth at this higher Sr
content and the particles have a well-defined shape (Battisha et al.,2009,Gong et al.,(2007, Willander et al.,
2012 and Nur et al., 2012).
Fig. 5a showed the magnetic property of BSTCO nanostructure measured at room temperature. The result
reported by Kaur J et .al, indicated that the pure BaTiO3 was intrinsic diamagnetic (Kaur et.al., 2011).From
Table 2 we can investigate that at 10% Sr doped BaTiO3 there are converted from diamagnetism to weak
ferromagnetism as a result different ionic radius of Sr2+ and Ba2+ (1.18 for Sr2+ and 1.35 for Ba2+ six
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coordination) (Shannon et al., 1976). Fig. 4b illustrated that ferromagnetic long-range order appeared in the
BSTCO nanostructure owing to the doping of Co, In the BSTCO the oxygen vacancies tend to appear in
proximity to Co2+ ions to keep charge neutrality, leading to the formation of bound magnetic polarons (BMP).
With a proper polaron concentration, the neighboring BMPs are likely to overlap and interact to create global
ferromagnetic .We can see that Co2+ made an enhancing of ferromagnetic properties with a saturation
magnetization of 127.58 memu/g, a retentivity of 19.2656 memu/g, and a coercivity of 112.54 Oe measured at
room temperature. We can see that the saturation magnetization Ms and Hc increases with the decrease in
particle size as a result of doping of the Co2+ ions may be due to different sizes between Co2+ and Ti4+ ions (0.65
(Ǻ)for Co2+ and.0.605 (Ǻ) for Ti4+; six coordination) (Shannon et al., 1976), and also in a multidomain system,
coercivity is related to motions of domains, the size dependence of coercivity when particle size decreases, the
coercivity increases (Culity et al.,2009).
Table 1: Dopant concentration, sample abbreviation, chemical formula and oxygen vacancy of Co concentration doped nano-structure BST
powder.
Dopant
Sample
Chemical formula
V
concentration
abbreviations
(Ǻ )3
Δ
a= b (Ǻ)
c (Ǻ)
X=0
B10STO
Ba0.9Sr0.1Ti1O3
3.9995
3.9985
63.9598
Δ =0
X=0,Y=0.1
B10ST10CO
Ba0.9 Sr0.1Ti0.9 Co0.1O2.9
Δ=0.1
Table 2: Magnetic Parameters at Room Temperature and particle size.
Sample abbreviations
particle size
Particle
Hc (coercivety
size(TEM)
(XRD)
B10ST
B10ST10CO
52 nm
31.3 nm
50 nm
30 nm
74.219 Oe
93.951 Oe
4.004586
4.0023
64.1837
Mr(remnant
magnetization)
Ms (saturation
magnetization)
4.9275 memu/g
19.656
memu/g
41.556 memu/g
127.58
memu/g
Conclusion:
Room-temperature ferromagnetism has been achieved in (Ba0.9Sr0.1)(Ti0.9Co0.1)O3 ceramics (x=0, and
10at%) using sol gel technique via partial Co substitution on the B site. The results show that all the samples are
a perovskite structure with the Co ions, ferromagnetism arising from the oxygen vacancies at the surface and
ferroelectricity from the core. The TEM was used to confirm the presence of the nano-structure phase giving 50
and 30 nm, respectively which is agree with that calculated data from XRD.
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