Journal of Magnetism and Magnetic Materials 324 (2012) 1305–1311

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

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

Magnetic and Microwave Absorbing Properties of ElectrospunBa(1�x)LaxFe12O19 Nanofibers

Cong-Ju Li a,b,n, Bin Wang a,b, Jiao-Na Wang a,b

a College of Material Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, PR Chinab Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing 100029, PR China

a r t i c l e i n f o

Article history:

Received 12 August 2011

Received in revised form

17 October 2011

Accepted 11 November 2011Available online 2 December 2011

Keywords:

Electrospinning

Barium ferrite

Lanthanum

Microwave absorption

53/$ - see front matter & 2011 Elsevier B.V. A

016/j.jmmm.2011.11.016

esponding author.

ail address: congjuli@gmail.com (C.-J. Li).

a b s t r a c t

Ba(1�x)LaxFe12O19 (0.00rxr0.10) nanofibers were fabricated via the electrospinning technique

followed by heat treatment at different temperatures for 2 h. Various characterization methods

including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer

(VSM), and microwave vector network analyzer were employed to investigate the morphologies,

crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The

SEM images indicate that samples with various values of x are of a continuous fiber-like morphology

with an average diameter of 110720 nm. The XRD patterns show that the main phase is M-type

barium hexaferrite without other impurity phases when calcined at 1100 1C. The VSM results show that

coercive force (Hc) decreases first and then increases, while saturation magnetization (Ms) reveals an

increase at first and then decreases with La3þ ions content increase. Both the magnetic and dielectric

losses are significantly enhanced by partial substitution of La3þ for Ba2þ in the M-type barium

hexaferrites. The microwave absorption performance of Ba0.95La0.05Fe12O19 nanofibers gets significant

improvement: The bandwidth below �10 dB expands from 0 GHz to 12.6 GHz, and the peak value of

reflection loss decreases from �9.65 dB to �23.02 dB with the layer thickness of 2.0 mm.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Due to the development of radar, microwave communicationstechnology and especially the need for the anti-electromagneticinterference coatings, microwave darkrooms and stealth technol-ogy, electromagnetic wave absorbers with the capability ofabsorbing unwanted electromagnetic signals have been investi-gated and become one of the most crucial high-tech materials inrecent years [1]. Ferrites are considered to be the best magneticmaterial for electromagnetic wave absorbers due to their excel-lent magnetic and dielectric properties. Among all the ferrites,barium ferrites (denoted as BaM), with hexagonal magnetoplum-bite structure, have been considerably studied as one of themost important microwave absorbing materials because of itshigh coercive force, large magneto crystalline anisotropy, highsaturation magnetization, as well as excellent chemical stabilityand corrosion resistivity [1,2]. Moreover, due to their largerintrinsic magnetocrystalline anisotropy field, BaM can be usedat much higher frequency than the ferrites with spinel and garnetstructure [3].

ll rights reserved.

In general, the resistivity of ferrite is very high, so the mainabsorbing mechanism is magnetic loss. Furthermore, the mag-netic loss of BaM derives from their intrinsic magnetic properties,the resonance absorption of moving magnetic domains, and spinrelaxation at the high-frequency alternating electromagneticfields [4,5]. It has been predicted that an improvement in theintrinsic magnetic properties of BaM can enhance their micro-wave absorbing ability. With a view to get high-power bariumferrite, there are several processes to synthesize single-domainbarium ferrites, such as sol–gel method [6], citrate precursor [7],chemical co-precipitation [8], glass crystallization [9], hydrother-mal methods [10], and microemulsion [11]. But, as far as weknow, most works are focused on the powders. However, thesehexagonal ferrites are quite heavy as in granular materials, whichlimit their practical applications in microwave absorbing andshielding materials. One of the ways to solve this problem is toproduce these magnetic absorbing materials with low-dimen-sional and large specific surface area, which can enhance theabsorbing ability per unit mass. The previously investigationshave proved that fibers can bring a much higher magneticpermeability than the same volume of materials in a nonfibrousform [12,13]. If the M-type barium ferrites are made intonanofibers, it will have a lower specific gravity, owing to a largerspecific surface area. Simultaneously, the large specific surfacearea is also beneficial to absorb electromagnetic wave. As is well

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–13111306

known, electrospinning technique is an advanced and highlyeffective method to fabricate organic and inorganic nanofiberswith large specific surface area, porous structure and uniformdiameter [14].

Currently, in order to tailor the electromagnetic parametersand ferromagnetic resonant frequency of the pure BaM ferrite,various ions doping have been investigated based on the fact thatBaM ferrite’s structure and ferromagnetic resonant frequency areclosely related to the chemical composition and the arrangementof ions in the crystal unit [4]. In early research, the magneticproperties of BaM can be changed by substituting Ba2þ with rareearth ions (such as La3þ , Pr3þ , Ce3þ , Sm3þ , and Gd3þ) due totheir good electric and magnetic properties [15–20]. In all theserare earth ions, La3þ has attracted much attention because of itscomparable radius with Ba2þ . The improvement of both magne-tocrystalline anisotropy or coercive force and magnetization offerrites can be achieved by substituting Ba2þ with La3þ , whichalters the Fe3þ–O–Fe3þ superexchange interaction and gives riseto spin canting, along with changing Fe3þ to Fe2þ on 2a site,in which the strong anisotropy of Fe2þ ions attribute to theincrease in magnetocrystalline anisotropy or coercive force (Hc).Nevertheless, up to now, there is no report on La-substitutedM-type BaFe12O19 nanofibers.

On the above basis, the present study proposes a method forsynthesizing Ba(1�x)LaxFe12O19 nanofibers with tailored magneticproperties and electromagnetic parameters. The special morphol-ogy and enhanced magnetic properties will increase the absorb-ing ability of GHz electromagnetic wave and the fittingelectromagnetic parameters will broaden the absorption band,which is desirable in the application of microwave absorbingmaterials. This paper employed a facile method to prepareBa(1�x)LaxFe12O19 (0.00r xr0.10) nanofibers followed by heattreatment at 1100 1C. Furthermore, this method can also enlargethe sort of electrospun nanofiber to other magnetic materials. Theresulted nanofibers were characterized by SEM, XRD, VSM, andmicrowave vector network analyzer.

Fig. 1. XRD patterns of samples: (a) the ferrite precursor calcined at 450 1C for 2 h,

(b)–(d) BaFe12O19/PVP composite nanofibers presintered at 450 1C for 2 h and then

calcined at 900–1100 1C for 2 h.

2. Experimental details

Ba(1�x)LaxFe12O19 nanofibers (x¼0.00, 0.02, 0.05, 0.08 and0.10) were prepared by the combination of citrate sol–gel methodand electrospinning technique followed by a thermal treatment.Appropriate amount (a molar ratio of Fe/(BaþLa) is 11) of bariumnitrate [Ba(NO3)2, AR], ferric nitrate [Fe(NO3)3 �9 H2O, AR] andlanthanum acetate [La(CH3COO)3, AR] were dissolved in 40.0 mldeionized water. And then citric acid was added to chelate Ba2þ

and Fe3þ in the solution under stirring (the molar ratio of citricacid to nitrate was 1:1). After dissolving the salts thoroughly,ammonium hydroxide was added to adjust the pH value to about7.0. Then the solution was heated at 60 1C on a hot plate for 4 h.As the water evaporated, the solution became viscous and finallyformed a rufous wet-gel of ferrite precursor.

The gel was added into a transparent polymer solution(22.5 wt%) which contains 3.0 g poly-(vinyl pyrrolidone) (PVP,Mr¼10,000, Tientsin Damao Chemical Reagents Co.) and 10.0 mlacetic acid for electrospinning. After being magnetically stirredfor 12 h, this mixture was loaded into a plastic capillary equippedwith a stainless steel spinneret. The spinneret used as the positiveelectrode was connected to a high-voltage supply (DW-D303-2AC, Tianjin Dongwen High Voltage Power Supply Plant). Duringthe electrospinning, the applied voltage was kept at þ19.0 kV,when the distance between spinneret and a plate collector wasoptimized and around 15.0 cm to collect the as-spun nanofibers.The as-spun Ba(1�x)LaxFe12O19/PVP composite fibers collectedwere dried first and then calcined at 450 1C and 900–1100 1C for

2 h, respectively, in ambient atmosphere. Finally, the mixtureswere naturally cooled to room temperature and the designedBa(1�x)LaxFe12O19 nanofibers were synthesized. As a complement,part of ferrite precursor was directly calcined at 450 1C for 2 h.

The crystalline structure was determined by X-ray diffrac-tion (XRD, D/MAX-IIIA, Rigaku, Japan) using Cu-Ka radiation(l¼0.15405 nm at 40 kV and 200 mA) in the range of 20–801for 2y, with a scanning speed of 101/min. The morphology of thenanofibers was characterized using a scanning electron micro-scopy (SEM, JSM-6360LV). The magnetic measurements werecarried out with a vibrating sample magnetometer (VSM, LakeShore 7410) at room temperature. The resulting nanofibers weremixed with paraffin wax at ratio of 1:1, and then pressed into acylinder with the outer diameter of 7.0 mm, the inner diameter of3.04 mm and the thickness of about 2.0 mm at 350 MPa. Thecomplex permittivity and permeability were measured directlywith the use of Agilent HP-8722 ES microwave vector networkanalyzer in the frequency range from 2 to 18 GHz.

3. Results and discussion

3.1. X-ray diffraction (XRD) analysis

Fig. 1 shows the XRD patterns of the nitrate-citrate gelcalcined at 450 1C and BaFe12O19 nanofibers calcined underdifferent calcination temperatures from 900 to 1100 1C. It canbe seen from Fig. 1(a), the peaks belong to the g-Fe2O3 (PDF no.4-755), which is of a cubic structure similar to the ‘‘S’’ block inBaM. This result indicates that the presintering process canobviously suppress the formation of a-Fe2O3 phase and is advan-tageous to obtain pure BaFe12O19 phase at a relatively lowertemperature [21]. When the calcination temperature is 900 1C(Fig. 1(b)), the diffraction peaks and relative intensities can be ingood accordance with the standard peaks of a-Fe2O3 (PDF no.5-637) and BaFe12O19 (PDF no.43-0002). This can be attributed tothe following two reactions. One is that g-Fe2O3 may react withBaCO3 to form BaFe12O19 phase and the other is that part ofg-Fe2O3 may directly transform to a-Fe2O3 phase at high tem-perature. With calcination temperature increasing, the amount ofBaFe12O19 increases monotonically and the diffraction peaksbecome narrower and higher. From Fig. 1(d), all the diffractionpeaks are in accordance with that of standard hexagonal

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–1311 1307

BaFe12O19 (PDF no.43-0002). When calcination temperaturereached 1000 1C (Fig. 1(c)), there are still a few impurities ofa-Fe2O3 in the sample, indicating that a-Fe2O3 is not fullytransformed into BaFe12O19. This is in accordance with the resultof other research, which reveals that completely transformationof a-Fe2O3 into BaFe12O19 needs a higher temperature [22].

Fig. 2 shows the XRD patterns of the Ba(1�x)LaxFe12O19 nano-fibers (x¼0.00, 0.02, 0.05, 0.08, and 0.10) obtained at 1100 1C for2 h. As can be seen from Fig. 2(a), all the XRD peaks belong to theBaFe12O19 phase, excluding any intermediate phase such asBaFe2O4, La2O3, and LaFeO3, which provided good evidence tothat La3þ ions are all incorporate into the lattice of BaFe12O19.Furthermore, the peaks do not change obviously with theincrease of La content, while alter the relative intensity. As Ba2þ

ions were substituted by La3þ ions, the XRD diffraction peaksbecame broader and weaker along with the shifts in the peakpositions (Fig. 2(b)), indicating that the crystallinity decreasedand the grains became smaller. The average grain size (Dhkl) ofvarious Ba(1�x)LaxFe12O19 nanofibers calcined at 1100 1C for 2 hare calculated from the full-width at half-maximum (FWHM)of the XRD reflection (1 0 7) and (1 1 4) planes as shown inFig. 2(a) using Scherrer’s equation (i.e. Dhkl¼0.89l/bcosy, where

Fig. 2. (a) XRD patterns of Ba(1�x)LaxFe12O19 (0.00rxr0.10) nanofibers calcined

at 1100 1C for 2 h; (b) partially enlarged XRD patterns of (a).

l is the incident wavelength of the X-ray radiation, b is the FWHMof the relevant diffraction peak and y is the diffraction angle).Simultaneously, the lattice parameters of these samples arecalculated from the values of d(h k l) corresponding to (1 0 7) and(1 1 4) peaks according to the following equation (i.e. 1/d2

(hkl)¼

4(h2þk2þ l2)/3a2

þ l2/c2). The results are summarized in Table 1. Itis clear that the lattice parameter ‘‘a’’ slightly increases and ‘‘c’’decreases with the substitution amount x increasing. This slightchange of the lattice parameter is largely associated with the La3þ

(0.106 nm) substitution for Ba2þ (0.135 nm), and on 2a site,some Fe3þ (0.067 nm) converted into Fe2þ (0.076 nm) in orderto compensate the excess positive charges due to the replacementof Ba2þ by La3þ . This result shows that the crystal structures ofsamples were contracted after being doped by La ions. Ascompared to undoped pure ferrite, with La3þ substitution contentincreasing, there is a relatively large decrease in the grains sizefrom 46.677 nm (x¼0.00) to 38.778 nm (x¼0.10) for Ba(1�x)

LaxFe12O19. It can be ascribed to that the radii of La3þ is onlyslightly different from that of Ba2þ , so a small amount doping willnot affect the crystal structure but the lattice will be distorted andthe internal stress will be induced by lattice distortion hinders thegrowth of grains [23].

3.2. Morphology analysis

Fig. 3(a) shows a scanning electron microscope (SEM) image ofthe as-spun undoped BaFe12O19/PVP nanofibers. Each individualnanofiber was smooth and uniform in cross-section. The contin-uous fibers were collected as randomly oriented with the averagediameter of 200720 nm. As shown in Fig. 3(b), some fibersbecome discontinuous and the average diameter decrease to110720 nm due to the complete decomposition of PVP fromthe nanofibers and the crystallization of BaFe12O19. Additionally,comparing with the nanoparticles, the specific surface of theprepared nanofibers with porous structure is larger, which isbenefit to microwave absorbing.

3.3. Magnetic measurements

Small amount of La3þ doping does not significantly affectnanofiber morphology but have a great influence on magneticproperties. The hysteresis loops of La-substituted hexaferrite,calcined at 1100 1C, show a behavior of the hard magneticmaterials with high coercive force, as shown in Fig. 4(a). Thevariations of Ms and Hc values of Ba(1�x)LaxFe12O19 are also shownin Fig. 4(b). As x increases, the value of Ms first increases reachinga maximum of 77.188 A m2/kg at x¼0.05, and then decreases,which coincides with some reports in the literature [18–20,24].Firstly, the increase in Ms could be contributed by the enhance-ment of hyperfine fields at 12k and 2b sites as strengtheningin the Fe3þ–O–Fe3þ superexchange interaction giving highernet magnetization [18]. As x increases, the effect of magneticdilution is occurred with the changing of the Fe3þ (high spin)valence state to Fe2þ (low spin) state on 2a site by substitutionof the Ba2þ with La3þ ions. Furthermore, the Fe3þ–O–Fe3þ

Table 1Effects of La substitution on the lattice parameters and the average crystallite size

for Ba(1�x)LaxFe12O19.

Sample a (A) c (A) c/a D (nm)

x¼0.00 5.8956 23.2316 3.9405 46.677

x¼0.02 5.8999 23.2270 3.9368 44.449

x¼0.05 5.8968 23.2117 3.9363 43.930

x¼0.08 5.8969 23.2103 3.9360 40.898

x¼0.10 5.8968 23.2117 3.9363 38.778

Fig. 3. (a) SEM image of BaFe12O19/PVP composite nanofibers; SEM images of Ba(1�x)LaxFe12O19 nanofibers with different contents (x): (b) x¼0.00; (c) x¼0.02; (d) x¼0.05;

(e) x¼0.08; (f) x¼0.10.

Fig. 4. (a) Hysteresis loops for the Ba(1�x)LaxFe12O19 nanofibers calcined at

1100 1C for 2 h; (b) effect of x on values of Ms and Hc of Ba(1�x)LaxFe12O19

nanofibers calcined at 1100 1C for 2 h.

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–13111308

superexchange interaction is disrupted and weakened by Fe2þ

ions and spin canting[20]. Therefore, the magnetic dilutioneffect and spin canting are main two reasons for lower Ms valuesin doped samples (when x40.05). Simultaneously, Fig. 4(b)illustrates that the Hc values of Ba(1�x)LaxFe12O19 nanofibers firstdecrease reaching a minimum of 283.81 kA/m at x¼0.05, and

increase remarkably. The Hc value of pure BaFe12O19 nanofibers(Hc¼344.12 kA/m) in this work is much larger than that ofthe bulk samples obtained by classical ceramic method(Hc¼175 kA/m), but still lower than the theoretically estimatedvalue (Hc¼533.32 kA/m) [25]. When the value of x exceeds 0.05,with Fe3þ ions change to Fe2þ ions on 2a site as usually found inrare earth substitutions, the magnetocrystalline anisotropy ishigher. It is inferred that the enhancement of Hc is due to theeffect of Fe2þ anisotropy on the 2a site. So as x increases, moreFe3þ ions on 2a site are changed to Fe2þ , resulted in the largermagnetocrystalline anisotropy and coercive force.

3.4. Complex permittivity and permeability of nanofibers

In order to investigate the intrinsic reasons for microwaveabsorption properties, we studied complex permittivity andpermeability of Ba(1�x)LaxFe12O19 nanofibers at different x values(x¼0.00, 0.05, and 0.10). Complex permittivity (er¼e0-je00) andcomplex permeability (mr¼m0-jm00) represent the dielectric anddynamic magnetic properties of magnetic materials. The realparts (e0 and m0) of the complex permittivity and permeabilitysymbolized the storage capability of electric and magnetic energy.The imaginary parts (e00 and m00) represent the loss of electric andmagnetic energy. The mechanisms of energy loss in magneticmaterials are due to dielectric and magnetic properties, whichdepend on the imaginary part of the complex permittivity andcomplex permeability, especially the imaginary part of complexpermeability.

Fig. 5(a) and (b) exhibits the frequency dependence of the real(e0) and imaginary (e00) components of the relative complexpermittivity for Ba(1�x)LaxFe12O19 nanofiber (x¼0.00, 0.05, and0.10) composites over 2–18 GHz. For the undoped sample, weobserve that the e0 of complex permittivity is nearly constant overthe whole frequency range, but the e00 increases and shows fourpeaks at about 6.0, 9.5, 15.5 and 18.0 GHz respectively. It is clearthat values of real part (e0) and imaginary part (e00) are larger forthe sample with La ions doped than those of undoped sample.In the plot of x¼0.05, the real part (e0) value declines from 4.19 to3.82 while the frequency increases from 2 to 18 GHz and theimaginary value (e00) keep stable at 0.29 within the wholefrequency range. In the plot of x¼0.10, compared to undopedsample, the values of e0 is slightly larger while of e00 is similar. Inthe La doped samples, the increase in the values of e0 and e00 maybe attributed to the substitution of La3þ for Ba2þ which isexpected to convert some of the Fe3þ at the octahedron sites toFe2þ at the tetrahedron sites in the ferrite structure to maintain

Fig. 5. Frequency dependence of the real (a) and imaginary part (b) of the complex

permittivity for different samples (x¼0.00, 0.05 and 0.10) at a layer thickness of

2.0 mm.

Fig. 6. Frequency dependence of the real (a) and imaginary part (b) of the complex

permeability for different samples (x¼0.00, 0.05 and 0.10) at a layer thickness of

2.0 mm.

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–1311 1309

charge neutrality. The electron hopping between Fe3þ and Fe2þ

ions resulted in the enhancement of dielectric loss, and the excessFe2þ strengthened the interfacial polarization for the La dopedsamples. Furthermore, radius of La3þ is different from that ofother cations, which will form extra intrinsic electric moment.Thus, values of e0 and e00 get larger with La3þ substitution for Ba2þ

in the M-type hexaferrite.Fig. 6(a) and (b) show the real (m0) and imaginary (m00) parts of

the relative complex permeability for Ba(1�x)LaxFe12O19 nanofiber(x¼0.00, 0.05, and 0.10) composites over 2–18 GHz respectively.The m0 and m00 are unity and nearly constant in the wholefrequency range for the pure BaFe12O19 and Ba0.9La0.1Fe12O19

nanofiber composites. The reasons for this phenomenon is theirnearly the same coercive force and magnetic saturation (Fig. 4(b)).Compared to the undoped sample, the values of m0 and m00 arelarger for the Ba0.95La0.05Fe12O19 composite and the values of m0decreases in the whole frequency range while m00 spectra haveshown two slightly peaks at around 4.74 and 11.71 GHz. Themaximum value of m00 as obtained at resonance frequency is 0.42at 5.08 GHz for the La3þ doped sample and 0.05 at 3.72 GHz forthe undoped sample, which indicates that La3þ doping canimprove the magnetic loss.

3.5. Microwave absorption properties of nanofibers

According to transmission line theory, the reflection loss ofelectromagnetic radiation, RL (dB), under normal wave incidenceat the surface of a single-layer material backed by a perfectconductor was calculated by the following equations (i.e.

RLðdBÞ ¼ 20log10Zin�1Zinþ1

������ and Zin ¼

mr

er

� �1=2tan h j 2pf d

c

� �ðmrerÞ

1=2h i

,

where Zin is the input impedance of absorber, d is the thicknessof the absorber, c is the velocity of electromagnetic wave (EW) infree space and f is the frequency of the incidence EW wave).According to equations, when the RL is �10, �20 dB and theattenuation of microwave absorption materials achieves 90%, and99%, respectively. The variation of the reflection loss versusfrequency for Ba(1�x)LaxFe12O19 nanofiber (x¼0.00, 0.05, and0.10) composites at the thickness of 2.0 mm are shown in Fig. 7.Here, the bandwidth is defined as the frequency width, in whichthe reflection loss is less than �10 dB. It can be seen that theabsorption performance get better with the substitute amount xincreasing from 0 to 0.05; but with the further increase of thesubstitution amount to 0.10, the absorption performance will beworse due to the decline of the magnetic properties. Compared to

Fig. 7. Reflection loss as a function of frequency for different samples (x¼0.00,

0.05 and 0.10) at a layer thickness of 2.0 mm.

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–13111310

the undoped sample, it can be seen that the minimum RL

decreases largely and the bandwidth is broadened significantlyfor the sample with x¼0.05. The minimum reflection lossdecreases from �9.65 dB of pure sample to �23.02 dB ofBa0.95La0.05Fe12O19 nanofiber composite. This is because thevalues of e00 and m00 are both larger than that of undoped sample,which implies more incident electromagnetic wave will beattenuated by means of dielectric loss and magnetic loss. Butfor the sample with x¼0.10, the reflection loss is not improvedinstead declined, which indicates that rare earth ions doping isrequired for an appropriate value [23]. The bandwidth for puresample with a thickness of 2.0 mm is only 0 GHz, while that forBa0.95La0.05Fe12O19 nanofiber composite with the same thicknessexpands to 12.6 GHz. At the same time, for the Ba0.95La0.05Fe12O19

nanofiber composite, the microwave absorption become worsewith the frequency increasing, while the plot of the reflection lossfor pure BaFe12O19 nanofiber composite has a peak value of�9.65 dB at 4.57 GHz. These results imply that the doping ofLa3þ ions can shift the maximum attenuation to the lowerfrequency region, increase microwave absorption and also enlargethe absorbing bandwidth. The improvement of microwaveabsorption performance for La-doped ferrite sample can beexplained by the following reasons: firstly, the grain size shrink-age induced by the difference of radius between La3þ and Ba2þ

can make the surface state and grain surface energy level changeobviously. The enhanced interface polarization and repetitiousreflection will cause more energy absorption when the micro-wave spreads in the ferrites [23]. Secondly, when the La3þ

substituted Ba2þ , partially Fe3þ will convert to Fe2þ to compen-sate the valence imbalance in BaFe12O19. It is well known that theelectrons hopping between ions with different valence induce theelectric dipole polarization [26] and therefore the dielectric losscan be improved after La substitution. Thirdly, the enhancedsaturation magnetization (see the Fig. 4(b)) caused by substitu-tion of La3þ ions will give rise to larger magnetic loss (e.g. thesample with x¼0.05).

4. Conclusion

In summary, Ba(1�x)LaxFe12O19 nanofibers (0.00rxr0.10)have been successfully fabricated via the electrospinning techni-que from a sol–gel solution containing a molar ratio of Fe/(BaþLa)of 11 as the precursors, poly-(vinyl pyrrolidone) and acetic acid as

a spinning aid, followed by heat treatment at different tempera-tures for 2 h. The XRD patterns show that the main phase of BaMferrite forms without other intermediate phase. The resultingfibers, with average diameter ca. 110710 nm, were ferrimagneticwith high saturation magnetization and coercive force as com-pared to bulk. It has been observed that the magnetic loss of theresulted hexaferrite nanofibers can be enhanced by partial sub-stitution of La3þ at Ba2þ sites due to the superexchange interac-tion of Fe3þ–O–Fe3þ gets strengthened. The dielectric loss is alsosignificantly enhanced for the La3þ doped sample comparing tothat of without La3þ due to the electron hopping between Fe3þ

and Fe2þ as well as the interfacial polarization for the La3þ dopedsample was strengthened by the excess Fe2þ . The bandwidthbelow �10 dB is as wide as 12.6 GHz, and the peak value ofreflection loss is �23.02 dB of Ba0.95La0.05Fe12O19 nanofibercomposite when the absorber layer thickness is only 2.0 mm.Thus the microwave absorption performance of La3þ dopedsamples is enhanced and shows significant promise as buildingblocks for effective microwave absorbers.

Acknowledgment

This study was partly supported by the Natural ScienceFoundation of China (grant no. 51073005), the Beijing NaturalScience Foundation (grant nos. 2112013, KZ201010012012),PHR (IHLB), the 973 Project (grant no. 2010CB933501), BeijingMunicipal Science and Technology Development Program (grantno. Z111100066611004) and Textile Vision Science & EducationFund.

References

[1] A. Ghasemi, A. Hossienpour, A. Morisako, A. Saatchi, M. Salehi, Electromag-netic properties and microwave absorbing characteristics of doped bariumhexaferrite, Journal of Magnetism and Magnetic Materials 302 (2006)429–435.

[2] H. Cho, S. Kim, M-hexaferrites with planar magnetic anisotropy and theirapplication to high-frequency microwave absorbers, Magnetics, IEEE Trans-actions on 35 (2002) 3151–3153.

[3] R. Dosoudil, M. Usakova, J. Franek, A. Gruskova, J. Slama, Dispersion ofcomplex permeability and EM-wave absorbing characteristics of polymer-based composites with dual ferrite filler, Journal of Magnetism and MagneticMaterials 320 (2008) e849–e852.

[4] J. Qiu, M. Gu, H. Shen, Microwave absorption properties of Al- and Cr-substituted M-type barium hexaferrite, Journal of Magnetism and MagneticMaterials 295 (2005) 263–268.

[5] S. Sugimoto, K. Haga, T. Kagotani, K. Inomata, Microwave absorption proper-ties of Ba M-type ferrite prepared by a modified coprecipitation method,Journal of Magnetism and Magnetic Materials 290-291 (2005) 1188–1191.

[6] T.-H. Ting, K.-H. Wu, Synthesis, characterization of polyaniline/BaFe12O19

composites with microwave-absorbing properties, Journal of Magnetism andMagnetic Materials 322 (2010) 2160–2166.

[7] A. Mali, A. Ataie, Structural characterization of nano-crystalline BaFe12O19

powders synthesized by sol–gel combustion route, Scripta Materialia 53(2005) 1065–1070.

[8] P. Shepherd, K.K. Mallick, R.J. Green, Magnetic and structural properties ofM-type barium hexaferrite prepared by co-precipitation, Journal of Magnet-ism and Magnetic Materials 311 (2007) 683–692.

[9] L. Rezlescu, E. Rezlescu, P. Popa, N. Rezlescu, Fine barium hexaferrite powderprepared by the crystallisation of glass, Journal of Magnetism and MagneticMaterials 193 (1999) 288–290.

[10] X. Liu, J. Wang, L.-M. Gan, S.-C. Ng, Improving the magnetic properties ofhydrothermally synthesized barium ferrite, Journal of Magnetism and Mag-netic Materials 195 (1999) 452–459.

[11] X. Liu, J. Wang, L.-M. Gan, S.-C. Ng, J. Ding, An ultrafine barium ferrite powderof high coercivity from water-in-oil microemulsion, Journal of Magnetismand Magnetic Materials 184 (1998) 344–354.

[12] S.K. Nataraj, B.H. Kim, M. dela Cruz, J. Ferraris, T.M. Aminabhavi, K.S. Yang,Free standing thin webs of porous carbon nanofibers of polyacrylonitrilecontaining iron-oxide by electrospinning, Materials Letters 63 (2009)218–220.

[13] C. Shao, H. Guan, Y. Liu, X. Li, X. Yang, Preparation of Mn2O3 and Mn3O4

nanofibers via an electrospinning technique, Journal of Solid State Chemistry177 (2004) 2628–2631.

C.-J. Li et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1305–1311 1311

[14] C.-J. Li, J.-N. Wang, Electrospun SrRe0.6Fe11.4O19 magnetic nanofibers: Fabri-cation and characterization, Materials Letters 64 (2010) 586–588.

[15] C. Sun, K. Sun, Sol–gel synthesis of Ce-substituted BaFe12O19, Journal ofMaterials Science 42 (2007) 5676–5679.

[16] W. Lixi, H. Qiang, M. Lei, Z. Qitu, Influence of Sm3þ substitution onmicrowave magnetic performance of barium hexaferrites, Journal of RareEarths 25 (2007) 216–219.

[17] G. Litsardakis, I. Manolakis, K. Efthimiadis, Structural and magnetic proper-ties of barium hexaferrites with Gd-Co substitution, Journal of Alloys andCompounds 427 (2007) 194–198.

[18] S. Ounnunkad, Improving magnetic properties of barium hexaferrites by La orPr substitution, Solid State Communications 138 (2006) 472–475.

[19] Y. Liu, M.G.B. Drew, Y. Liu, J. Wang, M. Zhang, Preparation and magneticproperties of La-Mn and La-Co doped barium hexaferrites prepared via animproved co-precipitation/molten salt method, Journal of Magnetism andMagnetic Materials 322 (2010) 3342–3345.

[20] S. Ounnunkad, P. Winotai, S. Phanichphant, Effect of La doping on structural,magnetic and microstructural properties of Ba1�xLaxFe12O19 ceramics preparedby citrate combustion process, Journal of Electroceramics 16 (2006) 357–361.

[21] W. Zhong, W. Ding, N. Zhang, J. Hong, Q. Yan, Y. Du, Key step in synthesis ofultrafine BaFe12O19 by sol–gel technique, Journal of Magnetism and MagneticMaterials 168 (1997) 196–202.

[22] P. Ren, J. Guan, X. Cheng, Influence of heat treatment conditions on thestructure and magnetic properties of barium ferrite BaFe12O19 hollowmicrospheres of low density, Materials Chemistry and Physics 98 (2006)90–94.

[23] N. Chen, K. Yang, M. Gu, Microwave absorption properties of La-substitutedM-type strontium ferrites, Journal of Alloys and Compounds 490 (2010)609–612.

[24] X. Liu, W. Zhong, S. Yang, Z. Yu, B. Gu, Y. Du, Influences of La3þ substitutionon the structure and magnetic properties of M-type strontium ferrites,Journal of Magnetism and Magnetic Materials 238 (2002) 207–214.

[25] U. Topal, H. Ozkan, H. Sozeri, Synthesis and characterization of nanocrystal-line BaFe12O19 obtained at 850 1C by using ammonium nitrate melt, Journalof Magnetism and Magnetic Materials 284 (2004) 416–422.

[26] Y. Wu, Z.W. Li, L. Chen, S.J. Wang, C.K. Ong, Effect of doping SiO2 on high-frequency magnetic properties for W-type barium ferrite, Journal of AppliedPhysics 95 (2004) 4235–4239.