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Synthesis and characterization of Ni1−xCoxFe2O4 nanoparticles
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 3132–3137
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Synthesis and characterization of Ni1xCoxFe2O4 nanoparticles M.K. Shobana , S. Sankar Department of Physics, CEG Campus, Anna University, Chennai 600025, Tamil Nadu, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 4 February 2009 Received in revised form 6 May 2009 Available online 15 May 2009
Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) nanoparticles have been synthesized with various crystallite sizes depending on the thermal treatments and composition (cobalt content) using the sol–gel combustion method. The size of nanoparticles has been controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of Ni1xCoxFe2O4 have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The presence of functional group has been identified by Fourier Transform Infrared (FTIR) spectra. From TGA–DTA studies, the weight gains of Ni1xCoxFe2O4 nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 1C. Magnetic properties of Ni1xCoxFe2O4 particles have been analysed using VSM and it is found that saturation magnetization (Ms) has increased with particle size and has coercivity (Hc) increased initially and then decreased. The Ms and Hc values decreased with the increase of content of cobalt in Ni1xCoxFe2O4. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic material Sol–gel combustion Thermal property Saturation magnetization
1. Introduction In the recent past, the nanomagnetic materials have gained remarkable scientific interest owing to their interesting properties and a variety of applications [1]. The high coercivity of these magnetic nanoparticles makes them interesting for applications in the fields of high-density magnetic media, recording color imaging, ferrofluids, high-frequency devices and magnetic refrigeration [2,3]. The interesting and useful magnetic properties of spinel ferrites depend on the choice of the cations along with Fe2+ and Fe3+ ions and their distribution between tetrahedral (A) and octahedral (B) sites of the spinel lattice. Finite size effect on the structural and magnetic properties of sol–gel-synthesized NiFe2O4 particle was reported by George et al. [4]. Chander et al. [5] reported the magnetic behaviour of Ni0.5Co0.5Fe2O4 nanoparticles prepared using two different routes. In this study, we report the finite size effects on the structural, thermal and magnetic properties of Ni1xCox Fe2O4 nanoparticles of different sizes.
2. Experimental details Nickel cobalt ferrite (Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9)) nanoparticles with different Co contents were prepared by the Corresponding author. Tel.: +91 44 22203159; fax: +91 44 22203160.
E-mail address: [email protected] (M.K. Shobana). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.05.018
sol–gel combustion method [6,7]. Analytical grade nickel nitrate, cobalt nitrate, ferric nitrate and citric acid have been used as source material for the preparation of Ni1xCox Fe2O4 nanoparticles. Here, water is used as a solvent. The pH of the sol was maintained in between 8 and 9. The yielded ash containing Ni1xCoxFe2O4 nanoparticles (ashes) were calcinated at 500, 700 and 900 1C for 1 h.
3. Characterization The prepared nanoparticles were characterized by powder Xray diffraction (XRD) using Philips 1710-X-ray diffractometer with CuKa radiation (1.5406 A˚). Morphological studies of the ferrites were carried out by employing an Atomic Force Microscope (Digital Nanoscope II, USA). The size of the powders were confirmed using transmission electron microscopy (TEM) (JEOL 100CXII). The powdered samples were sprinkled with acetone to fix on a glass plate (2 2 mm). It was air dried for 30 min and used for Atomic Force Microscopy (AFM) studies. Fourier transform infrared spectroscopic study was carried out in KBr medium using Shimadzu, FTIR-8400 in the wavenumber range 400–4000 cm1 with a resolution of 1–5 cm1. Thermal decomposition of the samples was studied from room temperature to 1000 1C using a thermogravimeter (Perkin Elmer analyzer). Vibrating sample magnetometer (VSM) was used to evaluate the room-temperature magnetic properties of the ferrites. The samples were weighed and kept in a plastic tube, and put on the sample holder with the help of a teflon tape.
ARTICLE IN PRESS M.K. Shobana, S. Sankar / Journal of Magnetism and Magnetic Materials 321 (2009) 3132–3137
4. Results and discussion 4.1. Structural and morphological studies The XRD patterns of all the samples of Ni0.4Co0.6Fe2O4 nanoparticles synthesized by the sol–gel combustion technique are depicted in Fig. 1. All the characteristic peaks of Ni0.4Co0.6Fe2O4 nanoparticles (NCF1, NCF2, NCF3 and NCF4 corresponding to the calcination temperatures 300, 500, 700 and 900 1C) are present in the diffraction pattern. It is found that all diffraction peaks can be perfectly indexed of the cubic spinel structure, and no peaks corresponding to impurities are detected in the XRD pattern. Likewise, the other two compositions Ni0.2Co0.8Fe2O4 and Ni0.1 Co0.9Fe2O4 nanoparticles calcined at 300, 500, 700 and 900 1C are illustrated in Fig. 2. The particle sizes of these samples have been calculated by Scherrer formula [8], viz., D¼
Kl b cos y
3133
diffraction peaks and decreasing of width. Fig. 2 illustrates the variation of particle sizes of Ni1xCoxFe2O4 nanoparticles at different concentrations of cobalt. From all compositions of Ni1xCoxFe2O4 nanoparticles, furthermore, we have observed that the particle size depends not only on composition but also on temperature, milling time and pH value of the solutions. This observation also agrees well with previous report of Malinofsky and Babbitt [12]. The present results depict that the increasing
Table 1 Particle sizes of Ni1xCoxFe2O4 at different calcination temperature. Temperature (1C)
300 500 700 900
Particle size (nm) Ni0.4Co0.6Fe2O4
Ni0.2Co0.8Fe2O4
Ni0.1Co0.9Fe2O4
18 22 48 65
16 22 43 58
14 19 41 52
(NCF1) (NCF2) (NCF3) (NCF4)
(2NCF1) (2NCF2) (2NCF3) (2 NCF4)
(3NCF1) (3NCF2) (3NCF3) (3NCF4)
where b is the full width half maximum (rad), l the wavelength of the X-ray, y the angle between the incident and diffracted beams (degree) and D the particle size of the sample (nm). The average particle size is found to be in the range 15–65 nm and is given in Table 1. The crystalline nature of the Ni0.4Co0.6Fe2O4 increased sharply, as observed, as the firing temperature was increased. This clearly shows that the particle size has increased with increase of calcination temperature in a manner similar to these observed by Roy et al. [9], Zhou et al. [10] and Dey et al. [11]. The crystalline property improves with the increase of intensity of X-ray
Intensity (arb. Units)
(311)
(511)
(200) (222)
(400)
(422)
(440) (d)
0.5 1.0
(c)
1.5
μm
(b)
(a) 30
40
50 2θ degrees
60
Fig. 1. XRD pattern of Ni0.2Co0.8Fe2O4 nanoparticles at different temperatures: (a) 300 1C, (b) 500 1C, (c) 700 1C and (d) 900 1C.
0.2 0.4 0.6 0.8 Fig. 2. Variation of crystallite size Ni1xCoxFe2O4 nanoparticles with calcination temperatures.
μm
Fig. 3. Topographic view of Ni0.2Co0.8Fe2O4 nanoparticles: (a) 300 1C and (b) 900 1C.
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Chemical and structural changes take place during calcination and can be monitored by spectroscopic analysis. Infrared spectroscopic studies were performed aiming to ascertain the metal oxygen nature of the product and follow the dehydration. Fig. 5 shows a characteristic peak at 584.45 cm1 for Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9). Krysewski et al. [14] reported that the peaks at 550, 793 and 895 cm1 could be due to the deformation of iron oxide lattice and OH groups bound to the surface of the Fe3O4 particles. The bands around 1384 and 570 cm1 are attributed to stretching vibrations of the anti-symmetric NO1 3 and tetrahedral complexes of the ferrite. In the as-prepared sample (300 1C) peaks, around 3420 cm1 and 1620–1630 cm1 show the presence of water molecule [15]. The ferrite stretching is shifted to higher wavenumber when compared with low temperature, and is due to the increase of calcination temperature. The shifted bands such as 590, 1383, and 1647 cm1 are compared to those at low temperatures [16]. The sharp line shows that the particle is in crystalline phase at high temperature. The peaks around 1384, 570 and 1590 cm1 disappear or decrease by heat treatments and represent the removal of NO1 3 ions and carboxyl group takes part in the reaction during combustion [17]. 4.2. Thermal studies
Fig. 4. Topographic view of Ni0.4Co0.6Fe2O4 nanoparticles: (a) 500 1C and (b) selected area diffraction pattern at 500 1C.
concentration of cobalt (Co) decreases the particle size. This may be due to migration of a small number of Co2+ ions in the midst of Co3+ ions in B-sites. This observation is similar to that of Caltun et al. [13] for CoMn ferrite. The morphology and particle sizes of the Ni1xCoxFe2O4 nanoparticles are determined by AFM (Fig. 3) and TEM (Fig. 4(a,b)). Fig. 3 shows the micrograph of the Ni0.4Co0.6Fe2O4 particles. It is evident from the AFM images that for the samples NCF1 and NCF4 calcinated at 300 and 900 1C, respectively, confirm the cubic morphology of the particles. The average particle of size of the sample is found to be 55 nm. Fig. 3(b) shows the existence of the Ni0.4Co0.6Fe2O4 nanoparticles with agglomeration. This may be due to evaporation of the polymer at high temperature. Agglomeration, one of the main constrains in the synthesis of magnetic nanomaterials, can be avoided by the use of polymer matrix to isolate the particles. Agglomeration of the prepared fine particles can be clearly noticed at higher temperatures (900 1C). Evaporation of the polymer above 500 1C is unavoidable. It is inferred that an increase in calcination temperature increases the particle size and it is also consistent with XRD results.
The experimental observation showed that the nitrate–citrate gels with all three molar ratios of metal nitrates to citric acid exhibited self-propagating combustion behaviour. When the dried gels were ignited at any point, the combustion rapidly propagated forward until all gels were completely burnt out to form powders. It was also observed that the combustion rate is associated with the ratio of nitrates of citric acid. The autocatalytic nature of the combustion process of gels has been studied by thermal analysis (DTA and TGA) of the dried gels. The TGA and DTA thermograms of the Ni1xCoxFe2O4 (x ¼ 0.6, 0.8, 0.9) sample decomposing in nitrogen atmosphere are shown in Fig. 6. The observed weight loss below 100 1C of Ni1xCoxFe2O4 (Wg for Co (x ¼ 0.6) ¼ 1.2%, Wg for Co (x ¼ 0.8) ¼ 1.1%, Wg for Co (x ¼ 0.9) ¼ 1%) is attributed to the loss of physically or chemically absorbed OH groups. Fig. 6 for all compositions of Ni1xCoxFe2O4 illustrates the weight gain above 200 1C for the sample prepared at 300 1C [18]. This could be due to the formation of surface carbonate as the capped magnetic particles interact with oxygen at temperature above 200 1C. An exothermic peak at 921.82, 886.68 and 842.53 1C for Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) respectively. No more exothermic peaks have been found and it represents the completion of ferritization or crystallization. It is inferred further that the completion of ferritization is shifted to low temperature by increasing Co content. These are much lower than the values reported by Sindhu et al. [19] for Ni ferrites prepared by conventional methods. 4.3. Magnetic studies The various magnetic properties like saturation magnetization and coercivity are estimated from the hysteresis curve. The variation of saturation magnetization is illustrated in Fig. 7 and coercivity in Fig. 8. The saturation magnetization of the nanosized Ni1xCoxFe2O4 (x ¼ 0.6, 0.8, 0.9) has increased with the increase of grain size. This observation can be due to many factors: cation distribution, the existence of surface spins or the formation of spin glass structure, etc. The presence of dead layer has also been thought to be one of the reasons for the reduced magnetization in the ultrafine region [20]. Moreover, the value of saturation magnetization depends not only on calcination temperature, but also on the composition (content of cobalt). The variation of
ARTICLE IN PRESS M.K. Shobana, S. Sankar / Journal of Magnetism and Magnetic Materials 321 (2009) 3132–3137
3135
f
e
% Transmittance
d
c
b
1383.01
3446.91
584.45
2924.18
1641.48
a
4000
3000
2000 Wavenumber (cm-1)
1000
500
Fig. 5. FTIR spectra of Ni1xCoxFe2O4: (a) 300 1C for x ¼ 0.6, (b) 500 1C for x ¼ 0.6, (c) 300 1C for x ¼ 0.8, (d) 500 1C for x ¼ 0.8, (e) 300 1C for x ¼ 0.9 and (f) 500 1C for x ¼ 0.9.
saturation magnetization (Ms) depends on the cation distribution in a spinel lattice. Moreover, high coercive field is one of the advantages of nanosize particles for the magnetic applications. Coercivity values are found to be smaller for the particles calcined at 300 1C (smaller crystallite size) and at 900 1C (higher crystallite size) when compared to these for the particles calcined at 500 1C (intermediate crystallite size). This observation is similar to that of Monte et al. [21] in Co ferrite. So the magnetic behaviour strongly depends on the variation of the particle size [22,23]. Further increase in doping levels decreases the strength of the exchange interactions, and leads to the lower value of the saturation magnetization [13,24].
5. Conclusions Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) nanoparticles were synthesized with various crystallite sizes depending on the
thermal treatments and composition (cobalt content) using the sol–gel combustion method. The size of nanoparticles is controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of the Ni1xCox Fe2O4 have been studied by using AFM and TEM studies. The presence of functional group has been identified by Fourier transform infrared spectra (FTIR). From TGA–DTA studies, the weight gains of the Ni1xCoxFe2O4 nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 1C. Magnetic properties of the Ni1xCoxFe2O4 particles have been analysed using VSM and it is found that saturation magnetization (Ms) has increased with particle size and has coercivity (Hc) increased initially and then decreased. The Ms and Hc values decreased with increase of content of cobalt in Ni1xCox Fe2O4. This might be due to migration of Co2+ ions in the B-sublattice.
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DTA
101.2 100.8 100.4 X=0.6
99.6
Weight (%)
0
200
400 600 Temperature (°C)
800
1000
103 102.5 102 101.5 101 100.5 100 99.5
X=0.8 0
200
400 600 Temperature (°C)
800
1000
Weight (%)
100.5 100 99.5 99 X=0.9 98.5 0
200
400 600 Temperature (°C)
0
0
200
400
600
800
1000
-5 -10 -15 -20 -25 X=0.6
-30 Temperature (°C)
800
1000
Heat flow endo down (mw)
100
Heat flow endo down (mw)
Weight (%)
101.6
Heat flow endo down (mw)
TGA 102
0
0
200
400
600
800
1000
-5 -10 -15 -20 X=0.8
-25 0 -2
Temperature (°C) 0
200
400
600
800
1000
-4 -6 -8 -10 -12
X=0.9
-14 Temperature (°C)
Fig. 6. TGA–DTA of Ni1xCoxFe2O4 nanoparticles for different Co content.
Acknowledgement The authors are thankful to Dr. Ponpandian, Bharathiar University for his valuable discussions and moral support. References [1] [2] [3] [4] [5] Fig. 7. Variation of saturation magnetization as a function of calcination temperature Ni1xCoxFe2O4 nanoparticles.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 8. Variation of coercivity as a function of calcination temperature for Ni1xCoxFe2O4 nanoparticles.
[18]
Schultz, K. Schnitzke, J. Welker, Appl. Phys. Lett. 56 (1990) 868–870. M. Kishimoto, Y. Sakurai, T. Ajima, J. Appl. Phys. 76 (1994) 7506–7509. L. Lu, M.L. Sui, K. Lu, Science 287 (2000) 1463–1466. Mathew George, Asha Mary John, Swapna S. Nair, P.A. Joy, M.R. Antharaman, J. Magn. Magn. Mater. 302 (2006) 190–195. Subhash Chander, Bipin K. Srivastava, Anjali Krishnamurthy, Indian J. Pure Appl. Phys. 42 (2004) 366–370. M.K. Shobana, V. Rajendran, M. Jeyasubramanian, N. Suresh Kumar, Mater. Lett. 612 (2007) 2616–2619. M.K. Shobana, S. Sankar, V. Rajendran, Mater. Chem. Phys 113 (2009) 10–13. B.D. Cullity, ‘Introduction to Magnetic Materials, Addison-Wesley, Massachusetts, 1972. M.K. Roy, Bidyut Haldar, H.C. Verma, Nanotechnology 17 (2006) 232–237. L. Zhou, Y. Cui, Y. Hua, L. Yu, W. Jin, S. Feng, Mater. Lett. 60 (2006) 104–108. S. Dey, A. Roy, J. Ghose, R.N. Bhowmic, R. Ranganathan, J. Appl. Phys. 90 (2001) 4138–4142. W.W. Millnofsky, R.W. Babbit, J. Appl. Phys. 35 (1964) 1014–1017. Ovidiu Caltun, G.S.N. Rao, K.H. Rao, B.P. Rao, Ioan Dumitru, Choung-Oh Kim, CheolGi Kim, J. Magn. Magn. Mater. 316 (2007) e618–e620. M. Kryszewski, J.K. Jeszka, Synth. Met. 94 (1998) 99–104. Shifeng Yan, Jianeing Geng, Li Yin, Enu Zhou, J. Magn. Magn. Mater. 277 (2004) 84–89. Vijay A. Hiremath., A. Venkataraman, Bull. Chem. Soc. 26 (2003) 391–396. Zhenxing Yue, Ji Zhou, Longtuli, Hongguo Zhang, Zhilun Gui, J. Magn. Magn. Mater. 208 (2000) 55–60. X. Huang, Z. Chen, J. Magn. Magn. Mater. 280 (2004) 37–43.
ARTICLE IN PRESS M.K. Shobana, S. Sankar / Journal of Magnetism and Magnetic Materials 321 (2009) 3132–3137
[19] S. Sindhu, S. Jegadesan, A. Parthiban, S. Valiyaveettil, J. Magn. Magn. Mater. 296 (2006) 104–113. [20] M. Zheng, X. C. Wu, B.S. Zou and Y.J. Wang, 183(1998) 152–156. [21] F. del Monte, M.P. Morales, D. Levy, A. Fernandez, M. Ocana, E. Roig, K. Molis, C.J. O’Grady, Serna, Langmuir 13 (1997) 3627–3634.
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[22] H.K. Wu, Y.U. Chang, Y.S. Chiu, T.R. Wu, J. Magn. Magn. Mater. 283 (2004) 380–384. [23] C.S. Kim, W.C. Kim, S. Yan, S.W. Lee, J. Magn. Magn. Mater. 215–216 (2000) 213–216. [24] O. Caltun, H. Chirac, Lupu., I. Dumitru., P.B. Rao, J. Opto. Adv. Mater. 9 (2007) 1158–1160.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and characterization of Ni1xCoxFe2O4 nanoparticles M.K. Shobana , S. Sankar Department of Physics, CEG Campus, Anna University, Chennai 600025, Tamil Nadu, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 4 February 2009 Received in revised form 6 May 2009 Available online 15 May 2009
Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) nanoparticles have been synthesized with various crystallite sizes depending on the thermal treatments and composition (cobalt content) using the sol–gel combustion method. The size of nanoparticles has been controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of Ni1xCoxFe2O4 have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The presence of functional group has been identified by Fourier Transform Infrared (FTIR) spectra. From TGA–DTA studies, the weight gains of Ni1xCoxFe2O4 nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 1C. Magnetic properties of Ni1xCoxFe2O4 particles have been analysed using VSM and it is found that saturation magnetization (Ms) has increased with particle size and has coercivity (Hc) increased initially and then decreased. The Ms and Hc values decreased with the increase of content of cobalt in Ni1xCoxFe2O4. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic material Sol–gel combustion Thermal property Saturation magnetization
1. Introduction In the recent past, the nanomagnetic materials have gained remarkable scientific interest owing to their interesting properties and a variety of applications [1]. The high coercivity of these magnetic nanoparticles makes them interesting for applications in the fields of high-density magnetic media, recording color imaging, ferrofluids, high-frequency devices and magnetic refrigeration [2,3]. The interesting and useful magnetic properties of spinel ferrites depend on the choice of the cations along with Fe2+ and Fe3+ ions and their distribution between tetrahedral (A) and octahedral (B) sites of the spinel lattice. Finite size effect on the structural and magnetic properties of sol–gel-synthesized NiFe2O4 particle was reported by George et al. [4]. Chander et al. [5] reported the magnetic behaviour of Ni0.5Co0.5Fe2O4 nanoparticles prepared using two different routes. In this study, we report the finite size effects on the structural, thermal and magnetic properties of Ni1xCox Fe2O4 nanoparticles of different sizes.
2. Experimental details Nickel cobalt ferrite (Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9)) nanoparticles with different Co contents were prepared by the Corresponding author. Tel.: +91 44 22203159; fax: +91 44 22203160.
E-mail address: [email protected] (M.K. Shobana). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.05.018
sol–gel combustion method [6,7]. Analytical grade nickel nitrate, cobalt nitrate, ferric nitrate and citric acid have been used as source material for the preparation of Ni1xCox Fe2O4 nanoparticles. Here, water is used as a solvent. The pH of the sol was maintained in between 8 and 9. The yielded ash containing Ni1xCoxFe2O4 nanoparticles (ashes) were calcinated at 500, 700 and 900 1C for 1 h.
3. Characterization The prepared nanoparticles were characterized by powder Xray diffraction (XRD) using Philips 1710-X-ray diffractometer with CuKa radiation (1.5406 A˚). Morphological studies of the ferrites were carried out by employing an Atomic Force Microscope (Digital Nanoscope II, USA). The size of the powders were confirmed using transmission electron microscopy (TEM) (JEOL 100CXII). The powdered samples were sprinkled with acetone to fix on a glass plate (2 2 mm). It was air dried for 30 min and used for Atomic Force Microscopy (AFM) studies. Fourier transform infrared spectroscopic study was carried out in KBr medium using Shimadzu, FTIR-8400 in the wavenumber range 400–4000 cm1 with a resolution of 1–5 cm1. Thermal decomposition of the samples was studied from room temperature to 1000 1C using a thermogravimeter (Perkin Elmer analyzer). Vibrating sample magnetometer (VSM) was used to evaluate the room-temperature magnetic properties of the ferrites. The samples were weighed and kept in a plastic tube, and put on the sample holder with the help of a teflon tape.
ARTICLE IN PRESS M.K. Shobana, S. Sankar / Journal of Magnetism and Magnetic Materials 321 (2009) 3132–3137
4. Results and discussion 4.1. Structural and morphological studies The XRD patterns of all the samples of Ni0.4Co0.6Fe2O4 nanoparticles synthesized by the sol–gel combustion technique are depicted in Fig. 1. All the characteristic peaks of Ni0.4Co0.6Fe2O4 nanoparticles (NCF1, NCF2, NCF3 and NCF4 corresponding to the calcination temperatures 300, 500, 700 and 900 1C) are present in the diffraction pattern. It is found that all diffraction peaks can be perfectly indexed of the cubic spinel structure, and no peaks corresponding to impurities are detected in the XRD pattern. Likewise, the other two compositions Ni0.2Co0.8Fe2O4 and Ni0.1 Co0.9Fe2O4 nanoparticles calcined at 300, 500, 700 and 900 1C are illustrated in Fig. 2. The particle sizes of these samples have been calculated by Scherrer formula [8], viz., D¼
Kl b cos y
3133
diffraction peaks and decreasing of width. Fig. 2 illustrates the variation of particle sizes of Ni1xCoxFe2O4 nanoparticles at different concentrations of cobalt. From all compositions of Ni1xCoxFe2O4 nanoparticles, furthermore, we have observed that the particle size depends not only on composition but also on temperature, milling time and pH value of the solutions. This observation also agrees well with previous report of Malinofsky and Babbitt [12]. The present results depict that the increasing
Table 1 Particle sizes of Ni1xCoxFe2O4 at different calcination temperature. Temperature (1C)
300 500 700 900
Particle size (nm) Ni0.4Co0.6Fe2O4
Ni0.2Co0.8Fe2O4
Ni0.1Co0.9Fe2O4
18 22 48 65
16 22 43 58
14 19 41 52
(NCF1) (NCF2) (NCF3) (NCF4)
(2NCF1) (2NCF2) (2NCF3) (2 NCF4)
(3NCF1) (3NCF2) (3NCF3) (3NCF4)
where b is the full width half maximum (rad), l the wavelength of the X-ray, y the angle between the incident and diffracted beams (degree) and D the particle size of the sample (nm). The average particle size is found to be in the range 15–65 nm and is given in Table 1. The crystalline nature of the Ni0.4Co0.6Fe2O4 increased sharply, as observed, as the firing temperature was increased. This clearly shows that the particle size has increased with increase of calcination temperature in a manner similar to these observed by Roy et al. [9], Zhou et al. [10] and Dey et al. [11]. The crystalline property improves with the increase of intensity of X-ray
Intensity (arb. Units)
(311)
(511)
(200) (222)
(400)
(422)
(440) (d)
0.5 1.0
(c)
1.5
μm
(b)
(a) 30
40
50 2θ degrees
60
Fig. 1. XRD pattern of Ni0.2Co0.8Fe2O4 nanoparticles at different temperatures: (a) 300 1C, (b) 500 1C, (c) 700 1C and (d) 900 1C.
0.2 0.4 0.6 0.8 Fig. 2. Variation of crystallite size Ni1xCoxFe2O4 nanoparticles with calcination temperatures.
μm
Fig. 3. Topographic view of Ni0.2Co0.8Fe2O4 nanoparticles: (a) 300 1C and (b) 900 1C.
ARTICLE IN PRESS 3134
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Chemical and structural changes take place during calcination and can be monitored by spectroscopic analysis. Infrared spectroscopic studies were performed aiming to ascertain the metal oxygen nature of the product and follow the dehydration. Fig. 5 shows a characteristic peak at 584.45 cm1 for Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9). Krysewski et al. [14] reported that the peaks at 550, 793 and 895 cm1 could be due to the deformation of iron oxide lattice and OH groups bound to the surface of the Fe3O4 particles. The bands around 1384 and 570 cm1 are attributed to stretching vibrations of the anti-symmetric NO1 3 and tetrahedral complexes of the ferrite. In the as-prepared sample (300 1C) peaks, around 3420 cm1 and 1620–1630 cm1 show the presence of water molecule [15]. The ferrite stretching is shifted to higher wavenumber when compared with low temperature, and is due to the increase of calcination temperature. The shifted bands such as 590, 1383, and 1647 cm1 are compared to those at low temperatures [16]. The sharp line shows that the particle is in crystalline phase at high temperature. The peaks around 1384, 570 and 1590 cm1 disappear or decrease by heat treatments and represent the removal of NO1 3 ions and carboxyl group takes part in the reaction during combustion [17]. 4.2. Thermal studies
Fig. 4. Topographic view of Ni0.4Co0.6Fe2O4 nanoparticles: (a) 500 1C and (b) selected area diffraction pattern at 500 1C.
concentration of cobalt (Co) decreases the particle size. This may be due to migration of a small number of Co2+ ions in the midst of Co3+ ions in B-sites. This observation is similar to that of Caltun et al. [13] for CoMn ferrite. The morphology and particle sizes of the Ni1xCoxFe2O4 nanoparticles are determined by AFM (Fig. 3) and TEM (Fig. 4(a,b)). Fig. 3 shows the micrograph of the Ni0.4Co0.6Fe2O4 particles. It is evident from the AFM images that for the samples NCF1 and NCF4 calcinated at 300 and 900 1C, respectively, confirm the cubic morphology of the particles. The average particle of size of the sample is found to be 55 nm. Fig. 3(b) shows the existence of the Ni0.4Co0.6Fe2O4 nanoparticles with agglomeration. This may be due to evaporation of the polymer at high temperature. Agglomeration, one of the main constrains in the synthesis of magnetic nanomaterials, can be avoided by the use of polymer matrix to isolate the particles. Agglomeration of the prepared fine particles can be clearly noticed at higher temperatures (900 1C). Evaporation of the polymer above 500 1C is unavoidable. It is inferred that an increase in calcination temperature increases the particle size and it is also consistent with XRD results.
The experimental observation showed that the nitrate–citrate gels with all three molar ratios of metal nitrates to citric acid exhibited self-propagating combustion behaviour. When the dried gels were ignited at any point, the combustion rapidly propagated forward until all gels were completely burnt out to form powders. It was also observed that the combustion rate is associated with the ratio of nitrates of citric acid. The autocatalytic nature of the combustion process of gels has been studied by thermal analysis (DTA and TGA) of the dried gels. The TGA and DTA thermograms of the Ni1xCoxFe2O4 (x ¼ 0.6, 0.8, 0.9) sample decomposing in nitrogen atmosphere are shown in Fig. 6. The observed weight loss below 100 1C of Ni1xCoxFe2O4 (Wg for Co (x ¼ 0.6) ¼ 1.2%, Wg for Co (x ¼ 0.8) ¼ 1.1%, Wg for Co (x ¼ 0.9) ¼ 1%) is attributed to the loss of physically or chemically absorbed OH groups. Fig. 6 for all compositions of Ni1xCoxFe2O4 illustrates the weight gain above 200 1C for the sample prepared at 300 1C [18]. This could be due to the formation of surface carbonate as the capped magnetic particles interact with oxygen at temperature above 200 1C. An exothermic peak at 921.82, 886.68 and 842.53 1C for Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) respectively. No more exothermic peaks have been found and it represents the completion of ferritization or crystallization. It is inferred further that the completion of ferritization is shifted to low temperature by increasing Co content. These are much lower than the values reported by Sindhu et al. [19] for Ni ferrites prepared by conventional methods. 4.3. Magnetic studies The various magnetic properties like saturation magnetization and coercivity are estimated from the hysteresis curve. The variation of saturation magnetization is illustrated in Fig. 7 and coercivity in Fig. 8. The saturation magnetization of the nanosized Ni1xCoxFe2O4 (x ¼ 0.6, 0.8, 0.9) has increased with the increase of grain size. This observation can be due to many factors: cation distribution, the existence of surface spins or the formation of spin glass structure, etc. The presence of dead layer has also been thought to be one of the reasons for the reduced magnetization in the ultrafine region [20]. Moreover, the value of saturation magnetization depends not only on calcination temperature, but also on the composition (content of cobalt). The variation of
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saturation magnetization (Ms) depends on the cation distribution in a spinel lattice. Moreover, high coercive field is one of the advantages of nanosize particles for the magnetic applications. Coercivity values are found to be smaller for the particles calcined at 300 1C (smaller crystallite size) and at 900 1C (higher crystallite size) when compared to these for the particles calcined at 500 1C (intermediate crystallite size). This observation is similar to that of Monte et al. [21] in Co ferrite. So the magnetic behaviour strongly depends on the variation of the particle size [22,23]. Further increase in doping levels decreases the strength of the exchange interactions, and leads to the lower value of the saturation magnetization [13,24].
5. Conclusions Ni1xCoxFe2O4 (x ¼ 0.6, 0.8 and 0.9) nanoparticles were synthesized with various crystallite sizes depending on the
thermal treatments and composition (cobalt content) using the sol–gel combustion method. The size of nanoparticles is controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of the Ni1xCox Fe2O4 have been studied by using AFM and TEM studies. The presence of functional group has been identified by Fourier transform infrared spectra (FTIR). From TGA–DTA studies, the weight gains of the Ni1xCoxFe2O4 nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 1C. Magnetic properties of the Ni1xCoxFe2O4 particles have been analysed using VSM and it is found that saturation magnetization (Ms) has increased with particle size and has coercivity (Hc) increased initially and then decreased. The Ms and Hc values decreased with increase of content of cobalt in Ni1xCox Fe2O4. This might be due to migration of Co2+ ions in the B-sublattice.
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Fig. 6. TGA–DTA of Ni1xCoxFe2O4 nanoparticles for different Co content.
Acknowledgement The authors are thankful to Dr. Ponpandian, Bharathiar University for his valuable discussions and moral support. References [1] [2] [3] [4] [5] Fig. 7. Variation of saturation magnetization as a function of calcination temperature Ni1xCoxFe2O4 nanoparticles.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 8. Variation of coercivity as a function of calcination temperature for Ni1xCoxFe2O4 nanoparticles.
[18]
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