Doping effect of multiwall carbon nanotubes on the microwaveelectromagnetic properties of NiCoZn spinel ferrites

Mangui Hana� and Longjiang DengState Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Scienceand Technology of China, Chengdu 610054, China

�Received 13 October 2006; accepted 2 December 2006; published online 3 January 2007�

NiCoZn ferrites have been found exhibiting two well separated resonance peaks. One is due todomain wall movement at 1.76 GHz. One is due to spin rotation at 6.80 GHz. With increasing thecontent of multiwall carbon nanotubes �MWCNTs� in the NiCoZn ferrite/MWCNT/wax hybridcomposites, both resonance peaks are maintained, but their peak positions are found dependent onthe MWCNT content. The damping factor for spin rotation is found decreasing as the content ofMWCNT in composites increases. The dielectric loss of hybrid composites is also foundsignificantly increased by increasing the content of MWCNT. The doping effects of MWCNT arethought due to the interaction between the ferrite and MWCNT. The microwave permeability ofpure MWCNT has also been investigated, and it is believed due to the residual nanosized Niparticles. © 2007 American Institute of Physics. �DOI: 10.1063/1.2429020�

Since the discovery of carbon nanotubes �CNTs� in 1991by Iijima,1 the intensive investigations on CNT have shownthat CNT has a series of unique mechanical, electrical, mag-netic, optical, and thermal properties.2–6 Multiwall carbonnanotube �MWCNT�/epoxy resin composites are consideredgood candidate materials for microwave applications due toits large dielectric loss, such as antireflection, electromag-netic interference shielding, or microwave absorbing.7,8 Ameasurable induced magnetic moment has been observedwhen CNTs are contacting with magnetic materials, espe-cially with half metal magnetic materials, for instance,Fe3O4. The induced magnetic moment was believed due tothe spin polarized charge transfer between CNTs and mag-netic material.9,10 Except for this induced magnetism, thereare always some nanosized residual magnetic metals �Fe orNi� left on CNT during the CNT manufacture process, andenable the CNT to exhibit a weak ferromagnetism. A mag-netoresistance effect also has been found in CNTs.11 As weknow, electromagnetic wave absorbers work by dissipatingelectromagnetic energy into heat via magnetic and dielectriclosses. Spinel ferrites are one of the frequently used electro-magnetic wave absorbing materials, but they suffer from thelower spin rotation resonance compared with other ferrites,such as hexagonal ferrites. In this letter, we report the dopingeffect of MWCNT on the resonance frequencies of NiCoZnspinel ferrites.

�Ni0.4Co0.24Zn0.36�Fe2O4 ferrites with average particlesize of 2–3 �m have been synthesized by sintering the oxidemixtures of Fe2O3, NiO, ZnO, and Co2O3 at 1250 °C for2 h. The crystal structure of prepared ferrite checked byx-ray diffraction shows a typical spinel structure. MWCNTshave been prepared by chemical vapor deposition using Ni asthe main catalyst. The average length of MWCNT is about50 �m. The outer diameter of MWCNT is about 10–30 nm.The purity of MWCNT has been checked by energy disper-sive x-ray �EDX� measurement. Three composites ofNiCoZn ferrite and MWCNT have been prepared by adjust-ing the MWCNT contents: Sample 0 contains 0 wt %

MWCNT, sample 1 contains 5.2 wt % MWCNT, smple 2contains 10.4 wt % MWCNT, and sample 3 contains15.6 wt % MWCNT. In each composite sample for micro-wave measurements, the weight of wax added for binding is16 wt %. The ferrite, MWCNT, and wax are carefully mixedto ensure that the samples are homogenous and isotropic.The shape of samples for microwave measurements on anAgilent 8720 vector network analyzer is toroidal with theinner diameter of 3 mm and the outer diameter of 7 mm. Thefrequency range selected for microwave measurements is0.5–14 GHz. The microwave permeability ��=��− j��� ofpure MWCNT also has been measured.

The EDX measurement shows that the impurities ofMWCNT include Ni �0.5 wt % �, Al �0.1 wt % �, and O�5.5 wt % �. Ni, Al, and O in MWCNT are residual sub-stances due to the MWCNT fabrication process. As shown inFig. 1�a�, two peaks are found in the spectrum of ��-f forsample 0 �NiCoZn ferrite/wax composite�. For these twopeaks, one is at 1.76 GHz and the other one is at 6.80 GHz.It is well accepted that there are two mechanisms responsiblefor the permeability dispersion of power ferrites: domainwall motion and spin rotation. The resonance frequency ofdomain wall motion �i. e., fdw� is lower than the one for thespin rotation �i. e., fr�. The permeability dispersion curve ofMWCNT is shown as the inset in Fig. 1�d�. The observedpermeability of MWCNT is believed due to the residualnanosized Ni particles on MWCNT. For samples 1, 2, and 3,the magnetic materials contributing to the permeability spec-trum shape include ferrite and Ni. For such hybrid magneticcomposites containing two ferromagnetic substances, the dis-persion curve is extremely difficult to be expressed by super-posing the magnetic spectrum of each magnetic material.Similar situation has been reported in permalloy-NiZn ferritehybrid composite materials by Kasagi et al.12 Also, if thepermeability dispersion curve is badly distorted, the disper-sion curve would be very difficult to be fitted by assuming itcomprises two component spectra: one is for domain wallmovement; one is for spin resonance, for instance, the per-meability spectra in Fig. 1�d�.a�Electronic mail: mangui@gmail.com

APPLIED PHYSICS LETTERS 90, 011108 �2007�

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Based on the spectrum shape, we learn that the spectra inFig. 1 are the relaxation type because no negative �� valueshave been observed. Therefore, the resonances at lower fre-quencies in Figs. 1�a�–1�c� are due to the domain wall move-ment mechanism. The resonances at higher frequencies aredue to the spin rotation mechanism. In Fig. 1�a�, fdw and fsare 1.76, and 6.80 GHz, respectively for sample 0. InFig. 1�b�, fdw and fs are 1.28, and 8.03 GHz for sample 1.In Fig. 1�c�, fdw and fs are 1.98, and 11.08 GHz for sample 2.Clearly, the doping effect of MWCNT on the resonance fre-quency of spin rotation is much significant than that of do-main wall movement. For the domain wall resonance with arelaxation-type spectrum, the real and imaginary parts of per-meability can be expressed by Debye’s dispersion law,13

�dw� = 1 + �d01

�1 − �f/fdw�2�, �1�

�dw� = �d0f/fdw

�1 − �f/fdw�2�, �2�

where �d0 is the static susceptibility for domain wall move-ment. The resonance frequency fdw above can be expressedby physical parameters as13 fdw= �9��wall�� / �8a�0

2Ms2�,

where �wall is the density of domain wall energy, a is theaverage distance of impurities in a ferrite particle which im-pede the domain wall movements, � is the electric resistivityof ferrite, and Ms is the saturation magnetization. In our case,the ferrites in different composite samples are same. Hence,�wall can be considered as a constant. The particle size offerrites is about 2–3 �m. Multidomains can exist in eachferrite particle, so the parameter a can be assumed as a con-stant. Then, as shown in this equation, fdw is dependent on �and Ms. It is well known that MWCNT is a substance withvery low resistivity and weak ferromagnetism. Doped withMWCNT, both � and Ms of ferrite-wax composite will drop.Qualitively speaking, for sample 1, we can conjure that thedrop in � is stronger than the drop in Ms, hence fdw is lowerthan the one for sample 0. For sample 2, the effect of the

drop in Ms is stronger than the drop in �, then fdw is higherthan samples 0 and 1.

As for the spin resonance frequency fs, it depends on14

fs= �� /2�� HA, where � is the gyromagnetic ratio and HA isthe effective magnetic anisotropic field. In our case, the per-meability spectrum due to the spin rotation mechanism is arelaxation type. Accordingly it can be expressed as15

�s� = 1 +�s0�s

2���s2 − �2� + �2�2�

��s2 − �2�1 + �2��2 + 4�2�s

2�2 , �3�

�s� =�s0�s����s

2 + �2�1 + �2����s

2 − �2�1 + �2��2 + 4�2�s2�2 , �4�

where �s0 is the static susceptibility for spin rotation, �s isthe angular resonance frequency, � is the damping factor forspin rotation, and � is the angular frequency of alternatingmagnetic field.

Fitting the experimental curves in Fig. 1 with combinedequations for ��-f �Eqs. �1��3�� and ��-f �Eqs. �2��4�� isfound extremely difficult. Therefore, we have separately fit-ted the peaks for spin rotation component �designated as“spin”� and domain wall component �designated as “dw”� inthe spectra: Firstly, we fit each peaks in the ��-f curves.Secondly, we calculate the ��-f curves by using the obtainedfitting parameters and corresponding �� expressions�Eqs. �1� or �3��. The fitting results are shown inFigs. 1�a�–1�c�. The fitting parameters are listed in Table I.fs�exp� is the frequency at which the �� value of the spinrotation component reaches a maximum value on the experi-mental curve. fdw�exp� has a similar definition for the do-main wall component on the experimental curve. As shownclearly in Table I, the damping factor � of spin rotationmechanism is monotonously decreased with the increase ofthe MWCNT contents in the composites. For spin rotationresonance, the maximum frequency fs�max�, which is de-fined as the frequency at which �� reaches a maximumvalue, can be derived from Eq. �4� as15 fs�max�= fs /��2+1.With the obtained fitting parameters, the calculated fs�max�values are also listed in Table I. The small difference be-tween the experimental fs�exp� values and the fitted fs�max�values indicate that our fitting method makes sense.

Tsutaoka reported that in ferrite/resin composites, bothfdw and fs increased with the decrease of the volume fractionof Ni–Zn ferrites.15 They ascribed this to the gap parameter� /D� in ferrite composites. According to their theory, asphere ferrite particle �or a particle cluster� with a diameterof D is supposed to be enclosed by a layer of nonmagnetic

FIG. 1. Microwave permeability spectra of NiCoZn ferrite/MWCNT/waxhybrid composites, “dw” denotes the domain wall component. “spin” de-notes the spin rotation component. �� �max� denotes the maximum value of�� found on the experimental curve. �a� is for sample 0 containing 0 wt %MWCNT. �b� is for sample 1 containing 5.2 wt % MWCNT. �c� is forsample 2 containing 10.4 wt % MWCNT. �d� is for sample 3 containing15.6 wt % MWCNT. The inset in �c� denotes the �-f spectrum forMWCNT/wax composite.

TABLE I. Fitting parameters for permeability dispersion curves. fdw �exp�and fs �exp� denote the frequencies at which the �� values on experimentalcurves reach maximum for domain wall motion component and spin rotationcomponent, respectively. fs �max� is the calculated value, please see itsdefinition in the text. fdw, fs, �d0, �s0, and � are the fitting parameters.

Samples

Domain wall motion Spin rotation

fdw

�GHz�fdw �exp�

�GHz� �d0

fs

�GHz�fs �exp��GHz�

fs �max��GHz� �s0 �

No. 0 1.57 1.76 1.49 9.42 6.80 6.39 1.167 1.083No. 1 1.20 1.28 1.26 10.73 8.03 8.12 1.411 0.863No. 2 1.63 1.98 1.83 11.37 11.08 10.20 0.881 0.493

011108-2 M. Han and L. Deng Appl. Phys. Lett. 90, 011108 �2007�

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substance, such as epoxy resin. The thickness of nonmag-netic layer is /2. fdw and fr can be expressed, respectivelyas fdw= fB-dw�1+�B-dw� /D��1/2 and fs= fB-s�1+�B-s� /D��,where �B-dw �fB-dw� and �B-s �fB-s� are the static susceptibility�resonance frequency� of bulk ferrite for domain wall motionand spin rotation mechanisms, respectively. As the concen-tration of nonmagnetic material increases �i.e., the concen-tration of magnetic material decreases�, the /D value in-creases. It is clear that both fdw and fs increase with thedecrease of ferrite content. However, in our case, the fdwvalue is not monotonously increased with the decrease of theferrite content �83, 78, and 72.9 wt % for samples 0, 1, and2, respectively�. For instance, fdw in sample 1 is lower thanthose in samples 0 and 2, see Table I. Furthermore, we alsofound that with more MWCNT �15.6 wt % � added in thecomposite studied �sample 3�, the dispersion curve has beenseriously distorted, as shown in Fig. 1�d�, where an addi-tional peak appears. In Fig. 1�d�, since the point A in theinset and the A� point have same frequency; therefore webelieve that the peak A� is due to the effect of MWCNT. Theother two peaks are due to the spin rotation component�fs�max�=7.06 GHz� and domain wall component�fdw�max�=0.85 GHz�. Compared with sample 2 inFig. 1�c�, both fdw�max� and fs�max� in Fig. 1�d� have

shifted toward lower frequencies. Therefore, from the per-spective of MWCNT doping effect on resonance frequency,the appropriate doping range is 0–10.4 wt %. In addition,the permeability behaviors of the MWCNT/NiCoZn ferrites/wax hybrid composites are different from the one reportedby Tsutaoka15 Such an unusual behavior is believed due tothe interactions between the MWCNTs and the NiCoZn fer-rite particles. Interactions maybe occur between the residualnanosized Ni particles in MWCNT and NiCoZn ferrite par-ticles, or between the MWCNT and NiCoZn particles. Fur-ther investigation will be conducted in the future. The varia-tion of permeability peak shapes also indirectly manifests theexistence of such an interaction.

The permittivity spectra for NiCoZn/MWCNT/wax hy-brid composites are shown in Fig. 2. For sample withoutMWCNT �sample 0�, the � values are almost constant withinthe measurement frequency range: �� is about 5 and �� isabout 0, showing a typical feature of ferrite in gigahertzrange. With increasing the MWCNT contents in composites�samples 1–3�, both �� and �� increase in the measurementfrequency range, and both are gradually decreased with in-creasing frequency. Especially, for sample 2 and 3, there is aclear inverse proportion relationship between �� and fre-quency. This is a typical feature of a conducting media andshows that with more MWCNT added into the studied com-posite, it will become a conducting composite due to the highconductivity of MWCNT. At lower frequency range of0.5–6 GHz, both samples 2 and 3 show a significant dielec-tric loss, please see the �� /�� values in the inset in Fig. 2�b�.

In summary, for NiCoZn ferrite/MWCNT/wax hybridcomposites, fdw and fs are found dependent on the contentsof MWCNT, especially for the spin rotation resonance fre-quency fs. From the perspective of increasing resonance fre-quency, the suitable doping range is 0–10.4 wt %. An in-creased dielectric loss also can be obtained. Employed as anelectromagnetic wave absorber, the reported results in thisletter offer us an alternative to adjust its working frequencyrange and dielectric loss.

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FIG. 2. Microwave dielectric properties of NiCoZn ferrite/MWCNT/waxcomposites. The inset in �b� is the dependence of dielectric loss on thedifferent MWCNT loadings.

011108-3 M. Han and L. Deng Appl. Phys. Lett. 90, 011108 �2007�

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