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Effect of Cu substitution on the magnetic and electrical properties of Ni–Zn ferrite synthesised by soft chemical method
Materials Chemistry and Physics 59 (1999) 1±5
Effect of Cu substitution on the magnetic and electrical properties of Ni±Zn ferrite synthesised by soft chemical method J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, A. Mitra, S.R. Sainkar, P.S. Anil Kumar1, S.K. Date* Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received 2 June 1998; received in revised form 2 December 1998; accepted 2 January 1999
Abstract Copper substituted Ni±Zn ferrite powders of Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 composition are synthesised at 808C by soft chemical route. Their electrical, magnetic, and microstructural properties are studied using XRD, VSM and SEM techniques. These studies revealed that the bulk density, dc electrical resistivity and initial permeability increased considerably with the optimum copper concentration of x 0.2. The major achievement was the decrease in its loss factor in the MHz frequency region. The improvement in the electrical and magnetic properties was attributed to the uniform grain-growth/microstructure of the copper substituted ferrite achieved at a lower sintering temperature T 10008C. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Ni±Zn ferrite; Soft chemical method; Magnetic properties
1. Introduction The pure and substituted Ni±Zn ferrites have been the subject of extensive studies because of their applications in electronic devices [1,2]. Efforts are being made to develop a low-power loss material operating in the MHz region in accordance with the miniaturisation of electronic devices. The magnetic properties of the polycrystalline ferrites are determined by their chemical composition and the microstructure (grains, grain boundaries and pores). The studies on microstructure and composition related magnetic properties have been reported for Ni±Zn ferrites by several researchers [3±7] in the ®eld of materials. These ferrites are produced by conventional ceramic processes which are known to have some inherent drawbacks resulting in nonreproducibility and deterioration in their magnetic properties. Wet-chemical or solution methods have overcome these problems to improve the performance of Ni±Zn ferrites synthesised at low temperature [8]. In our laboratory, we have succeeded in preparing active powders of Ni±Zn ferrite at 808C by controlled chemical coprecipitation method and have extensively characterised by various techniques to *Corresponding author. Tel.: +91-020-393849; e-mail: [email protected] 1 Centre for Advanced Studies in Material Science and Solid State Physics, Department of Physics, University of Pune, Pune 411 007, India.
study their structural, electrical and magnetic properties [9]. These studies revealed the correlation of structure property ± processing in the ferrite system. The frequency dependence of the initial permeability (i) and loss factor (tan /i) of the different samples of Ni±Zn ferrite composition Ni0.8Zn0.2Fe2O4 sintered under different set of conditions, were explained on the basis of their microstructural features. The low-temperature synthesis of Ni±Zn ferrite at 808C produced small ferrite particles (50 nm) which gave sintered compacts of bulk density 4.5 g cmÿ3 at a sintering temperature of 11008C/8 h. However, the maximum grain size obtained at 11008C was 700 nm which was insuf®cient to give permeability values higher than 50 in 20± 70 MHz frequency range. This was obviously due to the absence of contribution from domain wall motion to the permeability value as explained in our earlier communication [10]. Nakamura [11] has reported very recently, the lowtemperature sintering of Ni±Zn±Cu ferrite at 9008C resulting in magnetic permeability over 200 at 1 MHz and density greater than 4.5 gcmÿ3. The high permeability has been attributed to the domain wall contribution which is controlled not only by the higher post sintering density but also by increased grain size as suggested by Globus [12]. In the present work we have synthesised Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 at 808C and sintered in the device form. The choice of Cu is based on the fact that: (1) it
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(99)00019-X
2
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
increases the density and decreases the sintering temperature; (2) it increases the resistivity of the material. At an optimum doping of Cu we have succeeded in developing a low-power loss material operating in the MHz frequency region. 2. Experimental 2.1. Synthesis The ultra®ne ferrite powders of various compositions Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 were prepared using concentrated solutions of LR grade chemicals ± ferric nitrate, zinc nitrate, nickel nitrate, copper sulphate and sodium hydroxide ± by solution technique reported earlier [9]. The substitution of Ni by Cu in the required proportion was achieved using optimum amount of copper sulphate solution during precipitation. The precipitate was washed ÿ free from NOÿ 3 and SO4 ions and dried at 808C/36 h in air using an electrically heated oven followed by calcination at 2508C/4 h. The powders, thus, obtained were characterised by XRD and magnetisation measurements. The calcined powder was used to make toroids and pellets using 2% PVA as binder and compacting on a hydraulic press. The compacts (greens) were sintered at 10008C/4 h in air under the optimised sintering conditions. The initial heating rate was 508C/h upto 6008C to burn out the binder slowly, then the rate was increased to 1008C/h to reach the sintering temperature and soaked for 4 h before the furnace was switched off. The furnace was allowed to cool at its natural rate to room temperature. The bulk densities of the sintered samples were measured along with the other properties ± the saturation magnetisation (Ms), Curie temperature (Tc), dc electrical resistivity and magnetic permeability/loss factor. 2.2. Characterisation X-ray diffractograms were recorded of the as-dried (808C), calcined (2508C) and the sintered (10008C) ferrite samples using Philips PW 1730 X-ray diffractometer with Cu Ka radiation. Saturation magnetisation measurements were carried out using a EG & G-PAR model 4500 vibrating sample magnetometer (USA) at room temperature. The Curie temperature (Tc) was recorded from the temperature dependence of magnetisation. The dc resistivity measurements were done on the General Radio LCR bridge model 1608A using sintered ferrite pellets. Silver paste was applied to both the faces of the pellet for improved ohmic contacts before it was sandwiched between the two electrodes of the sample holder. XRD patterns are given in Fig. 1 while saturation magnetisation, Curie temperature, initial permeability, dc resistivity and bulk density data are given in Table 1. The initial permeability (i) and loss factor were evaluated using HP Q-meter model 4342a in the frequency range of 20±70 MHz on toroidal ferrite samples. Micro-
Fig. 1. X-ray diffraction patterns for the sintered compositions of Ni0.8ÿxCuxZn0.2Fe2O4, 0 x 0.4.
structural features of the sintered samples (x 0 to 0.4) were obtained with the help of Leica-Cambridge 440 scanning electron microscope. The micrographs were taken on the fractured surface of the sintered pellets (Fig. 2). 3. Results and discussion X-ray diffractograms of the as-dried (808C) and calcined (2508C) ferrite powders showed broad peaks indicating ®ne particle nature of the particles. The patterns matched well with the characteristic re¯ections of Ni±Zn ferrite [9] with no unidenti®ed extra lines. XRD patterns of the sintered (10008C) ferrite samples showed intense sharp peaks showing well-crystalline single phase ferrite for 0 x 0.2 compositions (Fig. 1). A second phase of CuO and CuFe2O4
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
3
Table 1 Electrical and magnetic properties of Ni0.8ÿxCuxZn0.2Fe2O4 Cu content x
0.0 0.1 0.2 0.3 0.4
Magnetization (Ms) (emu gÿ1) 2508C calcined
10008C sintered
46.7 38.7 38.7 38.0 38.2
72.2 68.6 67.8 66.0 66.3
Density (g cmÿ3)
Tc (C)
dc resistivity (ohm cm)
3.84 4.69 5.02 4.92 4.93
490 474 457 446 422
9.6 106 6.4 107 7.3 107 4.2 106 1.5 105
i
35 63 100 54 47
Fig. 2. Scanning electron micrographs of the samples of Cu substituted Ni±Zn ferrite.
Loss factor at flossmin
2.72 10ÿ4 3.65 10ÿ4 9.53 10ÿ5 1.26 10ÿ3 2.62 10ÿ3
4
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
appeared in the patterns of ferrite composition with x 0.3 as shown by the additional characteristic re¯ections at 2 38.28 and 36.07, respectively, with a relatively small intensity. The XRD results clearly indicate that the substitution of Ni by copper takes place only upto x 0.2, while for higher values of x, Cu forms a second phase of CuO/CuFe2O4 as evidenced by the XRD results. Another effect of increasing copper substitution in Ni±Zn ferrite (x 0 to 0.4) is increased grain growth as seen from the scanning electron micrographs (Fig. 2). Uniform grains of 500 nm are seen for x 0 composition and are progressively increased to 2 mm for x 0.4 composition. The magnetisation measurements of as-dried calcined, and sintered samples showed the characteristic hysteresis loops indicating ferrimagnetic nature of the ferrites. The saturation magnetisation (Ms) of these materials progressively increased at the higher temperatures of calcination (2508C) and sintering (10008C). The values of Ms of calcined and sintered samples are given in Table 1. Nickel ferrite is reported [13] to be 80% inverse spinel with equivalent Ni2 ions on B site. The decrease in Ms may be attributed to the substitution of Ni by Cu having a lower magnetic moment. The temperature variation of magnetisation was carried out to record the Curie temperature (Tc) of all the samples. The values of Tc are given in Table 1 which clearly show that the Tc has progressively decreased from 4908C to 4228C for the compositions 0 x 0.4. This result con®rms the substitution of Ni by Cu and its effect on AB and BB interactions resulting in the decrease of magnetisation and Curie temperature of Ni±Zn ferrite. The substitution of nickel by copper has also affected the bulk density and the resistivity of the ferrite. Table 1 shows the bulk density of the unsubstituted ferrite (x 0) 3.84 g cmÿ3 as against 5.02 g cmÿ3 of Cu-substituted
ferrite with x 0.2. This is the maximum density achieved for this composition on sintering at 10008C. The increase is attributed to the increase in grain size and reduction of the pores in the ferrite microstructure as seen in scanning electron micrograph (Fig. 2) of the respective samples. The increased density also helps in explaining the higher resistivity value obtained for composition with x 0.2. The high density ferrite containing Ni2 ions is expected to resist the oxidation of Ni2 to Ni3 (which normally takes place during the cooling cycle of the sintering step) and helps in decreasing the hopping conduction of electrons from Ni2 to Ni3 thereby increasing its resistivity. However, the conduction is increased with higher values of x 0.3 which may be attributed to the formation of a second phase namely CuO and CuFe2O4 for x 0.3 as detected by XRD. It is reported [14] that at a temperature little over 10008C CuO decomposes to Cu2O, and due to the formation of Cu1 and Cu2 states, the conduction is increased decreasing its resistivity value. Besides, CuFe2O4 has much lower resistivity of 103±104 ohm cm as compared to that of Ni±Zn ferrite composition having 106 ohm cm. Moreover, as CuO segregates out from the ferrite phase the remaining material is expected to be an iron excess compound which will decrease the resistivity of the material. An increase in permeability (i) is observed for all the substituted compositions and highest value obtained was 100 for the composition with x 0.2. The permeability can be expressed as 1 spin dw, where spin is the susceptibility due to spins and dw susceptibility due to domain wall motion [15]. In single-domain particles the domain wall motion is absent and in the unsubstituted sample the particles are single domain as they are of 500 nm size. Hence, the low value of permeability can be realised. But for the Cu substituted samples, multidomain
Fig. 3. Variation of loss factor with frequency for Ni0.8ÿxCuxZn0.2Fe2O4, 0 x 0.4.
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
particles of 3m size [16] are obtained which result in higher permeability values due to domain wall contribution. Another important factor which gives rise to an increase in the permeability is the increase in the density of the composition x 0.2. An increase in the density results in the reduction of demagnetizing ®eld due to pores which in turn increases the permeability. The values of i, at flossmin are given in Table 1. The flossmin is the frequency at which the minimum value of loss factor is obtained for a given ferrite composition. The decrease in the permeability for x 0.3 as compared to that of x 0.2, can be a manifestation of the formation of copper ferrite. In Fig. 3 the frequency dependence of the loss factor is given. The values of loss factor revealed that: (1) they are nearly constant for x 0 and 0.1, (2) reached a minimum value of 9.53 10ÿ5 for x 0.2 and (3) increased much faster (10ÿ3) with higher values of x. The increase in loss factor for higher values of x may be attributed to the second phase (CuO/CuFe2O4) which acts as a microstructural imperfection to promote magnetic losses. The substantial reduction in the loss factor for the x 0.2 composition can be attributed to the increase in resistivity. An increase in resistivity reduces the eddy current loss which is the main loss mechanism at MHz frequency region. 4. Conclusion The limiting amount of copper substitution is favourable for the grain growth of Ni±Zn ferrite. It helps in increasing the bulk density and improve the magnetic and electrical properties of Ni0.8Zn0.2Fe2O4 composition at lower sintering temperature of 10008C. Major achievements are: (1) the synthesis of Ni±Cu±Zn ferrite at 808C and (2) the devel-
5
opment of a low-power loss material operating in the MHz frequency region. Acknowledgements P.S.A. Kumar is grateful to UGC, India, for ®nancial support. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Abraham, Am. Ceram. Soc. Bull. 73 (1994) 62. T. Ochiai, Proc. ICF-7 Abstracts 3-2A1, Bordeaux, France, 1996, p. 12. D.C. Khan, M. Misra, A.R. Das, J. Appl. Phys. 53 (1982) 2722. M. Pal, P. Brahma, D. Chakravorty, J. Magn. Magn. Mater. 152 (1996) 370. J.H. Nam, W.G. Hur, J.H. Oh, J. Appl. Phys. 81 (1997) 4794. H. Igarashi, K. Okazaki, J. Am. Ceram. Soc. 60 (1977) 51. J.J. Shrotri, A.G. Bagul, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Ferrites: Proc. ICF6, in: T. Yamaguchi, M. Abe (Eds.), The Japan Society of Powder and Powder Metallurgy, Tokyo/Kyoto, 1992, p. 129. T. Kodama, Y. Wada, T. Yamamoto, M. Tsuji, Y. Tamaura, J. Mater. Chem. 5 (1995) 1413. P.S. Anil Kumar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Mater. Lett. 27 (1996) 293. P.S. Anil Kumar, J.J. Shrotri, C.E. Deshpande, S.K. Date, J. Appl. Phys. 81 (1997) 4788. T. Nakamura, J. Magn. Mater. 168 (1997) 285. A. Globus, J. Phys. Suppl. C 1 (1977) 1. J.M. Hastings, L.M. Corliss, Rev. Mod. Phys. 102 (1953) 1460. L. G van Uitert, J. Appl. Phys. 27 (1956) 723. P.S. Anil Kumar, S.R. Sainkar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, J. Appl. Phys. 83 (1998) 6864. P.J. van der Zaag, P.J. van der Valk, M.Th. Rekveldt, Appl. Phys. Lett. 69 (1996) 2927.
Effect of Cu substitution on the magnetic and electrical properties of Ni±Zn ferrite synthesised by soft chemical method J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, A. Mitra, S.R. Sainkar, P.S. Anil Kumar1, S.K. Date* Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received 2 June 1998; received in revised form 2 December 1998; accepted 2 January 1999
Abstract Copper substituted Ni±Zn ferrite powders of Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 composition are synthesised at 808C by soft chemical route. Their electrical, magnetic, and microstructural properties are studied using XRD, VSM and SEM techniques. These studies revealed that the bulk density, dc electrical resistivity and initial permeability increased considerably with the optimum copper concentration of x 0.2. The major achievement was the decrease in its loss factor in the MHz frequency region. The improvement in the electrical and magnetic properties was attributed to the uniform grain-growth/microstructure of the copper substituted ferrite achieved at a lower sintering temperature T 10008C. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Ni±Zn ferrite; Soft chemical method; Magnetic properties
1. Introduction The pure and substituted Ni±Zn ferrites have been the subject of extensive studies because of their applications in electronic devices [1,2]. Efforts are being made to develop a low-power loss material operating in the MHz region in accordance with the miniaturisation of electronic devices. The magnetic properties of the polycrystalline ferrites are determined by their chemical composition and the microstructure (grains, grain boundaries and pores). The studies on microstructure and composition related magnetic properties have been reported for Ni±Zn ferrites by several researchers [3±7] in the ®eld of materials. These ferrites are produced by conventional ceramic processes which are known to have some inherent drawbacks resulting in nonreproducibility and deterioration in their magnetic properties. Wet-chemical or solution methods have overcome these problems to improve the performance of Ni±Zn ferrites synthesised at low temperature [8]. In our laboratory, we have succeeded in preparing active powders of Ni±Zn ferrite at 808C by controlled chemical coprecipitation method and have extensively characterised by various techniques to *Corresponding author. Tel.: +91-020-393849; e-mail: [email protected] 1 Centre for Advanced Studies in Material Science and Solid State Physics, Department of Physics, University of Pune, Pune 411 007, India.
study their structural, electrical and magnetic properties [9]. These studies revealed the correlation of structure property ± processing in the ferrite system. The frequency dependence of the initial permeability (i) and loss factor (tan /i) of the different samples of Ni±Zn ferrite composition Ni0.8Zn0.2Fe2O4 sintered under different set of conditions, were explained on the basis of their microstructural features. The low-temperature synthesis of Ni±Zn ferrite at 808C produced small ferrite particles (50 nm) which gave sintered compacts of bulk density 4.5 g cmÿ3 at a sintering temperature of 11008C/8 h. However, the maximum grain size obtained at 11008C was 700 nm which was insuf®cient to give permeability values higher than 50 in 20± 70 MHz frequency range. This was obviously due to the absence of contribution from domain wall motion to the permeability value as explained in our earlier communication [10]. Nakamura [11] has reported very recently, the lowtemperature sintering of Ni±Zn±Cu ferrite at 9008C resulting in magnetic permeability over 200 at 1 MHz and density greater than 4.5 gcmÿ3. The high permeability has been attributed to the domain wall contribution which is controlled not only by the higher post sintering density but also by increased grain size as suggested by Globus [12]. In the present work we have synthesised Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 at 808C and sintered in the device form. The choice of Cu is based on the fact that: (1) it
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(99)00019-X
2
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
increases the density and decreases the sintering temperature; (2) it increases the resistivity of the material. At an optimum doping of Cu we have succeeded in developing a low-power loss material operating in the MHz frequency region. 2. Experimental 2.1. Synthesis The ultra®ne ferrite powders of various compositions Ni0.8ÿxCuxZn0.2Fe2O4 with 0 x 0.4 were prepared using concentrated solutions of LR grade chemicals ± ferric nitrate, zinc nitrate, nickel nitrate, copper sulphate and sodium hydroxide ± by solution technique reported earlier [9]. The substitution of Ni by Cu in the required proportion was achieved using optimum amount of copper sulphate solution during precipitation. The precipitate was washed ÿ free from NOÿ 3 and SO4 ions and dried at 808C/36 h in air using an electrically heated oven followed by calcination at 2508C/4 h. The powders, thus, obtained were characterised by XRD and magnetisation measurements. The calcined powder was used to make toroids and pellets using 2% PVA as binder and compacting on a hydraulic press. The compacts (greens) were sintered at 10008C/4 h in air under the optimised sintering conditions. The initial heating rate was 508C/h upto 6008C to burn out the binder slowly, then the rate was increased to 1008C/h to reach the sintering temperature and soaked for 4 h before the furnace was switched off. The furnace was allowed to cool at its natural rate to room temperature. The bulk densities of the sintered samples were measured along with the other properties ± the saturation magnetisation (Ms), Curie temperature (Tc), dc electrical resistivity and magnetic permeability/loss factor. 2.2. Characterisation X-ray diffractograms were recorded of the as-dried (808C), calcined (2508C) and the sintered (10008C) ferrite samples using Philips PW 1730 X-ray diffractometer with Cu Ka radiation. Saturation magnetisation measurements were carried out using a EG & G-PAR model 4500 vibrating sample magnetometer (USA) at room temperature. The Curie temperature (Tc) was recorded from the temperature dependence of magnetisation. The dc resistivity measurements were done on the General Radio LCR bridge model 1608A using sintered ferrite pellets. Silver paste was applied to both the faces of the pellet for improved ohmic contacts before it was sandwiched between the two electrodes of the sample holder. XRD patterns are given in Fig. 1 while saturation magnetisation, Curie temperature, initial permeability, dc resistivity and bulk density data are given in Table 1. The initial permeability (i) and loss factor were evaluated using HP Q-meter model 4342a in the frequency range of 20±70 MHz on toroidal ferrite samples. Micro-
Fig. 1. X-ray diffraction patterns for the sintered compositions of Ni0.8ÿxCuxZn0.2Fe2O4, 0 x 0.4.
structural features of the sintered samples (x 0 to 0.4) were obtained with the help of Leica-Cambridge 440 scanning electron microscope. The micrographs were taken on the fractured surface of the sintered pellets (Fig. 2). 3. Results and discussion X-ray diffractograms of the as-dried (808C) and calcined (2508C) ferrite powders showed broad peaks indicating ®ne particle nature of the particles. The patterns matched well with the characteristic re¯ections of Ni±Zn ferrite [9] with no unidenti®ed extra lines. XRD patterns of the sintered (10008C) ferrite samples showed intense sharp peaks showing well-crystalline single phase ferrite for 0 x 0.2 compositions (Fig. 1). A second phase of CuO and CuFe2O4
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
3
Table 1 Electrical and magnetic properties of Ni0.8ÿxCuxZn0.2Fe2O4 Cu content x
0.0 0.1 0.2 0.3 0.4
Magnetization (Ms) (emu gÿ1) 2508C calcined
10008C sintered
46.7 38.7 38.7 38.0 38.2
72.2 68.6 67.8 66.0 66.3
Density (g cmÿ3)
Tc (C)
dc resistivity (ohm cm)
3.84 4.69 5.02 4.92 4.93
490 474 457 446 422
9.6 106 6.4 107 7.3 107 4.2 106 1.5 105
i
35 63 100 54 47
Fig. 2. Scanning electron micrographs of the samples of Cu substituted Ni±Zn ferrite.
Loss factor at flossmin
2.72 10ÿ4 3.65 10ÿ4 9.53 10ÿ5 1.26 10ÿ3 2.62 10ÿ3
4
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
appeared in the patterns of ferrite composition with x 0.3 as shown by the additional characteristic re¯ections at 2 38.28 and 36.07, respectively, with a relatively small intensity. The XRD results clearly indicate that the substitution of Ni by copper takes place only upto x 0.2, while for higher values of x, Cu forms a second phase of CuO/CuFe2O4 as evidenced by the XRD results. Another effect of increasing copper substitution in Ni±Zn ferrite (x 0 to 0.4) is increased grain growth as seen from the scanning electron micrographs (Fig. 2). Uniform grains of 500 nm are seen for x 0 composition and are progressively increased to 2 mm for x 0.4 composition. The magnetisation measurements of as-dried calcined, and sintered samples showed the characteristic hysteresis loops indicating ferrimagnetic nature of the ferrites. The saturation magnetisation (Ms) of these materials progressively increased at the higher temperatures of calcination (2508C) and sintering (10008C). The values of Ms of calcined and sintered samples are given in Table 1. Nickel ferrite is reported [13] to be 80% inverse spinel with equivalent Ni2 ions on B site. The decrease in Ms may be attributed to the substitution of Ni by Cu having a lower magnetic moment. The temperature variation of magnetisation was carried out to record the Curie temperature (Tc) of all the samples. The values of Tc are given in Table 1 which clearly show that the Tc has progressively decreased from 4908C to 4228C for the compositions 0 x 0.4. This result con®rms the substitution of Ni by Cu and its effect on AB and BB interactions resulting in the decrease of magnetisation and Curie temperature of Ni±Zn ferrite. The substitution of nickel by copper has also affected the bulk density and the resistivity of the ferrite. Table 1 shows the bulk density of the unsubstituted ferrite (x 0) 3.84 g cmÿ3 as against 5.02 g cmÿ3 of Cu-substituted
ferrite with x 0.2. This is the maximum density achieved for this composition on sintering at 10008C. The increase is attributed to the increase in grain size and reduction of the pores in the ferrite microstructure as seen in scanning electron micrograph (Fig. 2) of the respective samples. The increased density also helps in explaining the higher resistivity value obtained for composition with x 0.2. The high density ferrite containing Ni2 ions is expected to resist the oxidation of Ni2 to Ni3 (which normally takes place during the cooling cycle of the sintering step) and helps in decreasing the hopping conduction of electrons from Ni2 to Ni3 thereby increasing its resistivity. However, the conduction is increased with higher values of x 0.3 which may be attributed to the formation of a second phase namely CuO and CuFe2O4 for x 0.3 as detected by XRD. It is reported [14] that at a temperature little over 10008C CuO decomposes to Cu2O, and due to the formation of Cu1 and Cu2 states, the conduction is increased decreasing its resistivity value. Besides, CuFe2O4 has much lower resistivity of 103±104 ohm cm as compared to that of Ni±Zn ferrite composition having 106 ohm cm. Moreover, as CuO segregates out from the ferrite phase the remaining material is expected to be an iron excess compound which will decrease the resistivity of the material. An increase in permeability (i) is observed for all the substituted compositions and highest value obtained was 100 for the composition with x 0.2. The permeability can be expressed as 1 spin dw, where spin is the susceptibility due to spins and dw susceptibility due to domain wall motion [15]. In single-domain particles the domain wall motion is absent and in the unsubstituted sample the particles are single domain as they are of 500 nm size. Hence, the low value of permeability can be realised. But for the Cu substituted samples, multidomain
Fig. 3. Variation of loss factor with frequency for Ni0.8ÿxCuxZn0.2Fe2O4, 0 x 0.4.
J.J. Shrotri et al. / Materials Chemistry and Physics 59 (1999) 1±5
particles of 3m size [16] are obtained which result in higher permeability values due to domain wall contribution. Another important factor which gives rise to an increase in the permeability is the increase in the density of the composition x 0.2. An increase in the density results in the reduction of demagnetizing ®eld due to pores which in turn increases the permeability. The values of i, at flossmin are given in Table 1. The flossmin is the frequency at which the minimum value of loss factor is obtained for a given ferrite composition. The decrease in the permeability for x 0.3 as compared to that of x 0.2, can be a manifestation of the formation of copper ferrite. In Fig. 3 the frequency dependence of the loss factor is given. The values of loss factor revealed that: (1) they are nearly constant for x 0 and 0.1, (2) reached a minimum value of 9.53 10ÿ5 for x 0.2 and (3) increased much faster (10ÿ3) with higher values of x. The increase in loss factor for higher values of x may be attributed to the second phase (CuO/CuFe2O4) which acts as a microstructural imperfection to promote magnetic losses. The substantial reduction in the loss factor for the x 0.2 composition can be attributed to the increase in resistivity. An increase in resistivity reduces the eddy current loss which is the main loss mechanism at MHz frequency region. 4. Conclusion The limiting amount of copper substitution is favourable for the grain growth of Ni±Zn ferrite. It helps in increasing the bulk density and improve the magnetic and electrical properties of Ni0.8Zn0.2Fe2O4 composition at lower sintering temperature of 10008C. Major achievements are: (1) the synthesis of Ni±Cu±Zn ferrite at 808C and (2) the devel-
5
opment of a low-power loss material operating in the MHz frequency region. Acknowledgements P.S.A. Kumar is grateful to UGC, India, for ®nancial support. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Abraham, Am. Ceram. Soc. Bull. 73 (1994) 62. T. Ochiai, Proc. ICF-7 Abstracts 3-2A1, Bordeaux, France, 1996, p. 12. D.C. Khan, M. Misra, A.R. Das, J. Appl. Phys. 53 (1982) 2722. M. Pal, P. Brahma, D. Chakravorty, J. Magn. Magn. Mater. 152 (1996) 370. J.H. Nam, W.G. Hur, J.H. Oh, J. Appl. Phys. 81 (1997) 4794. H. Igarashi, K. Okazaki, J. Am. Ceram. Soc. 60 (1977) 51. J.J. Shrotri, A.G. Bagul, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Ferrites: Proc. ICF6, in: T. Yamaguchi, M. Abe (Eds.), The Japan Society of Powder and Powder Metallurgy, Tokyo/Kyoto, 1992, p. 129. T. Kodama, Y. Wada, T. Yamamoto, M. Tsuji, Y. Tamaura, J. Mater. Chem. 5 (1995) 1413. P.S. Anil Kumar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Mater. Lett. 27 (1996) 293. P.S. Anil Kumar, J.J. Shrotri, C.E. Deshpande, S.K. Date, J. Appl. Phys. 81 (1997) 4788. T. Nakamura, J. Magn. Mater. 168 (1997) 285. A. Globus, J. Phys. Suppl. C 1 (1977) 1. J.M. Hastings, L.M. Corliss, Rev. Mod. Phys. 102 (1953) 1460. L. G van Uitert, J. Appl. Phys. 27 (1956) 723. P.S. Anil Kumar, S.R. Sainkar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, J. Appl. Phys. 83 (1998) 6864. P.J. van der Zaag, P.J. van der Valk, M.Th. Rekveldt, Appl. Phys. Lett. 69 (1996) 2927.