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Proceedings of the "International Conference on Advanced Nanomaterials & Emerging Engineering Technologies" (ICANMEET-20 13) =-_I!l.lIWIorganIzed by Sathyabama UnIVersIty, Chennm, IndIa m associatIOn wIth DRDO, New Delhi, India, 24th _26th, July, 2013. .L �
Structural, Raman, Dielectric, Magnetic and Magnetoelectric Properties of Ba and Mn Doped BiFe03 Nanoparticles Sunil Chauhan #1, Manoj Kumal2, San deep Chhoker#3 S. C. Katyal#4 #
Department of Physics & Material Science & Engineering,
Jaypee institute of information Technology, Noida-20i307, Uttar Pradesh, india
[email protected] [email protected] [email protected] [email protected] Abstract- Ba and Mn doped BiFe03 nanoparticles were synthesized by sol-gel method. X-ray diffraction and Raman results
spectroscopy
showed
the
presence
of
distorted
rhombohedral structure for Bio.8sBao'JsFe03 nanoparticles and the substitution
induced
phase
transition
(rhombohedral
to
orthorhombic) phase for Bio.8sMno'ISFe03 nanoparticles. FESEM images of both the samples revealed grain size in the range from 50 to 100 nm. Magnetic measurement showed room temperature ferromagnetic
behavior,
which
may
be
attributed
antiferromagnetic core and the ferromagnetic surface
to
the
of the
nanoparticles, together with the structural distortion caused by Ba and Mn substitution in BiFe03 samples. coupling
was
evidenced
by
the
The magnetoelectric
observation
of the dielectric
anomaly in the dielectric constant near antiferromagnetic Neel temperature in both the samples.
Keywords-
Multiferroics,
Sol-gel,
N anoparticles,
Raman
spectroscopy, Dielectric properties, Magnetic Properties
I.
INTRODUCTION
Magnetoelectric multiferroics materials are currently attracting much attention. Multiferroics are noteworthy for their unique and strong coupling of electric, magnetic, and structural order parameters, giving rise to simultaneous ferroelectricity, ferromagnetism, and ferroelasticity. Among the known magnetoelectric multiferroics, currently bismuth ferrite (BiFe03 or BFO) has been a scientifically and technologically important material because of its vast potential applications in the emerging field of spintronics, data storage media, sensors and multi-state memories. Nevertheless, BFO nanostructures continue to be regarded as primary building blocks for a range of future nanodevices due to their unique properties, such as photocatalytic properties, gas-sensing properties, and the photovoltaic property [1-6]. However, naturally occurring multiferroic materials are rare as the ferroelectricity in AB03 type of perovskite needs dOness electronic configuration of transition metal ions, whereas magnetism needs partially filled d orbitals of transition metal ions. These two requirements for the exhibition of ferroelectricity and ferromagnetism within a single phase are mutually exclusive to each other. BiFe03 belongs to AB03 type of compounds that shows ferroelectricity and
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antiferromagnetism in the single phase at room temperature and has rhombohedrally distorted perovskite structure with space group R3c. BiFe03 display coexistence of ferroelectricity (Tc 1099-1118 K) and antiferromagnetism (TN 643 K) over a wide temperature range above room temperature. BiFe03 is a rhombohedrally distorted perovskite belonging to the space group R3c with rhombohedral lattice 59.35°, or alternatively, 5.63 A, o.r parameters ar hexagonal parameters ahex 5.58 A, Chex 13.87 A. In BFO, 2 ferroelectricity arises due to stereochemically active 6s lone 3+ ions, while magnetic behavior pair electrons of the Bi 3 appears due to the Fe + ions. The antiferromagnetic ordering in bulk BiFe03 is G-type below TN' A canted spin structure gives a spiral modulation with a periodicity of 62 nm, incommensurate with the crystal lattice [5, 6]. Enhanced magnetization in small particles has recently been reported and attributed to surface-induced magnetization and ferromagnetism caused by apparent oxygen deficiency. It is widely expected that the strategies based on nanotechnology and divalent cation, transition metal ion and rare-earth metal ion substitution will have a great impact on their magnetic, electric, optical and magnetoelectric properties [7]. In the present communication, we report the influence of Ba and Mn doping on structural, magnetic, dielectric, magnetoelectric properties of BFO nanoparticles. =
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=
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II. EXPERIMENTAL Bio8sBao' ISFe03 and Bio8sMno'lsFe03 nanoparticles were prepared by the sol-gel route. Briefly, in a typical process, calculated amounts of bismuth nitrate Bi(N03k 5H20, ferric nitrate Fe(N03)3.9H20 and barium nitrate Ba(N03)Z' 6H20 (Manganese acetate Mn(CH3COO)z) were used as precursor dissolved in deionized water. The stoichiometric amount of tartaric acid (C6H607) was further added for the complete combustion of the nitrates [8]. The yellow transparent solutions were stirred vigorously for 12 hrs over a hot plate kept at 333 K. Further the transparent solutions were dried for two days in an oven maintained at 393 K to get the fluffy gels.
275
I
�
Proceedings of the
"International Conference on Advanced Nanomaterials & Emerging Engineering Technologies" (ICANMEET-20 13)
.� . .. ·.:I . ..... organIzed by Sathyabama University, Chennai, India in association with DRDO, New Delhi, India, 24th _26th, July, 2013. L...._ Finally, the fluffy gels were calcined at 873 K for two and half hrs in an air ambience resulting in the phase formation Ba and Mn doped BFO nanoparticles. The powder X-ray diffraction (XRD) was employed to study the crystal structure of the samples using Shimadzu XRD-6000 diffractometer with CuKa ("-=l.5406 A) radiation. Raman spectroscopy was carried out in the backscattering configuration (LabRAM HR) with charge coupled device detector and 514.5 nm Laser beam. The Laser power was kept below 2 mW in order to avoid any sample heating. The dielectric measurements were done on silver coated pellets using HIOKI 3522-50 LCR Hi-Tester. III. RESULTS AND DISCUSSION The effect of the Ba and Mn doping on the crystal structure of BFO nanoparticles was analyzed Rietveld refinement tool 'FullProf [9]. The Rietveld refinement was carried out by considering R3c space group with ionic positions of Bi/Ba at 6a, Fe at 6a and 0 at 18b for Bio8sBao'lsFe03 nanoparticles and Pbnm space group for Bio8SMno'lsFe03 nanoparticles with ionic positions, Bi at 4c, Fe/Mn at 4a, 01 at 4c and 02 at 8d. The Bragg peaks were modeled with Thompson-Cox-Hastings pseudo-Voigt function and the background was estimated by linear interpolation between selected background points. Fig. 1 features the observed, calculated and difference XRD profiles for Bio8sBao'lsFe03 and Bio8SMno'lsFe03 samples after final cycle of refinement. The profile fitting procedure 2 was adapted to minimize the X function. The refined structural parameters were found to be a=b=5.5871 A, c=13.8653 A, and V=374.120 N for Bio8SBao'lsFe03 sample: 3 a=5.5757, b=5.56174 A, c=7.8954 A, and V=247.2914 A for Bio.8sMno. !sFe03 sample. These structural parameters 3 2 confirmed that the Bi + ions were substituted by the Ba + ions in BFO lattice. The unit cell volume increases with Ba content in BFO and decreases with Mn content in BFO samples, since 3 2 the ionic radius of Ba + (l.49 A) is greater than that of Bi + 3+ 2+ (1.17 A) and the ionic radius of Mn is slightly less than Fe . Also the Fe-O-Fe bond angles increase with Ba content and decreases with Mn content, indicating the distortion in 2 2 rhombohedral structure caused by the Ba + and Mn + substitution in BFO. The structural phase transition in Mn doped BFO sample was also confirmed from the Raman spectroscopy result. (a) Bio.asBaO,1SFeO,nanoparticles R3c space
I
Yobs
BrBg�sltlon
mil
II
•
11111 30
40
50 60 29 (Degree)
70
Fig. 2. FESEM images (a) Bio85Bao15Fe03 and (b) Bio85Mno15Fe03 - Bio.asBao.1le03 nanoparticles
N
g
- Bi o' Mno.tleO] nanoparticles O S
w
200
400
600
800
1000
1200
1400
Raman shift (em"')
Fig.3 Raman spectra of Ba and Mn doped BFO nanoparticles •
II
20
Fig. I. Rietveld refinement of XRD patterns of doped BFO samples.
Fig. 2 shows the FESEM images of Bio8sBao'lsFe03 and Bio8sMno. !sFe03 sample sintered at 600 °C. It shows the dense and uniform distribution of particles having grain size in the range 50-100 nm. The room temperature Raman spectra of Bio8sBaoisFe03 and Bio8sMnoisFe03 nanoparticles are shown in the Fig. 3. The assignment of the observed vibration modes are based on first principal calculations carried out by Hermit et al [10] and experimentally reported Raman data by Porporati et al [11] and IR data by Chen et al [12]. As shown in Fig. 3, we have observed eleven TO vibrational modes (three A I (TO) + eight E(TO)) in the Raman spectra of Bio8sBaoisFe03 samples. Since the Raman spectra are sensitive to atomic displacements, the shift and suppression in the Raman modes with Ba and Mn content in BFO samples can provide valuable information about ionic substitution, lattice distortion and electric polarization.
80
90
The suppression in the E(T02), E(T03) and enhancement in A(T04) for Mn doped BFO sample shows the structural phase transition (rhombohedral (R3C) to orthorhombic (Pbnm)). The lower wavenumber vibration modes below 170 cm-! give the information about Bi atoms and the oxygen I motion strongly dominates in the modes above 267 cm- . The Fe atoms are mainly involved in the modes between 152 and I 261 cm- and also contribute to some higher wavenumber modes. The Raman modes in the range 700-1400 cm-! are the second phonon modes. The temperature dependence of r! at different frequencies for Bio8sBaoisFe03 and Bio8SMnoisFe03 samples are shown in Fig. 4. The dielectric constant £/ vs temperature shows anomaly in the vicinity of antiferromagnetic Neel temperature
276
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�
Proceedings of the
"International Conference on Advanced Nanomaterials & Emerging Engineering Technologies" (ICANMEET-20 13)
.� . .. ·"" . ..... organized by Sathyabama University, Chennai, India in association with DRDO, New Delhi, India, 24th _ 26th, July, 2013. L.......
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