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Structural, magnetic and dielectric properties of nanocrystalline cobalt ferrite by wet hydroxyl chemical route
Materials Science in Semiconductor Processing 16 (2013) 1695–1700
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural, magnetic and dielectric properties of nanocrystalline cobalt ferrite by wet hydroxyl chemical route T. Boobalan a, N. Suriyanarayanan b,n, S. Pavithradevi a a b
Park College of Engineering and Technology, Coimbatore 641659, India Government College of Technology, Coimbatore 641013, India
a r t i c l e i n f o
abstract
Available online 11 July 2013
Nanopowders of CoFe2O4 are synthesized via wet chemical co-precipitation processing at pH 8. The synthesized nanoferrite powders are annealed at various temperatures (350 1C, 700 1C and 1050 1C) and are characterized. X-ray diffraction (XRD) patterns indicate the crystalline nature of CoFe2O4 nanopowders. Transmission electron microscope (TEM) investigations show, anisotropic shapes like cubic, hexagonal and spherical morphology of nanoparticles with average particle size 38–85 nm. Dielectric constant decreases as the frequency increases. Low value of dielectric loss at higher frequencies suggests the material is suitable for high frequency applications. AC conductivity increases with frequency. The saturation magnetization (Ms), remanant magnetism (MR) and coercivity (HC) increases with applied field. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Co-precipitation Vibrational sample magnetometer FTIR Dielectric properties AC conductivity X-Ray diffraction
1. Introduction Cobalt ferrite is a cubic spinel ferrite with interesting magnetic properties useful in many technological applications [1–4]. Much attention has recently been devoted to the controlled preparation of nanosized ferrites [5,6] because of their potential applications in high density magnetic recording [7], electronic devices [8,9] and medicines [10,11]. Such particles have been the object of the intensive fundamental research to explore the size, composition and surface related effects on the magnetic properties [5,6,12–15]. The tunabilty of the particle size and composition allows fine modification of the magnetic properties. Nanosized Cobalt ferrites have been prepared using a variety of physical methods, including mechanochemical [16], post-laser deposition [17], combustion [18] and thermal decomposition [19]. Wet chemical methods are particularly interesting for their versatility, low-preparation temperature and potential for production scale up. Cobalt ferrite nanoparticles have been obtained by sonochemical [20], hydrothermal
n
Corresponding author. Tel.: +91 2432221 (0422). E-mail addresses: [email protected], [email protected] (N. Suriyanarayanan). 1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.04.013
[21,22], sol–gel [23], co-precipitation [24,25] and micelle microemulsion [26,27] approaches. It is also possible to stabilize these particles in organic [28] and inorganic [23] polymeric matrices. Various common methods for the preparation of ferrites have tendency of becoming impure while grinding, poor compositional control and chemical inhomogeneity, which could be overcome by wet chemical co-precipitation processing [24,25].This technique results in better homogeneity of the ferrite nanoparticles. Almost single phase crystalline structure is obtained by this method. The prepared CoFe2O4 samples show high value of coercivity and moderate saturation magnetization. Such nanoferrites are used in medical field especially in hyperthermia, magnetic resonance imaging (MRI), magnetic separation and drug delivery of cobalt ferrite nanoparticles [29]. 2. Experimental 2.1. Sample preparation Nanocrystalline CoFe2O4 is prepared by wet chemical co-precipitation processing at pH 8. The chemicals used are of high purity of A.R. grade cobalt chloride, ferric chloride. Each material is weighed and are carefully
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T. Boobalan et al. / Materials Science in Semiconductor Processing 16 (2013) 1695–1700
dissolved into 500 ml doubly distilled water according to the formula of COx [Fe2x O4]. Then, the solution is mixed together by constant stirring for 6 h. The precipitate is carried out by adding NaOH solution drop wise continuously till the pH reaches 8. The precipitate is washed several times with distilled water until the washings are free from impurities. The product is dried on a hot plate at 120 1C to so that the water content is evaporated and eventually converted to black powder. The resultant dried product is powdered in an agate mortar. The chemical reactions proceed as follows. CoCl2+2Fecl3+4NaOH+2H2O-CoFe2O4 2H2O+4Nacl +4HCl
(1)
Synthesized CoFe2O4 samples are annealed at different temperatures 350 1C, 700 1C and 1050 1C respectively for 3 h. Phase purity of the powder is confirmed by XRD studies using PW3701 Philips diffractometer. The powder morphology is studied using a JEOL model 1200 EX transmission electron microscope (TEM). Selected powders are characterized for FTIR by SHIMADZU. Magnetic measurements like retentivity, coercivity, and saturation magnetization are performed using vibrating sample magnetometer (VSM 7407) at room temperature. Sintered disc shaped samples with silver electrodes are subjected to the measurements of dielectric constant and dielectric loss using HP4194 analyzer at room temperature. Fig. 1. XRD of CoFe2O4 as prepared and annealed at different temperatures, (a) as prepared, b.350 1C, (c) 700 1C, and (d) 1050 1C.
3. Results and discussions
with a significant decrease in Fe3+ ions. Fe2+ ions have larger ionic radius (0.77 Å) than Fe3+ ions (0.63 Å) [32].
3.1. Phase identification Fig. 1 shows the XRD patterns of synthesized CoFe2O4 nanoparticles. All the peaks correspond to CoFe2O4 phase which show the formation of cobalt ferrite structure. Increase in annealing temperature leads to a gradual growth of the crystallite size and no additional phases are detected. The reflections from the XRD patterns depicts the characteristic peaks (220), (400), (511), (440) of cobalt ferrite structure with preferred orientation along (311) plane and it well agrees with the JCPDS (#22-1086) data. As the annealing temperature increases, the preferred orientation along (311) plane also increases and almost single phase cubic spinel structure is obtained which has a space group of Fd3m.The small peaks in XRD pattern indicate the existence of fine nanocrystalline cobalt ferrite nanoparticles [30,31]. The average grain size of the samples are calculated using the Debye–Scherrer formula D¼
0:9λ βcos θ
ð2Þ
where λ is the X ray wavelength, θ is the Bragg's angle and β is the full width of the diffraction line at the half maximum intensity. The average crystallite size is found to be in the range of 40–76 nm. The particle size increases as the annealing temperature increases. The increase in particle size is due to the agglomeration of particles and as the annealing temperature increases, more Fe2+ ions are formed
3.2. FTIR analysis Fig. 2 shows the FTIR spectra of CoFe2O4 nanoparticles annealed at 1050 1C recorded between 3500 cm−1 and 400 cm−1. The O–H stretching vibrations interacting through H bonds are observed at 3186 cm−1, 2345 cm−1, 2923 cm−1 and the absorption band present at about 1630 cm−1 is due to the bending of the absorbed water molecules. The intensities of absorption band at 1026 cm−1and 918 cm−1 are the characteristic of cobalt ferrite system and this may be due to the residual FeOOH. The absorption bands present at about 594 cm−1, and 401 cm−1 are due to the stretching vibrations of metal oxide in octahedral group complex CO2+– O2− and Fe3+–O2− tetrahedral group complex of the cobalt ferrite phase respectively which proves the existence of spinel ferrite [33,34]. 3.3. Morphology and microstructure Fig. 3(a,b) shows the TEM images of CoFe2O4 nanoparticles annealed at 700 1C and 1050 1C. Most of the particles present in the TEM images are seen to be anisotropic shapes like cubic, hexagonal, spherical with average particle size 38–85 nm, and it is in good agreement with XRD results. In certain regions of the TEM micrograph, dark areas are noticed due to congregation of nanoparticles. This occurs
T. Boobalan et al. / Materials Science in Semiconductor Processing 16 (2013) 1695–1700
due to high surface energy, large specific surface area and dipolar attractive interactions of the magnetic nanoparticles. Some moderately agglomerated particles as well as separated particles are present in the images. Agglomeration increases linearly with annealing temperature [35]. The selected area electron diffraction (SAED) patterns for the CoFe2O4 powder appears as concentric circular rings, that indicates the high crystallinity cubic spinel structure and the presence of nano-sized crystals (Fig. 4). 3.4. Dielectric properties The variation of dielectric constant for the samples annealed at various temperatures as a function of frequency is shown in Fig. 5. The dielectric constant decreases as the frequency increases. The decrease is rapid at lower frequency region and remains constant at higher frequencies. The decrease takes place when the jumping frequency of electric charge carriers could not follow the alternation of applied AC electric field beyond a certain critical frequency. Dielectric variation in ferrite molecules can be explained on the basis of space charge polarization, which is a result of the presence of higher conductivity phases (grains) in the insulation of matter (grain boundaries) of a dielectric causing localized accumulation of
Fig. 2. FTIR spectrum of CoFe2O4 annealed at 1050 1C.
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charges under the influence of an electric field. The samples show variation due to Maxwell Wagner type interfacial polarization in agreement with Koops phenomenological theory. The large value of dielectric constant at lower frequency is attributed to different type of polarizations (electronic, atomic, interfacial, and ionic) and as the frequency increases, ionic and orientation sources of polarizability decreases and finally disappear due to inertia of the molecules and ions. The mechanism of this dielectric polarization may also be attributed to the dipoles resulting from the change in valence of cations, such as Fe3 + /Fe2+. The polarization at lower frequencies may result from the electron hopping between Fe3+/Fe2+ ions in ferrite lattice [36–38]. Fig. 6 shows variation of tan δ with frequency for the ferrites, which shows a similar variation as that of the dielectric constant with frequency. The low value of loss tangent indicates that the synthesized ferrites can be used in high frequency applications. In the case of conduction mechanism, the type of polarons responsible for conduction and the variation of AC conductivity as a function of frequency as represented in Fig. 7. In most of the disordered solids, AC conductivity is directly proportional to the frequency. It is also well known that in large polaron hopping, the AC conductivity decreases with the frequency, whereas, in small polaron
Fig. 4. SAED image of CoFe2O4 annealed at 1050 1C.
Fig. 3. (a,b) TEM image of CoFe2O4 annealed at (a) 700 1C, (b) 1050 1C.
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Fig. 5. Variation of dielectric constant with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, and (d) 1050 1C.
Fig. 6. Variation of dielectric loss with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, and (d) 1050 1C.
hopping, it increases with the frequency [39]. The electrical conduction mechanism in terms of electron and polaron hopping model has been discussed by Austin and Mott [40,41]. In the present case, the plots for AC conductivity measurements are indicating that the concentration is due to small polarons. It has been shown that for ionic solids, concept of small polaron is valid [42,43]. As the frequency of the applied field increases, the conductive grains become more active thereby promoting electron hopping between two adjacent octahedral sites (B sites) in the spinel lattice and a transition between Fe2+ and Fe3+ ions. Therefore increase in conductivity is observed with frequency. 3.5. Magnetic properties Fig. 8 depicts the room temperature hysteresis loop of CoFe2O4 nanoparticles and Table 1 shows the magnetic
Fig. 7. Variation of a c conductivity with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, (d) 1050 1C.
Fig. 8. Magnetic properties of CoFe2O4 as prepared and annealed at various temperatures, (a) as prepared, (b) 350 1C, (c) 700 1C, (d) 1050 1C.
data obtained. It is observed that the samples annealed at 350 1C and 700 1C show approximately linear applied field dependence with small S-shape behavior even at 15 kOe. The value of saturation magnetization of the synthesized nanoparticles are low compared to bulk materials (Ms for bulk is 80 emu/gm) and as the annealing temperature increases it approaches to the bulk value (Table 1). It is observed that the smaller size particles exhibit low value of Ms due to surface disorder and modified cationic distribution. The surface of the nanoparticles consists of some canted or disordered spins that prevent the core spins from aligning along the field direction resulting in decrease of saturation magnetization of the nanoparticles [31]. Further, the low value of remanant magnetization at room temperature in M–H curve indicates the super paramagnetic behavior of the nanoparticles, which relax back their spins by rotation on removal of applied magnetic field so as to give a very low or nearly zero magnetic
T. Boobalan et al. / Materials Science in Semiconductor Processing 16 (2013) 1695–1700
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Table 1 Magnetic Properties of CoFe2O4 annealed at various temperatures. S. no. 1 2 3 4
Sample As synthesized CoFe2O4 CoFe2O4 annealed at 350 1C CoFe2O4 annealed at 700 1C CoFe2O4 annealed at 1050 1C
Average particle size D (nm) 40 48 64 76
Coercive field Hc (emu/g) 278.23 789.38
Saturation magnetization Ms (emu/g) 1.0515
Remanent magnetization MR (emu/g) 0.08253
11.961
3.4747
1262.6
19.049
9.5247
2219.3
60.046
Fig. 9. Variation of Coercivity with annealing temperatures.
moment [44,45]. At the same time, the non-saturated magnetization suggests, the existence of strong antiferromagnetic inter-cluster interactions mixed with ferromagnetic interactions inside clusters [46–48]. For the sample annealed at 1050 1C, the value of magnetization sharply increases with the external magnetic field strength and it reaches saturation at 15 kOe. The sample exhibits hysteresis behavior with a coercivity value of 2219.3 emu/g. Figs. 9 and 10 shows that, when annealing temperature increases particle size increases which in turn increases the saturation magnetization and coercive field. The enormous increase in coercivity value (Hc) with increase in particle size can be explained on the basis of domain structure, critical diameter of the nanoparticle and anisotropy of the crystal. As the size of the crystal grows, the entire domains combine to form a single domain in order to have large magnetization energy [49–52]. 4. Conclusions Magnetic cobalt ferrite nanoparticles are successfully synthesized by wet chemical. co-precipitation processing at pH 8. XRD studies reveal the crystalline phase of CoFe2O4. The particle size and crystallinity increases with annealing temperatures and it is found to be in the range of 40–76 nm. FTIR shows the high crystalline nature and presence of
23.960
Fig. 10. Variation of saturation magnetization with annealing temperatures.
CoFe2O4. TEM and SAED patterns confirm the particle size and high crystallinity respectively. From VSM, it is observed that the cobalt ferrite nanoparticle contains a typical system of non-interacting single domain exhibit uniaxial anisotropy. The effective uniaxial anisotropy arising in cobalt ferrite nanoparticles are from surface effects that lead to large anisotropy energy in nanoparticles. The dielectric constant decreases with annealing temperature and all the samples exhibit usual dielectric variation, which is due to the Maxwell–Wagner type interfacial polarization. The AC conductivity measurement suggests that the conduction is due to small polaron hopping. References [1] M. Grigorova, H.J. Blythe, V. Bloskov, V. Rusanov, V. Petkov, V. Masheva, D. Nihtianova, Ll.M. Martinez, J.S. Munoz, M. Mikhov, Journal of Magnetism and Magnetic Materials 183 (1998) 163. [2] E.S. Murdock, et al., IEEE Transactions on Magnets 28 (1992) 3078. [3] R. Valenzuela, Magnetic Ceramics, Cambridge University press, Cambridge, 1994. [4] S.N. Okuno, et al., Journal of Applied Physics 71 (1992) 5926. [5] M. Sugimoto, Journal of the American Ceramic Society 82 (1990) 269–280. [6] M.P. Pileni, Advanced Functional Materials 11 (2011) 323–335. [7] L. Gunther, Physics World 2 (1990) 28. [8] F. Mazaleyrat, L.K. Varga., Journal of Magnetism and Magnetic Materials 215–216 (2000) 253–259. [9] J. Petzold, Journal of Magnetism and Magnetic Materials 215–216 (2002) 84.
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[10] M.A. Riley, A.D Walmsley, J.D. Speight, I.R. Harris, Materials Science and Technology 18 (2002) 1–12. [11] C.R. Martin, D.T. Mitchell, Analytical Chemistry (1998) 322A–327A. [12] D.L. Leslie-Pelecky, R. Rieke, Chemistry of Materials 8 (1998) 1770–1783. [13] A.T. Ngo, P. Bonville, M.P. Pileni, Journal of Applied Physics 89 (2001) 3370–3376. [14] F. Bodker, S. Morup, S. Linderoth, Physical Review Letters 72 (1994) 282–285. [15] X. Battle, A. Labarta, Journal of Physics D: Applied Physics (35) R 15 (2002). [16] Y. Shi, J. Ding, H. Yin, Journal of Alloys Compounds 308 (2002) 290–295. [17] G. Hu, V.G. Harris, Y. Suzuki, IEEE Transactions on Magnets 37 (2001) 2347–2349. [18] C.H. Yan, Z.G. Xu, F.X. Cheng, Z.M. Wing, L.D. Sun, C.S. Liao, J.T. Jia, Solid State Communications 111 (1999) 287–291. [19] T. Heyon, Y. Chung, J. Park, S.S. Lee, Y.W. Kim, B.H. Park, Journal of Physical Chemistry 106 (2002) 6831–6833. [20] K.V.P.M. Shafi, A. Gedanken, R. Prozorov, J. Balogh, Chemistry of Materials 10 (1998) 3445–3450. [21] S. Komarneni, M.C. D’Arrigo, C. Leonelli, G.C. Pellacani, Journal of the American Ceramic Society 81 (1998) 3041–3043. [22] A. Cabanas, M. Poliakoff, Journal of Materials Chemistry 11 (2001) 1408–1416. [23] J.G. Lee, J.Y. Park, C.S. Kim, Journal of Materials Chemistry 33 (1998) 3965–3968. [24] J. de Vicente, A.V. Delgado, R.C. Plaza, J.D.G. Duran, F. Gonzalez Caballero, Langumir 16 (2000) 7954–7961. [25] M. Rajendiran, R.C. Pullar, A.K. Battacharya, D. Das, S.N. Chintalapudi, C.K. Majumada, Journal of Magnetism and Magnetic Materials 232 (2001) 71–83. [26] S. Li, L. Liu, V.T. John, C.J. O’Connor, V.G. Harris, IEEE Transactions on Magnets 37 (2001) 2350–2352. [27] X. Li, C. Kutal, Journal of Alloys and Compounds 349 (2003) 264–268. [28] S. Neveu, A. Bee, M. Robineau, D. Talbot, Journal of Colloid and Interface Science 255 (2002) 293–298. [29] S. Amiri, H Shokrollahi, Materials science and Engineering C 33–1 (2013) 1–8. [30] M.S. Kandhekar, R.C. Kambale, J.Y. Patil, Y.D. Kolekar, S.S. Suryavanshi, Journal of Alloys and Compounds 509 (2011) 1861–1865.
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Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural, magnetic and dielectric properties of nanocrystalline cobalt ferrite by wet hydroxyl chemical route T. Boobalan a, N. Suriyanarayanan b,n, S. Pavithradevi a a b
Park College of Engineering and Technology, Coimbatore 641659, India Government College of Technology, Coimbatore 641013, India
a r t i c l e i n f o
abstract
Available online 11 July 2013
Nanopowders of CoFe2O4 are synthesized via wet chemical co-precipitation processing at pH 8. The synthesized nanoferrite powders are annealed at various temperatures (350 1C, 700 1C and 1050 1C) and are characterized. X-ray diffraction (XRD) patterns indicate the crystalline nature of CoFe2O4 nanopowders. Transmission electron microscope (TEM) investigations show, anisotropic shapes like cubic, hexagonal and spherical morphology of nanoparticles with average particle size 38–85 nm. Dielectric constant decreases as the frequency increases. Low value of dielectric loss at higher frequencies suggests the material is suitable for high frequency applications. AC conductivity increases with frequency. The saturation magnetization (Ms), remanant magnetism (MR) and coercivity (HC) increases with applied field. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Co-precipitation Vibrational sample magnetometer FTIR Dielectric properties AC conductivity X-Ray diffraction
1. Introduction Cobalt ferrite is a cubic spinel ferrite with interesting magnetic properties useful in many technological applications [1–4]. Much attention has recently been devoted to the controlled preparation of nanosized ferrites [5,6] because of their potential applications in high density magnetic recording [7], electronic devices [8,9] and medicines [10,11]. Such particles have been the object of the intensive fundamental research to explore the size, composition and surface related effects on the magnetic properties [5,6,12–15]. The tunabilty of the particle size and composition allows fine modification of the magnetic properties. Nanosized Cobalt ferrites have been prepared using a variety of physical methods, including mechanochemical [16], post-laser deposition [17], combustion [18] and thermal decomposition [19]. Wet chemical methods are particularly interesting for their versatility, low-preparation temperature and potential for production scale up. Cobalt ferrite nanoparticles have been obtained by sonochemical [20], hydrothermal
n
Corresponding author. Tel.: +91 2432221 (0422). E-mail addresses: [email protected], [email protected] (N. Suriyanarayanan). 1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.04.013
[21,22], sol–gel [23], co-precipitation [24,25] and micelle microemulsion [26,27] approaches. It is also possible to stabilize these particles in organic [28] and inorganic [23] polymeric matrices. Various common methods for the preparation of ferrites have tendency of becoming impure while grinding, poor compositional control and chemical inhomogeneity, which could be overcome by wet chemical co-precipitation processing [24,25].This technique results in better homogeneity of the ferrite nanoparticles. Almost single phase crystalline structure is obtained by this method. The prepared CoFe2O4 samples show high value of coercivity and moderate saturation magnetization. Such nanoferrites are used in medical field especially in hyperthermia, magnetic resonance imaging (MRI), magnetic separation and drug delivery of cobalt ferrite nanoparticles [29]. 2. Experimental 2.1. Sample preparation Nanocrystalline CoFe2O4 is prepared by wet chemical co-precipitation processing at pH 8. The chemicals used are of high purity of A.R. grade cobalt chloride, ferric chloride. Each material is weighed and are carefully
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T. Boobalan et al. / Materials Science in Semiconductor Processing 16 (2013) 1695–1700
dissolved into 500 ml doubly distilled water according to the formula of COx [Fe2x O4]. Then, the solution is mixed together by constant stirring for 6 h. The precipitate is carried out by adding NaOH solution drop wise continuously till the pH reaches 8. The precipitate is washed several times with distilled water until the washings are free from impurities. The product is dried on a hot plate at 120 1C to so that the water content is evaporated and eventually converted to black powder. The resultant dried product is powdered in an agate mortar. The chemical reactions proceed as follows. CoCl2+2Fecl3+4NaOH+2H2O-CoFe2O4 2H2O+4Nacl +4HCl
(1)
Synthesized CoFe2O4 samples are annealed at different temperatures 350 1C, 700 1C and 1050 1C respectively for 3 h. Phase purity of the powder is confirmed by XRD studies using PW3701 Philips diffractometer. The powder morphology is studied using a JEOL model 1200 EX transmission electron microscope (TEM). Selected powders are characterized for FTIR by SHIMADZU. Magnetic measurements like retentivity, coercivity, and saturation magnetization are performed using vibrating sample magnetometer (VSM 7407) at room temperature. Sintered disc shaped samples with silver electrodes are subjected to the measurements of dielectric constant and dielectric loss using HP4194 analyzer at room temperature. Fig. 1. XRD of CoFe2O4 as prepared and annealed at different temperatures, (a) as prepared, b.350 1C, (c) 700 1C, and (d) 1050 1C.
3. Results and discussions
with a significant decrease in Fe3+ ions. Fe2+ ions have larger ionic radius (0.77 Å) than Fe3+ ions (0.63 Å) [32].
3.1. Phase identification Fig. 1 shows the XRD patterns of synthesized CoFe2O4 nanoparticles. All the peaks correspond to CoFe2O4 phase which show the formation of cobalt ferrite structure. Increase in annealing temperature leads to a gradual growth of the crystallite size and no additional phases are detected. The reflections from the XRD patterns depicts the characteristic peaks (220), (400), (511), (440) of cobalt ferrite structure with preferred orientation along (311) plane and it well agrees with the JCPDS (#22-1086) data. As the annealing temperature increases, the preferred orientation along (311) plane also increases and almost single phase cubic spinel structure is obtained which has a space group of Fd3m.The small peaks in XRD pattern indicate the existence of fine nanocrystalline cobalt ferrite nanoparticles [30,31]. The average grain size of the samples are calculated using the Debye–Scherrer formula D¼
0:9λ βcos θ
ð2Þ
where λ is the X ray wavelength, θ is the Bragg's angle and β is the full width of the diffraction line at the half maximum intensity. The average crystallite size is found to be in the range of 40–76 nm. The particle size increases as the annealing temperature increases. The increase in particle size is due to the agglomeration of particles and as the annealing temperature increases, more Fe2+ ions are formed
3.2. FTIR analysis Fig. 2 shows the FTIR spectra of CoFe2O4 nanoparticles annealed at 1050 1C recorded between 3500 cm−1 and 400 cm−1. The O–H stretching vibrations interacting through H bonds are observed at 3186 cm−1, 2345 cm−1, 2923 cm−1 and the absorption band present at about 1630 cm−1 is due to the bending of the absorbed water molecules. The intensities of absorption band at 1026 cm−1and 918 cm−1 are the characteristic of cobalt ferrite system and this may be due to the residual FeOOH. The absorption bands present at about 594 cm−1, and 401 cm−1 are due to the stretching vibrations of metal oxide in octahedral group complex CO2+– O2− and Fe3+–O2− tetrahedral group complex of the cobalt ferrite phase respectively which proves the existence of spinel ferrite [33,34]. 3.3. Morphology and microstructure Fig. 3(a,b) shows the TEM images of CoFe2O4 nanoparticles annealed at 700 1C and 1050 1C. Most of the particles present in the TEM images are seen to be anisotropic shapes like cubic, hexagonal, spherical with average particle size 38–85 nm, and it is in good agreement with XRD results. In certain regions of the TEM micrograph, dark areas are noticed due to congregation of nanoparticles. This occurs
T. Boobalan et al. / Materials Science in Semiconductor Processing 16 (2013) 1695–1700
due to high surface energy, large specific surface area and dipolar attractive interactions of the magnetic nanoparticles. Some moderately agglomerated particles as well as separated particles are present in the images. Agglomeration increases linearly with annealing temperature [35]. The selected area electron diffraction (SAED) patterns for the CoFe2O4 powder appears as concentric circular rings, that indicates the high crystallinity cubic spinel structure and the presence of nano-sized crystals (Fig. 4). 3.4. Dielectric properties The variation of dielectric constant for the samples annealed at various temperatures as a function of frequency is shown in Fig. 5. The dielectric constant decreases as the frequency increases. The decrease is rapid at lower frequency region and remains constant at higher frequencies. The decrease takes place when the jumping frequency of electric charge carriers could not follow the alternation of applied AC electric field beyond a certain critical frequency. Dielectric variation in ferrite molecules can be explained on the basis of space charge polarization, which is a result of the presence of higher conductivity phases (grains) in the insulation of matter (grain boundaries) of a dielectric causing localized accumulation of
Fig. 2. FTIR spectrum of CoFe2O4 annealed at 1050 1C.
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charges under the influence of an electric field. The samples show variation due to Maxwell Wagner type interfacial polarization in agreement with Koops phenomenological theory. The large value of dielectric constant at lower frequency is attributed to different type of polarizations (electronic, atomic, interfacial, and ionic) and as the frequency increases, ionic and orientation sources of polarizability decreases and finally disappear due to inertia of the molecules and ions. The mechanism of this dielectric polarization may also be attributed to the dipoles resulting from the change in valence of cations, such as Fe3 + /Fe2+. The polarization at lower frequencies may result from the electron hopping between Fe3+/Fe2+ ions in ferrite lattice [36–38]. Fig. 6 shows variation of tan δ with frequency for the ferrites, which shows a similar variation as that of the dielectric constant with frequency. The low value of loss tangent indicates that the synthesized ferrites can be used in high frequency applications. In the case of conduction mechanism, the type of polarons responsible for conduction and the variation of AC conductivity as a function of frequency as represented in Fig. 7. In most of the disordered solids, AC conductivity is directly proportional to the frequency. It is also well known that in large polaron hopping, the AC conductivity decreases with the frequency, whereas, in small polaron
Fig. 4. SAED image of CoFe2O4 annealed at 1050 1C.
Fig. 3. (a,b) TEM image of CoFe2O4 annealed at (a) 700 1C, (b) 1050 1C.
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Fig. 5. Variation of dielectric constant with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, and (d) 1050 1C.
Fig. 6. Variation of dielectric loss with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, and (d) 1050 1C.
hopping, it increases with the frequency [39]. The electrical conduction mechanism in terms of electron and polaron hopping model has been discussed by Austin and Mott [40,41]. In the present case, the plots for AC conductivity measurements are indicating that the concentration is due to small polarons. It has been shown that for ionic solids, concept of small polaron is valid [42,43]. As the frequency of the applied field increases, the conductive grains become more active thereby promoting electron hopping between two adjacent octahedral sites (B sites) in the spinel lattice and a transition between Fe2+ and Fe3+ ions. Therefore increase in conductivity is observed with frequency. 3.5. Magnetic properties Fig. 8 depicts the room temperature hysteresis loop of CoFe2O4 nanoparticles and Table 1 shows the magnetic
Fig. 7. Variation of a c conductivity with frequency for samples as prepared and annealed at (a) as prepared, (b) 350 1C, (c) 700 1C, (d) 1050 1C.
Fig. 8. Magnetic properties of CoFe2O4 as prepared and annealed at various temperatures, (a) as prepared, (b) 350 1C, (c) 700 1C, (d) 1050 1C.
data obtained. It is observed that the samples annealed at 350 1C and 700 1C show approximately linear applied field dependence with small S-shape behavior even at 15 kOe. The value of saturation magnetization of the synthesized nanoparticles are low compared to bulk materials (Ms for bulk is 80 emu/gm) and as the annealing temperature increases it approaches to the bulk value (Table 1). It is observed that the smaller size particles exhibit low value of Ms due to surface disorder and modified cationic distribution. The surface of the nanoparticles consists of some canted or disordered spins that prevent the core spins from aligning along the field direction resulting in decrease of saturation magnetization of the nanoparticles [31]. Further, the low value of remanant magnetization at room temperature in M–H curve indicates the super paramagnetic behavior of the nanoparticles, which relax back their spins by rotation on removal of applied magnetic field so as to give a very low or nearly zero magnetic
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Table 1 Magnetic Properties of CoFe2O4 annealed at various temperatures. S. no. 1 2 3 4
Sample As synthesized CoFe2O4 CoFe2O4 annealed at 350 1C CoFe2O4 annealed at 700 1C CoFe2O4 annealed at 1050 1C
Average particle size D (nm) 40 48 64 76
Coercive field Hc (emu/g) 278.23 789.38
Saturation magnetization Ms (emu/g) 1.0515
Remanent magnetization MR (emu/g) 0.08253
11.961
3.4747
1262.6
19.049
9.5247
2219.3
60.046
Fig. 9. Variation of Coercivity with annealing temperatures.
moment [44,45]. At the same time, the non-saturated magnetization suggests, the existence of strong antiferromagnetic inter-cluster interactions mixed with ferromagnetic interactions inside clusters [46–48]. For the sample annealed at 1050 1C, the value of magnetization sharply increases with the external magnetic field strength and it reaches saturation at 15 kOe. The sample exhibits hysteresis behavior with a coercivity value of 2219.3 emu/g. Figs. 9 and 10 shows that, when annealing temperature increases particle size increases which in turn increases the saturation magnetization and coercive field. The enormous increase in coercivity value (Hc) with increase in particle size can be explained on the basis of domain structure, critical diameter of the nanoparticle and anisotropy of the crystal. As the size of the crystal grows, the entire domains combine to form a single domain in order to have large magnetization energy [49–52]. 4. Conclusions Magnetic cobalt ferrite nanoparticles are successfully synthesized by wet chemical. co-precipitation processing at pH 8. XRD studies reveal the crystalline phase of CoFe2O4. The particle size and crystallinity increases with annealing temperatures and it is found to be in the range of 40–76 nm. FTIR shows the high crystalline nature and presence of
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Fig. 10. Variation of saturation magnetization with annealing temperatures.
CoFe2O4. TEM and SAED patterns confirm the particle size and high crystallinity respectively. From VSM, it is observed that the cobalt ferrite nanoparticle contains a typical system of non-interacting single domain exhibit uniaxial anisotropy. The effective uniaxial anisotropy arising in cobalt ferrite nanoparticles are from surface effects that lead to large anisotropy energy in nanoparticles. The dielectric constant decreases with annealing temperature and all the samples exhibit usual dielectric variation, which is due to the Maxwell–Wagner type interfacial polarization. The AC conductivity measurement suggests that the conduction is due to small polaron hopping. References [1] M. Grigorova, H.J. Blythe, V. Bloskov, V. Rusanov, V. Petkov, V. Masheva, D. Nihtianova, Ll.M. Martinez, J.S. Munoz, M. Mikhov, Journal of Magnetism and Magnetic Materials 183 (1998) 163. [2] E.S. Murdock, et al., IEEE Transactions on Magnets 28 (1992) 3078. [3] R. Valenzuela, Magnetic Ceramics, Cambridge University press, Cambridge, 1994. [4] S.N. Okuno, et al., Journal of Applied Physics 71 (1992) 5926. [5] M. Sugimoto, Journal of the American Ceramic Society 82 (1990) 269–280. [6] M.P. Pileni, Advanced Functional Materials 11 (2011) 323–335. [7] L. Gunther, Physics World 2 (1990) 28. [8] F. Mazaleyrat, L.K. Varga., Journal of Magnetism and Magnetic Materials 215–216 (2000) 253–259. [9] J. Petzold, Journal of Magnetism and Magnetic Materials 215–216 (2002) 84.
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