Temperature and frequency-dependent dielectric properties of Zn substituted Li–Mg ferrites

https://doi.org/10.1016/S0304-8853(99)00138-9Get rights and content

Abstract

The DC and AC resistivity (ρDC,ρAC), dielectric constant (ε′) and dielectric loss tangent (tan δ) of Zn substituted Li–Mg ferrites having the general formula LixMg0.4 Zn0.6−2xFe2+xO4 (where x=0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) have been investigated as a function of composition, temperature and frequency. The compositional variation of DC resistivity and dielectric constant show the inverse trend with each other. The sample with x=0.15 (Zn=0.3) shows lowest DC resistivity and highest dielectric constant. The dielectric constant increases slowly with temperature in the beginning and then abruptly at about 473 K and above. The AC resistivity and dielectric constant of all the samples decrease with increase in frequency exhibiting normal ferrimagnetic behaviour. The variation of dielectric loss tangent with frequency showed maxima in the 2–40 kHz frequency range. These maxima are also found to shift towards low-frequency region as the content of Zn increases. All the variations are explained on the basis of Fe2+ and Fe3+ concentrations on octahedral sites and electronic hopping frequency between Fe2+ and Fe3+ ions.

Introduction

The dielectric properties of ferrites are dependent upon several factors namely, chemical composition, method of preparation, grain size etc. The ferrites behave as inhomogeneous dielectric materials in which individual high-conducting grains are separated by either air gaps or low-conducting layers. The dielectric constants as high as 105 at low frequencies are observed in case of ferrites. For microwave applications, high resistivity with low dielectric losses are needed.

In the early days, Mg–Zn, Ni–Zn and Mn–Zn ferrites were used for the microwave devices, which were later replaced by garnets having high resistivity and low dielectric losses. But because of low Curie temperature, high stress sensitivity and high costs of garnets, once again the physicists and technologists have returned to ferrites especially Li ferrites which have become important materials for microwave applications because of their low costs, squareness of hysteresis loop along with high Curie temperature and low dielectric losses. Many researchers have studied Li–Cd [1], [2], Li–Zn [3], Li–Mg [4] and Li–Ni [5] ferrites. However, no reports have been found in the literature about Zn substituted Li–Mg ferrites so far. Therefore, in the present communication, we report on the dielectric properties of these ferrites.

Section snippets

Experimental procedure

Zn-substituted Li–Mg ferrites having the general formula LixMg0.4Zn0.6−2xFe2+xO4 (where x=0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) were prepared by conventional double sintering ceramic method. The presintering of the samples was carried at 873 K for 12 h and the final sintering at 1273 K for 24 h. The single-phase formation of the ferrites was confirmed by X-ray diffraction patterns obtained by using PHILIPS PW-1710 diffractometer with Cu Kα radiation (λ=15.418 nm). The DC resistivity (ρDC) and

Compositional variation of ε′ and ρDC

Fig. 1 shows the variations of dielectric constant and DC resistivity with zinc content. From this figure it is clear that ε′ and ρDC have inverse trend with each other. The DC resistivity initially found to decrease while the dielectric constant found to increase with increase in Zn content up to 0.3. The sample with Zn =0.3 has minimum resistivity ρDC=2.5×105 Ω- cm and maximum dielectric constant ε′=2.6×104. Further, with the addition of Zn beyond 0.3, ρDC found to increase while ε′ found to

Acknowledgements

The authors thank Prof. Rohini C. Ayyer, Microwave Lab, Department of Physics, University of Poona, Pune (India) for extending their helping hands during experimental measurements.

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