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Enhancement in Dielectric and Magnetic Properties of Substituted Ni-Zn Nano-Ferrites by Coprecipitation Method Atul Thakur , Preeti Thakur , and Jen-Hwa Hsu Department of Physics, National Taiwan University, Taipei 106, Taiwan Physics Department, Himachal Pradesh University, Shimla 171005, India Indium substituted nano nickel zinc ferrites with 0, 0.1, 0.2 and 0.3 are synthesized by a coprecipitation method. Magnetic and dielectric properties are studied over a frequency range 1–30 MHz. Initial permeability is found to be in the range of 9–19 with a magnetic loss tangent of – . The dielectric constant is noticed in the range of 11–17 along with low di– ). The composition with is found to have the highest specific saturation magnetization of 78.8 electric loss tangent ( emu/g. Appreciable dielectric and magnetic values along with low losses can be correlated to small grain size and better compositional stoichiometry obtained as a result of processing by coprecipitation method. These fascinating dielectric and magnetic properties of these materials reveal a direction for high-frequency applications. Index Terms—Dielectric devices, ferrites, magnetic losses, permeability.
I. INTRODUCTION
I
N the last decade, due to the progress in synthesis techniques, a great interest was paid to the ultra-fine soft nanoferrite materials. The powders with the particle size in the nanometric scale expand their scope for use in device applications. Increased surface/volume ratio, surface morphology and dissolution of higher concentrations of impurities in nanocrystalline materials compared to their bulk counterparts could be the possible reasons for advantages in these materials [1]. Ni-Zn nanoferrites have spinel structure and have fascinating properties like high permeability, high electrical resistivity, low eddy current losses, high Curie temperature, mechanical hardness, chemical stability and reasonable cost. These properties of ferrites are highly sensitive to the cation distribution which in turn depends on the method of preparation and doping. Thus, proper selection of dopant and controlled synthesis of nanocrystallites enable us to tune their fundamental properties, which will lead to a high performance material. ions are quite effective to replace ions and are expected to reduce anisotropy constant as well as magnetostriction in ferrites [2]. Therefore, in the present study, substituted Ni-Zn nano-ferrites were prepared by a coprecipitation method and an enhancement in dielectric and magnetic properties were noticed. II. EXPERIMENTAL DETAILS with 0, 0.1, 0.2, and 0.3 were prepared by a coprecipitation method [3]. High purity chemicals nickel chloride hexahydrate, zinc chloride, indium chloride hexahydrate and iron (III) chloride hexahydrate were taken in proper stoichiometric proportion and dissolved in the boiling solution of 0.40 M NaOH under vigorous stirring for 40 minutes.
Manuscript received February 21, 2011; revised April 21, 2011; accepted April 27, 2011. Date of current version September 23, 2011. Corresponding author: J.-H. Hsu (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2156394
After the suspension was cooled to ambient temperature, precipitate was washed carefully with distilled water several times to remove impurities and centrifuged. The residue was dried in an electrical oven at 80 C for overnight. The powders were pre-sintered at 500 C in air for 3 h at a heating and cooling rate of 200 C/h. Then, they were pressed into torroids and pellets under a pressure of 55 MPa using a uniaxial hydraulic press. These samples were sintered in air at 900 C for another 3 h at a heating and cooling rate of 200 C/h in box type furnace. X-ray diffraction (XRD) measurements were performed on a Philips PW 1729 diffractometer using radiation. Transmission electron microscopy (TEM) images were obtained from Jeol JEM-100CX. Magnetic and dielectric properties were measured over 1–30 MHz by using an Agilent Technologies 4285A precision LCR meter. Magnetization curves were recorded on vibrating sample magnetometer (VSM). All the measurements were performed at room temperature. III. RESULTS AND DISCUSSION The crystal structure of prepared nano-ferrites is checked by XRD as shown in Fig. 1. A typical spinel structure is noticed which is maintained upon ion substitution in Ni-Zn ferrites. This indicates that ions are properly dissolved in the lattice structure of Ni-Zn ferrite system. An average crystallite diameter determined by using Scherrer’s formula is found to increase from 19 nm to 32 nm with ions substitution which may be preferably due to the smaller size ions (0.067 nm) being successively replaced with ions of larger size (0.091 nm) [4]. Consequently, the values of lattice parameter, a, were found to be 8.345, 8.452, 8.512, and 8.634 with 0, 0.1, 0.2 and 0.3 respectively. Therefore, substitution of ions for ions also leads to expansion of unit cell [5]. Fig. 2 shows the TEM image of which indicates that the particles are spherical and narrowly distributed. From the figure, it is clear that the particles are spherical in shape and are agglomerated to some extent. This agglomeration can be attributed to the magnetic dipole interactions arising between ferrite particles [6]. The estimated average particle sizes are around 35 nm. This result agrees well with that estimated
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THAKUR et al.: ENHANCEMENT IN DIELECTRIC AND MAGNETIC PROPERTIES OF
Fig. 1. XRD patterns of
with
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0, 0.1, 0.2 and 0.3.
Fig. 3. (a) Variation of dielectric constant, and (b) dielectric loss tangent, , with frequency over a range 1–30 MHz.
Fig. 2. TEM image of
.
using XRD measurements. The average crystallite diameter is found to be quite consistent with TEM results. The dielectric constant is found to be in the range of 11–17 at room temperature, as shown in Fig. 3(a). The dielectric constant of any material, in general, is due to dipolar, electronic, ionic, and interfacial polarizations. At low frequencies, dipolar and interfacial polarizations are known to play the most important role. At high frequencies, electronic and ionic polarizations are the main contributors [3]. Therefore, in the present study, consistent and appreciable values of dielectric constant are due to electronic and ionic polarizations. Sivakumar et al. [7] have obtained a value of about 10 for dielectric constant at room temperature and at 1 MHz, for the 14 nm grains of ferrites. In our measurement, the dielectric constant is found to be almost uniform and comparatively higher at the same temperature and frequency up to 30 MHz. The dielectric behaviour of ferrites is strongly related to their conduction mechanism [8]. The electron exchange between ions and ions, and hole transfer between ions and ions at the octahedral sites can result in the local displacement of charge carriers in the direction of applied electric field, which determines the polarization of ferrites [9]. Comparatively high value of dielectric constant may be explained in terms of bigger grain size and better compositional stoichiometry with a relatively smaller concentration of easily polarizable ions. This is attributed to the lower sintering temperature made possible by the coprecipitation technique, which results in small grain sizes and re-
duced Zn loss compared to the conventional ceramic route [7], where the processing temperature and grain size are relatively higher. These observations show that low dielectric constants are associated with stoichiometric ferrites. The dielectric constant is found to decrease with increasing ions concentration. This decrease in dielectric constant can be explained as ions are substituted for ions, the concentration of ions is decreased and the rate of hopping process is reduced. Exceptional behavior is noticed at . The observed trends of dielectric constant indicate a possible presence of resonance with a peak occurring at a frequency above 30 MHz. The dielectric loss tangent is found to be of the order of – as shown in Fig. 3(b). Dielectric loss in polycrystalline ferrites is a result of the lag in polarization vis-á-vis the alternating electric field. This lag increases due to the presence of impurities and imperfections in the ferrite structure, thereby increasing the dielectric loss. A major contribution to dielectric losses in ferrites comes from electron hopping between the and ions [10]. Dielectric losses obtained in the present work are of the order of – , which are lower by at least one to two orders of magnitude compared with those obtained for the conventionally processed samples [11]. It is normally difficult to obtain low losses, especially in ferrites containing zinc. This is due to the volatile nature of zinc that results in defect structures. The lower values of dielectric loss obtained in the present work can be attributed to the curtailing of the ions on account of the coprecipitation process, resulting in better compositional stoichiometry and crystal structure [12]. Initial permeability, , is found to be in the range of 9–19 with a magnetic loss tangent of – as shown in Fig. 4(a) and (b), respectively. Permeability is known to be dependent on density and porosity [13]. Microstructural features such as porosity and grain size and its distribution determine the magnetic properties of the Ni-Zn ferrites. It is well known that the initial permeability characteristics depend not only on chemical composition but also on the microstructure of the
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
Fig. 5. Hysteresis loops for 0.3. Inset depicts measurements close to origin.
Fig. 4. (a) Variation of initial permeability, , and (b) magnetic loss tangent, , with frequency over a range 1–30 MHz.
with
0, 0.1, 0.2, and
A or B-sites. It is well known that the net magnetization is the result of difference in magnetization between A and B-sites by the relation (2)
sintered body. Desired magnetic properties of ferrite can be achieved by the control of microstructures. All the samples are found to obey Snoek’s law [3]
As diamagnetic indium ions are substituted for iron ions, they will replace iron at A-site up to . Moreover, the permeability of ferrite materials is related to saturation magnetization and coercivity, via relation
(1) is the saturation magnetization, is the gyromagnetic where ratio and is the static permeability that can be defined as the real permeability far below the relaxation frequency. Small and uniform grain size is favorable to obtain low power loss, but large grain size is favourable to get high permeability [13]. The observed values in the present study may be attributed to the rotational contributions only. As compositional stoichiometry and small grain size play a crucial role in modifying the ferrite properties, coprecipitation technique could be especially advantageous in processing ferrites intended for high frequency applications. is found to be appreciable as the sample is formed at a low temperature and the grain size is found to be small and uniform, which results in a single-domain structure with uniform magnetization [3]. The resonance peak, which occurs when the frequency of applied field equals the Larmor precession of electron spins, could not be observed in the present technique. The main reason is that, as the grain size becomes smaller and uniform, the resonance character vanish [14]. Similar trend of results was found by Rado et al. [15] and Snoek [16]. The other reason for the absence of resonance peak may be because they probably lie beyond the measurable frequency range. -H curves at room temperature are recorded for all the samples as shown in Fig. 5. Specific saturation magnetization is found to improve with the substitution of ions. The substitution with ions is expected to reduce anisotropy constant as well as magnetostriction in ferrites. The observation of hysteresis loops at room temperature is consistent with the occurrence of a ferrimagnetic ordering in these nanomaterials. An increase of 25% in for is observed. This may be attributed to cation distribution of doped Ni-Zn ferrite over
(3) Therefore, for , higher will lead to an increased value of [see Fig. 4(a)]. The increased value of is quite suitable to get higher cut-of frequency and this material can be suitable for high frequency applications. Beyond , the decrease in may be explained as indium will replace iron at B-site, which in turn decrease and hence, . The coercive field of the samples at 0, 0.1, 0.2 and 0.3 are 95.9, 81.2, 62.3 and 75.8 Oe, respectively. These low values of coercive field are in agreement with the well established soft magnetic character of nanoferrites [17]. These results, especially for , make this material suitable for high frequency applications. IV. CONCLUSION The experimental results show that the substitution of indium ions for iron ions in is found to play a crucial role on the electromagnetic properties of final products. The particular composition with has sustained and higher permeability than permittivity and fairly low loss tangents over the desired frequency range. Our experimental data demonstrate that the proposed nanoferrite prepared by a co-precipitation method with improved properties could be a good candidate as a magnetodielectric material for the high frequency applications. ACKNOWLEDGMENT This work was supported by the National Science Council of Taiwan under Contract NSC-99-2112-M-002-020-MY3
THAKUR et al.: ENHANCEMENT IN DIELECTRIC AND MAGNETIC PROPERTIES OF
and the Ministry of Economic Affairs of Taiwan under Grant 99-EC-17-A-08-S1-006.
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[7] N. Sivakumar, A. Narayanasamy, N. Ponpandian, and G. Govindaraj, “Grain size effect on the dielectric behavior of nanostructured ,” J. Appl. Phys, vol. 101, p. 084116, 2007. and some [8] K. Iwauche, “Dielectric properties of fine particles of ferrites,” Japan J. Appl. Phys., vol. 10, pp. 1520–1528, 1971. [9] C. Parkash and J. S. Baijal, “Dielectric behaviour of tetravalent titanium-substituted Ni-Zn ferrites,” J. Less-Common Met., vol. 107, pp. 51–57, 1985. [10] L. G. Van Uitert, “Dielectric properties of and conductivity in ferrites,” Proc. IRE, vol. 44, pp. 1294–1303, 1956. [11] A. Thakur and M. Singh, “Preparation and characterization of nanosize ferrite by citrate precursor method,” Ceram. Int., vol. 29, pp. 505–511, 2003. [12] D. W. Johnson, Jr. and B. B. Ghate, “Scientific approach to processing of ferrites,” in Advances in Ceramics, F. Y. Wang., Ed. Columbus, OH: American Ceramic Society, 1985, vol. 15, Fourth International Conference on Ferrites, pp. 27–38. [13] T. Y. Byun, S. C. Byeon, K. S. Hong, and C. K. Kim, “Factors affecting initial permeability of Co-substituted Ni-Zn-Cu ferrites,” IEEE Trans. Magn., vol. 35, no. 5, pp. 3445–3447, May 1999. [14] P. Mathur, A. Thakur, and M. Singh, “Low temperature processing of Mn-Zn nanoferrites,” J. Mater Sci., vol. 42, pp. 8189–8192, 2007. [15] G. T. Rado, “Magnetic spectra of ferrites,” Rev. Mod Phys., vol. 25, pp. 81–89, 1953. [16] J. L. Snoek, New Developments in Ferromagnetic Materials. New York: Elsevier Publishing, 1947. [17] S. Chkoundali, S. Ammar, N. Jouini, F. Fiévet, P. Molinié, M. Danot, F. Villain, and J.-M. Grenèche, “Nickel ferrite nanoparticles: Elaboration in polyol medium via hydrolysis, and magnetic properties,” J. Phys.: Condens. Matter, vol. 16, p. 4357, 2004.
