magnetic and microwave absorbing properties of magnetite

Journal of Magnetism and Magnetic Materials 322 (2010) 3401–3409

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

fax: +6

E-m

journal homepage: www.elsevier.com/locate/jmmm

Magnetic and microwave absorbing properties of magnetite–thermoplasticnatural rubber nanocomposites

Ing Kong a,n, Sahrim Hj Ahmad a, Mustaffa Hj Abdullah a, David Hui b,Ahmad Nazlim Yusoff c, Dwi Puryanti a

a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysiab Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USAc Diagnostic Imaging and Radiotherapy Programme, Faculty of Allied Health Sciences, Universiti Kebangsaan Malaysia, 50300 Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:

Received 27 April 2010

Received in revised form

7 June 2010Available online 15 June 2010

Keywords:

Magnetite

Thermoplastic natural rubber

Microstructure

Magnetic property

Microwave absorbing property

53/$ - see front matter & 2010 Elsevier B.V. A

016/j.jmmm.2010.06.036

esponding author. Tel.: +60 3 89215891(offic

0 3 89213777.

ail address: [email protected] (I. Ko

a b s t r a c t

Magnetic and microwave absorbing properties of thermoplastic natural rubber (TPNR) filled magnetite

(Fe3O4) nanocomposites were investigated. The TPNR matrix was prepared from polypropylene (PP),

natural rubber (NR) and liquid natural rubber (LNR) in the ratio of 70:20:10 with the LNR as the

compatibilizer. TPNR-Fe3O4 nanocomposites with 4–12 wt% Fe3O4 as filler were prepared via a Thermo

Haake internal mixer using a melt-blending method. XRD reveals the presence of cubic spinel structure

of Fe3O4 with the lattice parameter of a¼8.395 A. TEM micrograph shows that the Fe3O4 nanoparticles

are almost spherical with the size ranging 20–50 nm. The values of saturation magnetization (MS),

remanence (MR), initial magnetic susceptibility (wi) and initial permeability (mi) increase, while the

coercivity (HC) decreases with increasing filler content for all compositions. For nanocomposites, the

values of the real (er0) and imaginary permittivity (er

0 0) and imaginary permeability (mr0 0) increase, while

the value of real permeability (mr0) decreases as the filler content increases. The absorption or minimum

reflection loss (RL) continuously increases and the dip shifts to a lower frequency region with the

increasing of both filler content in nanocomposites and the sample thickness. The RL is �25.51 dB at

12.65 GHz and the absorbing bandwidth in which the RL is less than �10 dB is 2.7 GHz when the filler

content is 12 wt% at 9 mm sample thickness.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The high frequency electromagnetic wave is drawing moreattention, due to the explosive growth in the utilization oftelecommunication devices in industrial, medical and militaryapplications [1]. Many devices such as AC motors, digital compu-ters, calculators, point-of-sale terminals, printers, modems, electro-nic typewriters, digital circuitry and cellular phones are capable ofcreating electromagnetic interference (EMI) that may cause inter-ruption to those applications. EMI can cause severe interruption onelectronically controlled devices such as system malfunctions,generating false images, increase clutters on radar and reduceperformance because of system-to-system coupling [2,3]. Further-more, there are ongoing controversies world wide over the potentialhealth hazards to the human body associated with exposure toelectromagnetic field. With the aim of controlling the problemscreated by EMI, electromagnetic wave absorbers with the capabilityof absorbing unwanted electromagnetic signals were investigated.

ll rights reserved.

e), +60 12 5731025(mobile);

ng).

Research on their electromagnetic and absorption properties is stillbeing carried out. In producing microwave absorbing materials,several parameters needed to be taken into consideration, such asthe weight, thickness, filler content, types of filler, environmentalresistance and mechanical strength [4].

Ferrites are considered to be the best magnetic material forelectromagnetic wave absorbers due to their excellent magneticand dielectric properties, but they are expensive and heavy. Onthe other hand, the use of polymers to protect the electronicdevices from EMI is popular due to the light weight, flexibility andcost effectiveness. However, polymers are electricallyinsulating and transparent to electromagnetic wave. In order toeffectively suppress EMI, ferrite materials are incorporated intopolymer matrices [5]. Many works have been done on polymer-based composites filled with magnetic materials in micrometer-size, such as Ba-ferrite [6], iron-fibre [7], NiZn-ferrite [8] andFe3O4/YIG [9]. However, these polymer-based composites filledwith conventional magnetic particles have difficulty in meetingthe criterion in thin and light weight microwave absorber due tohigh filler content and material thickness are needed to exhibit alow reflection coefficient over a wide frequency range.

In this paper, we report the magnetic, electromagnetic andmicrowave absorbing properties of polymer-based nanocomposites

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I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3401–34093402

consisting of thermoplastic natural rubber (TPNR) as the matrix andFe3O4 nanoparticles as the fillers. TPNR exhibits imtermediateproperties between those of natural rubber and thermoplastics. It isthe carrier of microwave absorbents. It can make the microwaveabsorber soft, flexible and easy to be clipped. Fe3O4 nanoparticle, amember of spinel-type ferrite, was selected mainly because of itsunique and novel physiochemical properties that can be attainedaccording to their particle size (quantum size effect), shape,morphology and engineering form (film/self-assembled nanocrys-tals and ferrofluids) [10]. TPNR-Fe3O4 nanocomposites with variousfiller content were prepared by the melt-blending method and theyare expected to show better microwave absorption properties withlower filler content and material thickness than those of theconventional composites. The microstructure and morphology ofthe nanocomposites were also studied.

2. Experimental methods

2.1. Materials

Fe3O4 nanoparticles, with the particle size ranging 20–30 nm,were obtained from commercial suppliers in powder form(Nanostructured & Amorphous Materials Inc., USA). Naturalrubber (NR) and polypropylene (PP) were supplied by RubberResearch Institute of Malaysia (RRIM) and Mobile (M) Sdn. Bhd.,respectively. Liquid natural rubber (LNR) was prepared by thephotosynthesized degradation of the NR in visible light.

2.2. Preparation of the composites samples

TPNR filled Fe3O4 nanocomposites with 4–12 wt% of Fe3O4

were prepared by melt-blending technique using laboratorymixer (Model Thermo Haake 600p). The weight ratio of PP, NRand LNR is 70:20:10 with the LNR as the compatibilizer for themixture. Blending was carried out with a mixing speed of 100 rpmat 180 1C for 13 min. The NR was initially melted in an internalmixer. The LNR, which was previously mixed with Fe3O4

nanoparticles was then added into the internal mixer 3 min afterthe blending started. NR, LNR and the nanoparticles were allowedto mix for 4 min before PP was charged into the internal mixer.Once a homogeneous mixture is assumed after 13 min, the blendwas removed from the internal mixer and subsequently pressedat 185 1C under 45 MPa of pressure for about 2 min using a hotpress (Carver Laboratory Press) into thin sheets of about 3 mmthick from which test specimens were cut. Toroidal-shapedsamples were prepared using injection moulding (Model Ray-Run) to fit closely into a coaxial measurement cell (outer diameter�3.5 mm, inner diameter �1.5 mm).

2.3. Measurements

The microstructure of the nanocomposites was studied using theX-ray diffraction (XRD) technique (Siemens D5000 diffractometer)with CuKa1 radiation (l¼1.541 A) in the 2y range 10–601, in stepsof 0.021. The magnetic properties were measured using a vibratingsample magnetometer (VSM model LDJ 9600) at room tempera-tures (25 1C). The measurements were carried out in a maximumfield of 5 kOe. Magnetic parameters such as saturation magnetiza-tion (MS), remanence (MR), coercivity (HC), initial magneticsusceptibility (wi) and initial permeability (mi) were determined.The scattering parameters of the toroidal samples corresponding tothe reflection (S11

* and S22* ) and transmission (S21

* and S12* ) of a

transverse electromagnetic (TEM) wave were measured using aHewlett Packard 8720D microwave vector network analyzer

(MVNA). The measurements were performed at the frequencyrange 1–20 GHz. The toroids were tightly fit into a 3.5 mm coaxialmeasurement cell. The inner dimension of the air-line is 1.5 mm.Each sample was positioned exactly in the middle of the sampleholder by means of a tiny metal rod. After the sample was insertedinto one end of the airline, it was pushed until it reached the desiredposition indicated by a displacement marker on the rod. A full two-port calibration was initially performed on the test setup, in order toremove errors due to the directivity, source match, load match,isolation and frequency response in both the forward and reversemeasurements. The real and imaginary components of the complexdielectric permittivity and magnetic permeability were determinedfrom the complex scattering parameters using the Nicolson–Rossmodel for magnetic material and the precision model for non-magnetic material. The instrument was calibrated by measuring thecomplex permittivity and permeability of air using the air-filledsample holder, where the results show that er

0 ¼mr0E0 and

er00¼mr

00 ¼1. The dependence of the absorption characteristics onthe frequency and thickness were obtained based on a model, inwhich an electromagnetic wave is normally incident on the surfaceof the materials backed by a perfect conductor.

3. Results and discussion

3.1. Microstructure

The X-ray diffractogram of TPNR, pure Fe3O4 nanoparticles andnanocomposites with different filler content are shown in Fig. 1.For pure Fe3O4 nanoparticles, there are characteristic peaks at2y¼30.341, 35.621, 43.181, 53.661 and 57.221 which can beassigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2) and (5 1 1) planes ofFe3O4, respectively (JCPDS 01-1111). The d values calculated fromthe XRD patterns are well indexed to the cubic spinel phase ofFe3O4 with the lattice parameter of a¼8.395 A with no impurityphases detected. The uncorrected crystallite size, D, calculatedfrom the XRD peak broadening using Debye–Scherrer’s formula(D¼kl/b cos y. k¼0.9, l is the X-ray wavelength, b is the FWHMof the (3 1 1) peak and y is its peak position) is 22 nm. The X-raydiffraction patterns of the nanocomposites comprise of twophases, which are the crystalline and amorphous phases. It canbe seen that the crystallinity of the composites increased withincreasing of Fe3O4 filler. This was due to the added crystallineFe3O4 phase that migrates into the amorphous phase of TPNR,thus, reducing the amorphous domains of the TPNR sample [11].The diffractogram also indicates that the structure of Fe3O4 in thenanocomposites is maintained.

3.2. Morphology observation

Fig. 2 shows the TEM micrograph of magnetite nanoparticles.The particles are almost spherical with diameter ranging 20–50 nm.Particles were polydisperse and some of them agglomerated due tomagneto-dipole interactions between particles.

Fig. 3 shows the SEM micrograph of the cross section ofnanocomposites containing 12 wt% of Fe3O4. There are largeamounts of Fe3O4 nanoparticles throughout the TPNR matrix andthe nanoparticles are well dispersed in the matrix. The Fe3O4

nanoparticles are visible as white spots inside or outside the TPNRmatrix.

3.3. Magnetic properties

The hysteresis of pure Fe3O4 nanoparticles and nanocompo-sites with different filler content was measured at room

10 20 30 40 50 60

Inte

nsity

(a.u

.)

2 theta (degree)

(511

)

(220

)

(311

)

(422

)

(400

)

Fe3O4

12 wt%

8 wt%

4 wt%

TPNR

Fig. 1. X-ray diffraction patterns of pure TPNR, pure Fe3O4 nanoparticles and nanocomposites with different filler contents.

Fig. 2. TEM photograph of the Fe3O4 nanoparticles.

Fig. 3. SEM micrograph of the nanocomposites with 12 wt% Fe3O4 nanoparticles.

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3401–3409 3403

temperature. Fig. 4 exhibits the loops for the four samples with 4,8, 12 and 100 wt% of Fe3O4. The corresponding results from VSMfor all samples are listed in Table 1. It can be inferred from theloops that the Fe3O4 nanoparticles and nanocomposites aremagnetically soft at room temperature as their coercivities arein the range of 75–90 Oe [12]. The saturation magnetization (MS)and remanence (MR) increase, while the coercive force (HC)decreases with increasing filler content. The results obtained inthis study agreed well with that reported by Ahmad et al. [13] onbarium ferrite polymeric composites and Stefcova et al. [14] onmagnetic silicon rubber. For samples with lower filler content, theincrease in HC indicates that the TPNR matrix is resistive towardsthe alignment of the magnetic moment of the filler. Therefore, thenanocomposites with lower filler content are hardlydemagnetized compared to those nanocomposites with higherfiller loading. The initial magnetic susceptibility (wi) and initialpermeability (mi) of the samples also increased with increasingfiller content. This result is consistent with previous reports onother magnetic polymers [15–17].

At lower filler content (o10 wt%), the filler can be modeled asisolated non-interacting spheres. In this regime, the effectivemagnetic permeability of the composites with isolated particlesshows a linear relationship with filler concentration demon-strated by Wagner’s equation

miðØÞ ¼ 1þAØ, ð1Þ

where mi(Ø) is the initial magnetic permeability of granularcomposites, Ø is the weight fraction of the filler and A is acoefficient dependent on magnetic properties, geometry andvolume of the filler [16]. The measured initial permeability valuesare plotted in Fig. 5 to compare with the values given by Eq. (1).The measured values were very nearly equal to those predicted byEq. (1) at low weight fraction of Øo0.1.

3.4. Microwave electromagnetic properties

The complex dielectric permittivity and magnetic permeabilityrepresent the dynamic dielectric and magnetic properties ofelectromagnetic materials. The real components (er

0 and mr0) of

-10

-8

-6

-4

-2

0

2

4

6

8

10

-5 -4 -3 -2 -1 0 1 2 3 4 5

Mag

netiz

atio

n (e

mu/

g)

Field (kOe)

Fe3O4 (x 8)

12 wt%

8 wt%

4 wt%

Fig. 4. Hysteresis loops for pure nanoparticles and nanocomposites with different filler contents.

Table 1Magnetic properties of nanocomposites with increasing filler content.

Samples wi (70.001) mi (70.01) HC (70.05 Oe) MS (70.05 emu/g) MR (70.05 emu/g)

4 wt% 0.006 1.08 85.93 2.38 0.30

8 wt% 0.013 1.16 83.12 4.50 0.64

12 wt% 0.018 1.22 80.46 6.09 0.84

Fe3O4 0.032 1.40 73.02 63.79 11.57

1.00

1.05

1.10

1.15

1.20

1.25

1.30

2184

Initi

al P

erm

eabi

lity

(�i)

Filler Content (wt%)

µi (Experimental) µi (Wagner's Equation)

Fig. 5. The experimental and Wagner’s equation predicted initial magnetic

permeability values as a function of the weight fraction of the Fe3O4 nanoparticles.


the complex dielectric permittivity and magnetic permeabilitysymbolize the storage capability of electric and magnetic energy.The imaginary components (er

00 and mr00) represent the loss of the

electric and magnetic energy. The mechanisms of energy loss inmagnetic materials are due to dielectric and magnetic properties,which depend on the imaginary part of the complex permittivityand complex permeability. The average dielectric energy loss isgiven by /WES¼1/2oer

00Eo2, where er

00 ¼[sdc/(oeo)+eac00], sdc is

the direct current conductivity, o is the angular frequency, eo isthe permittivity of free space and eac

00 is the alternating current

loss contribution at high frequencies [3,18,19]. The averagemagnetic energy loss is given by /WMS¼1/2omr

00Ho2. For

microwave absorbers, high imaginary components of the complexdielectric permittivity and magnetic permeability are expected[20].

Fig. 6(a) and (b) shows the real (er0) and imaginary (er

00)components of the relative complex dielectric permittivity (er

*) forpure TPNR, pure Fe3O4 nanoparticles and nanocompositescontaining 4, 8 and 12 wt% of Fe3O4 in 1–20 GHz frequencyrange. It can be seen that the er

0 for pure TPNR is nearly constantat 2.6 and the er

00 is almost zero, indicating that the dielectriclosses are very small. For the nanocomposites and the pure Fe3O4,the real values er

0 of the samples are almost constant in the wholefrequency range, but the imaginary values er

00 decreasedramatically from 1 to 5 GHz before being gradually decreasedfor frequencies above 5 GHz. The values er

0 and er00 of the

nanocomposites increase as the filler content increases. All thesamples showed a constant er

0 value throughout the wholefrequency range used in this work. The constancy in the valuesof er

0 indicates that there was a domination of one type ofpolarization process, where the oscillation of the electric dipolemoments was in phase or slightly out of phase with themicrowave frequency. The most possible mechanism in thisfrequency range is orientational polarization. This interpretationis supported by the fact that neither relaxation nor resonant typebehavior is present in the er

0 versus frequency plot. Furthermore,the atomic and electronic polarizations occur at a period shorterthan the period of a microwave. The dielectric loss in the samplescan be described as due to the contributions from both the direct

2.5

3.0

3.5

4.0

4.5

5.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TPNR 4 wt% 8 wt% 12 wt% Fe3O4

� r

Frequency (GHz)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TPNR 4wt% 8 wt% 12 wt% Fe3O4

�r

Frequency (GHz)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


�r

/�r

Frequency (GHz)

Fig. 6. (a) Real, (b) imaginary and (c) tan loss permittivity curves plotted against frequency for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites.


current conductivity and the alternating current conductivity orion jump and dipole relaxation base on the expression er

00 ¼[sdc/(oeo)+eac

00]. The expression shows that direct current conductionloss is inversely proportional to the frequency, hence, the reason

for the increase in er’’ for the materials with decreasing frequencyin the low-frequency regime.

The dissipation factors represented by dielectric loss (tande¼er

00/er0) are illustrated in Fig. 6(c). Tan de Increased as the filler


content increased. Tan de initially decreased sharply beforereaching an almost constant value as the frequency was furtherincreased.

Fig. 7(a) and (b) shows the real and imaginary components ofthe relative complex magnetic permeability (mr

0 and mr00) for pure

TPNR, pure Fe3O4 nanoparticles and nanocomposites containing 4,8 and 12 wt% of Fe3O4 in 1–20 GHz frequency range. The values ofmr0 and mr

00 are, respectively, unity and zero in the wholefrequency range for the nonmagnetic TPNR sample, while astrong decrease with an increase in frequency for both quantitiesis observed at the whole frequency range for the pure Fe3O4

nanoparticles. For the nanocomposites, the values of mr0 decreases

while mr00 increases as the filler content increases. Both the mr

0 andmr00 for the nanocomposites decrease in the whole frequency range

used in this study. The effects of incorporating the Fe3O4

nanoparticles into the matrix of TPNR is to raise mr0 above unity

at low frequencies and lower mr0 at high frequencies, while mr

00 isslightly increased above zero throughout the whole frequencyrange. The magnetic permeability for the TPNR matrix is asexpected since it is non-magnetic. A sharp decrease in mr

0 and mr00

with the frequency from 1 GHz for the ferrite constitutes a part forthe resonance peak due to domain wall resonance, which issupposed to occur at lower frequency [3]. The pure TPNR sampleexhibits no wall resonance as expected.

The magnetic dissipation factors, tan dm (mr00/mr

0), are shown inFig. 7(c). The loss tangent of the magnetic permeability increases asthe filler content increases. The value, however, decreased slowly asthe frequency increases. Generally, microwave absorption proper-ties of the ferrite are determined by the dielectric and magneticlosses [21–23]. The dielectric and magnetic losses increase whenthe concentration of Fe3O4 nanoparticles increase, which may resultin an improvement of the microwave absorption properties.

3.5. Microwave absorption properties

According to the transmission line theory [24], for a micro-wave absorbing material backed by a perfect conductor, the inputimpedance (Zin) at the air–material interface is given by

Zin ¼ Z0

ffiffiffiffiffim*

r

e*r

stanh gt

� �, ð2Þ

where mr*¼mr

0 � jmr00, er

*¼er

0–jer00, Zo¼(mo/eo)1/2

¼376.7 O, is theintrinsic impedance of the free space, g¼[jo(mr

*er*)1/2]/c is the

propagation factor in the material, o is the angular frequency, c isthe speed of light and t is the thickness of the sample. Thereflection coefficient (G) is defined as G¼(Zin/Zo–1)/(Zin/Zo+1)¼[(mr

*/er*)1/2 tanh(gt)–1]/[(mr

*/er*)1/2 tanh(gt)+1]. The power

reflectivity or the reflection loss (RL), in decibel (dB) of thenormally incident electromagnetic wave at the absorber surfacecan be calculated using the following equation:

RL ¼ 20log109G9: ð3Þ

The matching condition for a perfect absorption is given byZin¼Zo. According to Eqs. (2) and (3), the matching condition hasbeen found to be determined by the combination of the sixparameters er

0, er00, mr

0, mr00, o and t.

The variation of the minimum reflection loss (RL) for pure TPNR,pure Fe3O4 nanoparticles and nanocomposites containing 4, 8 and12 wt% Fe3O4 at the thickness of 9 mm are shown in Fig. 8. Here,the bandwidth is defined as the frequency width, in which thereflection loss is less than �10 dB. The RL curves were obtained byassuming a normal or nearly perpendicular incident of themicrowave field onto the surface of a specular absorber backedby a perfect conductor, where the absorber is assumed to have aconstant thickness. RL was calculated from a computer simulation

using the values of mr* and er

* previously obtained. The reflectionloss minimum or the dip in RL is equivalent to the occurrence ofminimal reflection of the microwave power for the particularthickness or destructive interference between the reflected waves.

From Fig. 8, it is found that there are only two shallow dips forpure TPNR at 5.25 and 14.15 GHz. TPNR almost does not absorbmicrowave when there is no filler in it and it can be considered asweak microwave absorber. For nanocomposites, the results revealthe existence of two matching conditions, at low and highfrequencies, for the same thickness of the samples. These twomatching conditions are associated with an odd number multipleof a quarter wavelength thickness (t) of the material, i.e., t¼nl/4(n¼1, 3, 5, 7, 9, y), where n¼1 corresponds to the first dip at lowfrequency. The propagating wavelength in a material (lm) isexpressed by lm¼lo/(9mr

*99er*9)1/2, where lo is the free space

wavelength and 9mr*9 and 9er

*9 are the moduli of mr* and er

*,respectively. At the particular thickness, the incident and reflectedwaves in the material are out of phase by 1801, resulting in a totalcancellation of the reflected waves in the material. It can be seenthat the first and second matching conditions are associated withl/4 and 3l/4, respectively. The dips in RL versus frequency plotimply low reflectivity or good absorption. The dips are shifted to alower frequency with increasing filler content. The frequency ofthe high frequency dip decreases from 14.35 GHz for sample with4 wt% filler to 12.65 GHz for sample with 12 wt% filler. It wasreported in the literatures that the values of er

0 and er00 increase

with increasing filler concentration, resulting in the position of thedips moving to a lower frequency [25]. This shows that the dipfrequency of the nanocomposites can be manipulated easilyby changing the filler concentration. As the filler content increases,the reflection loss increases from �10.79 dB for sample with4 wt% filler to �25.51 dB for sample with 12 wt% filler. Theabsorbing bandwidth of the nanocomposites is also proportionalto the filler content. The bandwidth for sample with 4 wt% fillerwith a thickness of 9 mm is only 0.6 GHz, while the one for12 wt% filler with the same thickness is 2.7 GHz. It implies that theincrease of filler content in the nanocomposites can shift themaximum attenuation to a lower frequency, increasemaximum absorbing effect and also enlarge the absorbingbandwidth. The pure Fe3O4 nanoparticles showed a minimumreflection loss of �32.19 dB at 3.65 GHz and �10.77 dB at11.65 GHz, respectively. The first matching condition at lowfrequency in some ferrites has been interpreted as due to thespin rotational resonance frequency. Nevertheless, in this study,the spin rotational resonance frequency for the ferrites could notbe determined due to the absence of the resonance peak on themagnetic loss spectrum. It has also been suggested that maximumabsorption or minimum reflection of microwave power by anabsorber backed by a perfect conductor would occur when9mr

*9¼9er*9 regardless of whether there exists resonance or other-

wise [3,18]. The maximum absorption at 9mr*9¼9er

*9, depends on themagnitudes of both the magnetic and dielectric losses. Themaximum absorption at that particular frequency is consistentwith the reflection coefficient

G¼ðZin�ZoÞ

ðZinþZoÞ¼ðZin=ZoÞ�1� �

Zin=Zo

� �þ1

� � � 0 or Zin � Zo: ð4Þ

This is achievable only if tanh(gt)E1 or gt must be largeror either g or t is large so that tanh(gt) remains unity. Sinceg is proportional to (mr

*er*)1/2, the higher values of the two

quantities will contribute to the higher absorption.Table 2 shows the calculated and experimental matchingfrequencies for both f1(n¼1) and f2(n¼3). The results indicatethat the experimental and the calculated values are in goodagreement.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


�r

Frequency (GHz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


�r

Frequency (GHz)

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


�r

/�r

Frequency (GHz)

Fig. 7. (a) Real, (b) imaginary and (c) tan loss permeability curves plotted against frequency for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites.


Fig. 9 shows the frequency dependence of the reflection lossfor the nanocomposites with 12 wt% filler at various thickness(t¼6–10 mm). The dips of the minimum reflection loss increases

from �18.61 dB for t¼6 mm to �29.85 dB for t¼10 mm and thefrequency of the maximum absorption also shifts from 16.95 GHzfor t¼6 mm to 11.55 GHz for t¼10 mm with increasing thickness

-35

-30

-25

-20

-15

-10

-5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

RL

(dB

)

Frequency (GHz)


Fig. 8. Frequency dependences of RL for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites at the thickness of 9 mm.

Table 2Microwave absorption properties of nanocomposites with increasing filler content at the thickness of 9 mm.

Samples Calculated matching frequency (70.05 GHz) Experimental matching frequency (70.05 GHz) Reflection loss (70.01 dB) Bandwidth (70.05 GHz)

f1 f2 f1 f2 RL1 RL2 BW1 BW2

TPNR 4.62 14.05 5.25 14.15 �1.99 �2.29 – –

4 wt% 4.63 14.23 4.85 14.35 �7.81 �10.79 – 0.6

8 wt% 4.12 13.02 4.25 13.15 �9.37 �16.10 – 2.0

12 wt% 3.93 12.53 3.95 12.65 �11.26 �25.51 0.8 2.7

Fe3O4 3.58 11.50 3.65 11.65 �32.19 �10.77 1.6 1.1

-35

-30

-25

-20

-15

-10

-5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

RL

(dB

)

Frequency (GHz)

6 mm 7 mm 8 mm 9 mm 10 mm

Fig. 9. Frequency dependences of RL for the nanocomposites with 12 wt% Fe3O4 at different thicknesses.



[1,20]. As mentioned above, the dip in RL indicates the occurrenceof absorption or minimal reflection of the microwave power. Theintensity and the frequency at the reflection loss minimal,therefore, depend on the properties and thickness of thematerials. It is evident that the dips correspond to n¼1, 3, 5, yare suppressed and shifted to a higher frequency. Since theoccurrence of the dip reflects the geometrical cancellation ofwaves at the surface of an absorber with a particular thickness, itis thought that the matching thickness is reduced while thematching frequency is increased.

4. Conclusions

In this work, TPNR filled Fe3O4 nanocomposites with differentfiller weight percent were prepared as microwave absorbingmaterials. The most possible mechanism to explain the constancyin the values of er

0 is orientational polarization. The dielectric lossin the samples at low frequencies is very much influenced bydirect current conductivity, whereas the loss at higher frequenciesis attributed to alternating current conductivity. The ferritesexhibited domain wall resonance in the low-frequency regimewith increase in magnetic loss. The microwave absorptionproperties analyzed by using a specular absorber method revealthe existence of two matching conditions of minimum reflectionat low and high frequencies for the same thickness of the samples.The first matching condition at low frequency is due to 9mr

*9¼9er*9,

and the second matching condition is due to the geometricalcancellation of the incidence and reflected waves at the surface ofthe absorber. The increment of filler content in the nanocompo-sites can shift the maximum attenuation to a lower frequency,increase maximum absorbing effect and also enlarge the absorb-ing bandwidth. The minimum reflection loss was found increasingand moving towards the lower frequency region with increasingof sample thickness. Conclusively, the microwave absorbingproperties of nanocomposites were determined by the thicknessand the composition of filler. The dip frequency, microwaveabsorbing properties and the absorbing bandwidth of thenanocomposites can be manipulated easily by changing the fillerconcentration and thickness of the sample.

Acknowledgments

This work was supported by the Scientific Advancement FundAllocation (SAGA), STGL-010-2006, of the Academy of ScienceMalaysia and Science Fund 03-01-02-SF0059 from the Ministry ofScience, Technology and Innovation, Malaysia (MOSTI). Theauthors would like to acknowledge Professor Roslan Abd. Shukorfrom School of Applied Physics, Universiti Kebangsaan Malaysiafor the revision in English writing and Mr. Mohd. Razali fromScience and Technology Research Institute for Defense (STRIDE)for the microwave measurements.

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