© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 205, No. 10, 2464–2468 (2008) / DOI 10.1002/pssa.200723525 p s sapplications and materials science

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Electrical properties of yttrium-doped Zn and Ni–Zn ferrites

Goran Stojanovic*, 1, Vladimir Srdic2, and Marija Maletin2

1 Department of Electronics, Faculty of Technical Sciences, University of Novi Sad, T. D. Obradovica 6, 21000 Novi Sad, Serbia 2 Department of Materials Engineering, Faculty of Technology, University of Novi Sad, B. C. Lazara 1, 21000 Novi Sad, Serbia

Received 28 October 2007, revised 6 April 2008, accepted 25 May 2008

Published online 16 July 2008

PACS 68.37.Hk, 75.50.Gg, 75.50.Tt, 77.22.Ch

* Corresponding author: e-mail sgoran@uns.ns.ac.yu, Phone: +381 21 485 25 52, Fax: +381 21 475 05 72

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction With the rapid development of mobile communication and information technology, electronic components with small size, low cost and high perform-ance are in demand. The size, performance and reliability make surface mounting device (SMD) chip inductors very attractive for a wide range of applications, such as EMI suppression in universal series bus (USB), low-voltage dif-ferential signalling and in other high-speed digital inter-faces incorporated in notebooks and personal computers, digital cameras and scanners. These chip inductors are typically fabricated by coating ferrite and electrical paste alternately and then co-firing. Ni–Zn ferrite is usually used as a magnetic material for multilayer chip inductors due to its low sintering temperature and dominant electrical prop-erties at high frequencies. The high resistivity, low dielec-tric loss, mechanical hardness, high Curie temperature and chemical stability are also very useful factors for many modern technological applications. The physical properties of such ferrites depend on the method of preparation, sintering time and temperature and the amount and type of substituent. The selection of an ap-propriate method of synthesis is essential in obtaining good-quality ferrites. In order to use Ni–Zn ferrites in mul-tilayer chip components, the sintering temperature must

not be over the melting point of Ag. Therefore, several low-temperature synthesis methods have been developed to obtain nanostructured Ni–Zn ferrites: the citrate precur-sor method [1, 2], reverse micelle technique [3–6], hydro-thermal technique [7], sol–gel method [8], co-precipitation and calcinations [9]. Each method has some advantages and disadvantages. For example, the citrate precursor wet chemical method has unique advantages in terms of obtain-ing nanoparticles that can be densified easily at lower tem-perature; the reverse micelle chemical synthesis method is a popular technique which permits a narrow distribution of nanocrystalline particles to be obtained; hydrothermal pro-cessing has been found to be a very effective method to prepare nanophase ferrite in a controlled manner; and sol–gel techniques offer low-temperature processing and short fabrication times at comparatively low cost. In the work reported in this paper a low-temperature chemical co-precipitation method was used to synthesize different com-positions of nanostructured Ni–Zn ferrites. Recently, many authors [10–13] have analysed the in-fluence of Fe substitution by rare earth cations into spinel structure of Ni–Zn ferrite. Their investigations show im-portant modifications of the electrical and magnetic prop-erties. Moreover, some authors have dealt with the dielec-

Three types of zinc ferrite samples ZnFe2–x

YxO

4 (x = 0, 0.3

and 0.6) and two of nickel–zinc ferrites Ni0.5

Zn0.5

Fe2–x

YxO

4

(x = 0 and 0.3) were prepared by a low-temperature chemical

co-precipitation method in order to examine the influence of

yttrium ions on the electrical properties of these nanostruc-

tured ferrites. Plots for the real and imaginary parts of the

permittivity, tangent loss and resistivity of these samples were

obtained as a function of frequency using capacitance meas-

urements of an impedance analyser in the frequency range

from 100 Hz to 40 MHz at room temperature. It is observed

that with appropriate addition of yttrium ions, dielectric con-

stant and dielectric loss tangent can be decreased and resistiv-

ity increased.

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tric behaviour of Mn–Zn ferrites [14–16] or with the mag-netic properties of nanostructured ferrites [17–21] for high-frequency or microwave applications. Apart from magnetic properties, a very important role from a practical point of view is played by the electrical properties of these ferrites. The effect of atmosphere on the characteristics of Ni–Zn ferrites with different stoichiometry has been inves-tigated in the past [22]. Authors have shown that the resis-tivity is reduced by decreasing iron excess in high oxygen leading to lower Fe2+ level. Ni–Zn ferrites for high-fre-quency switching power supply applications have also been reviewed [23]. Changes in density, grain size and mi-crostructure of these ferrites were correlated to zinc loss at elevated temperature. Moreover, the dielectric behaviour of the Ni–Zn ferrites (where 0 ≤ x ≤ 1) as a function of frequency, composition and temperature has been reported [24]. It was shown that the real dielectric constant and dielectric loss factor increase as the Zn2+ ion substitution increases while the number of Ni2+ ions decreases. Re-sistivity of Ni–Zn ferrites of different composition, Ni1–xZn

xFe2O4 (with x = 0.2, 0.35, 0.5 and 0.6), has been

studied as a function of composition [25], and the tempera-ture dependence for the same samples has been investi-gated [26]. Additionally, electrical conductivity and dielec-tric measurements have been performed for nanocrystalline NiFe2O4 spinel for four different average grain sizes, rang-ing from 8 nm to 97 nm [27]. The main conclusions from these studies are (i) resistivity generally increases with de-creasing grain size; (ii) high resistivity makes these ferrites suitable for high-frequency applications where eddy cur-rent losses are required to be low; and (iii) ferrites substi-tuted with rare earth ions have an improved microwave be-haviour compared to non-substituted samples. The present work reports the synthesis of zinc and nickel–zinc ferrite samples of composition ZnFe2–xYx

O4 (x = 0, 0.3 and 0.6) and Ni0.5Zn0.5Fe2–xYx

O4 (x = 0 and 0.3) with ultrafine nanoparticles using a low-temperature co-precipitation technique. In addition, the possibility of introducing small amounts of yttrium ions into the lattice, using the same preparative method, is also investigated. Furthermore, an important objective is to study the effect of Y dopant on the real and imaginary parts of the dielec-tric constant (ε) as well as dielectric loss factor (tan δ) in the frequency range from 100 Hz to 40 MHz for these fer-rite systems.

2 Experimental 2.1 Sample preparation Nanoparticles of compo-sition ZnFe2–xYx

O4 (with x = 0, 0.3 and 0.6) and Ni0.5Zn0.5Fe2–xYx

O4 (where x = 0 and 0.3) were prepared by a low-temperature chemical co-precipitation method [28] using aqueous solutions of nitrate precursors. Stoichio-metric amounts of Fe(NO3)3 ⋅ 9H2O (Merck, Germany), Zn(NO3)2 ⋅ 6H2O (Merck, Germany) and Ni(NO3)2 ⋅ 6H2O (Fluka, Germany) were dissolved in distilled water and mixed with appropriate amounts of aqueous solution of YCl3 (Merck, Germany). The co-precipitation reaction was

Figure 1 Representative SEM micrograph of a pressed pellet.

carried out at 80 °C for 60 min under continuous stirring in the presence of sodium hydroxide as a precipitating alkali. Excess sodium hydroxide was not used for the precipita-tion in order to avoid the formation of soluble sodium zin-cate and thus disturb the stoichiometry of synthesized fer-rite nanopowders. The precipitates were separated from the slurry by centrifuging and washed a number of times with distilled water and then with absolute ethanol. The nanoparticles formed were dried at 120 °C for 1 day and finally dry milled in a mortar. Green pellets having a thickness of 1 mm and a diameter of 10 mm were prepared from as-synthesized nanopowders by uniaxially pressing, with a classic hydraulic press, in a hard metal at room tem-perature and 500 MPa. The pressed pellets obtained had densities of around 48% and very fine microstructures, as can be seen in Fig. 1. The surfaces of samples were grounded and polished to be parallel and smooth and after that coated with silver paste as a contact material and con-nected with very short lead copper wires in order to per-form electrical measurements.

2.2 Measuring technique Several techniques can be used for experimental determination of electrical properties of ferrite materials and they very depend on the frequency range. In this study, capacitance measurements were car-ried out with a HP-4194A impedance/gain-phase analyser using a HP-16047A test fixture, in the frequency range from 100 Hz to 40 MHz at room temperature. A personal computer with in-house-built software was used for acqui-sition of measured data. The real and imaginary parts of dielectric constant (ε′, ε″), dielectric loss factor (tan δ), and resistivity (ρ) were calculated from the measured data. Fer-rite samples prepared in the capacitor form can be consid-ered electrically equivalent to a capacitance Cp in parallel with a resistance Rp as illustrated in Fig. 2. These values, Cp and Rp, were measured directly using the impedance analyser and important electrical parameters

2466 G. Stojanovic et al.: Electrical properties of yttrium-doped Zn and Ni–Zn ferrites

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

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can be calculated using the following formulas:

p

0

,

C

Cε =¢ (1)

p 0

1,

R Cε

ω

=¢¢ (2)

tan ,ε

δε

″

′

= (3)

0

1,ρ

ωε ε″= (4)

where C0 is the sample capacitance in vacuum (C0 = Aε0/d, which depends on the electrode spacing (d) and the elec-trode area (A)), ω is the angular frequency of the applied field and ε0 is the permittivity in vacuum equal to 8.85 × 10–2 pF/cm.

3 Results and discussion In our earlier work [29] we have already presented the effect of yttrium and indium doping on the structure of zinc ferrite nanoparticles. It was found that Y-doped zinc ferrite nanoparticles, with a single spinel phase and a size of a few nanometres, are formed in the whole range of investigated yttrium concentration (ZnFe2–xYx

O4; x ≤ 0.6). It is well known that Zn and Ni–Zn ferrites represent suitable candidates among several ferrite materials with good potentials for high-frequency applica-tions. Therefore it is very important to know the frequency dependence of electrical parameters of these nanocrystal-line ferrite green pellets in a wide frequency range. The conductivity in ferrites occurs mainly due to hop-ping of electrons between ions of the same element exist-ing in different valence state, distributed over crystal-lographically equivalent lattice sites. In the investigated ferrites, Fe2+ ions were created during sample preparation providing an increase in electron hopping between the Fe ions in B sites in 2+ and 3+ valence states. The variation of resistivity of the ferrite samples under investigation with frequency is shown in Fig. 3. It can be seen that the resistivity decreases with increasing fre-quency. At low frequency, where the resistivity has the highest value and the grain boundary effect is dominant, more energy is required for exchange electrons between

Fe2+ and Fe3+ ions located on the grain boundaries and as a result dielectric losses are high. The pure ZnFe2O4 pellet has the lowest resistivity, which can be increased with the addition of Ni. It is known that Ni2+ ions occupy octahedral B sites, then Fe3+ ions move from B to A sites. As a conse-quence, the probability of electron hopping between Fe3+ and Fe2+ at the B sites decreases, resulting in an increase of resistivity. Addition of Y3+ ions also increases resistivity (Fig. 3). Y3+ ions have a high preference for octahedral B sites substituting the Fe3+. Therefore the number of Fe3+ ions at B sites decreases, the probability of hopping is re-duced and thus resistivity increases. Due to low processing temperature, these samples possess smaller grain size (nanoparticles) and consequently a greater number of grain boundaries. The grain boundaries are the regions of mismatch between energy states of adjacent grains and hence act as a barrier to the flow of electrons. This phe-nomenon leads to the high value of resistivity for the sam-ples under investigation in this paper and accordingly low eddy currents and dielectric losses. The resistivity for the Ni0.5Zn0.5Fe2O4 sample at low frequency of around

Figure 3 (online colour at: www.pss-a.com) Variation of

resistivity (ρ) with frequency for five nanostructured ferrite sam-

ples.

Figure 2 Schematic representation of prepared

sample and its equivalent circuit.

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Figure 4 (online colour at: www.pss-a.com) Variation of the real

part of permittivity (ε′) with frequency. 108 Ω cm is comparable with the values for dc resistivity presented in [25] and much higher then those (around 106 Ω cm) reported [30, 31] for Ni–Zn ferrites prepared by the conventional ceramic method. The variation of the real part of the dielectric constant with frequency is depicted in Fig. 4. The real part of the dielectric constant decreases continuously with increasing frequency. This is a consequence of the fact that beyond a certain frequency of applied electric field, the electronic exchange between Fe2+ and Fe3+ cannot follow the alternat-ing field. Starting from low frequency, curves for ε ′ decrease sharply, and the decreasing trend becomes slower at high frequencies. This decreasing trend at higher frequencies can be explained by the Maxwell–Wagner theory, which is a result of the inhomogeneous nature of dielectric struc-ture. The maximum value of ε ′ in the range between 80 (for Ni0.5Zn0.5Fe1.7Y0.3O4) and 550 (for ZnFe1.4Y0.6O4) has been obtained at 100 Hz whereas the ZnFe2O4 sample has the biggest values of ε′ in the whole frequency range. The real dielectric permittivity in the range between 4.5 and 20 is obtained for frequencies greater than 1 MHz for samples analysed in this paper. The similar frequency behaviour of dielectric constant has been reported earlier [32] for the Ni0.5Zn0.5Fe2O4 sample. However, a higher value around 80 (using flash combustion technique) was observed at a fre-quency of 1 MHz compared to approximately 20 for the same frequency in this paper. The imaginary part of the dielectric constant as a func-tion of the frequency is shown in Fig. 5. It can be seen that ε″ decrease as the frequency increases. The curves show similar behaviour to the real part of the permittivity but with slightly higher values of the relative dielectric con-stant. This can be explained by a correlation between the

Figure 5 (online colour at: www.pss-a.com) Variation of the

imaginary part of permittivity (ε″) with frequency.

conduction mechanism (due to hopping of electrons be-tween Fe2+ and Fe3+) and dielectric properties of ferrites. The dielectric loss arises from the delay of the polarization behind the applied alternating electric field and is caused by the impurities and imperfections in the crystal lattice. Figure 6 shows a plot of the dielectric loss tangent (tan δ) versus frequency at room temperature. It can be seen that a loss peak with high intensity is observed for ZnFe2O4 at frequencies around 1 kHz (a peak around the same frequency has been reported earlier [24]) and after that for the Ni0.5Zn0.5Fe2O4 sample, around 120 Hz. These

Figure 6 (online colour at: www.pss-a.com) Variation of the di-

electric loss factor (tan δ) with frequency.

2468 G. Stojanovic et al.: Electrical properties of yttrium-doped Zn and Ni–Zn ferrites

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dielectric relaxation peaks take place when the jumping frequency of the localized electric charge carrier becomes approximately equal to that of the externally applied ac electric field. For samples with x = 0.3, the dielectric loss tangent has smaller values than for the other samples ana-lysed and the peak is shifted towards lower frequencies. The values of the dielectric loss for all presented samples are much lower in comparison with the Ni1–xZn

xFe2O4

(with 0 ≤ x ≤ 1) ferrite samples reported in [24], where tan δ has a value around 5 at frequencies around 100 kHz. It can be seen from Figs. 4–6 that the values of ε′, ε″ and tan δ of the samples decrease with increasing Y con-tent until some concentration (x), and it is obvious that above-mentioned parameters have higher values for x = 0.6 than for x = 0.3. These results are in accordance with con-clusions in [15], where it has been illustrated that beyond x = 0.4, these electrical parameters show an increase with an increase of the rare earth metal content. However, the results presented in [15] are in the narrow frequency range 1 kHz–13 MHz, whereas this work presents values of all electrical parameters in the wider frequency range from 100 Hz to 40 MHz. 4 Conclusions This paper demonstrates that the low-temperature chemical co-precipitation method can be ap-plied for successful preparation of yttrium-doped Zn and Ni–Zn ferrites for high-frequency applications. The elec-trical parameters (real and imaginary part of permittivity, dielectric loss factor and resistivity) of uniaxially pressed samples were evaluated through impedance analyser me-asurements and studied as a function of frequency. High electrical resistivity and low dielectric loss are a result of appropriate control of chemical composition and nano-structure of the ferrites. Nanostructured samples presented in this paper can find various technological applications in electronics and telecommunications fields. The presented results would be very useful for selection of an appropriate ferrite material for the desired operating frequency.

Acknowledgements This work was supported by the Min-

istry of Science, Republic of Serbia, project number 142059.

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