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Electrical conductivity of gadolinium substituted Mn–Zn ferrite
June 2000
Materials Letters 44 Ž2000. 253–255 www.elsevier.comrlocatermatlet
Electrical conductivity of gadolinium substituted Mn–Zn ferrite D. Ravinder ),1, K. Suresh Department of Physics, Osmania UniÕersity, Hyderabad 500 007 (A.P.), India Received 19 September 1999; accepted 14 January 2000
Abstract Electrical conductivity of polycrystalline gadolinium substituted manganese–zinc ŽMn–Zn. ferrite was investigated from room temperature to well beyond the Curie temperature. Plots of logŽ s T . vs. 1000rT show 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 these ferrites. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Mn–Zn ferrites; Electrical conductivity; Activation energies; Hopping of electrons; Conduction mechanism
1. Introduction Recently, many efforts have been made in developing low-power loss materials of manganese–zinc ŽMn–Zn. ferrites at high frequency, in accordance with the demand for miniaturization of electrical devices w1,2x. Mn–Zn ferrites are soft ferrites characterised by high magnetic permeabilities and low losses and have numerous electronic applications over a wide range of frequencies. Mn–Zn ferrites are mainly used as inductor and transformer cores and in switch mode power supplies ŽSMPS.. Previously, we have investigated the electrical w3,4x and elastic w5x studies of mixed Mn–Zn ferrites as a function of composition and temperature. In view of their extensive applications, it is thought desirable to study the influence of rare earth ion substituting iron ions in Mn 0.58 Zn 0.37 Fe 2.05 O4
ferrite on electrical conductivity. The results of such a study are presented in this paper. 2. Experimental 2.1. Materials Polycrystalline samples of gadolinium substituted Mn–Zn ferrites having the chemical formulae Mn 0.58 Zn 0.37 Fe 2.05yxGd xO4 where x s 0 and 0.4 were prepared by the conventional double sintering method using ferric oxide, zinc oxide, manganese carbonate and gadolinium oxide. The samples were pre-sintered for 4 h in air at 8008C. Final sintering of the specimens were carried out for 4 h at 12008C. X-ray diffractometer studies of the samples using CuK a radiation confirmed the spinel formation. 2.2. Methods
)
Corresponding author. Department of Physics, Post-Graduate College of Science, Saifabad, Hyderabad-4, A.P., India. 1
Electrical conductivity and thermoelectric power measurements were made by the two probe w6x and differential w7x methods, respectively.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 3 8 - 0
D. RaÕinder, K. Sureshr Materials Letters 44 (2000) 253–255
254
3. Results and discussion
3.1. Electrical conductiÕity at room temperature The electrical conductivity values of Mn–Zn and Mn–Zn–Gd ferrites are given in Table 1. It can be seen from the table that when 0.4 mol of Gd is added, the conductivity decreases from 4.61 = 10y6 Vy1 cmy1 to 1.08 = 10y8 Vy1 cmy1 . The observed variation of electrical conductivity with Gd substitution could be explained on the basis of Verwey mechanism w8x and hopping mechanism w9x. The addition of Gd substitutes on the octahedral sites leading to replacement of Fe 3q ions. The Gd ions reduce the number of Fe 3q ions at the octahedral sites, which reduces the number of hopping electrons between Fe 3q and Fe 2q ions leading to reduced conductivity.
nism for n-type is predominantly due to hopping of electrons w9x from Fe 2q to Fe 3q ions: Fe 3qq e
3.2. Thermoelectric power at room temperature The thermoelectric power Ž S . is measured according to equation: Ss
Fig. 1. Plot of log Ž s T . vs. 1000r T for Mn 0.58 Zn 0.37 Fe 2.05 O4 .
m Fe
2q
3.3. Variation of electrical conductiÕity with temperature
V DT
where V is the thermo-voltage determined from the polarity of the cold end of the specimen as the charge carriers diffuse from the hot to the cold point. The Seebeck coefficient ŽTable 1. for the above ferrites under investigation is negative indicating that electrons are majority carriers. On the basis of its negative sign, these ferrites have been classified as n-type semiconductors. Thus, the conduction mecha-
The variation of electrical conductivity with temperature is shown in Figs. 1 and 2. It can be understood from the plots of log Ž s T . vs. 1000rT that the electrical conductivity increases with increasing temperature continuously with a change of slope at the magnetic transition temperature Ts ŽK., which is marked by arrows. The values of Curie temperature Tc ŽK. obtained by the gravity method are also given in Table 1. It can be seen from the table that the values of Ts ŽK. and Tc ŽK. are in good agreement
Table 1 Experimental data on Mn–Zn–Gd ferrite system at room temperature Sl. no.
Ferrite composition
Seebeck coefficient Ž S . ŽmVrK.
Electrical conductivity Ž s . Ž Vy1 cmy1 .
Ts ŽK.
Tc ŽK.
Activation energy Ferri-magnetic Para-magnetic region ŽeV. region ŽeV.
1 2
Mn 0.58 Zn 0.37 Fe 2.05 O4 Mn 0.58 Zn 0.37 Fe1.65 Gd 0.4 O4
y3 y27
4.61 = 10y6 1.08 = 10y8
471 443
474 445
0.32 0.36
0.44 0.48
D. RaÕinder, K. Sureshr Materials Letters 44 (2000) 253–255
255
to 0.48 eV and high activation energy goes hand in hand with a low conductivity of the ferrites w13–15x. Acknowledgements The authors are grateful to the Department of Science and Technology, New Delhi for financial assistance. The authors also thank Dr. P. Judson, Prof. K.S.N. Murthy for their encouragement. References Fig. 2. Plot of log Ž s T . vs. 1000r T for Mn 0.58 Zn 0.37 Fe1.65 Gd 0.4 O4 .
with each other with in the limits of experimental errors thereby indicating that the change of slope may be due to the ferri- to para-magnetic transition. Similar transitions in the neighbourhood of Curie point have also been observed in various ferrite systems w10–12x. The temperature variation of electrical conductivity is given by:
ss
A T
ž
exp y
E KT
/
where A is a constant, K is Boltzmann’s constant and E is the activation energy. The activation energies in the ferrimagnetic region and paramagnetic regions are calculated from the slopes of log Ž s T . vs. 1000rT. The calculated values of the activation energies are given in Table 1. It is evident that the values of activation energies are ranging from 0.32
w1x D. Ravinder, K. Latha, J. Appl. Phys. 75 Ž1994. 6118. w2x Y. Sakaki, M. Yoshida, T. Sato, IEEE Trans. Magn. MAG-29 Ž1993. 3517. w3x D. Ravinder, A.V. Ramana Reddy, Mater. Lett. 38 Ž1999. 265. w4x K. Latha, D. Ravinder, Phys. Status Solidi A 139 Ž1993. K109. w5x D. Ravinder, T. Alivelumanga, Mater. Lett. 37 Ž1998. 51. w6x V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi A 38 Ž1976. 647. w7x V.D. Reddy, M.A. Malik, P.V. Reddy, Mater. Sci. Eng., B 8 Ž1991. 295. w8x E.J.W. Verwey, J.H. De Boer, Rec. Thav. Chim. Phys-Beas. 55 Ž1935. 531. w9x L.G. Van Uitert, Physica 23 Ž1955. 1883. w10x M. Pal, P. Brahma, D. Chakravorthy, J. Magn. Magn. Mater. 152 Ž1996. 370. w11x M.A. Elihiti, J. Magn. Magn. Mater. 136 Ž1994. 138. w12x D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 Ž1990. 963. w13x A.A. Samokhralov, A.G. Rustmov, Sov. Phys. Solid State 7 Ž1965. 961. w14x D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 Ž1990. 1079. w15x P. Venugopal Reddy, T. Seshagiri Rao, S.M.D. Rao, J. Less-Common Met. 90 Ž1983. 243.
Materials Letters 44 Ž2000. 253–255 www.elsevier.comrlocatermatlet
Electrical conductivity of gadolinium substituted Mn–Zn ferrite D. Ravinder ),1, K. Suresh Department of Physics, Osmania UniÕersity, Hyderabad 500 007 (A.P.), India Received 19 September 1999; accepted 14 January 2000
Abstract Electrical conductivity of polycrystalline gadolinium substituted manganese–zinc ŽMn–Zn. ferrite was investigated from room temperature to well beyond the Curie temperature. Plots of logŽ s T . vs. 1000rT show 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 these ferrites. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Mn–Zn ferrites; Electrical conductivity; Activation energies; Hopping of electrons; Conduction mechanism
1. Introduction Recently, many efforts have been made in developing low-power loss materials of manganese–zinc ŽMn–Zn. ferrites at high frequency, in accordance with the demand for miniaturization of electrical devices w1,2x. Mn–Zn ferrites are soft ferrites characterised by high magnetic permeabilities and low losses and have numerous electronic applications over a wide range of frequencies. Mn–Zn ferrites are mainly used as inductor and transformer cores and in switch mode power supplies ŽSMPS.. Previously, we have investigated the electrical w3,4x and elastic w5x studies of mixed Mn–Zn ferrites as a function of composition and temperature. In view of their extensive applications, it is thought desirable to study the influence of rare earth ion substituting iron ions in Mn 0.58 Zn 0.37 Fe 2.05 O4
ferrite on electrical conductivity. The results of such a study are presented in this paper. 2. Experimental 2.1. Materials Polycrystalline samples of gadolinium substituted Mn–Zn ferrites having the chemical formulae Mn 0.58 Zn 0.37 Fe 2.05yxGd xO4 where x s 0 and 0.4 were prepared by the conventional double sintering method using ferric oxide, zinc oxide, manganese carbonate and gadolinium oxide. The samples were pre-sintered for 4 h in air at 8008C. Final sintering of the specimens were carried out for 4 h at 12008C. X-ray diffractometer studies of the samples using CuK a radiation confirmed the spinel formation. 2.2. Methods
)
Corresponding author. Department of Physics, Post-Graduate College of Science, Saifabad, Hyderabad-4, A.P., India. 1
Electrical conductivity and thermoelectric power measurements were made by the two probe w6x and differential w7x methods, respectively.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 3 8 - 0
D. RaÕinder, K. Sureshr Materials Letters 44 (2000) 253–255
254
3. Results and discussion
3.1. Electrical conductiÕity at room temperature The electrical conductivity values of Mn–Zn and Mn–Zn–Gd ferrites are given in Table 1. It can be seen from the table that when 0.4 mol of Gd is added, the conductivity decreases from 4.61 = 10y6 Vy1 cmy1 to 1.08 = 10y8 Vy1 cmy1 . The observed variation of electrical conductivity with Gd substitution could be explained on the basis of Verwey mechanism w8x and hopping mechanism w9x. The addition of Gd substitutes on the octahedral sites leading to replacement of Fe 3q ions. The Gd ions reduce the number of Fe 3q ions at the octahedral sites, which reduces the number of hopping electrons between Fe 3q and Fe 2q ions leading to reduced conductivity.
nism for n-type is predominantly due to hopping of electrons w9x from Fe 2q to Fe 3q ions: Fe 3qq e
3.2. Thermoelectric power at room temperature The thermoelectric power Ž S . is measured according to equation: Ss
Fig. 1. Plot of log Ž s T . vs. 1000r T for Mn 0.58 Zn 0.37 Fe 2.05 O4 .
m Fe
2q
3.3. Variation of electrical conductiÕity with temperature
V DT
where V is the thermo-voltage determined from the polarity of the cold end of the specimen as the charge carriers diffuse from the hot to the cold point. The Seebeck coefficient ŽTable 1. for the above ferrites under investigation is negative indicating that electrons are majority carriers. On the basis of its negative sign, these ferrites have been classified as n-type semiconductors. Thus, the conduction mecha-
The variation of electrical conductivity with temperature is shown in Figs. 1 and 2. It can be understood from the plots of log Ž s T . vs. 1000rT that the electrical conductivity increases with increasing temperature continuously with a change of slope at the magnetic transition temperature Ts ŽK., which is marked by arrows. The values of Curie temperature Tc ŽK. obtained by the gravity method are also given in Table 1. It can be seen from the table that the values of Ts ŽK. and Tc ŽK. are in good agreement
Table 1 Experimental data on Mn–Zn–Gd ferrite system at room temperature Sl. no.
Ferrite composition
Seebeck coefficient Ž S . ŽmVrK.
Electrical conductivity Ž s . Ž Vy1 cmy1 .
Ts ŽK.
Tc ŽK.
Activation energy Ferri-magnetic Para-magnetic region ŽeV. region ŽeV.
1 2
Mn 0.58 Zn 0.37 Fe 2.05 O4 Mn 0.58 Zn 0.37 Fe1.65 Gd 0.4 O4
y3 y27
4.61 = 10y6 1.08 = 10y8
471 443
474 445
0.32 0.36
0.44 0.48
D. RaÕinder, K. Sureshr Materials Letters 44 (2000) 253–255
255
to 0.48 eV and high activation energy goes hand in hand with a low conductivity of the ferrites w13–15x. Acknowledgements The authors are grateful to the Department of Science and Technology, New Delhi for financial assistance. The authors also thank Dr. P. Judson, Prof. K.S.N. Murthy for their encouragement. References Fig. 2. Plot of log Ž s T . vs. 1000r T for Mn 0.58 Zn 0.37 Fe1.65 Gd 0.4 O4 .
with each other with in the limits of experimental errors thereby indicating that the change of slope may be due to the ferri- to para-magnetic transition. Similar transitions in the neighbourhood of Curie point have also been observed in various ferrite systems w10–12x. The temperature variation of electrical conductivity is given by:
ss
A T
ž
exp y
E KT
/
where A is a constant, K is Boltzmann’s constant and E is the activation energy. The activation energies in the ferrimagnetic region and paramagnetic regions are calculated from the slopes of log Ž s T . vs. 1000rT. The calculated values of the activation energies are given in Table 1. It is evident that the values of activation energies are ranging from 0.32
w1x D. Ravinder, K. Latha, J. Appl. Phys. 75 Ž1994. 6118. w2x Y. Sakaki, M. Yoshida, T. Sato, IEEE Trans. Magn. MAG-29 Ž1993. 3517. w3x D. Ravinder, A.V. Ramana Reddy, Mater. Lett. 38 Ž1999. 265. w4x K. Latha, D. Ravinder, Phys. Status Solidi A 139 Ž1993. K109. w5x D. Ravinder, T. Alivelumanga, Mater. Lett. 37 Ž1998. 51. w6x V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi A 38 Ž1976. 647. w7x V.D. Reddy, M.A. Malik, P.V. Reddy, Mater. Sci. Eng., B 8 Ž1991. 295. w8x E.J.W. Verwey, J.H. De Boer, Rec. Thav. Chim. Phys-Beas. 55 Ž1935. 531. w9x L.G. Van Uitert, Physica 23 Ž1955. 1883. w10x M. Pal, P. Brahma, D. Chakravorthy, J. Magn. Magn. Mater. 152 Ž1996. 370. w11x M.A. Elihiti, J. Magn. Magn. Mater. 136 Ž1994. 138. w12x D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 Ž1990. 963. w13x A.A. Samokhralov, A.G. Rustmov, Sov. Phys. Solid State 7 Ž1965. 961. w14x D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 Ž1990. 1079. w15x P. Venugopal Reddy, T. Seshagiri Rao, S.M.D. Rao, J. Less-Common Met. 90 Ž1983. 243.