Abbas 2007 Journal of Magnetism and Magnetic Materials

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Journal of Magnetism and Magnetic Materials 309 (2007) 2024

Complex permittivity, complex permeability and microwave absorption

properties of ferritepolymer composites

S.M. Abbasa,b,1, A.K. Dixitb, R. Chatterjeea,, T.C. Goelc

aDepartment of Physics, IIT Delhi, Hauz Khas, New Delhi 110016, IndiabDMSRDE, G. T. Road, Kanpur 208013, India

cBITS, PilaniGoa Campus, Zuari Nagar, Goa 403726, India

Received 24 December 2005; received in revised form 30 May 2006

Available online 5 July 2006

Abstract

The complex permittivity (e0je00), complex permeability (m0jm00) and microwave absorption properties of ferritepolymer composites

prepared with different ferrite ratios of 50%, 60%, 70% and 80% in polyurethane (PU) matrix have been investigated in X-band

(8.212.4 GHz) frequency range. The M-type hexaferrite composition BaCo+20.9 Fe+20.05Si

+40.95Fe

+310.1O19 was prepared by solid-state reaction

technique, whereas commercial PU was used to prepare the composites. At higher GHz frequencies, ferrites permeabilities are drastically

reduced, however, the forced conversion of Fe+3 to Fe+2 ions that involves electron hopping, could have increased the dielectric losses in

the chosen composition. We have measured complex permittivity and permeability using a vector network analyzer (HP/Agilent model

PNA E8364B) and software module 85071. All the parameters e0, e00, m0 and m00 are found to increase with increased ferrite contents.

Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a

single-layered plane wave absorber backed by a perfect conductor. The composite with 80% ferrite content has shown a minimum

reflection loss of 24.5 dB (499% power absorption) at 12 GHz with the 20 dB bandwidth over the extended frequency range of

1113 GHz for an absorber thickness of 1.6 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic

interference (EMI) and reduction of radar signatures (stealth technology).r 2006 Elsevier B.V. All rights reserved.

Keywords: Ferritepolymer composite; Hexaferrite; Complex permeability; Reflection loss; Microwave absorption

1. Introduction

The microwaves in higher GHz ranges are increasingly

exploited by circuitry designers in the industries dealing

with wireless telecommunication systems, radar, local area

network, medical equipments, etc., due to saturation in

lower frequency bands. This is, however, posing a seriousproblem of electromagnetic interference (EMI) and elec-

tromagnetic compatibility (EMC) in higher GHz range

[1,2]. In order to provide solution to EMI/ EMC problem

in higher GHz range, microwave absorbers in higher GHz

range are in high demand [3,4]. An appropriate microwave

absorber in electronic equipments controls the excessive

self-emission of electromagnetic waves and also ensures the

undisturbed functioning of the equipment in presence of

external electromagnetic wave. Microwave absorbers are

also highly demanded in defence and aerospace industries,

as the application of microwave-absorbing coating on the

exterior surfaces of military aircraft and vehicles helps toavoid detection by the radar (Stealth technology) [5].

M-type hexaferrites are the most promising materials and

extensively used for the development of microwave

absorbers in higher GHz range because of their large

tunable anisotropy field (Ha 2 K/Ms) causing magnetic

resonance in 252 GHz [6-12]. However, due to reduced

magnetic permeabilities of ferrites in higher GHz range,

resonance absorption is weak, and also in narrow

frequency range. Alternatively, ferrites can be engineered

to show enhanced dielectric losses in wide frequency

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Corresponding author. Tel.:+91 11 2659 1354; fax: +91 11 2658 1114.

E-mail address: [email protected] (R. Chatterjee).1Registered for Ph.D. degree in IIT Delhi, Hauz Khas,

New Delhi 110016, India.
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range by appropriate choice of composition and heat

treatment. In the chosen hexaferrite composition,

BaCoxFe+2

ySix+yFe+3122x2yO19 (x 0.9 and y 0.05),

Fe+3 ions are forced to convert into Fe+2 ions by the

excess Si+4 (0.05) ion substitution. The electron hopping

between Fe+3 and Fe+2 ions is expected to enhance the

dielectric losses. In the present study, we have preparedferritepolymer composites based on different ratios of the

synthesized ferrite in polyurethane (PU) matrix and

measured the complex permittivity and complex perme-

ability in X-band frequencies (8.212.4 GHz). The reflec-

tion losses in the composite samples were also determined

at different X- band frequencies and sample thicknesses,

using a computer simulation based on a model of a single-

layered plane wave absorber backed by a perfect conductor

[13]. The matching frequencies for minimum reflection and

the corresponding matching thicknesses for the best sample

(80% ferrite) are also presented.

2. Experimental work

2.1. Synthesis of ferrite and preparation of composite

samples

Ba-hexaferrite was synthesized by standard ceramic

route. The raw materials used, were barium carbonate

(99%, CDH, India), silicon dioxide (98%, CDH, India),

cobalt oxide (98%, CDH, India) and ferric oxide (97%,

CDH, India). The stoichiometric proportion of powders

were weighed and wet mixed in acetone medium for 8 h

using a ball mill. This powder mixture was thermally

treated at 1290 1C for 4 h in air to convert it into ferritephase. Heat-treated powder was finally ground and sieved

to 400 mesh to get particles size o50mm. The hexaferrite

structure was checked by X-ray diffraction (XRD)

technique. The XRD results revealed that the homoge-

neous phase of M-type Ba-hexaferrite was obtained.

Ferritepolymer composites were prepared by thoroughly

mixing the ferrite in different weight ratios, 50%, 60%,

70% and 80% in two-pack PU matrix consisting polyol-8

(Ciba-Geigy, Switzerland) and hexamethylene diisocynate

(E-Merck, Germany) taken in equal proportion. Thick

ferritePU paste was poured in a suitable mould and then

cured at 701C. The prepared composite samples are

designated as FPU1, FPU2, FPU3 and FPU4 for increas-

ing ferrite content from 50% to 80%.

2.2. Microwave measurements

The samples were shaped to fit exactly into rectangular

X-band wave-guide (WR90) for microwave measure-

ments. The complex scattering parameters that correspond

to the reflection (S11 or S22) and transmission (S21 or S12)

in the composite samples were measured using a vector

network analyzer (HP/Agilent, PNA E8364B). Full two-

port calibrations were 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

direction. The complex permittivity and permeability were

then determined from the measured scattering parameters

using Agilent software module 85071, based on the

procedure given in HP product note [14].

3. Results and discussion

3.1. Permittivity spectra

Figs. 1 and 2 show the complex permittivity spectra,

real (er0) and imaginary (er

00) parts, respectively, for the

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8 9 10 11 12 13

4

6

8

10

12

14

16

FPU4

FPU3

FPU2

FPU1

'

Frequency (GHz)

Fig. 1. Real part of permittivity spectra of composite samples FPU1,

FPU2, FPU3 and FPU4.

8 9 10 11 12 13

0.0

0.4

0.8

1.2FPU4

FPU3

FPU2

FPU1

"

Frequency (GHz)

Fig. 2. Imaginary part of permittivity spectra of composite samples

FPU1, FPU2, FPU3 and FPU4.

S.M. Abbas et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 2024 21


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composite samples FPU1, FPU2, FPU3 and FPU4 in

the frequency range of 8.212.4 GHz. The er0 spectra of

all the samples show insignificant variation in the whole

frequency range used in the present work. However, the

er0 values have increased from $5.3 for FPU1 to $14.3

for FPU4 with increasing ferrite content from 50% to

80%. The er00

spectra for FPU3 and FPU4 also showinsignificant variation with frequency, but in case of

FPU1 and FPU2 some wavy behaviour is observed.

There is a gradual increase in er00 values around 0.2 for

FPU1 to $1.2 for FPU4 with increasing ferrite percen-

tage. The M-type Ba-hexaferrite (BaFe12O19) is a well-

studied crystal [15]. It has a two-formula hexagonal unit

cell that can be described as RSR*S*. The S and S* are

called the spinel blocks that consist of two oxygen layers,

separated by a block R containing the barium ion. R*

and S* are obtained from blocks R and S, respectively,

by rotation over 1801 around the c axis. The structure

possesses three kinds of interstitials sites viz., tetrahe-

dral, octahedral and trigonal bi-pyramid with coordina-

tion of four, six and five O2 ions. All the 12 Fe+3 ions

are distributed in these three interstitial sites in a specific

manner. The positive ions Ba+2 and Fe+3 at their

respective positions form the electric dipoles with the

surrounding negative O2 ions, contributing to dielectric

constant (er0) through dipolar polarization and by dipole

relaxation to dielectric loss (er00). When the pair of Co+2

and Si+4 ions substitute some of the Fe+3 ions more

dipoles are generated, thereby, increasing both the

dielectric constant and loss. The excessive Si+4

(y 0.05) ions in the present composition force the

conversion of Fe+3

ions into Fe+2

ions, furtherincreasing the dielectric constant and loss. Since the

present non-stoichiometric composition, BaCoxFe+2

y

Six+yFe+3122x2yO19 (x 0.9 and y 0.05) has variety

of positive ions of different valences, having different

coordination with O2 ions; dipoles of different strength

are formed. These different dipoles have different

relaxation time, giving rise to different relaxation

frequencies. The electron hopping between Fe+3 and

Fe+2 ions also contribute to the dielectric loss due to

enhanced conduction mechanisms giving rise to another

relaxation frequency [16,17]. In case of a ferritepolymer

composite, the contribution to dielectric constant and

dielectric loss also occur due to interfacial polarization

and its relaxation as the semiconducting ferrite particles

separated by insulating matrix molecules giving rise to

heterogeneity. Different relaxation frequencies of var-

ious dipoles formed in the ferrite structure, hopping of

electrons and the relaxation due to interfacial polariza-

tion all are responsible for oscillatory behaviour of

absorption in the samples. However, as the ferrite

content in the composite is increased high and smooth

loss curves are obtained. This can be attributed to the

overlapping of individual relaxation peaks of different

diploes and dominance of relaxation due to interfacial

polarization.

3.2. Permeability spectra

Figs. 3 and 4 show the complex permeability spectra, real

(mr0) and imaginary (mr

00) parts, respectively, for all the

composite samples. In general, both mr0 and mr

00 values

increase as the ferrite percentage is increased in the

composite (barring FPU2). The mr0

spectra for all thesamples show a decreasing trend with increasing frequency.

The mr00 spectra, however, show small resonance peaks for

FPU1 and FPU2 at lower side of X-band frequency,

whereas no such peak is observed for FPU3 and FPU4

samples. Ba-hexaferrite (BaFe12O19) with its large aniso-

tropic field (Ha 1.36 MA/m) exhibits a ferrimagnetic

resonance peak at 47.6 GHz.The resonance peak, however,

can be shifted to any frequency region by appropriate pair

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8 9 10 11 12 130.04

0.06

0.08

0.10

0.12

0.14

FPU4

FPU2

FPU3

FPU1

"

Frequency (GHz)

Fig. 4. Imaginary part of permeability spectra of composite samples

FPU1, FPU2, FPU3 and FPU4.

8 9 10 11 12 13

1.04

1.08

1.12

1.16

1.20

1.24

1.28

FPU4

FPU3

FPU2

FPU1

'

Frequency (GHz)

Fig. 3. Real part of permeability spectra of composite samples FPU1,

FPU2, FPU3 and FPU4.

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of divalent and tetravalent ions substitution in place of

Fe+3 ions [18]. In the present ferrite composition,

substitution by Co+2Si+4 (x 0.9) ions pair lowers the

anisotropy field, which has resulted the resonance peak at

8.5 GHz, which is evident from mr00 spectra of FPU1 and

FPU2. The peak, probably, shifted beyond X-band

towards lower side (not observed) with increased ferritecontents in FPU3 and FPU4. Kim et al. [19] had also

observed the similar effect that is shifting of resonance

peak towards low-frequency side with increased ferrite

volume fraction for MnZn ferrite. The lower values of mr0

in case of FPU2 as compared to FPU1 are not understood.

The oscillatory behaviour ofmr00 can be understood on the

basis of the precession motion of the magnetization vector.

In a virgin hexaferrite, 12 Fe+3 ions are distributed in three

different interstitial sites viz., tetrahedral, octahedral and

one bi-pyramidal. The seven octahedral ions and one

bipyramidal ion are oppositely coupled to two octahedral

ions and two tetrahedral ions, giving a net magnetization of

4 Fe+3 pointing along the c axis. In this situation,

magnetization vector on interaction with microwave

precise in a stable manner yielding a smooth resonance

curve. While, some of the Fe+3 ions are substituted by a

pair of Co+2 and Si+4 magnetization vector shift towards

basal plane. Bending towards basal plane is increased on

higher substitution as in the present case. In this situation

the magnetization vector precess in a zigzag manner,

giving rise to oscillatory behaviour of absorption. Such

behaviour is also observed by Meshram et al. [11].

However, at higher ferrite content, overlapping of precise

motion of several crystallites results in a smoothened

absorption curve.

3.3. Microwave absorbing properties

Measured values ofer0, er

00, mr0 and mr

00 as shown in Figs.

14 are used to determine the reflection loss in the

composite samples based on a model of a single-layered

plane wave absorber proposed by Naito and Suetake [13].

In this model, the wave impedance (Z) at airabsorber

interface is given as Z Zo(mr/er)1/2 tanh [(j2p/c)

(mrer)1/2fd], where mr mr

0jmr00 and er er

0j er00 are the

relative complex permeability and permittivity of the

absorber medium, respectively. Zo 377O and f are wave

impedance and frequency, respectively, in free space, c is

the velocity of light and d is the sample thickness. The

reflection loss (RL) in decibels (dB) is then determined

as RL 20 log10 [j(ZZo)/(Z+Zo) j]. The impedance-

matching condition representing the perfectly absorbing

properties is given by Z Zo. This condition is satisfied at

a particular matching thickness (tm) and a matching

frequency (fm), where minimum reflection loss occurs.

Fig. 5 shows the X-band absorption spectra for

composites FPU1, FPU2, FPU3 and FPU4. The dip in

reflection loss is equivalent to the occurrence of minimum

reflection or maximum absorption of the microwave power

for a particular sample thickness. The thicknesses of these

composites are optimized to bring the dip at the central

frequency of X-band. FPU1 shows the minimum reflection

loss of7 dB for sample thickness of 3.2 mm, FPU2 shows

12 dB at thickness of 2.6 mm, FPU3 shows 12.6 dB at

thickness of 2.3 mm and FPU4 shows 21 dB at thickness

of 1.85 mm.

Fig. 6 shows, separately, the X-band absorption spectra

for the composite FPU4 (80% ferrite) at different sample

thicknesses: 1.4, 1.6, 1.8, 2 and 2.2 mm. This shows a

minimum RL 24.5 dB at fm 12 GHz with the 20dB

bandwidth over the extended frequency range of

1113 GHz for tm 1.6 mm, a minimum RL 21.8 dB

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8 9 10 11 12 13

-25

-20

-15

-10

-5

0

FPU4

FPU3

FPU2

FPU1t =3.2mm

t=2.6mm

t =2.3mm

t =1.85mm

Frequency (GHz)

ReflectionLo

ss(dB)

Fig. 5. Comparison of reflection loss for FPU1, FPU2, FPU3 and FPU4

at centre frequency of X-band.

7 9 10 11 12 13

-25

-20

-15

-10

-5

0

t=2.2mm

t=2.0mm

t=1.8mm

t=1.6mm

t=1.4mm

Frequency (GHz)

8

ReflectionLoss(dB)

Fig. 6. Reflection loss of the composite FPU4 (80% ferrite) at different

sample thicknesses.

S.M. Abbas et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 2024 23


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at fm 10.5GHz with a 20dB bandwidth over the

frequency range of 9.911GHz for tm 1.8 mm and

19 dB between 9.1 and 9.5 GHz for tm 2.0 mm.

The minimum RL or matching condition is shifted beyond

X-band for 2.0 mm otmo1.6mm.

It can be noticed in Fig. 5 that by increasing the ferrite

content in the composite, the absorption in the sample can

be increased and it also reduces the sample thickness.

In Fig. 6 it can, further, be noticed that the dip showing

minimum RL shifts towards a higher frequency side if the

sample thickness is reduced. Contrary to the normalbehaviour, however, absorption is increasing as the

thickness is decreased. This can be understood based on

quarter-wave principle [20]. 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 1801 and cancel

each other at airabsorber interface for absorbers satisfy-

ing the quarter-wave thickness criteria: t lo/4 (jmrjjerj)1/2,

where lo c/f is the free-space wavelength of incident

wave and jmrj and jerj are the moduli of mr and er. As

explained earlier, increased ferrite content in the composite

increases the values of jmrjjerj, hence the thickness is

reduced. Since, the thickness is inversely proportional to

frequency, the above criterion is satisfied at reduced sample

thickness for higher frequencies. The higher absorption at

reduced thickness is the result of total cancellation,

satisfying the above criteria perfectly. In case of absorber

thicknesses not satisfying the criteria, only partial cancella-

tion occurs, giving lower absorption. The matching

thickness and matching frequencies for minimum reflection

are identified in case of FPU4 (80% ferrite) and their

relationship is shown in Fig. 7. The matching frequency

decreases almost linearly with the increase of absorber

thickness.

4. Conclusion

The ferritepolymer composites with different ferrite

ratios have been successfully prepared in PU matrix. The

complex permittivity, permeability and their relationship

with microwave absorption properties were investigated.

It is found that the absorption properties in the compositesare greatly improved with increasing ferrite contents in the

polymer matrix. The composite with 80% ferrite content

has shown a minimum reflection loss of 24.5dB at

12 GHz with a 20 dB bandwidth over the extended

frequency range of 1113 GHz in a quite thin sample with

a thickness of only 1.6 mm. A relationship between a

matching thickness and a matching frequency has been

established. It is found that the matching frequency for

minimum reflection decreases with increasing thickness of

the composite. The prepared composites have potential

applications in EMI shielding and reduction of radar

signatures (Stealth technology).

Acknowledgements

The authors are grateful to the Director DMSRDE,

Kanpur for extending the facility for microwave measure-

ment.

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1.4 1.6 1.8 2.0 2.2

8

9

10

11

12

13

Matchingfrequency(GHz)

Matching thickness (mm)

Fig. 7. Relationship between matching frequency and matching thickness

for composite FPU4 (80% ferrite).

S.M. Abbas et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 202424

Abbas 2007 Journal of Magnetism and Magnetic Materials

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