Electromagnetic and microwave absorption properties

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The hexaferrite compositions were synthesized by solid-state reaction technique, ... 2007 American Institute of Physics. ... 1114; electronic mail: rmala@physics.iitd.ac.in ..... Singh, V. K. Babbar, A. Razdan, R. K. Puri, and T. C. Goel, J. Appl.
JOURNAL OF APPLIED PHYSICS 101, 074105 共2007兲

Electromagnetic and microwave absorption properties of „Co2+ – Si4+… substituted barium hexaferrites and its polymer composite S. M. Abbas and R. Chatterjeea兲 Department of Physics, IIT Delhi, Hauz Khas, New Delhi-110016, India

A. K. Dixit and A. V. R. Kumar DMSRDE, G. T. Road, Kanpur-208013, India

T. C. Goel BITS, Pilani-Goa Campus, Zuari Nagar, Goa-403726, India

共Received 27 October 2006; accepted 1 February 2007; published online 4 April 2007兲 The electromagnetic 共EM兲 and microwave absorption properties of 共Co2+ – Si4+兲 substituted barium +2 4+ +3 hexaferrite compositions BaCo2+ x Fey Six+y Fe12−2x−2y O19 共x = 0.9 and y = 0.0, 0.05, and 0.2兲 and its polymer composites prepared from hexaferrite, polyaniline, and carbon powders dispersed in polyurethane matrix have been investigated at the microwave frequency range of the X band 共8.2– 12.4 GHz兲. The hexaferrite compositions were synthesized by solid-state reaction technique, whereas polyaniline, by chemical route. The permeabilities of a ferrite are drastically reduced at higher gigahertz frequencies. The permittivities, however, can be enhanced by appropriate choice of composition and processing temperature. In the present ferrite composition, silicon content is taken in excess so as to convert some of the Fe3+ ions to Fe2+ ions. This conversion has been shown to enhance EM and absorption properties. Mössbauer spectroscopy on the samples establishes that addition of excess Si4+ converts some of the Fe3+ to Fe2+. The sintered ferrites have shown resonance phenomena, but the composites do not. The EM parameters ␧⬘, ␧⬙, ␮⬘, and ␮⬙ were measured using a vector network analyzer 共Agilent, model PNA E8364B兲. These measured EM parameters were used to determine the absorption spectra at different sample thicknesses based on a model of a single layered plane wave absorber backed by a perfect conductor. The sintered ferrite composition 共x = 0.9 and y = 0.05兲 showed the best absorption properties 关a minimum reflection loss of −17.7 to − 14.3 dB over the whole frequency range of the X band 共8.2–12.4兲 for a sample thickness of just 0.8 mm兴, and it is used in the composite absorbers in powder form along with other constituents. The optimized composite absorber has shown dielectric constant ␧⬘ ⬃ 11.5, dielectric loss ␧⬙ ⬃ 2.3, and a minimum reflection loss of −29 dB at 10.97 GHz with the −20 dB bandwidth over the frequency range of 9.7– 12.2 GHz for a sample thickness of 2.0 mm. The magnetic parameters ␮⬘ and ␮⬙ for the composite remained nearly 1 and 0, respectively, throughout the measured frequency range. Both sintered ferrite and composite absorbers can fruitfully be utilized for suppression of electromagnetic interference and reduction of radar signatures 共stealth technology兲. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2716379兴 I. INTRODUCTION

Due to saturation in the sub-gigahertz range, microwaves in the higher gigahertz range are being increasingly utilized in wireless communication, radar, local area network, etc. Along with the development of higher gigahertz electronics and the trend towards miniature circuitry, electromagnetic interference 共EMI兲 is also becoming a serious problem and a matter of crucial concern in higher gigahertz range.1 The reduction of electromagnetic backscattering using the microwave absorbing materials therefore has important implications in the field of electromagnetic compatibility2 共EMC兲. Microwave absorbers are also in high demand for defense use. Application of microwave absorbing coating on the exterior surfaces of military aircraft and vehicles helps avoid detection by radar.3 The development of electromagnetic a兲

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wave absorber to improve front to back ratio of a 2 GHz antenna can be traced back to the mid-1930s. Emerson4 had given a historical summary on the development of microwave absorbers in the early periods. Tiny iron balls, aluminum flakes, carbon black, etc., were early absorbing materials. In these materials, electromagnetic 共EM兲 and absorption properties are fixed, and final absorption tunability only depends on their contents. Contrary to this, M-type hexaferrites can provide tailor made properties; their large tunable anisotropy field 共Ha = 2K / M s兲 causing magnetic resonance in 2 – 52 GHz has been extensively exploited in recent years for preparation of microwave absorbers in higher gigahertz range.5–10 In general, ferrites exhibit absorption based on magnetic resonance phenomenon, which is either for selective frequencies or over a narrow frequency range. In higher gigahertz range, ferrite’s permeabilities reduce drastically, rendering it less effective for microwave absorption at higher gigahertz. However, ferrites can be engineered to show enhanced dielectric losses by appropriate choice of composi-

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tion and heat treatment. In this report we discuss the detailed results obtained for 共Co2+ – Si4+兲 doped Ba hexaferrite com+2 4+ +3 positions BaCo2+ x Fey Six+y Fe12−2x−2y O19 共x = 0.9 and y = 0.0, 0.05, and 0.2兲. The excess Si+4 共y = 0.05 and 0.2兲 ions are expected to convert some of the Fe+3 into Fe+2 in the ferrite structure to maintain charge neutrality. The electron hopping between Fe+3 and Fe+2 ions is expected to enhance the dielectric loss. The practical absorbers are generally backed by a metal conductor;11 an impedance matching at air to absorber interface is a major issue. Impedance matching is more flexibly achieved in ferrite-polymer composites rather than in sintered ferrites. EM and microwave absorption properties of prepared sintered ferrite compositions were initially investigated at X-band frequencies, and the best one was selected for composite absorber. Composite absorbers then were prepared using different ratios of ferrite, polyaniline, and carbon black. The optimized composite sample with appropriate ratio of its constituents has provided the broadband absorption over the wide frequency range. II. EXPERIMENTAL WORK

Ba hexaferrite powders were prepared by standard ceramic route at 1290 ° C, reported elsewhere,12 and sintered disk shaped samples of 1 in. diameter were also prepared at the same temperature. Polyaniline was synthesized by chemical oxidative polymerization of aniline monomer using ammonium per sulfate as oxidizer in the presence of 1M HCl as reported elsewhere.13 The composite samples were prepared by thoroughly mixing the appropriate amounts of ferrite, polyaniline, and carbon 共Senka Carbon, India兲 powders in two-pack polyurethane 共PU兲 matrix consisting of polyol-8 共Ciba-Geigy, Switzerland兲 and hexamethylene di-isocynate 共E-Merck, Germany兲 taken in equal proportion. The mix was poured in a suitable mold and then cured at 70 ° C. The hexaferrite structure was checked by x-ray diffraction 共XRD兲 technique. The XRD results revealed that the homogeneous phase of M-type Ba hexaferrite was obtained. The Mössbauer spectra on ferrite powder samples were recorded in the constant acceleration mode with the moving ␥-ray source 50 mCi Co57 共in rhodium matrix兲. The Doppler velocity was provided to the emitted ␥ rays by means of an electromechanical transducer. The proportional counter was employed to detect the ␥ rays that were passed through the ferrite sample. Data were collected into 512 channels, each containing ⬃共2 – 3兲 ⫻ 106 counts. This resulted in two mirrorsymmetric 256-channel spectra, one of each of which was preliminarly fitted to get representative spectra. The microwave measurements were carried out on sintered ferrites as well as on composite samples. The samples were shaped to fit exactly into a 0.4⫻ 0.9 in.2 rectangular X-band waveguide 共WR90兲. The complex scattering parameters that correspond to the reflection 共S11 or S22兲 and transmission 共S21 or S12兲 were measured using a vector network analyzer 共Agilent, PNA E8364B兲. Full two port calibration was initially done on the test setup in order to remove errors due to the directivity, source match, load match, isolation, etc., in both the forward and reverse directions. The complex permittivity and permeability were then determined from the

+3 FIG. 1. Mössbauer spectra of hexaferrite BaCoxFe+2 y Six+y Fe12−2x−2y O19: 共a兲 for 共x = 0.9; y = 0.0兲, 共b兲 for 共x = 0.9; y = 0.05兲, and 共c兲 for 共x = 0.9; y = 0.2兲.

measured scattering parameters using Agilent software module 85071 共version E兲, based on the procedure given in the HP product note.14 III. RESULT AND DISCUSSION

Figures 1共a兲–1共c兲 show the Mössbauer spectra for 2+ 4+ 2+ 4+ – Si0.9+0.05 兲, and 共Co0.9 – Si0.9+0.2 兲 sub共Co2+ – Si4+兲0.9, 共Co0.9 stituted barium hexaferrites. The recorded spectra exhibit typical characteristics of substituted M-type hexaferrites. The M-type barium hexaferrites with magnetoplumbite structure contain two formula unit cells, which are made of ten oxygen layers and can be described as RSR*S*, where R 2− = 共Ba2+Fe3+ is a barium containing hexagonal block 6 O11兲 2+ is a spinel with three oxygen layers, while S = 共Fe3+ 6 O 8兲 * * block with only two oxygen layers; R and S are obtained from the R and S blocks, respectively, by a rotation of 180° about the c axis.15 The 24 Fe3+ ions are distributed over five distinct sites:16 three octahedral sites 共12k, 4f VI, and 2a兲, one tetrahedral 共4f IV兲 site, and one bipyramidal site 共2b兲, in the ratio of 6:2:1:2:1, respectively. Magnetically, they form two

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+3 FIG. 2. Real permittivity spectra of hexaferrite BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.0, 0.05, and 0.2兲.

FIG. 3. Imaginary permittivity spectra of +3 BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.0, 0.05, and 0.2兲.

sublattice groups: spin up 共12k, 2a, and 2b兲 and spin down 共4f IV and 4f VI兲 sites along the c axis. The magnetic and dielectric properties of the substituted hexaferrites are strongly dependent on the electronic configuration of the substituting cations and their preferred locations in different sites.16 The final presence of Fe3+ or Fe2+ ions with their respective proportions in different sites gives rise to magnetic hyperfine sextets due to Zeeman and quadrupole splitting. The intensity of each sextet is directly proportional to the number of iron ions in that site. These sites are assigned according to their contributions in Fig. 1共a兲. Four of these may be visually seen. The weak 2a component is buried under the 4f IV peak. Comparing Fig. 1共a兲 with the spectra of pure17 hexaferrite BaFe12O19, one can easily identify that the substitution of Fe3+ by the cation pair 共Co2+ – Si4+兲0.9 reduces the strongest 12k component in the case of pure ferrite. This feature is more or less common to all the three spectra in the present work. Further, in Fig. 1共a兲, the central third and fourth peaks are symmetrical; this indicates that all iron atoms are in the 3+ states. But in Figs. 1共b兲 and 1共c兲 these peaks become asymmetric for cation pair substitutions 2+ 4+ 2+ 4+ – Si0.9+0.05 兲 and 共Co0.9 – Si0.9+0.2 兲 containing excess Si4+ 共Co0.9 ions. This asymmetry is increased with increased Si4+ content 共latter case兲. The asymmetry in the central peaks is attributed to the conversion of Fe3+ ions to Fe2+ for balancing the charge neutrality in the structure.18 Additionally, reduction in 2b component for excess Si4+ cases suggest the strong preference of Si4+ for 2b sites.19 Figures 2 and 3 show the X-band complex permittivity spectra, real 共␧r⬘兲 and imaginary 共␧r⬙兲 parts, respectively, for sintered samples of 共Co2+ – Si4+兲 substituted barium hexafer+2 4+ +3 rites BaCo2+ x Fey Six+y Fe12−2x−2y O19, where x = 0.9 is kept constant and y takes three values: no excess Si4+ 共y = 0.0兲, low excess Si4+ 共y = 0.05兲, and high excess Si4+ 共y = 0.2兲. The ␧r⬘ and ␧r⬙ spectra of all the three sintered ferrite samples have shown good dispersion relation between them and a general decreasing trend with increasing frequency. The maximum obtained values of ␧r⬘ and ␧r⬙ are 38.9 and 31.7 for the y = 0.0 case, 43.5 and 36.2 for the y = 0.05 case, and 63.4 and 57.6 for the y = 0.2 case, respectively. It is evident that both the values have increased with excess Si4+ ions. It is attributed to the conversion of Fe3+ to Fe2+, directly in proportion

to the excess Si4+ ions. This is in good agreement with the findings of the Mössbauer spectra shown in Figs. 1共a兲–1共c兲. From permittivity spectra 共Figs. 2 and 3兲, a dielectric resonance or relaxation phenomena are also evident, more clearly seen in high excess Si4+ case. Figures 2 and 3 indicate that both cases of ferrites with excess Si4+ y = 0.05 and y = 0.2 exhibit resonance at lower X-band frequencies or below it. Possibly this resonance was due to the matching frequency of electron hopping between Fe3+ to Fe2+ ions to the applied EM wave frequency. In the absence of excess Si4+ y = 0.0, the normal small relaxation peaks were observed at frequencies of 10.05 and 12.23 GHz. In a complex hexagonal structure of hexaferrites, positive and negative ions of different valences are separated at the varying bond lengths, generating dielectric moments of varying strengths, giving rise to dipolar polarization. Further, in polycrystalline ferrites, low resistive grains are separated by highly resistive grain boundaries, which creates heterogeneity, giving rise to interfacial polarization.20 Both the phenomena contribute to the dielectric constant 共␧r⬘兲. The dielectric loss 共␧r⬙兲 in the ferrites, however, depends on the number and nature of the different ions present to exhibit relaxation behavior. The presence of ferrous 共Fe2+兲 and ferric 共Fe3+兲 in the ferrites also contributes to the values of ␧r⬘ and ␧r⬙ due to enhanced conduction and electron hopping mechanisms.21 Moreover, when the frequency of electron hopping between Fe3+ to Fe2+ ions matches that of microwave, dielectric resonance phenomenon occurs, which is responsible for the high dielectric loss.22 In the present ferrite composition, this phenomenon is clearly evident in the cases with excess Si+4 ions 共y = 0.05 and 0.2兲, due to the high conversion of Fe3+ ions to Fe2+ ions. Figures 4 and 5 show the X-band complex permeability spectra, real 共␮r⬘兲 and imaginary 共␮r⬙兲 parts, respectively, for the same sintered ferrite compositions 共x = 0.9; y = 0.0, 0.05, and 0.2兲 as described in the previous section. Here again, both ␮r⬘ and ␮r⬙ spectra have shown good dispersion relation, exhibiting magnetic resonance. The maximum ␮r⬙ values as obtained at resonance frequencies are 0.83 at 9.12 GHz for the y = 0.0 case, 0.5 at 9.63 GHz for the y = 0.05 case, and 0.32 at 9.21 GHz for the y = 0.2 case. ␮r⬙ values have decreased with increased Si+4 ion contents. The ␮r⬘ spectra for

hexaferrite

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FIG. 4. Real permeability spectra of +3 BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.0, 0.05, and 0.2兲.

hexaferrite

all the three cases show a decreasing trend with increasing frequency. The maximum ␮r⬘ values as obtained at the lower frequency end are 1.19 for the y = 0.0 case, 2.22 for the y = 0.2 case, and the highest, 2.81, for the y = 0.05 case. M-type barium hexaferrite 共BaFe12O19兲 with its large anisotropic field 共Ha = 17 kOe兲 exhibits a ferrimagnetic resonance f r = ␥Ha at 47.6 GHz, where ␥ = 2.8 is the gyromagnetic ratio. The resonance frequency, however, can be tuned to any desired frequency region between 2 and 52 GHz by substitution of an appropriate pair of divalent and tetravalent ions in place of Fe3+ ions.23 In the present ferrite compositions, substitution by 共Co2+ – Si4+兲 ions pair for x = 0.9 lowers the anisotropy field to the extent so as to bring the resonance frequency in the X band 共8.2– 12.4 GHz兲. The observed ␮r⬙ spectra as shown in Fig. 5 are in good agreement with this kind of mechanism of natural magnetic resonance involving spin or precession motion of magnetization vector. The slight shift in resonance frequency for excess Si+4 共y = 0.05 and 0.2兲 cases can be attributed to the presence of Fe2+ 共as discussed above兲 causing fluctuations of spins. Figures 6–8 show the absorption spectra for the substituted sintered ferrite compositions for y = 0.0, 0.05, and 0.2 at the optimum thickness and thicknesses below and above the optimum thickness. The dip in reflection loss is equivalent to the occurrence of minimum reflection or maximum absorption of the microwave power for a particular sample thick-

FIG. 5. Imaginary permeability spectra of +3 BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.0, 0.05, and 0.2兲.

hexaferrite

+3 FIG. 6. Absorption behavior of hexaferrite BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.0兲.

ness. These are obtained using measured values of ␧r⬘, ␧r⬙, ␮r⬘, and ␮r⬙ based on a model of a single layered plane wave absorber proposed by Naito and Suetake.24 In this model, the wave impedance 共Z兲 at the air-absorber interface is given by Z = Z0共␮r / ␧r兲1/2 tanh关共−j2␲ / c兲共␮r␧r兲1/2 fd兴, where ␮r = ␮r⬘ − j␮r⬙ and ␧r = ␧r⬘ − j␧r⬙ are the relative complex permeability and permittivity of the absorber medium, respectively. Z0 and f are the wave impedance and frequency, respectively, in free space. c is the velocity of light and d is the sample thickness. The reflection loss 共RL兲 in decibels is then determined as RL = −20 log10 关兩共Z − Z0兲 / 共Z + Z0兲兩兴. The impedance matching condition representing the perfectly absorbing properties is given by Z = Z0 = 377 ⍀. This condition is satisfied at a particular matching thickness 共tm兲 and a matching frequency 共f m兲, where minimum reflection loss or maximum absorption occurs. The sintered ferrite composition 共x = 0.9; y = 0.0兲 shows minimum RL of around −9.0 dB at frequencies of 8.2– 9.12 GHz for the optimum thickness tm = 1.2 mm. The composition 共x = 0.9; y = 0.2兲 shows minimum RL of around −11.0 dB at frequencies of 8.2– 9.21 GHz for the optimum thickness tm = 0.8 mm. The best ferrite composition 共x = 0.9; y = 0.05兲 has shown minimum RL between −17.77 and −14.3 dB over the whole frequency range of the X band 共8.2– 12.4 GHz兲 for the optimum thickness tm = 0.8 mm. These curves were obtained for large number of thicknesses starting from 0.1 to 3.0 mm at intervals of 0.2 mm, but for

+3 FIG. 7. Absorption behavior of hexaferrite BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.2兲.

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+3 FIG. 8. Absorption behavior of hexaferrite BaCoxFe+2 y Six+y Fe12−2x−2y O19 共x = 0.9; y = 0.05兲.

all the thicknesses below or above tm, poor absorption is achieved as evident from curves shown just below and above the tm values. Using the best ferrite composition 共x = 0.9; y = 0.05兲 in powder form and along other constituents such as polyaniline and carbon black, composite absorbers were also prepared. Results obtained in an optimized composite absorber are discussed here. Figure 9 shows the X-band spectra of ␧r⬘, ␧r⬙, ␮r⬘, and ␮r⬙ for optimized composite absorber. All the spectra exhibit insignificant variation with frequency. ␧r⬘ values for composite absorber lie from 11.77 at 8.2 GHz to 11.19 at 12.4 GHz, while ␧r⬙ values lie from 2.43 at 8.2 GHz to 2.23 at 12.4 GHz. The values of magnetic permeability 共␮r⬘兲 lie between ⬃1.2 and ⬃1.1, while magnetic loss 共␮r⬙兲 values are close to zero. In a composite absorber, properties depend on the nature and contents of its constituents. The prepared composite is a mixture of conducting polyaniline, carbon particles, and semiconducting ferrite separated by insulating PU molecules. Inclusions of different conductivities in a nonconductive matrix create heterogeneities of different levels in the composite sample. This, by interfacial polarization and its relaxation, contributes to the dielectric constant and dielectric loss, respectively, apart from their individual contributions. In addition to this, eddy current loss may also be expected due to the conducting nature of polyaniline and

FIG. 10. Absorption behavior of optimized composite absorber.

carbon black. The low values of ␮r⬘ and ␮r⬙ are understandably due to dilution and shielding of ferrite particles by the nonmagnetic polymer matrix and inclusions, which weaken the intergranular magnetic interaction.9 The reflection loss in the composite absorbers has also been calculated from the model of Naito and Suetake24 as described earlier. X-band absorption spectra of optimized composite absorber are shown in Fig. 10. This has shown a minimum RL of −29 dB at 10.97 GHz with a −20 dB bandwidth over the frequency range of 9.7– 12.2 GHz for an optimum thickness tm = 2.0 mm, a minimum RL of −29.4 dB at ⬃10 GHz with a −20 dB bandwidth over the frequency range of 8.7– 11.14 GHz for tm = 2.2 mm, and a minimum RL of −28 dB at 8.87 GHz with a −20 dB bandwidth over the extended frequency range of 7.5– 10.0 GHz for tm = 2.4 mm. It can be noticed that the dip showing minimum RL shifts towards a lower frequency side as the sample thickness is increased. This can be understood based on the quarter-wave principle.1 When an electromagnetic wave is incident on an absorber sample backed by a metal plate, it is partially reflected from air to absorber interface and partially reflected from absorber to metal interface. These two reflected waves are out of phase by 180° and cancel each other at the airabsorber interface for absorbers satisfying the quarter-wave thickness criteria t = ␭o / 4共兩␮r兩兩␧r兩兲1/2, where ␭o = c / f is the free space wavelength of incident wave and 兩␮r兩 and 兩␧r兩 are the moduli of ␮r and ␧r. Since the thickness is inversely proportional to frequency, the above criterion is satisfied at increased sample thickness for lower frequencies. IV. CONCLUSION

FIG. 9. Electromagnetic properties of optimized composite absorber.

Electromagnetic and microwave absorption properties of 共Co2+ – Si4+兲 substituted barium hexaferrites have been studied, and the effect of excess Si4+ content has been demonstrated. It has been shown that excess Si4+ content in ferrite composition enhances the permittivities due to conversion of Fe3+ to Fe2+ ions in the hexaferrite structure. The values of ␧r⬘ and ␧r⬙ have increased from 38.9 and 31.7 for no excess Si4+ case to 63.4 and 57.6 for high excess Si4+ case, respec2+ 4+ – Si0.9+0.05 兲 has tively. The sintered ferrite composition 共Co0.9 shown a minimum reflection loss of −17.7 to − 14.3 dB over the whole frequency range of the X band 共8.2− 12.4 GHz兲 for a sample thickness of just 0.8 mm. Such wide band absorp-

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tion of sintered ferrite in small thickness is not reported. The composite absorber, based on hexaferrite, polyaniline, and carbon black, have been fabricated and characterized for their electromagnetic and microwave absorption properties. In spite of poor magnetic properties, the composite absorber has shown high and broadband microwave absorption due to better impedance matching. The composite absorber with averaged dielectric constant of 11.5 and dielectric loss of 2.33 has shown a minimum reflection loss of −29 dB at 10.97 GHz with a −20 dB bandwidth over the frequency range of 9.7− 12.2 GHz in a sample thickness of 2.0 mm and −29.4 dB at ⬃10 GHz with a −20 dB bandwidth over the frequency range of 8.7− 11.14 GHz in a sample thickness of 2.2 mm. Both sintered ferrite and composite absorber have potential applications in EMI shielding and reduction of radar signatures 共stealth technology兲. ACKNOWLEDGMENT

The authors are grateful to Director DMSRDE, Kanpur, for extending the facility for microwave measurement. one of the authors 共S.M.A.兲 is a scientist from DMSRDE, Kanpur 共registered for Ph.D. at IIT Delhi兲. 1

A. N. Yusoff, M. H. Abdullah, S. H. Ahmad, S. F. Jusoh, A. A. Mansor, and S. A. A. Hamid, J. Appl. Phys. 92, 876 共2002兲. D. D. L. Chung, Carbon 39, 279 共2001兲. 3 R. A. Stonier, SAMPE J. 27, 9 共1991兲. 4 W. H. Emerson, IEEE Trans. Antennas Propag. 21, 484 共1973兲. 5 T. Kagotani, D. Fujiwara, S. Sugimoto, K. Inomata, and M. Homma, J. Magn. Magn. Mater. 272–276, e1813 共2004兲. 6 M. R. Meshram, N. K. Agarwal, B. Sinha, and P. S. Misra, J. Magn. Magn. Mater. 271, 207 共2004兲. 2

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Z. Haijun, L. Zhichao, M. Chenliang, Y. Xi, and Z. Liangying, Mater. Sci. Eng., B 96, 289 共2002兲. 8 P. Singh, V. K. Babbar, A. Razdan, S. L. Srivastava, and T. C. Goel, Mater. Sci. Eng., B 78, 70 共2000兲. 9 P. Singh, V. K. Babbar, A. Razdan, R. K. Puri, and T. C. Goel, J. Appl. Phys. 87, 4362 共2000兲. 10 M. B. Amin and J. R. James, Radio Electron. Eng. 51, 209 共1981兲. 11 V. B. Bregar, A. Znidarsic, D. Lisjak, and M. Drofenik, Materiali in Technologije 39共3兲, 89 共2005兲. 12 S. M. Abbas, A. K. Dixit, R. Chatterjee, and T. C. Goel, J. Magn. Magn. Mater. 309共1兲, 20 共2007兲. 13 S. M. Abbas, A. K. Dixit, R. Chatterjee, and T. C. Goel, Mater. Sci. Eng., B 123, 167 共2005兲. 14 Hewlett-Packard microwave network analyzer Catalog No. 8510 and Product Note No. 8510–3 and 共1987兲. 15 J. Smit and H. P. J. Wijn, Ferrites: Physical Properties of Ferromagnetic Oxides inRelation to their Technical Applications 共Philips Technical Library, Eindhoven, The Netherlands, 1959兲, p. 180. 16 X. Z. Zhou, A. H. Morrish, and Z. W. Li, IEEE Trans. Magn. 27, 4654 共1991兲. 17 D. G. Agresti and T. D. Shelfer, IEEE Trans. Magn. 25, 4069 共1991兲. 18 A. Morel, J. M. L. Breton, J. Kreisel, G. Wiesinger, F. Kools, and P. Tenaud, J. Magn. Magn. Mater. 242–245, 1405 共2002兲. 19 Q. A. Pankhurst, D. H. Jones, A. H. Morrish, X. Z. Zhou, and A. R. Corradi, Proceedings of the International Conference on Ferrites, IFC-5, edited by C. M. Srivastava and M. J. Patni 共Oxford and IBH, New Delhi, India, 1989兲, p. 323. 20 J. Smit and H. P. J. Wijn, Ferrites: Physical Properties of Ferromagnetic Oxides inRelation to their Technical Applications 共Philips Technical Library, Eindhoven, The Netherlands, 1959兲, p. 240. 21 S. Chikazumi and S. H. Charap, Physics of Magnetism 共Wiley, New York, 1964兲, p. 328. 22 Z. Haijun, L. Zhichao, M. Chenliang, Y. Xi, Z. Liangying, and W. Mingzhong, Mater. Chem. Phys. 80, 129 共2003兲. 23 H. Severin and J. P. Stoll, Z. Angew. Phys. 23, 209 共1967兲. 24 Y. Naito and K. Suetake, IEEE Trans. Microwave Theory Tech. MTT-19, 65 共1971兲.

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