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Magnetic and electrical properties of low-temperature sintered Mn-doped NiCuZn ferrites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
Magnetic and electrical properties of low-temperature sintered Mn-doped NiCuZn ferrites Zhenxing Yue*, Ji Zhou, Zhilun Gui, Longtu Li Department of Materials Science & Engineering, State key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China Received 30 October 2002; received in revised form 3 February 2003
Abstract Mn-doped NiCuZn ferrites with compositions of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 (where x ¼ 0:02; 0.04, 0.06, 0.08, 0.10) were prepared by usual ceramic method. Initial permeability (m0i and m00i ), DC resistivity (rDC ), dielectric constant (e0 ) and dielectric loss tangent (tan d) were measured as a function of frequency and temperature. With increasing MnO2 content, the resonant frequency and the Curie temperature decrease, whereas the m0i increases first and decreases after it takes a maximum at MnO2 content of 0.06. The e0 and tan d show a decrease with the increase of frequency for all the samples and the dielectric dispersion is enhanced with increasing MnO2 content. On increasing temperature, the e0 and tan d are increased significantly, especially for samples with higher MnO2 content. The rDC and conduction activation energy decrease with the increase of MnO2 content. The possible mechanism for the influence of MnO2 on the electromagnetic properties was discussed. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Magnetic materials; Magnetic properties; Electrical properties
1. Introduction Recently, NiCuZn ferrites have been extensively studied for multilyer chip inductor (MLCI) applications because of their good properties at high frequencies and low sintering temperatures [1–3]. In this application, NiCuZn ferrites need to be sintered at o950 C in order to co-fire with silver internal electrode during the manufacture of MLCIs. The low-melting compounds, such as Bi2O3, PbO, V2O5 and glass, are usually doped to promote the low-temperature sintering of *Corresponding author. Tel.: +86-10-6278-4579; fax: +8610-6277-1160. E-mail address: [email protected] (Z. Yue).
NiCuZn ferrites [4–6]. The permeability of the low-temperature sintered ferrites is usually lower than that of ferrites sintered at high temperature due to the incorporation of low-melting compounds. For the high performance MLCIs with high inductance and reliability, high permeability NiCuZn ferrites are required. There are several routes to increase the permeability values of NiCuZn ferrite, including (1) increasing grain size [7], (2) increasing magnetization or decreasing magnetostriction constant through the incorporation of some cations [8]. For the MLCI application, the fine-grained ferrites are required due to thin ferrite layer. So, the second route would be preferable. In our previous study, a high permeability NiCuZn ferrite with fine-grained
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00214-2
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
2. Experimental procedure NiCuZn ferrite powders with compositions of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 (where x ¼ 0:02; 0.04, 0.06, 0.08, 0.10) were prepared by usual ceramic method. The analytical grade Fe2O3, NiO, ZnO, CuO and MnO2 were weighed following above formula and mixed in a ball mill. The resulting powders were pre-heated at 850 C for 2 h to form ferrites, and then milled in a ball mill again. In order to attain low-temperature sintering, 2 wt% Bi2O3 was added for all compositions. The resulting ferrite powders were mixed with an appropriate amount of 5 wt% polyvinyl alcohol as a binder, and granulated with a 60-mesh sieve. The granulated powders were uniaxially pressed to form green toroidal and pellet specimens. After binder burnt-out at 600 C for 1 h, the compacts were sintered at 900 C for 4 h in air. The permeability and the magnetic losses were measured on the wound toroid-shape samples as a function of frequency and temperature using HP4194 and HP4192 impedance analyzer. Sintered disc-shape samples with silver electrodes were subjected to the measurements of dielectric properties and electrical resistivity using HP4194 and HP4140B analyzer.
3. Results and discussion 3.1. Magnetic permeability and its dependence on frequency and temperature The measurements of the initial permeability of ferrites as a function of frequency are performed, and the values at 1 MHz are given in Table 1. The bulk density, initial permeability, resonant frequency, and Curie temperature for Mn-doped NiCuZn ferrites are also included in Table 1. The
Table 1 Density, initial permeability, resonant frequency and Curie temperature of Mn-doped NiCuZn ferrites MnO2 content x
Density (g/cm3)
mi at 1 MHz
fr (MHz)
Tc ( C)
0.02 0.04 0.06 0.08 0.10
5.33 5.32 5.30 5.31 5.28
218 258 315 241 246
11.07 9.95 8.03 8.03 7.21
134 122 109 97 86
500 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
400 300 µ'i
microstructure was prepared by a sol–gel autocombustion method through the incorporation of a small amount of MnO2 [9]. In this paper, we will investigate the effects of MnO2 on the magnetic and electrical properties of NiCuZn ferrites prepared by conventional ceramic method.
259
200 100 0 103
104
105 106 Frequency (Hz)
107
108
Fig. 1. Initial permeability (m0i ) of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 ferrites as a function of frequency.
dependence of initial permeability of Mn-doped NiCuZn ferrites on frequency is shown in Fig. 1. It is clearly seen from Table 1 and Fig. 1 that the initial permeability increases as Mn content is increased from 0.02 to 0.06, then decreases with further increase in MnO2 content. Thus, a maximum of initial permeability occurs at MnO2 content of 0.06. The occurrence of maximum permeability in the present Mn-doped NiCuZn ferrites is similar to the result reported by Nam et al. [10]. In their report, however, the maximum permeability occurs at MnO2 content of 0.01. The difference in MnO2 content may be attributed to the homogeneity in composition of prepared samples. Nam et al. [10] reported that the grain size and magnetostriction constant are decreased with the Mn-substitution in NiCuZn ferrites. These two factors are responsible for the change of permeability with MnO2 content. In one hand, the decrease in grain size reduces the permeability
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
260
by decreasing the contribution of domain wall. On the other hand, the decrease in magnetostriction constant induced by Mn substitution significantly increases the permeability of ferrites. Consequently, a maximum value of permeability occurs at MnO2 content, 0.06 in the present samples. Fig. 2 shows the imaginary permeability (m00i ) of ferrite samples as a function of frequency. The resonant frequency, at which the maximum m00i occurs, slightly decreases with increase in Mn concentration. Fig. 3 shows the real permeability as a function of temperature for Mn-doped ferrite samples. The decrease in the Curie temperature with increasing Mn content is observed from the
120 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
100
µ''i
80 60 40 20
0
105
106
107
108
Frequency (Hz) Fig. 2. Imaginary permeability (m00i ) of (Ni0.25xMnxCu0.25ZZn0.5)O(Fe2O3)0.98 ferrites as a function of frequency.
figure. This suggests that Mn ions may be incorporated into the lattice of ferrite, resulting in the weakening in A–B interaction in ferrite. Additionally, it has been shown that the sharpness of the permeability drop at the Curie point can be used as a measure of the degree of compositional homogeneity in the sample. The present ferrites show good homogeneity, as shown in Fig. 3, where an abrupt drop in permeability occurs within the temperature range less than 10 C near Curie temperature. 3.2. DC electrical resistivity and conductivity The DC electrical resistivity is an important property of low-temperature sintered ferrite for MLCI application, because resistivity remarkably affects the electroplating of devices. Fig. 4 shows the variation of the DC resistivity versus the Mn content (x) for samples at room temperature (25 C). It can be seen that resistivity is decreased by the addition of MnO2. This decrease in the resistivity may be attributed to the fact that in the case of Mn-doped NiCuZn ferrite, octahedral sites (B sites) are occupied by Ni2+, Fe3+ and Cu2+ ions. The conduction mechanism in ferrite is considered as the electron hopping between Fe2+ and Fe3+ in B site [11]. Obviously, the more the Fe2+ ions content the higher the conduction and consequently a decrease in the resistivity. Therefore, the observed decrease in the DC resistivity 10
500
Initial permeability
300 200 100
9 8 logρdc
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
400
7 6
0 -25
0
25
50 75 100 125 150 175 200 Temperature (°C)
Fig. 3. Temperature dependence of initial permeability (m0i ) for NiCuZn ferrites doped with MnO2.
5
0.02
0.04
0.06
0.08
0.10
Mn content, x Fig. 4. Variation of DC resistivity of NiCuZn ferrites with MnO2 content.
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
with the increase of MnO2 content may be attributed to the presence of increased Fe2+ ions content due to the incorporation of MnO2. In fact, in Mn-doped NiCuZn ferrite, the following equilibrium may exist during the sintering, Fe3þ þ Mn2þ 3Fe2þ þ Mn3þ : Because the trivalent state of Mn is stable at sintering temperature used in the present study, some Fe2+ can be formed for the charge equilibrium due to the incorporation of Mn in ferrite lattice. With increasing Mn content, more Fe2+ are formed, resulting in increasing the probability of electron hopping and decreasing resistivity, as shown in Fig. 4. The relationship between the DC conductivity sDC and temperature T is given according to Wilson’s equation as sAC ¼ s0 exp½E=kT;
ð1Þ
where s0 is a pre-exponential constant, E is the activation energy for electric conduction and k is Boltzmann’s constant. Fig. 5 shows the logarithm of sDC as function of 1=T for Mn-doped NiCuZn ferrites. It can be seen from Fig. 5 that the DC conductivity increases with increasing temperature for all samples. This indicates that Mn-doped NiCuZn ferrites have semiconductor-like behavior. The increase in the electrical conductivity as temperature increases may be related to the increase in drift mobility of the thermally activated
T3
-12
T2
-16
T1
-20 -24
1.5
2.0
2.5
3.0
Table 2 Conduction activation energy and dc electrical resistivity of Mn-doped NiCuZn ferrites MnO2 content, x
E1 (eV)
E2 (eV)
E3 (eV)
r (O cm) at 25 C
0.02 0.04 0.06 0.08 0.10
0.82 0. 62 0.53 0.49 0.49
0.61 0.50 0.47 0.43 0.43
0.68 0.54 0.50 0.47 0.45
1.56 109 1.15 107 6.25 106 2.28 106 7.46 105
Activation energy (eV)
lnσdc [Ω-1.cm-1]
-8
charge carriers (electron and hole) according to hopping conduction mechanism. The linear relation in Fig. 5 was broken to three lines at two temperatures, indicated as three temperature ranges in Fig. 5. It has been reported that the change in the slope of straight line is related to the change in conduction mechanism accompanied by transformation from the ferrimagnetic to paramagnetic state at Curie point and the magnetic phase transition in ferrimagnetic state [11–13]. The magnitude of the slope depends on the exchange interaction between the outer and inner electrons, which changes at the Curie temperature and the magnetic phase transition temperature. The activation energy for electric conduction E in the ferrimagnetic and paramagnetic ranges were determined from the slopes of these lines. The obtained values of activation energy for three temperature ranges are shown in Table 2. The DC electrical resistivities for all samples are also included in Table 2. Fig. 6 shows
0.9
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
-4
261
3.5
1000/T [K-1] Fig. 5. Temperature dependence of DC conductivity for NiCuZn ferrites doped with MnO2.
T1 region T2 region T3 region
0.8 0.7 0.6 0.5 0.4
0.02
0.04
0.06 0.08 Mn content, x
0.10
Fig. 6. Effect of MnO2 content on the activation energy.
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
262
the activation energy for conduction as a function of MnO2 content. It can be seen that the activation energy for all three temperature ranges decreases with increasing MnO2 content. This suggests that the incorporation of Mn into NiCuZn ferrite enhances the electron hopping conduction between Fe3+ and Fe2+. 3.3. Dielectric constants and dielectric loss tangents Figs. 7 and 8 show the dielectric constant and loss tangent as a function of frequency for Mndoped samples, respectively. The significant dispersion in dielectric constant and loss tangent is observed in low frequency range for all samples. 400
Dielectric constant
350 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
300 250 200 150 100 50 0 103
104
105 Frequency (Hz)
106
107
Fig. 7. Frequency dependence of dielectric constant for NiCuZn ferrites doped with MnO2.
Dielectric loss tangent
5 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
4 3 2 1 0
103
104
105 Frequency (Hz)
106
107
Fig. 8. Frequency dependence of dielectric loss tangent for NiCuZn ferrites doped with MnO2.
The extent of dispersion is largely affected by MnO2 content. With increasing MnO2 content, the frequency dispersion is enlarged significantly. The composition dependence of dielectric constant and dissipation factor can be explained by using the assumption that the mechanism of dielectric polarization is similar to that of conduction process. It has been concluded that the electron exchange between Fe2+ and Fe3+ results in the local displacement of charges in the direction of an electric field, which is responsible for polarization in ferrites [14,15]. With increasing the frequency of the externally applied electric field, the electronic exchange between Fe2+ and Fe3+ cannot follow the alternating field, resulting in decrease in dielectric constant. The magnitude of exchange depends on the concentration of Fe3+/Fe3+ ion pairs present on B site. For the present ferrites, the incorporation of Mn in B site of ferrite may increase the concentration of Fe3+/Fe3+ ion pairs. The dielectric loss in ferrites is considered to be originated from two mechanisms: electron hopping and charged defect dipoles. The former contributes to the dielectric loss mainly in the low frequency range. This is consistent with the DC conduction mechanism as discussed above. In the high frequency range, the dielectric loss mainly results from the response of defect dipoles to the field. These dipoles in ferrites are formed due to the change of cation valence state, such as Fe3+/ Fe2+, Ni3+/Ni2+, Cu2+/Cu1+ and Mn3+/Mn2+, during sintering under oxygen partial pressure. The relaxation of dipoles under electric field is decreased with increasing frequency, resulting in the decrease in the dielectric loss in high frequency range, as shown in Fig. 8. The consistence of dielectric polarization with conduction mechanism can be observed from the dependence of dielectric constant and loss tangent on temperature, shown in Figs. 9 and 10. The dielectric constant and loss tangent monotonously increase with increasing temperature for the samples with large content of MnO2. For the samples doped with small content of MnO2, the Debye dielectric relaxation can be observed in the plots of dielectric constant and loss tangent with temperature. This may be originated from the relaxation of dipoles in ferrites.
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
Dielectric constant
300 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
200
100
0
0
50
100
150
200
263
show a decrease with the increase of frequency for all the samples and the dielectric dispersion is enhanced with increasing MnO2 content. On increasing temperature, the dielectric constant and tan d are increased significantly, especially for samples with higher MnO2 content. The DC electrical resistivity and conduction activation energy decreases with the increase of MnO2 content. The obtained experimental results provide important information on improving properties of NiCuZn ferrites for multilayer chip inductor application.
Temperature (°C) Fig. 9. Temperature dependence of dielectric constant for NiCuZn ferrites doped with MnO2.
Dielectric loss tangent
1.0
This work has been financially supported by the National High-tech Development Plan of China, the National Natural Science Foundation of China (Grant No. 59995523), and the National Major Fundamental Research Project (Grant No. 2002CB613307).
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
0.8
Acknowledgements
0.6 0.4
References 0.2 0.0
0
50 100 Temperature (°C)
150
Fig. 10. Temperature dependence of dielectric loss tangent for NiCuZn ferrites doped with MnO2.
4. Conclusion The experimental results presented above indicate that MnO2 addition has significant effect on the magnetic and electrical properties of lowtemperature sintered NiCuZn ferrites. Mn substitution in NiCuZn ferrites also remarkably affects the dependencies of magnetic and electric properties on frequency and temperature. With increasing MnO2 content, the resonant frequency and the Curie temperature decrease, whereas the initial permeability increases first, takes a maximum at MnO2 content of 0.06 and then decreases. The dielectric constant and loss tangent (tan d)
[1] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285. [2] J.-Y. Hsu, W.-S. Ko, H.-D. Shen, C.-J. Chen, IEEE Trans. Magn. 30 (1994) 4875. [3] M. Fujimoto, J. Am. Ceram. Soc. 77 (11) (1994) 2873. [4] T. Nakamura, Y. Okano, S. Miura, J. Mater. Sci. 33 (1998) 1091. [5] T. Aoyama, K. Hirota, O. Yamaguchi, J. Am. Ceram. Soc. 79 (10) (1996) 2792. [6] S.F. Wang, Y.R. Wang, Thomas C.K. Yang, P.J. Wang, C.A. Lu, J. Magn. Magn. Mater. 217 (2000) 35. [7] R. Valenzuela, Magnetic Ceramics, Cambridge University Press, Cambridge, 1994. [8] G.F. Dionne, R.G. West, J. Appl. Phys. 61 (1987) 3868. [9] Z. Yue, J. Zhou, L. Li, Z. Gui, J. Magn. Magn. Mater. 233 (2003) 224. [10] J.H. Nam, W.G. Hur, J.H. Oh, J. Appl. Phys. 81 (8) (1997) 4794. [11] M.A. El Hiti, J. Magn. Magn. Mater. 136 (1994) 138. [12] B. Parvatheeswara Rao, K.H. Rao, J. Mater. Sci. 32 (1997) 6049. [13] A.M. Abdeen, J. Magn. Magn. Mater. 185 (1998) 199. [14] G. Ranga Mohan, D. Ravinder, A.V. Ramana Reddy, B.S. Boyanov, Mater. Lett. 40 (1999) 39. [15] S.C. Watawe, B.D. Sarwade, S.S. Bellad, B.D. Sutar, B.K. Chougula, J. Magn. Magn. Mater. 214 (2000) 55.
Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
Magnetic and electrical properties of low-temperature sintered Mn-doped NiCuZn ferrites Zhenxing Yue*, Ji Zhou, Zhilun Gui, Longtu Li Department of Materials Science & Engineering, State key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China Received 30 October 2002; received in revised form 3 February 2003
Abstract Mn-doped NiCuZn ferrites with compositions of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 (where x ¼ 0:02; 0.04, 0.06, 0.08, 0.10) were prepared by usual ceramic method. Initial permeability (m0i and m00i ), DC resistivity (rDC ), dielectric constant (e0 ) and dielectric loss tangent (tan d) were measured as a function of frequency and temperature. With increasing MnO2 content, the resonant frequency and the Curie temperature decrease, whereas the m0i increases first and decreases after it takes a maximum at MnO2 content of 0.06. The e0 and tan d show a decrease with the increase of frequency for all the samples and the dielectric dispersion is enhanced with increasing MnO2 content. On increasing temperature, the e0 and tan d are increased significantly, especially for samples with higher MnO2 content. The rDC and conduction activation energy decrease with the increase of MnO2 content. The possible mechanism for the influence of MnO2 on the electromagnetic properties was discussed. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Magnetic materials; Magnetic properties; Electrical properties
1. Introduction Recently, NiCuZn ferrites have been extensively studied for multilyer chip inductor (MLCI) applications because of their good properties at high frequencies and low sintering temperatures [1–3]. In this application, NiCuZn ferrites need to be sintered at o950 C in order to co-fire with silver internal electrode during the manufacture of MLCIs. The low-melting compounds, such as Bi2O3, PbO, V2O5 and glass, are usually doped to promote the low-temperature sintering of *Corresponding author. Tel.: +86-10-6278-4579; fax: +8610-6277-1160. E-mail address: [email protected] (Z. Yue).
NiCuZn ferrites [4–6]. The permeability of the low-temperature sintered ferrites is usually lower than that of ferrites sintered at high temperature due to the incorporation of low-melting compounds. For the high performance MLCIs with high inductance and reliability, high permeability NiCuZn ferrites are required. There are several routes to increase the permeability values of NiCuZn ferrite, including (1) increasing grain size [7], (2) increasing magnetization or decreasing magnetostriction constant through the incorporation of some cations [8]. For the MLCI application, the fine-grained ferrites are required due to thin ferrite layer. So, the second route would be preferable. In our previous study, a high permeability NiCuZn ferrite with fine-grained
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00214-2
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
2. Experimental procedure NiCuZn ferrite powders with compositions of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 (where x ¼ 0:02; 0.04, 0.06, 0.08, 0.10) were prepared by usual ceramic method. The analytical grade Fe2O3, NiO, ZnO, CuO and MnO2 were weighed following above formula and mixed in a ball mill. The resulting powders were pre-heated at 850 C for 2 h to form ferrites, and then milled in a ball mill again. In order to attain low-temperature sintering, 2 wt% Bi2O3 was added for all compositions. The resulting ferrite powders were mixed with an appropriate amount of 5 wt% polyvinyl alcohol as a binder, and granulated with a 60-mesh sieve. The granulated powders were uniaxially pressed to form green toroidal and pellet specimens. After binder burnt-out at 600 C for 1 h, the compacts were sintered at 900 C for 4 h in air. The permeability and the magnetic losses were measured on the wound toroid-shape samples as a function of frequency and temperature using HP4194 and HP4192 impedance analyzer. Sintered disc-shape samples with silver electrodes were subjected to the measurements of dielectric properties and electrical resistivity using HP4194 and HP4140B analyzer.
3. Results and discussion 3.1. Magnetic permeability and its dependence on frequency and temperature The measurements of the initial permeability of ferrites as a function of frequency are performed, and the values at 1 MHz are given in Table 1. The bulk density, initial permeability, resonant frequency, and Curie temperature for Mn-doped NiCuZn ferrites are also included in Table 1. The
Table 1 Density, initial permeability, resonant frequency and Curie temperature of Mn-doped NiCuZn ferrites MnO2 content x
Density (g/cm3)
mi at 1 MHz
fr (MHz)
Tc ( C)
0.02 0.04 0.06 0.08 0.10
5.33 5.32 5.30 5.31 5.28
218 258 315 241 246
11.07 9.95 8.03 8.03 7.21
134 122 109 97 86
500 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
400 300 µ'i
microstructure was prepared by a sol–gel autocombustion method through the incorporation of a small amount of MnO2 [9]. In this paper, we will investigate the effects of MnO2 on the magnetic and electrical properties of NiCuZn ferrites prepared by conventional ceramic method.
259
200 100 0 103
104
105 106 Frequency (Hz)
107
108
Fig. 1. Initial permeability (m0i ) of (Ni0.25xMnxCu0.25Zn0.5)O(Fe2O3)0.98 ferrites as a function of frequency.
dependence of initial permeability of Mn-doped NiCuZn ferrites on frequency is shown in Fig. 1. It is clearly seen from Table 1 and Fig. 1 that the initial permeability increases as Mn content is increased from 0.02 to 0.06, then decreases with further increase in MnO2 content. Thus, a maximum of initial permeability occurs at MnO2 content of 0.06. The occurrence of maximum permeability in the present Mn-doped NiCuZn ferrites is similar to the result reported by Nam et al. [10]. In their report, however, the maximum permeability occurs at MnO2 content of 0.01. The difference in MnO2 content may be attributed to the homogeneity in composition of prepared samples. Nam et al. [10] reported that the grain size and magnetostriction constant are decreased with the Mn-substitution in NiCuZn ferrites. These two factors are responsible for the change of permeability with MnO2 content. In one hand, the decrease in grain size reduces the permeability
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
260
by decreasing the contribution of domain wall. On the other hand, the decrease in magnetostriction constant induced by Mn substitution significantly increases the permeability of ferrites. Consequently, a maximum value of permeability occurs at MnO2 content, 0.06 in the present samples. Fig. 2 shows the imaginary permeability (m00i ) of ferrite samples as a function of frequency. The resonant frequency, at which the maximum m00i occurs, slightly decreases with increase in Mn concentration. Fig. 3 shows the real permeability as a function of temperature for Mn-doped ferrite samples. The decrease in the Curie temperature with increasing Mn content is observed from the
120 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
100
µ''i
80 60 40 20
0
105
106
107
108
Frequency (Hz) Fig. 2. Imaginary permeability (m00i ) of (Ni0.25xMnxCu0.25ZZn0.5)O(Fe2O3)0.98 ferrites as a function of frequency.
figure. This suggests that Mn ions may be incorporated into the lattice of ferrite, resulting in the weakening in A–B interaction in ferrite. Additionally, it has been shown that the sharpness of the permeability drop at the Curie point can be used as a measure of the degree of compositional homogeneity in the sample. The present ferrites show good homogeneity, as shown in Fig. 3, where an abrupt drop in permeability occurs within the temperature range less than 10 C near Curie temperature. 3.2. DC electrical resistivity and conductivity The DC electrical resistivity is an important property of low-temperature sintered ferrite for MLCI application, because resistivity remarkably affects the electroplating of devices. Fig. 4 shows the variation of the DC resistivity versus the Mn content (x) for samples at room temperature (25 C). It can be seen that resistivity is decreased by the addition of MnO2. This decrease in the resistivity may be attributed to the fact that in the case of Mn-doped NiCuZn ferrite, octahedral sites (B sites) are occupied by Ni2+, Fe3+ and Cu2+ ions. The conduction mechanism in ferrite is considered as the electron hopping between Fe2+ and Fe3+ in B site [11]. Obviously, the more the Fe2+ ions content the higher the conduction and consequently a decrease in the resistivity. Therefore, the observed decrease in the DC resistivity 10
500
Initial permeability
300 200 100
9 8 logρdc
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
400
7 6
0 -25
0
25
50 75 100 125 150 175 200 Temperature (°C)
Fig. 3. Temperature dependence of initial permeability (m0i ) for NiCuZn ferrites doped with MnO2.
5
0.02
0.04
0.06
0.08
0.10
Mn content, x Fig. 4. Variation of DC resistivity of NiCuZn ferrites with MnO2 content.
ARTICLE IN PRESS Z. Yue et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 258–263
with the increase of MnO2 content may be attributed to the presence of increased Fe2+ ions content due to the incorporation of MnO2. In fact, in Mn-doped NiCuZn ferrite, the following equilibrium may exist during the sintering, Fe3þ þ Mn2þ 3Fe2þ þ Mn3þ : Because the trivalent state of Mn is stable at sintering temperature used in the present study, some Fe2+ can be formed for the charge equilibrium due to the incorporation of Mn in ferrite lattice. With increasing Mn content, more Fe2+ are formed, resulting in increasing the probability of electron hopping and decreasing resistivity, as shown in Fig. 4. The relationship between the DC conductivity sDC and temperature T is given according to Wilson’s equation as sAC ¼ s0 exp½E=kT;
ð1Þ
where s0 is a pre-exponential constant, E is the activation energy for electric conduction and k is Boltzmann’s constant. Fig. 5 shows the logarithm of sDC as function of 1=T for Mn-doped NiCuZn ferrites. It can be seen from Fig. 5 that the DC conductivity increases with increasing temperature for all samples. This indicates that Mn-doped NiCuZn ferrites have semiconductor-like behavior. The increase in the electrical conductivity as temperature increases may be related to the increase in drift mobility of the thermally activated
T3
-12
T2
-16
T1
-20 -24
1.5
2.0
2.5
3.0
Table 2 Conduction activation energy and dc electrical resistivity of Mn-doped NiCuZn ferrites MnO2 content, x
E1 (eV)
E2 (eV)
E3 (eV)
r (O cm) at 25 C
0.02 0.04 0.06 0.08 0.10
0.82 0. 62 0.53 0.49 0.49
0.61 0.50 0.47 0.43 0.43
0.68 0.54 0.50 0.47 0.45
1.56 109 1.15 107 6.25 106 2.28 106 7.46 105
Activation energy (eV)
lnσdc [Ω-1.cm-1]
-8
charge carriers (electron and hole) according to hopping conduction mechanism. The linear relation in Fig. 5 was broken to three lines at two temperatures, indicated as three temperature ranges in Fig. 5. It has been reported that the change in the slope of straight line is related to the change in conduction mechanism accompanied by transformation from the ferrimagnetic to paramagnetic state at Curie point and the magnetic phase transition in ferrimagnetic state [11–13]. The magnitude of the slope depends on the exchange interaction between the outer and inner electrons, which changes at the Curie temperature and the magnetic phase transition temperature. The activation energy for electric conduction E in the ferrimagnetic and paramagnetic ranges were determined from the slopes of these lines. The obtained values of activation energy for three temperature ranges are shown in Table 2. The DC electrical resistivities for all samples are also included in Table 2. Fig. 6 shows
0.9
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
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261
3.5
1000/T [K-1] Fig. 5. Temperature dependence of DC conductivity for NiCuZn ferrites doped with MnO2.
T1 region T2 region T3 region
0.8 0.7 0.6 0.5 0.4
0.02
0.04
0.06 0.08 Mn content, x
0.10
Fig. 6. Effect of MnO2 content on the activation energy.
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the activation energy for conduction as a function of MnO2 content. It can be seen that the activation energy for all three temperature ranges decreases with increasing MnO2 content. This suggests that the incorporation of Mn into NiCuZn ferrite enhances the electron hopping conduction between Fe3+ and Fe2+. 3.3. Dielectric constants and dielectric loss tangents Figs. 7 and 8 show the dielectric constant and loss tangent as a function of frequency for Mndoped samples, respectively. The significant dispersion in dielectric constant and loss tangent is observed in low frequency range for all samples. 400
Dielectric constant
350 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
300 250 200 150 100 50 0 103
104
105 Frequency (Hz)
106
107
Fig. 7. Frequency dependence of dielectric constant for NiCuZn ferrites doped with MnO2.
Dielectric loss tangent
5 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
4 3 2 1 0
103
104
105 Frequency (Hz)
106
107
Fig. 8. Frequency dependence of dielectric loss tangent for NiCuZn ferrites doped with MnO2.
The extent of dispersion is largely affected by MnO2 content. With increasing MnO2 content, the frequency dispersion is enlarged significantly. The composition dependence of dielectric constant and dissipation factor can be explained by using the assumption that the mechanism of dielectric polarization is similar to that of conduction process. It has been concluded that the electron exchange between Fe2+ and Fe3+ results in the local displacement of charges in the direction of an electric field, which is responsible for polarization in ferrites [14,15]. With increasing the frequency of the externally applied electric field, the electronic exchange between Fe2+ and Fe3+ cannot follow the alternating field, resulting in decrease in dielectric constant. The magnitude of exchange depends on the concentration of Fe3+/Fe3+ ion pairs present on B site. For the present ferrites, the incorporation of Mn in B site of ferrite may increase the concentration of Fe3+/Fe3+ ion pairs. The dielectric loss in ferrites is considered to be originated from two mechanisms: electron hopping and charged defect dipoles. The former contributes to the dielectric loss mainly in the low frequency range. This is consistent with the DC conduction mechanism as discussed above. In the high frequency range, the dielectric loss mainly results from the response of defect dipoles to the field. These dipoles in ferrites are formed due to the change of cation valence state, such as Fe3+/ Fe2+, Ni3+/Ni2+, Cu2+/Cu1+ and Mn3+/Mn2+, during sintering under oxygen partial pressure. The relaxation of dipoles under electric field is decreased with increasing frequency, resulting in the decrease in the dielectric loss in high frequency range, as shown in Fig. 8. The consistence of dielectric polarization with conduction mechanism can be observed from the dependence of dielectric constant and loss tangent on temperature, shown in Figs. 9 and 10. The dielectric constant and loss tangent monotonously increase with increasing temperature for the samples with large content of MnO2. For the samples doped with small content of MnO2, the Debye dielectric relaxation can be observed in the plots of dielectric constant and loss tangent with temperature. This may be originated from the relaxation of dipoles in ferrites.
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Dielectric constant
300 x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
200
100
0
0
50
100
150
200
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show a decrease with the increase of frequency for all the samples and the dielectric dispersion is enhanced with increasing MnO2 content. On increasing temperature, the dielectric constant and tan d are increased significantly, especially for samples with higher MnO2 content. The DC electrical resistivity and conduction activation energy decreases with the increase of MnO2 content. The obtained experimental results provide important information on improving properties of NiCuZn ferrites for multilayer chip inductor application.
Temperature (°C) Fig. 9. Temperature dependence of dielectric constant for NiCuZn ferrites doped with MnO2.
Dielectric loss tangent
1.0
This work has been financially supported by the National High-tech Development Plan of China, the National Natural Science Foundation of China (Grant No. 59995523), and the National Major Fundamental Research Project (Grant No. 2002CB613307).
x=0.02 x=0.04 x=0.06 x=0.08 x=0.10
0.8
Acknowledgements
0.6 0.4
References 0.2 0.0
0
50 100 Temperature (°C)
150
Fig. 10. Temperature dependence of dielectric loss tangent for NiCuZn ferrites doped with MnO2.
4. Conclusion The experimental results presented above indicate that MnO2 addition has significant effect on the magnetic and electrical properties of lowtemperature sintered NiCuZn ferrites. Mn substitution in NiCuZn ferrites also remarkably affects the dependencies of magnetic and electric properties on frequency and temperature. With increasing MnO2 content, the resonant frequency and the Curie temperature decrease, whereas the initial permeability increases first, takes a maximum at MnO2 content of 0.06 and then decreases. The dielectric constant and loss tangent (tan d)
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