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Electrical conductivity of chromium substituted copper ferrites
Journal of Alloys and Compounds 370 (2004) L17–L22
Letter
Electrical conductivity of chromium substituted copper ferrites D. Ravinder∗ , K. Sathi Reddy, P. Mahesh, T. Bhaskar Rao, Y.C. Venudhar Department of Physics, Post-Graduate College of Science, Osmania University, Hyderabad 500 004, Andhra Pradesh, India Received 1 July 2003; received in revised form 23 September 2003; accepted 23 September 2003
Abstract Electrical conductivity of copper–chromium (Cu–Cr) ferrites of various compositions have been investigated as a function of composition and temperature. Plots of log (σT) versus 103 /T are almost linear and have shown a transition near the Curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region. An attempt is made to explain the conduction mechanism in Cu–Cr ferrites. © 2003 Elsevier B.V. All rights reserved. Keywords: Cu–Cr ferrites; Electrical conductivity; Curie temperature; Chromium
1. Introduction
3. Results and discussion
Mixed copper ferrites have been commercially used for many years as high-frequency devices such as radio frequency coils, transformer cores, rod antennas and magnetic cores of read-write heads for high speed digital tapes [1,2]. However, no information is available on electrical conductivity of copper–chromium (Cu–Cr) ferrites in the literature. Moreover, there is a need for a thorough study of electrical properties of these ferrites possessing desired application in microwave devices. Therefore, we have undertaken a systematic study of the electrical conductivity of Cu–Cr ferrites as a function of composition and temperature. The results of such studies are presented in this communication.
Experimental data for the mixed Cu–Cr ferrites are given in Table 1, which includes the compositional formulae of all the ferrites under investigation and the values of electrical conductivity at room temperature. It can be seen from the table that the values of electrical conductivity varies from 6.28 × 10−8 to 4.12 × 10−4 Ohm−1 cm−1 and decreases with increase of chromium content. This observation is in agreement with the result reported by Van Uitert [4] who found that the resistivity of Li–Zn ferrites increased with increase of zinc content. The temperature dependent on electrical conductivity of mixed Cu–Cr ferrites of various compositions has been investigated from room temperature to well beyond the Curie temperature. Plots of log (σT) versus temperature (103 /T) are shown in Figs. 1–5. It can be seen from the figures that the value of log (σT) increases linearly with increasing temperature up to a certain temperature Tσ (K) at which a change
2. Experimental details Mixed copper–chromium ferrites having the chemical formulae Cu1−x Crx Fe2 O4 (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the conventional double sintering ceramic technique. The samples were sintered for 6 h at 1100 ◦ C. The electrical conductivity measurements were carried out by two probe method [3].
∗ Corresponding author. Tel.: +91-40-27175257; fax: +91-40-27017944. E-mail address: [email protected] (D. Ravinder).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.126
Table 1 Electrical conductivity data on mixed Cu–Cr ferrites at room temperature Serial no.
Ferrite composition
Electrical conductivity (σ) (Ohm−1 cm−1 )
1 2 3 4 5
Cu0.9 Cr0.1 Fe2 O4 Cu0.8 Cr0.2 Fe2 O4 Cu0.7 Cr0.3 Fe2 O4 Cu0.6 Cr0.4 Fe2 O4 Cu0.5 Cr0.5 Fe2 O4
4.12 9.84 7.06 8.98 6.28
× × × × ×
10−4 10−5 10−6 10−7 10−8
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Fig. 1. Plot of log (σT) vs. temperature for Cu0.9 Cr0.1 Fe2 O4 .
Fig. 2. Plot of log (σT) vs. temperature for Cu0.8 Cr0.2 Fe2 O4 .
D. Ravinder et al. / Journal of Alloys and Compounds 370 (2004) L17–L22
Fig. 3. Plot of log (σT) vs. temperature for Cu0.7 Cr0.3 Fe2 O4 .
Fig. 4. Plot of log (σT) vs. temperature for Cu0.6 Cr0.4 Fe2 O4 .
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Fig. 5. Plot of log (σT) vs. temperature for Cu0.5 Cr0.5 Fe2 O4 .
of slope has occurred. The Curie temperatures for the Cu–Cr ferrite specimens under investigation have been determined by using a gravity method [5]. The transition temperatures Tσ (K) are given in Table 2 along with the Curie temperatures Tc (K). It can be seen from Table 2 that the transition Table 2 Transition temperatures for Cu–Cr ferrites Serial no.
Ferrite composition
Tc (K)
Tσ (K)
1 2 3 4 5
Cu0.9 Cr0.1 Fe2 O4 Cu0.8 Cr0.2 Fe2 O4 Cu0.7 Cr0.3 Fe2 O4 Cu0.6 Cr0.4 Fe2 O4 Cu0.5 Cr0.5 Fe2 O4
652 594 536 478 420
649 598 542 476 421
temperature Tσ (K) corresponds to the magnetic transition, since they are near the observed Curie temperature Tc (K) for all the ferrites under investigation. It can be noted from the figure that the value of Tc (K) decreases with the increase of chromium content. The decrease of Curie temperature with increase of chromium content can be explained on the basis of the number of magnetic ions present in the two sub-lattices and their mutual interactions. As Fe3+ ions are gradually replaced by chromium ions, the number of magnetic ions begin to decrease at both sides, which also weakens the strength of AB exchange interactions of the type 2− 3− Fe3+ A -O FeB . Thus, the thermal energy required to offset the spin alignment decreases, thereby decreasing the Curie temperature. Fig. 6 shows the plot of Curie temperature as a function of composition. It can be seen from the figure that
D. Ravinder et al. / Journal of Alloys and Compounds 370 (2004) L17–L22
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Fig. 6. Plot of Curie temperature vs. Chromium content.
the value of Curie temperature decreases with the increase of chromium content. A similar decrease of the Tc (K) with the composition was also observed by Ahmed et al. [6] in the case of Ni–Al ferrites and Ravinder in Mn–Cd [7] ferrites. The existence of the kinks or transitions in the neighbourhood of the Curie point has been explained by the theory given by Irkin and Turov [8]. It was shown theoretically that on passing through the Curie point a change must occur in the gradient of the straight line [9] and the magnitude of effect depends on the exchange interaction between the outer and inner electrons which alters at the Curie point. The experimental observation of the transition near the Curie point in the case of mixed Li–Mg ferrites is thus in conformity with the theory developed by Irkin and Turov [8]. Similar transitions in the neighbourhood of the Curie point have also been observed by Lanje and Kulkarni
in Ca–La [10] ferrites, and Ravinder and Ravi Kumar [11] in case of cerium substituted Mn–Zn ferrites. Generally, the change of slope is attributed to change in conductivity mechanism. The conduction at lower temperature (above Curie temperature) due to polaron hopping [12–14].
Acknowledgements The authors are grateful to Prof. M. Narsimha Chary, Head, Department of Physics and Prof. P. Venugopal Reddy, Principal, Dr. G. Prasad, Post-Graduate College of Science, Saifabad, Osmania University for their encouragement.
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References [1] J. Kulikouski, A. Lexiecoski, J. Magn. Magn. Mater. 19 (1980) 117. [2] E.E. Riches, in: J.G. Cook (Ed.), Ferrites: A Review of Materials and Applications, Miles and Boons, London, 1972, p. 17. [3] V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi A 38 (1976) 647. [4] L.G. Van Uitert, J. Chem. Phys. 23 (1955) 1883. [5] D. Ravinder, Thesis, Osmania University, Hyderabad, 1988. [6] M.A. Ahmed, M.K. El-Nimr, A. Tawfik, A.M. El-Hasab, Phys. Status Solidi A 123 (1991) 501.
[7] D. Ravinder, Mater. Lett. 44 (2000) 130. [8] Y.P. Irkin, E.A. Turov, Sov. Phys. JETP 33 (1957) 673. [9] J. Smith, H.P.J. Wijn, Ferrites, Philips Technical Library, London, 1959, p. 234. [10] N.Y. Lanje, D.K. Kulkarni, J. Magn. Magn. Mater. 234 (2001) 114. [11] D. Ravinder, B. Ravi Kumar, Mater. Lett. 53 (2003) 1738. [12] M.I. Kringer, Phys. Status Solidi B 79 (1979) 9. [13] M.I. Klings, J. Phys. C 8 (1975) 3595. [14] N.F. Mott, R.W. Gurey, Electronic Process in Ionic Crystals, Oxford University Press, Oxford, London, 1948.
Letter
Electrical conductivity of chromium substituted copper ferrites D. Ravinder∗ , K. Sathi Reddy, P. Mahesh, T. Bhaskar Rao, Y.C. Venudhar Department of Physics, Post-Graduate College of Science, Osmania University, Hyderabad 500 004, Andhra Pradesh, India Received 1 July 2003; received in revised form 23 September 2003; accepted 23 September 2003
Abstract Electrical conductivity of copper–chromium (Cu–Cr) ferrites of various compositions have been investigated as a function of composition and temperature. Plots of log (σT) versus 103 /T are almost linear and have shown a transition near the Curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region. An attempt is made to explain the conduction mechanism in Cu–Cr ferrites. © 2003 Elsevier B.V. All rights reserved. Keywords: Cu–Cr ferrites; Electrical conductivity; Curie temperature; Chromium
1. Introduction
3. Results and discussion
Mixed copper ferrites have been commercially used for many years as high-frequency devices such as radio frequency coils, transformer cores, rod antennas and magnetic cores of read-write heads for high speed digital tapes [1,2]. However, no information is available on electrical conductivity of copper–chromium (Cu–Cr) ferrites in the literature. Moreover, there is a need for a thorough study of electrical properties of these ferrites possessing desired application in microwave devices. Therefore, we have undertaken a systematic study of the electrical conductivity of Cu–Cr ferrites as a function of composition and temperature. The results of such studies are presented in this communication.
Experimental data for the mixed Cu–Cr ferrites are given in Table 1, which includes the compositional formulae of all the ferrites under investigation and the values of electrical conductivity at room temperature. It can be seen from the table that the values of electrical conductivity varies from 6.28 × 10−8 to 4.12 × 10−4 Ohm−1 cm−1 and decreases with increase of chromium content. This observation is in agreement with the result reported by Van Uitert [4] who found that the resistivity of Li–Zn ferrites increased with increase of zinc content. The temperature dependent on electrical conductivity of mixed Cu–Cr ferrites of various compositions has been investigated from room temperature to well beyond the Curie temperature. Plots of log (σT) versus temperature (103 /T) are shown in Figs. 1–5. It can be seen from the figures that the value of log (σT) increases linearly with increasing temperature up to a certain temperature Tσ (K) at which a change
2. Experimental details Mixed copper–chromium ferrites having the chemical formulae Cu1−x Crx Fe2 O4 (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the conventional double sintering ceramic technique. The samples were sintered for 6 h at 1100 ◦ C. The electrical conductivity measurements were carried out by two probe method [3].
∗ Corresponding author. Tel.: +91-40-27175257; fax: +91-40-27017944. E-mail address: [email protected] (D. Ravinder).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.126
Table 1 Electrical conductivity data on mixed Cu–Cr ferrites at room temperature Serial no.
Ferrite composition
Electrical conductivity (σ) (Ohm−1 cm−1 )
1 2 3 4 5
Cu0.9 Cr0.1 Fe2 O4 Cu0.8 Cr0.2 Fe2 O4 Cu0.7 Cr0.3 Fe2 O4 Cu0.6 Cr0.4 Fe2 O4 Cu0.5 Cr0.5 Fe2 O4
4.12 9.84 7.06 8.98 6.28
× × × × ×
10−4 10−5 10−6 10−7 10−8
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Fig. 1. Plot of log (σT) vs. temperature for Cu0.9 Cr0.1 Fe2 O4 .
Fig. 2. Plot of log (σT) vs. temperature for Cu0.8 Cr0.2 Fe2 O4 .
D. Ravinder et al. / Journal of Alloys and Compounds 370 (2004) L17–L22
Fig. 3. Plot of log (σT) vs. temperature for Cu0.7 Cr0.3 Fe2 O4 .
Fig. 4. Plot of log (σT) vs. temperature for Cu0.6 Cr0.4 Fe2 O4 .
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Fig. 5. Plot of log (σT) vs. temperature for Cu0.5 Cr0.5 Fe2 O4 .
of slope has occurred. The Curie temperatures for the Cu–Cr ferrite specimens under investigation have been determined by using a gravity method [5]. The transition temperatures Tσ (K) are given in Table 2 along with the Curie temperatures Tc (K). It can be seen from Table 2 that the transition Table 2 Transition temperatures for Cu–Cr ferrites Serial no.
Ferrite composition
Tc (K)
Tσ (K)
1 2 3 4 5
Cu0.9 Cr0.1 Fe2 O4 Cu0.8 Cr0.2 Fe2 O4 Cu0.7 Cr0.3 Fe2 O4 Cu0.6 Cr0.4 Fe2 O4 Cu0.5 Cr0.5 Fe2 O4
652 594 536 478 420
649 598 542 476 421
temperature Tσ (K) corresponds to the magnetic transition, since they are near the observed Curie temperature Tc (K) for all the ferrites under investigation. It can be noted from the figure that the value of Tc (K) decreases with the increase of chromium content. The decrease of Curie temperature with increase of chromium content can be explained on the basis of the number of magnetic ions present in the two sub-lattices and their mutual interactions. As Fe3+ ions are gradually replaced by chromium ions, the number of magnetic ions begin to decrease at both sides, which also weakens the strength of AB exchange interactions of the type 2− 3− Fe3+ A -O FeB . Thus, the thermal energy required to offset the spin alignment decreases, thereby decreasing the Curie temperature. Fig. 6 shows the plot of Curie temperature as a function of composition. It can be seen from the figure that
D. Ravinder et al. / Journal of Alloys and Compounds 370 (2004) L17–L22
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Fig. 6. Plot of Curie temperature vs. Chromium content.
the value of Curie temperature decreases with the increase of chromium content. A similar decrease of the Tc (K) with the composition was also observed by Ahmed et al. [6] in the case of Ni–Al ferrites and Ravinder in Mn–Cd [7] ferrites. The existence of the kinks or transitions in the neighbourhood of the Curie point has been explained by the theory given by Irkin and Turov [8]. It was shown theoretically that on passing through the Curie point a change must occur in the gradient of the straight line [9] and the magnitude of effect depends on the exchange interaction between the outer and inner electrons which alters at the Curie point. The experimental observation of the transition near the Curie point in the case of mixed Li–Mg ferrites is thus in conformity with the theory developed by Irkin and Turov [8]. Similar transitions in the neighbourhood of the Curie point have also been observed by Lanje and Kulkarni
in Ca–La [10] ferrites, and Ravinder and Ravi Kumar [11] in case of cerium substituted Mn–Zn ferrites. Generally, the change of slope is attributed to change in conductivity mechanism. The conduction at lower temperature (
Acknowledgements The authors are grateful to Prof. M. Narsimha Chary, Head, Department of Physics and Prof. P. Venugopal Reddy, Principal, Dr. G. Prasad, Post-Graduate College of Science, Saifabad, Osmania University for their encouragement.
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References [1] J. Kulikouski, A. Lexiecoski, J. Magn. Magn. Mater. 19 (1980) 117. [2] E.E. Riches, in: J.G. Cook (Ed.), Ferrites: A Review of Materials and Applications, Miles and Boons, London, 1972, p. 17. [3] V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi A 38 (1976) 647. [4] L.G. Van Uitert, J. Chem. Phys. 23 (1955) 1883. [5] D. Ravinder, Thesis, Osmania University, Hyderabad, 1988. [6] M.A. Ahmed, M.K. El-Nimr, A. Tawfik, A.M. El-Hasab, Phys. Status Solidi A 123 (1991) 501.
[7] D. Ravinder, Mater. Lett. 44 (2000) 130. [8] Y.P. Irkin, E.A. Turov, Sov. Phys. JETP 33 (1957) 673. [9] J. Smith, H.P.J. Wijn, Ferrites, Philips Technical Library, London, 1959, p. 234. [10] N.Y. Lanje, D.K. Kulkarni, J. Magn. Magn. Mater. 234 (2001) 114. [11] D. Ravinder, B. Ravi Kumar, Mater. Lett. 53 (2003) 1738. [12] M.I. Kringer, Phys. Status Solidi B 79 (1979) 9. [13] M.I. Klings, J. Phys. C 8 (1975) 3595. [14] N.F. Mott, R.W. Gurey, Electronic Process in Ionic Crystals, Oxford University Press, Oxford, London, 1948.