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Fabrication and Microwave Absorption Propertiesof Fe0.64Ni0.36-NiFe2O4 Nanocomposite

Xu Yan, Desheng Xue∗

(Received 15 September 2012; accepted 18 September 2012; published online 25 September 2012.)

Abstract: Fe0.64Ni0.36-NiFe2O4 nanocomposite was performed with partially reducing NiFe2O4 nanoparticlesin Ar/H2 ambient. The microwave and static magnetic properties were investigated. The results showed that

the nanocomposite was characterized with enhanced microwave absorption properties. The optimal reflection

loss (RL) of the nanocomposite reached −24.8 dB at 14 GHz for an absorber thickness of 1.5 mm. Meanwhile,

a broad bandwidth for RL

Nano-Micro Lett. 4 (3), 176-179 (2012)/ http://dx.doi.org/10.3786/nml.v4i3.p176-179

Cu Kα radiation. The nanoparticle images were takenby transmission electron microscope (TEM, JIM-2010).The static magnetic properties were investigated by Vi-brating Sample Magnetometer (VSM, Lakeshore 7300).The samples for microwave electromagnetic propertiesmeasurement were mixed with paraffin (the mass ratioof sample to paraffin is 1:1), and compressed into ringshape with 7.00 mm outer diameter, 3.00 mm innerdiameter and 2 mm thickness. The scattering param-eters (S11, S21) were measured by a network analyzer(Agilent Technologies E8363B) in the range of 0.1-18GHz. All of the measurements were performed underthe room temperature.

Results and discussion

Figure 1(a) shows the TEM image of sample A. Thenanoparticles’ average diameter is about 20 nm. TheTEM micrograph of sample B is shown in Fig. 1(b), anobvious increase of the average diameter is observed.Figure 1(c) shows the XRD patterns of the two sam-ples. Single spinel NiFe2O4 phase was observed in sam-ple A. After partially reducing process, fcc Fe0.64Ni0.36phase and spinel NiFe2O4 phase coexisted in sampleB. According to Scherrer’s formula and the full widthat half-maximum (FWHM) of (311) peak, the averagegrain size of NiFe2O4 was estimated to be 21 and 53nm for sample A and sample B, respectively. The in-crease of grain size between the two samples may be at-

tributed to the annealing in partially reducing process.Figure 1(d) illustrates the magnified image of nanopar-ticle in sample B. The inter-planar space of the particlein the Section I is 0.486 nm, which corresponds to the(111) plane of NiFe2O4 with spinel structure. However,with the inter-planar space of Section II and III can notbe distinguished. The high-resolution image containingSection II and III is shown in Fig. 1(e). The latticewith inter-planar spaces of 1.78 and 2.02 nm in Sec-tion II can be identified as the (111) and (200) planesof Fe0.64Ni0.36 respectively, and the lattice with inter-planar spaces of 2.51 nm in Section III can be identifiedas the (311) plane of NiFe2O4. The results well agreewith the XRD data. Thus, it can be speculated thatparts of NiFe2O4 nanoparticles in sample A merged intolarge particle in the heating process under partly reduc-tion, which led to the increase of average diameter andgrain size in sample B. When the large particles werein Ar/H2 atmosphere, the surfaces of these particleswere reduced to Fe0.64Ni0.36 alloy. After the partial re-ducing process, the sample was cooled down to roomtemperature. Then it was exposed in the air ambient.The surfaces of the Fe0.64Ni0.36 alloy were passivatedinto NiFe2O4, and the thickness of the passivated layeris about 4 nm. The reduced Fe0.64Ni0.36 phase wasembodied in the NiFe2O4 nanoparticles. Figure 1(f) il-lustrates the selected area electron diffraction (SAED)patterns of sample A and B, the results well agree withthe data of XRD.

(a) (b)

(d) (e) (f)

(c)

20 nm 20 nm

10 nm 5 nm 2 l/nm

20 40 60 802θ (degrees)

Inde

nsity

(a.u

.)

(111

) (220

)

(222

)(1

11)(3

11)

Fe0.64Ni0.36

NiFe2O4

NiFe2O4(111)

NiFe2O4(440)(422)(400)(311)(220)

Fe0.64Ni0.36(200)

Fe0.64Ni0.36NiFe2O4

d=4.86 d=1.78(111)

Sample A Sample B

d=2.02d=2.51

PartlyReducedNiFe2O4

(200

)(5

11)

(440

)

Section III

Section II

Section I

(111)(200)(511)

(622)

Fig. 1 TEM images of sample A (a) and B (b), XRD patterns of sample A and B (c), HRTEM images of sample B (d ande) and SAED patterns of sample A and B (f).

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Nano-Micro Lett. 4 (3), 176-179 (2012)/ http://dx.doi.org/10.3786/nml.v4i3.p176-179

Figure 2 shows the hysteresis loops of sample A andB. After partially reducing procedure, the saturationmagnetization (M

S) of the sample B increased from 32

to 78 emu/g, the coercivity was enhanced from 61 to195 Oe. The M

Sof Fe0.64Ni0.36 nanoparticles was re-

ported to be 126.8 emu/g [7]. According to the MS

ofsample A, B and Fe0.64Ni0.36 nanoparticles, the weightratio of Fe0.64Ni0.36 in the sample B was calculated tobe 48.5%.

195 Oe

61 Oe

0

0 500−500−10000

−70

70Sample A

Sample B

Mag

neti

zati

on (

emu/

g)

0

100000Applied field (Oe)

195

61 Oe

0

Sample A

Sample B

Fig. 2 The hysteresis loops of sample A and B, the inertindicates the enlarged part of coercivity.

Figure 3(a) exhibits the real and imaginary parts (ε′,ε′′) of complex permittivity spectra of the two sam-

ples. The solid square corresponds to the sample A,and the red circle presents the sample B. The ε′ and ε′′

of sample A both show a decrease when the frequencyis lower than 1 GHz and then keeps almost constant inthe following measured range. It can be noticed thatboth ε′ and ε′′ of the sample B are higher than thoseof the sample A throughout the measured range, that’sto say, the energy loss in the sample B is enhanced inwhich ε′′ represents dielectric loss. In this study, thepartially reduced Fe0.64Ni0.36 in the sample B increasesthe conductivity of the sample and contributes to theenhancement of the real part of complex permittivity.On the other hand, the free electrons can accumulateat the interface of the two phases, generating interfa-cial electric dipolar polarization which contributes tothe increasing of the imaginary part of the sample B.

Figure 3(b) exhibits the real and imaginary parts (μ′,μ′′) of complex permeability spectra of the sample Aand B. The μ′ of the sample A decreases from 1.8 to0.9 with as the frequency increases from 0.1 to 18 GHz,while the μ′ of the sample B decreases from 2.4 to 0.8.The μ′′ spectrum of sample A shows a single broadpeak at about 3 GHz which can be ascribed to the nat-ural resonance of NiFe2O4 [8]. In the μ

′′ spectrum ofthe sample B, the strongest peak appears at about 2.4GHz, the second peak is around 12 GHz and the thirdpeak stands at 17 GHz. The peak position of the firstpeak is close to that of the sample A, and the peak

25

20

15

10

5

0

Per

mit

tivi

ty

ε' sample A

ε" sample A

ε' sample B

ε" sample B

6.0G0 12.0G

(a)

Frequency (Hz)18.0G

ε' sample A'

ε" sample A"

ε' sample B'

ε" sample B"

(a)

Per

mea

bilit

y

μ' sample A

μ" sample A

μ' sample B

μ" sample B

3

2

1

0

6.0G0 12.0GFrequency (Hz)

18.0G

(b)μ' sample A'

μ" sample A"

μ' sample B'

μ" sample B"

( )

0Frequency (Hz)

Ref

lect

ion

loss

(dB

)

0

−5

−10

−15

−20

−25

−306.0G 12.0G 18.0G

(d)

0

(c)

Frequency (Hz)6.0G 12.0G 18.0G

Ref

lect

ion

loss

(dB

)

0

−5

−10

−15

−20

−25

−30

1.0 mm1.3 mm1.5 mm1.8 mm2.0 mm2.5 mm3.0 mm3.5 mm4.0 mm (c)

1.0 mm01.3 mm1.5 mm1.8 mm2.0 mm2.5 mm3.0 mm3.5 mm4.0 mm

Fig. 3 Frequency dependence of the complex permittivity (a) and permeability (b), the relationship of reflection loss andfrequency of the sample A (c) and B (d) for varied thickness.

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Nano-Micro Lett. 4 (3), 176-179 (2012)/ http://dx.doi.org/10.3786/nml.v4i3.p176-179

intensity is about two times more than that of as-prepared nanoparticles. The mass ratio of nickel fer-rite in sample B is smaller than that of sample A, thecontribution of natural resonance of NiFe2O4 to μ

′′ inthe sample B also is smaller than that of the sampleA. As a result, the strongest peak could be causedby natural resonance of both nickel ferrite and invaralloy. The second and third peak may result fromexchange mode of invar alloy [9]. The magnetic loss(μ′/μ′′) of two samples have been calculated, the val-ues of partly reduced sample were enhanced in compar-ison with those of NiFe2O4 nanoparticles throughoutthe measured range.

Figure 3(c) shows the calculated reflection loss (RL)of the sample A. The optimal RL is higher than 10 dB.The thickness dependent RL spectra of the sample Bare shown in Fig. 3(d). In comparison with the sampleA, the optimal RL of the sample B reached -24.8 dB at14 GHz for a thickness of 1.5 mm. A broad bandwidthfor RL