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Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite
Materials Research Bulletin 43 (2008) 2160–2165 www.elsevier.com/locate/matresbu
Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite V.L. Mathe *, R.B. Kamble Novel Materials Research Laboratory, Department of Physics, University of Pune, Ganeshkhind, Pune 411 007, MS, India Received 25 April 2007; received in revised form 12 August 2007; accepted 2 September 2007 Available online 11 September 2007
Abstract Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300–900 K is investigated. ln r versus 1/T plot shows four break resulting into five regions in 300–900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (e0 ) with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in e0 variation with temperature show frequency dependence. Electrical and dielectric properties of Ni0.4Co0.6Fe2O4 sample show unusual behavior in the temperature range 723–833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723– 833 K for Ni0.4Co0.6Fe2O4. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Chemical synthesis; C. X-ray diffraction; D. Electrical properties
1. Introduction Spinel ferrites having general formula AB2O4 are of vital importance from both basic science point of view and technological applications point of view. In ferrite ‘A’ is divalent metal ion and ‘B’ is trivalent metal ion. The structural and electrical properties are very much sensitive to the methodology adopted for the synthesis and synthesis parameters. Normally, in AB2O4 structure divalent ion is bigger in size than that of trivalent ion. Former occupies octahedral site while latter occupies tetrahedral site with some exceptions. In zinc and cadmium ferrite divalent metal ion (Zn2+ and Cd2+) occupies tetrahedral site and trivalent metal ion (Fe3+) occupies octahedral site, which forms normal spinel structure. In nickel and cobalt ferrite divalent metal ion (Co2+ and Ni2+) occupies octahedral site and Fe3+ ions occupies half at tetrahedral and octahedral each forms inverse spinel structure. This cation distribution decides the structural, electrical and magnetic properties for a particular ferrite system at and well above room temperature.
* Corresponding author. Tel.: +91 20 25692678; fax: +91 20 25691684. E-mail address: [email protected] (V.L. Mathe). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.09.001
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Recently theoretical studies indeed predict new fascinating properties for nickel ferrite and cobalt ferrite that depends upon cation distribution and electrical state of the transition metal ions of the spinel structure. In some condition half metallic character is expected. This property combined with the high Curie temperature of this ferrite would make them very useful as ferromagnetic electrode in magnetic tunnel junction [1] or in spintronic devices. It is well known that cobalt ferrite exhibit high corrosion resistivity, magnetocrystalline anisotropy, magnetostriction and magneto-optic properties; which make this material useful in many applications like recording media with optical wave guide, magnetic static wave devices and surface acoustic wave transducers [2–5]. In most of the cases reports are limited to chemical synthesis of the samples and only room temperature studies on dielectric properties [6–8]. A need exists to study the properties as a function of temperature as well, to investigate the resistivity mechanisms and polarization at and well above room temperature up to Curie temperature. In the present work, therefore, we choose a particular composition namely Ni0.4Co0.6Fe2O4; report its synthesis by the chemical co-precipitation method and their structural, morphological, electrical and dielectric properties well above curie temperature of Ni and Co ferrite. 2. Experimental The Ni–Co ferrite sample having composition Ni0.4Co0.6Fe2O4 was prepared using chemical co-precipitation method. In this reaction stoichiometric amounts of iron (III) nitrate, cobalt (II) acetate and nickel (II) nitrate were dissolved in distilled water. The solution was heated to 100 8C with continuous stirring and 2 M NaOH solution was added ml by ml till the pH of the solution become 12. Addition of NaOH resulted into brown coloured precipitate. This precipitate was washed several times with distilled water till the pH of the filtrate became 7. The wet brown coloured slurry was subjected to heat treatment in oven at 100 8C for 12 h to remove water content from the slurry. This has resulted in to fine powder and named ‘as prepared powder’. The as-prepared powder was then subjected to heat treatment of 600 8C for 8 h. After confirmation of phase formation the fine powder was shaped into pellets of diameter 10 mm and thickness 2 mm. The pellets were then subjected at 600 8C for 8 h for densification purpose. Silver paint was applied to the two plain faces of the pellet, which acts as electrode. This sample in the final form was used for electrical and dielectric measurements. DC electrical resistivity measurements were carried out using two-probe method, where, Keithley multimeter model 2000 was used to measure current at constant voltage, at different temperatures in the range 300–900 K. The dielectric measurements were carried out using Hioki 3532-50 LCR Hi tester at fixed frequencies as a function of temperature in the range 300–900 K. 3. Results and discussion The Ni–Co ferrite having nominal composition Ni0.4Co0.6Fe2O4 was prepared using chemical co-precipitation method. The powder of Ni0.4Co0.6Fe2O4 was characterized by X-ray diffraction (XRD) technique at several stages. The as-prepared powder was found to be amorphous. The powder head treated at 600 8C for 8 h was characterized by XRD technique. The X-ray diffractogram is shown in Fig. 1. It shows total seven peaks in the 2u range of 20–808. All the peaks were identified by comparing the ‘d’ spacing with that of JCPDS data of NiFe2O4 [9]. All peaks belong to spinel cubic structure only. The lattice parameter calculated by further analysis is ‘a’ = 8.362 Å. The value of lattice parameter is higher than that of NiFe2O4 and lower than that of CoFe2O4, which is expected due to bigger ionic size of Co2+ (0.082 nm) than that of Ni2+ (0.078 nm) ion. Reports in literature revealed that the ferrite samples prepared by chemical method show presence of a-Fe2O3 peak due to loss of divalent metal ion during washing and drying [10–12]. In the present case there is no unidentified peak in XRD pattern indicating the synthesis parameters are well optimized. The crystallite size was calculated from the XRD line width of 311 peak using Scherrer formula and is 14 nm. SEM picture of the sample treated at 600 8C for 8 h is shown in Fig. 2. It is seen from Fig. 2 that the grains are spherical in shape with average size of 50 nm. The size distribution is uniform and compact. The size obtained from SEM is high than that of calculated from X-ray data. Experimental density measurements carried out using liquid immersion method show good agreement with that of X-ray density. X-ray density was calculated using the formula D = 8M/Na3 for the spinel ferrite, where M is the molecular weight of ferrite, N the Avagadro’s number and ‘a’ is the lattice parameter. Experimental density is equal to 5.182 gm/cm3, and X-ray density is equal to 5.334 gm/cm3. The data reveals that the sample is highly dense and is of good quality for electrical and dielectric measurements. Fig. 3 shows plot of ln r versus 1/T for the Ni0.4Co0.6Fe2O4. In the present case measurements are carried out in the temperature range 300–900 K. In this plot change in slope of the plot indicates change in conduction mechanism. In
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Fig. 1. X-ray diffraction patter of Ni0.4Co0.6Fe2O4.
Fig. 2. SEM micrograph of Ni0.4Co0.6Fe2O4.
the present case, the whole region of plot shows five linear regions. In literature, it is found that the spinel ferrites prepared by solid state reaction technique show three linear regions with two times break in the plot in the temperature range from room temperature to well above Curie temperature [13–14]. The first region from room temperature to around 400 K is attributed to impurity, voids, etc. conduction. Above 400 K to the Curie temperature is second region, which is attributed to ferrimagnetic region of conduction. Above Curie temperature is third region, which is attributed to paramagnetic region. In the present case five regions are observed as follows: (i) the first region is from room temperature to 333 K, (ii) second; 333–573 K, (iii) third; 573–723 K, (iv) fourth; 723–833 K and (v) fifth above 833 K. For a while if third region is ignored then the plot comprise mainly three regions only, i.e. first, second and fifth. In the
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Fig. 3. Plot of ln r vs. 1/T for Ni0.4Co0.6Fe2O4.
light of literature it is expected that in the 333–833 K temperature range there should not be any break in the slope of the plot. But for the present sample in ferrimagnetic region an additional anomalous behavior is observed in the temperature range 573–723 K which need explanation. Such anomalous behavior is repeatedly observed even for other samples of Ni–Co and Ni–Zn family prepared by co-precipitation method. To answer this anomaly let us look into the aspect of cation distribution. It is expected that in Ni–Co ferrite both Ni2+ and Co2+ ions to occupy octahedral site and Fe3+ ions are expected to occupy tetrahedral and octahedral site. Kim et al., based on Mössbauer spectra analysis reported that part of divalent ions may occupy tetrahedral site in Ni–Co ferrite [15]. Altering the cation distribution between the chemically inequivalent sites will not lead to change crystal structure and chemical composition but will affect the electrical properties. The present study therefore indicates that a ferrite with a particular composition may have different cation distribution in its bulk phase that in its nanoparticle phase. When, temperature is increased above 500 K slow migration of Fe3+ ions from A site to the B site starts. Migration of Fe3+ ion to B site increase Fe3+–Fe2+ hopping probability at octahedral site, which results in decreased resistivity values. The amount of Fe3+ ions migration increases up to 700 K and afterwards it saturates [15]. This results an anomaly in the resistivity behavior in third region. The activation energy calculated in different regions is given in Table 1. The temperature 833 K is noted as Curie temperature of Ni0.4Co0.6Fe2O4, which is below Curie temperature of NiFe2O4 and above Curie temperature of CoFe2O4 [16,17]. Activation energy below 833 K, i.e. in ferrimagnetic region is small as compared to that of activation energy above 833 K, i.e. in paramagnetic region. This is because ferrimagnetic region is an ordered state, while paramagnetic is disordered state hence it requires more energy for the activation of the charge carrier. The variation of dielectric constant with temperature for Ni0.4Co0.6Fe2O4 is given Fig. 4. The value of dielectric constant is quite low at room temperature; which is about 104 times lower than those obtained for ferrite samples prepared by the conventional ceramic method [18,19]. The value of dielectric constant is high at low frequencies and low at high frequencies. Moreover, increase in value of e0 with increasing temperature is more at low frequencies, when compared it with high frequencies. The dielectric constant of any material, in general, is due to dipolar, electronic, ionic, and interfacial polarizations. At low frequencies dipolar and interfacial polarizations are known to play the most important role. Both these polarizations are strongly temperature-dependent. Whereas the interfacial polarization Table 1 Activation energy values of Ni0.4Co0.6Fe2O4 in different temperature regions Temperature range
Activation energy (eV)
I (300–333 K) II (333–573 K) III (573–723 K) IV (723–833 K) V (above 833 K)
0.157 0.657 0.817 0.675 0.822
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Fig. 4. Variation dielectric constant (e0 ) with temperature for Ni0.4Co0.6Fe2O4 at 1 kHz, 10 kHz, 100 kHz and 1 MHz.
increases with temperature due to creation of crystal defects, dipolar polarization decreases with increase in temperature. The rapid increase in the dielectric constant with increase in temperature at low frequencies suggests that the effect of temperature is more pronounced on the interfacial than on the dipolar polarization. At high frequencies, electronic and ionic polarizations are the main contributors and their temperature dependence is insignificant [20]. The bird eye-view of Fig. 4 shows following main features. (i) The e0 increases slowly at all frequencies up to 700 K; above 700 K, it increases rapidly. (ii) In the temperature range of measurements two peaks are observed; one at 773 K and other at 833 K. The peak at 833 K is dominant. (iii) The peak temperature show slight shift with change in frequency. Feature (i) can be explained as: up to 700 K the thermal energy given is insufficient to free the localized dipoles to be oriented in the direction of applied electric field. Above 700 K a large number of dipoles become free due to sufficiently high thermal energy and the applied electric field aligned them in its direction. Such behavior is observed by Ahmad et al. in case of lanthanum substituted Ni–Zn ferrite [21]. In the second feature the peak observed at 773 K can be explained as: the migration of Fe3+ ions increases electron exchange between thermally activated Fe3+–Fe2+; hole transfer between Co3+–Co2+ and Ni3+–Ni2+ at octahedral site, which results into increased polarization and dielectric constant. Afterwards, migration saturates resulting e0 peak with temperature. This observation is analogous to resistivity data in the range of temperature 723–833 K. The peak around 833 K is attributed to ferrimagnetic (magnetically ordered) to paramagnetic (disordered) transition of Ni0.4Co0.6Fe2O4. This is in agreement with dielectric behavior observed for other ferrites at Curie temperature [21–25]. The decrease in e0 above 833 K is attributed to decrease in internal viscosity of the system giving rise to more degree of freedom to the dipoles with the result of increasing the disorder in the system and hence decrease in e0 . This is in agreement with Ahmed et al. [21]. The third feature can be explained as: the behavior observed in Ni–Co is due to collective contribution of two type of charge carriers ‘p’ and ‘n’ to polarization. The appearance of ‘p’ type carriers in the present case is due to hole transfer in Co3+–Co2+ and Ni3+–Ni2+ while Fe2+–Fe3+ gives rise to ‘n’ type charge carriers. The polarization of ‘p’ type charge carriers is in the opposite direction to that of ‘n’ type charge carriers. Also the mobility of ‘p’ type charge carriers is low as compared to that of ‘n’ type charge carriers. Therefore, the shift in peak depends upon the majority charge carriers. In the present case the majority charge carriers are of ‘p’ type, confirmed from Seebeck coefficient data with temperature (not shown). The peak in e0 shifts to low temperature site with increasing temperature. In conclusion, nanocrystalline Ni0.4Co0.6Fe2O4 ferrite sample has been prepared successfully using chemical coprecipitation technique. The spinel cubic structure having lattice parameter a = 8.362 Å is confirmed from XRD data analysis. Nanocrystalline nature of the sample is confirmed from microscopic analysis. The anomalous electrical and dielectric behavior observed in the temperature range 723–833 K is explained on the basis of migration of Fe3+ ions from ‘A’ site to ‘B’ site. The behavior around 833 K is attributed to transition from ordered state (ferrimagnetic) to disordered state (paramagnetic).
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Acknowledgement VLM is thankful to UGC, New Delhi and DST, New Delhi for the financial support. References [1] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [2] M. Grigorova, H.J. Blythe, V. Blaskov, V. Rusanov, V. Petkov, V. Masheva, D. Nihtianova, L.I.M. Martinez, J.S. Muñoz, M. Mikhov, J. Magn. Magn. Mater. 183 (1998) 163. [3] S.N. Okuno, S. Hashimoto, K. Inomata, J. Appl. Phys. 71 (1992) 5926. [4] Y. Okamura, S. Yamamoto, J. Appl. Phys. 69 (1993) 4583. [5] A.D. Fisher, Circuits Syst. Signal Process 4 (1985) 265. [6] B. Parvatheeswara Rao, K.H. Rao, K. Trinadha, O.F. Caltun, J. Optoelect. Adv. Mater. 6 (2004) 951. [7] I.H. Gul, A.Z. Abbasi, F. Amin, M. Anis-ur-Rehman, A. Maqsood, J. Magn. Magn. Mater. 311 (2007) 494. [8] A.Y. Lipare, P.N. Vasambekar, A.S. Vaingankar, J. Magn. Magn. Mater. 279 (2004) 160. [9] JCPDC Card No. 10-325. [10] C. Rath, K.K. Sahu, S. Anand, S.K. Date, N.C. Mishra, R.P. Das, J. Magn. Magn. Mater. 202 (1999) 77. [11] O. Suwalka, R.K. Sharma, V. Sebastian, N. Lakshmi_, K. Venugopalan, J. Magn. Magn. Mater. 313 (2007) 198. [12] R.V. Mangalaraja, S. Ananthakmar, P. Manohara, F.D. Gnanama, M. Awanob, Mater. Sci. Eng. A 367 (2004) 301. [13] S. Balaji, R. Kalai Selvan, L. John Berchmans, S. Angappan, K. Subramanian, C.O. Augustin, Mater. Sci. Eng. B 119 (2005) 119. [14] M.G. Patil, V.C. Mahajan, A.K. Ghatage, S.A. Patil, Ind. J. Pure Appl. Phys. 34 (1996) 1665. [15] C.S. Kim, S.W. Lee, S.L. Park, J. Appl. Phys. 79 (1996) 5428. [16] R.N. Singha, N.K. Singha, J.P. Singha, G. Balajib, N.S. Gajbhiyeb, Int. J. Hydrogen Energy 31 (2006) 701. [17] B. Viswanathan, V.R.K. Murthy, Ferrite Materials: Science and Technology, Narosa Publishing House, New Delhi, India, 1990, p. 11. [18] L. Radhapiyari, S. Phanjoubam, H.N.K. Sarma, C. Prakash, Mater. Lett. 44 (2000) 65. [19] C.G. Koops, Phys. Rev. 83 (1951) 121. [20] L.L. Hench, J.K. West, Principles of Electronic Ceramics, Wiley, New York, 1990, p. 189. [21] M.A. Ahmed, E. Ateia, L.M. Salah, A.A. El-Gamal, Mater. Chem. Phys. 92 (2005) 310. [22] A. Thakur, P. Mathur, M. Singh, J. Phys. Chem. Solids 68 (2007) 378. [23] D. Ravinder, K. Vijaya Kumar, Bull. Mater. Sci. 24 (2001) 505. [24] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Appl. Phys. 91 (2002) 6626. [25] H.M. Zaki, J. Alloys Compd., in press.
Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite V.L. Mathe *, R.B. Kamble Novel Materials Research Laboratory, Department of Physics, University of Pune, Ganeshkhind, Pune 411 007, MS, India Received 25 April 2007; received in revised form 12 August 2007; accepted 2 September 2007 Available online 11 September 2007
Abstract Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300–900 K is investigated. ln r versus 1/T plot shows four break resulting into five regions in 300–900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (e0 ) with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in e0 variation with temperature show frequency dependence. Electrical and dielectric properties of Ni0.4Co0.6Fe2O4 sample show unusual behavior in the temperature range 723–833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723– 833 K for Ni0.4Co0.6Fe2O4. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Chemical synthesis; C. X-ray diffraction; D. Electrical properties
1. Introduction Spinel ferrites having general formula AB2O4 are of vital importance from both basic science point of view and technological applications point of view. In ferrite ‘A’ is divalent metal ion and ‘B’ is trivalent metal ion. The structural and electrical properties are very much sensitive to the methodology adopted for the synthesis and synthesis parameters. Normally, in AB2O4 structure divalent ion is bigger in size than that of trivalent ion. Former occupies octahedral site while latter occupies tetrahedral site with some exceptions. In zinc and cadmium ferrite divalent metal ion (Zn2+ and Cd2+) occupies tetrahedral site and trivalent metal ion (Fe3+) occupies octahedral site, which forms normal spinel structure. In nickel and cobalt ferrite divalent metal ion (Co2+ and Ni2+) occupies octahedral site and Fe3+ ions occupies half at tetrahedral and octahedral each forms inverse spinel structure. This cation distribution decides the structural, electrical and magnetic properties for a particular ferrite system at and well above room temperature.
* Corresponding author. Tel.: +91 20 25692678; fax: +91 20 25691684. E-mail address: [email protected] (V.L. Mathe). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.09.001
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Recently theoretical studies indeed predict new fascinating properties for nickel ferrite and cobalt ferrite that depends upon cation distribution and electrical state of the transition metal ions of the spinel structure. In some condition half metallic character is expected. This property combined with the high Curie temperature of this ferrite would make them very useful as ferromagnetic electrode in magnetic tunnel junction [1] or in spintronic devices. It is well known that cobalt ferrite exhibit high corrosion resistivity, magnetocrystalline anisotropy, magnetostriction and magneto-optic properties; which make this material useful in many applications like recording media with optical wave guide, magnetic static wave devices and surface acoustic wave transducers [2–5]. In most of the cases reports are limited to chemical synthesis of the samples and only room temperature studies on dielectric properties [6–8]. A need exists to study the properties as a function of temperature as well, to investigate the resistivity mechanisms and polarization at and well above room temperature up to Curie temperature. In the present work, therefore, we choose a particular composition namely Ni0.4Co0.6Fe2O4; report its synthesis by the chemical co-precipitation method and their structural, morphological, electrical and dielectric properties well above curie temperature of Ni and Co ferrite. 2. Experimental The Ni–Co ferrite sample having composition Ni0.4Co0.6Fe2O4 was prepared using chemical co-precipitation method. In this reaction stoichiometric amounts of iron (III) nitrate, cobalt (II) acetate and nickel (II) nitrate were dissolved in distilled water. The solution was heated to 100 8C with continuous stirring and 2 M NaOH solution was added ml by ml till the pH of the solution become 12. Addition of NaOH resulted into brown coloured precipitate. This precipitate was washed several times with distilled water till the pH of the filtrate became 7. The wet brown coloured slurry was subjected to heat treatment in oven at 100 8C for 12 h to remove water content from the slurry. This has resulted in to fine powder and named ‘as prepared powder’. The as-prepared powder was then subjected to heat treatment of 600 8C for 8 h. After confirmation of phase formation the fine powder was shaped into pellets of diameter 10 mm and thickness 2 mm. The pellets were then subjected at 600 8C for 8 h for densification purpose. Silver paint was applied to the two plain faces of the pellet, which acts as electrode. This sample in the final form was used for electrical and dielectric measurements. DC electrical resistivity measurements were carried out using two-probe method, where, Keithley multimeter model 2000 was used to measure current at constant voltage, at different temperatures in the range 300–900 K. The dielectric measurements were carried out using Hioki 3532-50 LCR Hi tester at fixed frequencies as a function of temperature in the range 300–900 K. 3. Results and discussion The Ni–Co ferrite having nominal composition Ni0.4Co0.6Fe2O4 was prepared using chemical co-precipitation method. The powder of Ni0.4Co0.6Fe2O4 was characterized by X-ray diffraction (XRD) technique at several stages. The as-prepared powder was found to be amorphous. The powder head treated at 600 8C for 8 h was characterized by XRD technique. The X-ray diffractogram is shown in Fig. 1. It shows total seven peaks in the 2u range of 20–808. All the peaks were identified by comparing the ‘d’ spacing with that of JCPDS data of NiFe2O4 [9]. All peaks belong to spinel cubic structure only. The lattice parameter calculated by further analysis is ‘a’ = 8.362 Å. The value of lattice parameter is higher than that of NiFe2O4 and lower than that of CoFe2O4, which is expected due to bigger ionic size of Co2+ (0.082 nm) than that of Ni2+ (0.078 nm) ion. Reports in literature revealed that the ferrite samples prepared by chemical method show presence of a-Fe2O3 peak due to loss of divalent metal ion during washing and drying [10–12]. In the present case there is no unidentified peak in XRD pattern indicating the synthesis parameters are well optimized. The crystallite size was calculated from the XRD line width of 311 peak using Scherrer formula and is 14 nm. SEM picture of the sample treated at 600 8C for 8 h is shown in Fig. 2. It is seen from Fig. 2 that the grains are spherical in shape with average size of 50 nm. The size distribution is uniform and compact. The size obtained from SEM is high than that of calculated from X-ray data. Experimental density measurements carried out using liquid immersion method show good agreement with that of X-ray density. X-ray density was calculated using the formula D = 8M/Na3 for the spinel ferrite, where M is the molecular weight of ferrite, N the Avagadro’s number and ‘a’ is the lattice parameter. Experimental density is equal to 5.182 gm/cm3, and X-ray density is equal to 5.334 gm/cm3. The data reveals that the sample is highly dense and is of good quality for electrical and dielectric measurements. Fig. 3 shows plot of ln r versus 1/T for the Ni0.4Co0.6Fe2O4. In the present case measurements are carried out in the temperature range 300–900 K. In this plot change in slope of the plot indicates change in conduction mechanism. In
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Fig. 1. X-ray diffraction patter of Ni0.4Co0.6Fe2O4.
Fig. 2. SEM micrograph of Ni0.4Co0.6Fe2O4.
the present case, the whole region of plot shows five linear regions. In literature, it is found that the spinel ferrites prepared by solid state reaction technique show three linear regions with two times break in the plot in the temperature range from room temperature to well above Curie temperature [13–14]. The first region from room temperature to around 400 K is attributed to impurity, voids, etc. conduction. Above 400 K to the Curie temperature is second region, which is attributed to ferrimagnetic region of conduction. Above Curie temperature is third region, which is attributed to paramagnetic region. In the present case five regions are observed as follows: (i) the first region is from room temperature to 333 K, (ii) second; 333–573 K, (iii) third; 573–723 K, (iv) fourth; 723–833 K and (v) fifth above 833 K. For a while if third region is ignored then the plot comprise mainly three regions only, i.e. first, second and fifth. In the
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Fig. 3. Plot of ln r vs. 1/T for Ni0.4Co0.6Fe2O4.
light of literature it is expected that in the 333–833 K temperature range there should not be any break in the slope of the plot. But for the present sample in ferrimagnetic region an additional anomalous behavior is observed in the temperature range 573–723 K which need explanation. Such anomalous behavior is repeatedly observed even for other samples of Ni–Co and Ni–Zn family prepared by co-precipitation method. To answer this anomaly let us look into the aspect of cation distribution. It is expected that in Ni–Co ferrite both Ni2+ and Co2+ ions to occupy octahedral site and Fe3+ ions are expected to occupy tetrahedral and octahedral site. Kim et al., based on Mössbauer spectra analysis reported that part of divalent ions may occupy tetrahedral site in Ni–Co ferrite [15]. Altering the cation distribution between the chemically inequivalent sites will not lead to change crystal structure and chemical composition but will affect the electrical properties. The present study therefore indicates that a ferrite with a particular composition may have different cation distribution in its bulk phase that in its nanoparticle phase. When, temperature is increased above 500 K slow migration of Fe3+ ions from A site to the B site starts. Migration of Fe3+ ion to B site increase Fe3+–Fe2+ hopping probability at octahedral site, which results in decreased resistivity values. The amount of Fe3+ ions migration increases up to 700 K and afterwards it saturates [15]. This results an anomaly in the resistivity behavior in third region. The activation energy calculated in different regions is given in Table 1. The temperature 833 K is noted as Curie temperature of Ni0.4Co0.6Fe2O4, which is below Curie temperature of NiFe2O4 and above Curie temperature of CoFe2O4 [16,17]. Activation energy below 833 K, i.e. in ferrimagnetic region is small as compared to that of activation energy above 833 K, i.e. in paramagnetic region. This is because ferrimagnetic region is an ordered state, while paramagnetic is disordered state hence it requires more energy for the activation of the charge carrier. The variation of dielectric constant with temperature for Ni0.4Co0.6Fe2O4 is given Fig. 4. The value of dielectric constant is quite low at room temperature; which is about 104 times lower than those obtained for ferrite samples prepared by the conventional ceramic method [18,19]. The value of dielectric constant is high at low frequencies and low at high frequencies. Moreover, increase in value of e0 with increasing temperature is more at low frequencies, when compared it with high frequencies. The dielectric constant of any material, in general, is due to dipolar, electronic, ionic, and interfacial polarizations. At low frequencies dipolar and interfacial polarizations are known to play the most important role. Both these polarizations are strongly temperature-dependent. Whereas the interfacial polarization Table 1 Activation energy values of Ni0.4Co0.6Fe2O4 in different temperature regions Temperature range
Activation energy (eV)
I (300–333 K) II (333–573 K) III (573–723 K) IV (723–833 K) V (above 833 K)
0.157 0.657 0.817 0.675 0.822
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Fig. 4. Variation dielectric constant (e0 ) with temperature for Ni0.4Co0.6Fe2O4 at 1 kHz, 10 kHz, 100 kHz and 1 MHz.
increases with temperature due to creation of crystal defects, dipolar polarization decreases with increase in temperature. The rapid increase in the dielectric constant with increase in temperature at low frequencies suggests that the effect of temperature is more pronounced on the interfacial than on the dipolar polarization. At high frequencies, electronic and ionic polarizations are the main contributors and their temperature dependence is insignificant [20]. The bird eye-view of Fig. 4 shows following main features. (i) The e0 increases slowly at all frequencies up to 700 K; above 700 K, it increases rapidly. (ii) In the temperature range of measurements two peaks are observed; one at 773 K and other at 833 K. The peak at 833 K is dominant. (iii) The peak temperature show slight shift with change in frequency. Feature (i) can be explained as: up to 700 K the thermal energy given is insufficient to free the localized dipoles to be oriented in the direction of applied electric field. Above 700 K a large number of dipoles become free due to sufficiently high thermal energy and the applied electric field aligned them in its direction. Such behavior is observed by Ahmad et al. in case of lanthanum substituted Ni–Zn ferrite [21]. In the second feature the peak observed at 773 K can be explained as: the migration of Fe3+ ions increases electron exchange between thermally activated Fe3+–Fe2+; hole transfer between Co3+–Co2+ and Ni3+–Ni2+ at octahedral site, which results into increased polarization and dielectric constant. Afterwards, migration saturates resulting e0 peak with temperature. This observation is analogous to resistivity data in the range of temperature 723–833 K. The peak around 833 K is attributed to ferrimagnetic (magnetically ordered) to paramagnetic (disordered) transition of Ni0.4Co0.6Fe2O4. This is in agreement with dielectric behavior observed for other ferrites at Curie temperature [21–25]. The decrease in e0 above 833 K is attributed to decrease in internal viscosity of the system giving rise to more degree of freedom to the dipoles with the result of increasing the disorder in the system and hence decrease in e0 . This is in agreement with Ahmed et al. [21]. The third feature can be explained as: the behavior observed in Ni–Co is due to collective contribution of two type of charge carriers ‘p’ and ‘n’ to polarization. The appearance of ‘p’ type carriers in the present case is due to hole transfer in Co3+–Co2+ and Ni3+–Ni2+ while Fe2+–Fe3+ gives rise to ‘n’ type charge carriers. The polarization of ‘p’ type charge carriers is in the opposite direction to that of ‘n’ type charge carriers. Also the mobility of ‘p’ type charge carriers is low as compared to that of ‘n’ type charge carriers. Therefore, the shift in peak depends upon the majority charge carriers. In the present case the majority charge carriers are of ‘p’ type, confirmed from Seebeck coefficient data with temperature (not shown). The peak in e0 shifts to low temperature site with increasing temperature. In conclusion, nanocrystalline Ni0.4Co0.6Fe2O4 ferrite sample has been prepared successfully using chemical coprecipitation technique. The spinel cubic structure having lattice parameter a = 8.362 Å is confirmed from XRD data analysis. Nanocrystalline nature of the sample is confirmed from microscopic analysis. The anomalous electrical and dielectric behavior observed in the temperature range 723–833 K is explained on the basis of migration of Fe3+ ions from ‘A’ site to ‘B’ site. The behavior around 833 K is attributed to transition from ordered state (ferrimagnetic) to disordered state (paramagnetic).
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