Microwave absorption properties of multiferroic BiFeO3 nanoparticles

Microwave absorption properties of multiferroic BiFeO 3nanoparticlesYu-Qing Kang a ,Mao-Sheng Cao a ,⁎,Jie Yuan b ,Xiao-Ling Shi aa School of Materials Science and Engineering,Beijing Institute of Technology,Beijing 100081,China bSchool of Information Engineering,Centre University for Nationality,Beijing 100081,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 19January 2009Accepted 6March 2009Available online 16March 2009Keywords:Multiferroic BiFeO 3Microwave absorption Sol –gelMultiferroic BiFeO 3(BFO)nanoparticles ranging from 60nm to 120nm were synthesized successfully by a sol –gel method,and the microwave absorption properties of BFO nanoparticles were investigated in the range of 12.4GHz to 18GHz.The re flection loss of BFO nanoparticles is more than 10dB (or more than 90%)in the 13.1GHz –18GHz range and reaches to 26dB at 16.3GHz,which indicated that the BFO is a good candidate for microwave absorption application.The excellent microwave absorption properties of BFO nanoparticles could be attributed to the good electromagnetic match as a consequence of the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles,which has been con firmed by electric and magnetic measurement.Moreover,the nanosize-con finement effect may also have contribution to the high re flection loss of BFO nanoparticles.©2009Elsevier B.V.All rights reserved.1.IntroductionMultiferroic BiFeO 3(BFO)has attracted much attention because it is one of the several compounds that exhibit coexistence of ferroelectricity and ferromagnetism at room temperature,due to its high ferroelectric Curie temperature (T C ~1103K)and G-type antiferromagnetic Néel temperature (T N ~647K)[1].So it is expected to form a new type of magnetoelectric devices by a combination of ferroelectric and ferro-magnetic properties [2–4].In addition to the potential applications as magnetoelectric devices,BFO might find applications as microwave absorption materials due to its magnetoelectric coupling.As the excellent microwave absorption generally results from the good electromagnetic match,i.e.,ef ficient complementarity between the relative permittivity and permeability [5,6].In recent years,extensive investigations have been carried out to fabricate microwave absorption materials with good electromagnetic match,such as CNTs/Fe,[7]CNTs/CoFe 2O 4,[8]and Ni(C)nanocapsules [9].These core-shell nanocompo-sites showed better microwave absorption than the pure core or shell materials.However,the complex fabrication process of these core-shell nanocomposites has been challenging for putting such materials into practical applications.Therefore,it is signi ficant to search other approaches to fabricate microwave absorption materials with good electromagnetic match.In this letter,multiferroic BFO nanoparticles were synthesize by a sol –gel method,and the microwave absorption properties were investigated in detail.2.Experimental detailsSol –gel method was employed to prepare BFO nanoparticles.Bismuth nitrate (Bi(NO 3)3·5H 2O)and iron nitrate (Fe(NO 3)3·9H 2O)in stoichio-metric proportions (1:1molar ratio)were dissolved in 2-methoxyetha-nol (C 3H 8O 2).The solution was adjusted to a pH value of about 4by adding 2-methoxyethanol and nitric acid.Then citric acid in 1:1molar ratio with respect to the metal nitrates was added to the solution,followed by polyethylene glycol as a dispersant.The mixture was stirred for 30min at 50°C to obtain the sol,which was then kept at 80°C for 48h to form the dried gel.The dried gel was then grinded into powders and calcined at 300°C.The calcined powders were sintered at 500°C for 2h,and then cooled rapidly to room temperature.The structure and morphology of the BFO nanoparticles were investigated by X-ray diffraction (XRD)(X'Pert PRO,Cu-Ka)and transmission electron microscope (TEM)(JEM-2010).Electric char-acterization was carried out on a ferroelectric tester (WS 2000).Materials Letters 63(2009)1344–1346⁎Corresponding author.Tel./fax:+861068914062.E-mail address:caomaosheng@ (M.-S.Cao).Fig.1.XRD pattern of as-prepared BiFeO 3nanoparticles.Inset shows the rhombohedral cell of BiFeO 3(O =red,Bi =green,and Fe =blue).(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)0167-577X/$–see front matter ©2009Elsevier B.V.All rights reserved.doi:10.1016/j.matlet.2009.03.010Contents lists available at ScienceDirectMaterials Lettersj o u r na l ho m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /m a t l e tMagnetic properties were measured using vibrating sample magnet-ometer (VSM)(LakeShore 7407).For the electromagnetic parameters measurements carried out using a vector network analyzer (ANRIGSU 37269D),the BFO nanoparticles were pressed into pellets with a dimension of 15.20mm×7.56mm×2.26mm and then annealed at 500°C for 30min.3.Results and discussionFig.1shows the XRD pattern of the as-prepared BFO nanoparticles.It is clear that the BFO nanoparticles are highly crystallized and all diffraction peaks of BFO nanoparticles can be perfectly indexed to BFO with JCPDS card No.86-1518.This result indicated that the as-prepared BFO is rhombohedrally distorted perovskite with space group R 3c (No.161),as shown in the inset.Fig.2represents the typical transmission electron microscope (TEM)image of the BFO nanopar-ticles.It can be seen that the BFO mainly consists of roughly spherical with an average size about 60–120nm.The corresponding energy dispersive spectroscopy (EDS)of a typical particle shows that the atomic ratio of Bi to Fe is approximately 1:1.The ferroelectric nature of BFO nanoparticles measured at room temperature is shown in Fig.3(a).The saturation polarization P s and remnant polarization P r are found to be 2.1μC/cm 2and 1.0μC/cm 2at a maximum applied electric field of 130kV/cm,respectively.Although these values are smaller than that reported for most of BFO thin film (it is comparable with that of BFO thin film with a P s value of 2.2μC/cm 2and a P r value of 0.83μC/cm 2[10]),they are higher than that of bulk materials [11].The magnetic hysteresis loops have been measured over ±1.5T at room temperature,as shown in Fig.3(b).It is clear that a saturated magnetic hysteresis loop is obtained with saturation magnetization M s of about 1.5emu/g.This result is consistent with a previous work [12],which reports that the M s decreases with increasing particle size of nanoscale BFO.Inset in Fig.3(b)gives the partially enlarged M –H curve,which reveals that the coercive field of the BFO nanoparticles is quite small,similar to BFO nanowires [13],nanotubes [14]and BFO thin film [15].In conclusion,the as-prepared BFO nanoparticles represent weak ferromagnetic order rather than superparamagnetism.Fig.4shows the complex permittivity and complex permeability of the BFO nanoparticles as a function of frequencies in the range of 12.4GHz to 18GHz.From Fig.4(a),the real permittivity (ɛ′)increases from ~13to ~19at 16.7GHz and then decreases slightlywithFig.2.TEM image of BiFeO 3nanoparticles.Fig.3.(a)Room temperature ferroelectric hysteresis loop of BiFeO 3nanoparticles.(b)M –H hysteresis loop measured at room temperature for BiFeO 3nanoparticles,and inset shows the partially enlargedcurve.Fig.4.(a)Complex permittivity and (b)complex permeability of BiFeO 3nanoparticles versus frequencies in the range of 12.4GHz to 18GHz.1345Y.-Q.Kang et al./Materials Letters 63(2009)1344–1346increasing frequency,while the imaginary permittivity (ɛ″)nearly increases with increasing frequency until a maximum value of ~7.2.From Fig.4(b),it can be seen that the real permeability (μ′)of BFO nanoparticles varies from ~1.3to ~0.8depending on the frequency,and the imaginary permeability (μ″)increases until attaining a maximum value and then decreases with an average value of about 0.1.The re flection loss,which directly determined the microwave absorption properties of materials,can be de fined as [7,8]:R dB ðÞ=20log j z in −1ðÞ=z in +1ðÞjWhere z in is the normalized input impedance could be obtained from the following expression:z in =ffiffiffiffiffiffiffiffiffiffiffiffiffiμr =e r p tanh j 2π=c ðÞffiffiffiffiffiffiffiffiffiffiffiffiffiμr =e r p fd ÂÃ,where c is the velocity of electromagnetic waves in free space,f is the frequency of microwaves,and d is the thickness of the absorber.Fig.5shows the re flection loss of BFO nanoparticles ranging from 12.4GHz to 18GHz (solid circles).It can be seen that the re flection loss of BFO is more than 10dB (or more than 90%)in the 13.1GHz –18GHz range with 3.5mm thickness layer.The maximum re flection loss reaches to 26dB at 16.3GHz,which is comparable with that of Fe/CNTs (more than 25dB)[7],and CNTs/CoFe 2O 4(18dB)[8].As for comparison,the re flection loss of bulk BFO was also investigated,as shown in Fig.5(hollow circles).The re flection loss of bulk BFO,with a maximum value of 9.8dB at 12.6GHz,is less than that of BFO nanoparticles.The high re flection loss of BFOnanoparticles could be attributed to the good electromagnetic match resulted from the coexistence of ferroelectric and weak ferromagnetic order.Moreover,the nanosize-con finement effect may also have contribution to the high re flection loss of BFO nanoparticles.4.ConclusionsMultiferroic BFO nanoparticles with particles size of 60–120nm have been successfully prepared by a sol –gel method.Electric and magnetic measurement con firmed the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles.The BFO nanoparti-cles showed high re flection loss,which could be attributed to the good electromagnetic match,as well as nanosize-con finement effect.In general,the high re flection loss of BFO nanoparticles is favorable to the application as microwave absorption materials.AcknowledgementThe authors thank the National Natural Science Foundation (Grant No.50872159),the National Defense Funds (Grant No.51318030302and A2220061080)and the Scienti fic Research Foundation of Graduate School of BIT (Grant No.AA200802)for providing the research grant.References[1]Fischer P,Polomska M,Sosnowska I,Szymanksi M.J Phys C 1980;13:1931.[2]Wang J,Neaton JB,Zheng H,Nagarajan V,Ogale SB,Liu B,et al.Science2003;299:1719–22.[3]Hur N,Park S,Sharma PA,Ahn JS,Guha S,Cheong SW.Nature 2004;429:392–5.[4]Sun T,Pan ZX,Dravid VP,Wang ZY,Yu MF,Wang J.Appl Phys Lett 2006;89:163117.[5]Wadhawan A,Garrett D,Perez JM.Appl Phys Lett 2003;83:2683–5.[6]Cao MS,Qin RR,Qiu CJ,Zhu J.Mater Design 2003;24:391–6.[7]Che RC,Peng LM,Duan XF,Chen Q,Liang XL.Adv Mater 2004;16:401–5.[8]Che RC,Zhi CY,Liang CY,Zhou XG.Appl Phys Lett 2006;88:033105.[9]Zhang XF,Dong XL,Huang H,Liu YY,Wang WN,Zhu XG,et al.Appl Phys Lett2006;89:053115.[10]Palkar VR,John J,Pinto R.Appl Phys Lett 2002;80:1628–30.[11]Kumar MM,Palkar VR,Srinivas K,Suryanarayana SV.Appl Phys Lett 2000;76:2764–6.[12]Mazumder R,Devi PS,Bhattacharya D,Choudhury P,Sen A,Raja M.Appl Phys 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