JOURNAL OF RARE EARTHS, Vol. 28, No. 3, Jun. 2010, p. 451

Foundation item: Project supported by the National Defence Fundamental Research (MKPT-232)

Corresponding author: ZHANG Qitu (E-mail: njzqt@126.com; Tel.: +86-25-83587246)

DOI: 10.1016/S1002-0721(09)60132-0

Microwave electromagnetic and absorbing properties of Dy3+ doped MnZn ferrites

SONG Jie ( )1, WANG Lixi ( )1, XU Naicen ( )1, ZHANG Qitu ( )1, 2 (1. College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China; 2. Jiangsu Provincial Key Laboratory of New Materials of Inorganic and its Composites, Nanjing University of Technology, Nanjing 210009, China)

Received 31 August 2009; revised 3 February 2010

Abstract: Dy3+ doped Mn-Zn ferrites Mn0.3Zn0.7Fe2–xDyxO4 x=0, 0.01, 0.02, 0.03, 0.04 were prepared by the conventional solid-state re-action. The crystal structure, surface morphology and electromagnetic properties of the calcined samples were characterized by X-ray diffrac-tion analysis(XRD), scanning electron microscopy(SEM) and network analyzer (Agilent 8722ET). All the XRD patterns showed the single phase of the spinel-type ferrite without other intermediate when x 0.03. The average crystallite size was about 44 56 nm. The microwave electromagnetic properties of the samples were studied at the frequency range from 2 GHz to 18 GHz. It was shown that small amounts of Dy3+ substitution could adjust microwave electromagnetic parameters magically. The tan exhibited a maximal peak when x=0.03, and the peak value was 0.56. It indicated that the microwave electromagnetic loss properties were excellent when x=0.03. Furthermore, the reasons were also discussed using electromagnetic theory. The reflection loss (RL) increased with the Dy3+ content when x<0.04. The Mn0.3Zn0.7Fe1.97Dy0.03O4 ferrite displayed excellent microwave absorption properties. The frequency (with respect to –10 dB) started from 11.9 GHz, and the bandwidth reached about 3.5 GHz. The peak value of RL was about –20.5 dB at a matching thickness of 2.7 mm.

Keywords: Mn-Zn ferrite; Dy-doped; permittivity; permeability; microwave absorbing properties; rare earths

Spinel-type ferrites with the general formula MFe2O4 (M is a divalent metal cation) are very important materials be-cause of their dielectric and magnetic properties[1,2]. As a soft magnetic material, MnZn ferrite is an important member of spinel family[3]. MnZn ferrites, capable of combining their high permeability, and high saturation magnetic flux density, are widely used as recording heads, communication pulse transformers and microwave absorbing materials, etc.[4–6].

It is known that the rare earth (RE) ions have unpaired 4f electrons and the strong spin-orbit coupling of the angular momentum. Moreover, 4f shell of rare earth ions is shielded by 5s25p6 and almost not affected by the potential field of surrounding ions. Doping rare earth ions into spinel-type fer-rites, the occurrence of 4f-3d couplings which determine the magnetocrystalline anisotropy in ferrites can also improve the electrical and magnetic properties of MnZn ferrites. A number of investigations have been reported on the effect of rare earth ions on magnetic properties of spine-type fer-rites[7,8], while the effect on microwave electromagnetic per-formance is seldom studied recently.

In the present study, an attempt has been made to study the Dy3+ substitution position in the ferrite system. Further-more, the effect of small amounts of Dy3+ substitution on the microwave electromagnetic properties of MnZn ferrite was also investigated.

1 Experimental

MnZn ferrites with a composition of Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) were prepared by the conven-tional solid-state reaction. The starting materials, MnO2, ZnO, Fe2O3 and Dy2O3 (Analytical Reagent) were stoichiometrically weighed and ball milled in an attritor with de-ionized water for 8 h. After drying, the mixture of oxide powder was homogenized and calcined at 1150 ºC for 5 h, and the calcined powders were ball milled in de-ionized wa-ter at a rotation speed of 300 r/min for 3 h again.

The X-ray diffraction (XRD) was carried out to check the phase purity with an ARL’ X-ray powder diffractometer us-ing Cu K radiation. A scanning electron microscope (SEM) was used to observe the natural surface of Dy3+ ions doped ferrite samples. A network analyzer (Agilent 8722ET) was employed to determine the values of ', '', ' and '' at the frequency range of 2–18 GHz using a reflection/transmission technique. The ferrite-paraffin wax compositions with 70 wt.% of ferrite were prepared by homogeneously mixing the ferrite powder and toroidal-shaped samples of 3.04 mm in-ner diameter, 7.0 mm outer diameter and 5 mm length (Fig. 1). The measured values of reflected and transmitted scat-tering parameters (S11, S21) were used to determine ', '', ' and ''[9].

2 Results and discussion

The XRD patterns was examined to identify the phase of Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) powders

452 JOURNAL OF RARE EARTHS, Vol. 28, No. 3, Jun. 2010

Fig. 1 Simple coaxial transmission lines hold the samples of mate-

rial under test

calcined at 1150 ºC for 5 h (Fig. 2). When the substituted amount x 0.03, XRD shows that the samples are the sin-gle-phase cubic spinel MnZn ferrites. No characteristic peaks of impurities are detected in the pattern. It indicates that the Dy3+ ions can be completely solved into the MnZn ferrite crystal lattice when the substitution content x 0.03. However, it is observed that the cubic spinel phase coexists with some amount of DyFeO3 phase when x=0.04. Because the ionic radius of Dy3+(0.104 nm) is larger than that of the Fe3+ (0.067 nm), the amount of Fe3+ ions substituted by Dy3+ ions is limited, thus redundant Dy3+ ions aggregates on the grain boundaries forming DyFeO3 phase[10]. It is shown that the maximum substitution content of Fe3+ by Dy3+ is 0.03 under our experiment condition.

For nanocrystalline materials, the size of primary nanopar-ticles can be estimated using the amount by which the X-ray line is broadened. The average crystallite size (D311) of Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) ferrites was calculated from the XRD line broadening of the (311) XRD-peaks by using Scherrer’s equation:

0.89

coshkl

i

D (1)

where is the incident wavelength of Cu K radiation of the XRD, i is the peak width at midheight, and is the consid-ered angle. It is clearly concluded that the synthesized pow-ders are nanocrystallite particles with crystallite size of 44–56 nm.

The typical SEM images of the as-prepared samples cal-cined at 1150 ºC for 5 h are shown in Fig. 3. The as-prepared powders are agglomerated and essentially consist of some irregularly cubic particles. The particles are somewhat ex-panded in dimension with the increase of Dy3+ substitution content in the samples, the average particle size is about 200 nm for the Mn0.3Zn0.7Fe2O4 sample, while that of Mn0.3Zn0.7Fe1.97Dy0.03O4 sample is the largest among these samples, because the replacement of limited amounts of Dy3+ ions occurs at x=0.03, and the expansion of the ferrite lattice reaches its maximum. When x=0.04, redundant Dy3+ ions form DyFeO3 phase along the grain boundaries, which inhibit the grain growth.

Fig. 2 XRD patterns of the Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02,

0.03, 0.04) ferrites calcined at 1150 ºC for 5 h

Fig. 3 SEM photograph of the natural surface of Mn0.3Zn0.7Fe2–xDyxO4 ferrites

(a) x=0; (b) x=0.01, (c) x=0.02, (d) x=0.03, (e) x=0.04

SONG Jie et al., Microwave electromagnetic and absorbing properties of Dy3+ doped MnZn ferrites 453

Complex permittivity ( r= ' j '') and complex permeabil-

ity ( r= ' j '') represent the dielectric and dynamic magnetic properties of magnetic materials. The real parts of complex permittivity and permeability symbolize the storage capabil-ity of electric and magnetic energy. The imaginary parts represent the loss of electric and magnetic energy. The mi-crowave electromagnetic properties of the powders were measured with an analyzer (Agilent 8722ET), and the com-plex permittivity and complex permeability computed from S-parameter values are clarified in Figs. 4 and 5.

As shown in Fig. 4(a), ' of the MnZn ferrites doped with Dy3+

exhibits a higher value than that of pure MnZn ferrite. From Fig. 4(b), it is found that '' has a shoulder peak at around 13 GHz frequency position. In addition, the values of '' also exhibit a higher value than that of pure MnZn ferrite,

and the '' increases with the Dy3+ content when x 0.03. It is reasoned that MnZn ferrites turn into solid solution after be-ing doped with little amounts of Dy3+ ions. It is shown that the crystal cell of MnZn ferrite expanded. There is the for-mation of intrinsic electric moment due to larger ionic radii of Dy3+ compared with that of Fe3+. Because the intrinsic electric moment occur the orientation polarization under ex-ternal electric field, which improves the dielectric loss[11].

It is shown in Fig. 5(a), the ' values of the MnZn ferrites doped with Dy3+

shows a higher value than that of pure MnZn ferrite, and 0.03 mol Dy3+ doped MnZn ferrite exhib-its the highest real part of complex permeability. All the ' values fluctuate between 0.85 and 1.30. As shown in Fig.

5(b), '' has a peak at around 13 GHz which is provoked by natural resonance[12]. In addition, the peak values of '' in-crease with Dy3+ content when x<0.04, while it decreases a little when x=0.04. According to the ferromagnetic theory[13], the nature resonance frequency is determined by magneto-crystalline anisotropy field (HA) and the peak of '' is con-nected with magnetization (Ms). Table 1 represents the structure and magnetic properties of some ions. From these parameters it shows that the ions magnetic moment (nB) of Dy3+ is larger than that of Fe3+. The enhanced peak of '' for Dy-substituted ferrite indicates that substituting Dy3+ ions for Fe3+ ions increases the Ms for ferrites. In the crystal structure of spinel-type ferrite, the electromagnetic properties of fer-rites depend on the distribution of basic metal ions. Having unpaired electrons in d orbit, the spin magnetic moment of Fe3+ ion (3d5) is 5 B. The magnetic moment of Dy3+ ion consists of two parts, orbital magnetic moment and spin magnetic moment. Orbital magnetic moment of Dy3+ ion ex-ists, because the radius of Dy3+ ion is so large that crystalline field has not much stricture to Dy3+ ion. Whereas, the asso-ciation of orbital magnetic moment and spin magnetic mo-ment is 10.63 B

[14]. The substitution of Dy3+ (10.63 B) for Fe3+ (5 B) causes the increase of Ms of the ferrite. Thus, the increase of r is obtained, as shown in Fig. 5(b), which has been verified by experiments.

The electromagnetic loss property of the materials can be described by tan ( is dielectric phase angle)[15]: tan =tan e+tan m= ''/ '+ ''/ ' (2)

Fig. 4 Complex permittivity of the Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) ferrites

(a) Frequency variation of the real part of complex permittivity ( '); (b) Frequency variation of the imaginary part of complex permittivity ( '')

Fig. 5 Complex permeability of the Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) ferrites

(a) Frequency variation of the real part of complex permeability ( '); (b) Frequency variation of the imaginary part of complex permeability ( '')

454 JOURNAL OF RARE EARTHS, Vol. 28, No. 3, Jun. 2010

Table 1 Structure and magnetic properties of ions

nB/ B Ions Electron

configuration

Ion radius/

(10–10 m) Calculated Measured

Fe3+ 3d5 0.64 5.92 5.9

Dy3+ 4f95s2p6 0.908 10.63 10.6

Fig.6 Microwave absorbing of the Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01,

0.02, 0.03, 0.04) ferrites

where tan e and tan m are the dielectric loss and magnetic loss, respectively. Fig. 6 shows the dependence of tan on frequency. The trend of tan is similar to the ''. In addition, the values increase significantly with small amounts of Dy3+ ions doped, and tan exhibits a maximal peak at the fre-quency of 13 GHz when x=0.03.

For a microwave absorbing layer, the reflection loss (RL) is given as[16]:

r r 0 r r

r r 0 r r

1 / tanh( 2RL 20 lg

1 / tanh( 2

j f d

j f d (3)

where f0 is frequency, d the absorber thickness, r the com-plex permittivity ( r= ' j ''), and r the complex permeabil-ity ( r= ' j ''). The electromagnetic wave absorption prop-erties were determined from the frequency dependence of RL at a thickness. Using the above formula and measured values of r and r, the matching thicknesses of Mn0.3Zn0.7Fe2–x DyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) ferrites were calculated to be 3.1, 2.8, 2.7, 2.7 and 2.9 mm, respec-tively.

Fig. 7 Calculated reflection loss curve for single-layer coatings at

matching thickness

The calculate RL curves of the single-layer ferrite com-pound for Dy3+ doped MnZn ferrites at matching thickness are displayed in Fig. 7. It is observed that Dy-substitution is useful to broaden the absorbing band. Fig. 7 also shows that the peak value of RL increases with the Dy3+ content when x<0.04, while it decreases a little when x=0.04. In addition, the peak of RL shifts to low frequency position. RL is totally decided by dielectric and magnetic loss. When the substi-tuted amount x=0.03, tan exhibits a maximal peak, indicating that RL reaches a maximum. The Mn0.3Zn0.7Fe1.97Dy0.03O4 fer-rite has excellent microwave absorption properties at a matching thickness of 2.7 mm. The peak value of RL is about –20.5 dB. The frequency (with respect to –10 dB) starts from 11.9 GHz, and the bandwidth reaches about 3.5 GHz.

3 Conclusions

The Dy3+ doped Mn-Zn ferrites Mn0.3Zn0.7Fe2–xDyxO4 (x=0, 0.01, 0.02, 0.03, 0.04) calcined at 1150 ºC for 5 h were pure phase of the spinel-type ferrite without other intermedi-ate phase when x 0.03, while the DyFeO3 phase appeared when x=0.04. The average crystallite size was about 44 56 nm. The electromagnetic loss properties of the prepared Mn-Zn ferrites was enhanced significantly by partial substitution of Dy3+ ions for Fe3+ sites. When x=0.03, the Dy-doped ferrite showed the best electromagnetic loss performance. The Mn0.3Zn0.7Fe1.97Dy0.03O4 ferrite exhibited excellent micro-wave absorption properties at a matching thickness of 2.7 mm. The frequency (with respect to –10 dB) started from 11.9 GHz, and the bandwidth reached about 3.5 GHz. The peak value of RL was about –20.5 dB. This indicated a po-tential to be used for thin ferrite absorber.

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