00Hexaferrite--FeCo nanocomposite

Hexaferrite–FeCo nanocomposite particles and their electrical and magnetic properties at high frequenciesC.Sudakar,G.N.Subbanna,a)and T.R.N.Kutty b)Materials Research Centre,Indian Institute of Science,Bangalore560012,India͑Received28April2003;accepted8August2003͒Nanocomposites are realized by chemical reduction whereby the conducting magnetic particles ofFe–Co alloy are generated within the insulating ferrimagnetic BaCo2Fe16O27or Ba2Co2Fe12O22hexaferrite matrix.Transmission electron microscopy revealed that metal nanoparticles precipitatecoherently as thinflakes along the a–b planes of the derivative magnetoplumbite lattice of thehexaferrites above the characteristic reduction temperature,T RϾ375°C in H2atmosphere.The coercivity increases with T R in the early stages of the solid-state precipitation and then decreaseswith the formation of larger fractions of Fe–Co alloy;a converse trend is noticed for magnetization.The complex permittivity increases with reduction toϳ50in the broad frequency range of4–18GHz.The complex permeability is also enhanced with the content of Fe–Co nanoparticles.It isproposed that the spin reorientation at the Fe–Co/hexaferrite interface gives rise to broadbandresponse,rendering these composite particles useful as electromagnetic microwave absorbers.©2003American Institute of Physics.͓DOI:10.1063/1.1614848͔I.INTRODUCTIONComposites,where the magnetic particles are usually embedded in either insulating or conducting matrix,are often employed in high speed electronic circuits to reduce the elec-tromagnetic interference͑EMI͒,decrease the noise level of signals and ensure the electromagnetic compatibility.1EM radiation shields,either reflection or absorption based,need be thin and lightweight,responsive to broader frequency range and nearly independent of the incident angle.As far as the thickness and working frequency bandwidth are con-cerned,the magnetic composites have definitive advantages over their dielectric analogues.The most commonly used fillers are spinel ferrites or hexaferrites.The latter with pla-nar magnetic anisotropy are of great interest for use as elec-tromagnetic energy dissipation materials in microwave fre-quency range.2BaCo2Fe16O27(WCo2)and Ba2Co2Fe12O22 (YCo2)hexaferrites are soft magnetic materials with planar anisotropy;they have relatively high resonance frequency and high permeability.However for broadband operation, magnetic nanocomposites are desirable.Nanosize iron par-ticles dispersed in an oxide matrix by reaction milling have been studied.3However composite materials with conducting magnetic particles generated in situ by chemical reactions within the ferrimagnetic hexaferrite insulating matrix are scarcely known.II.EXPERIMENTIn this article we report the structural,magnetic,and microwave absorption properties at very high frequencies ͑4–18GHz͒of the hexaferrite–FeCo alloy nanocomposites obtained by controlled chemical reduction of hexa-ferrite,WCo2,and YCo2.WCo2and YCo2hexaferrites with oriented grains were prepared by heat-treating atϳ1250°C the precursors obtained by the wet chemical gel-to-crystallite conversion.4Hexaferrite particles on controlled reduction in flowing hydrogen at350–450°C causes in situ precipitation of FeCo alloy͑iron rich͒nanoparticles within the hexaferrite matrix.Sample identifications used in the subsequent sec-tions have been denoted with the type of hexaferrite followed by the heat-treatment condition in H2.Thus,for example, WCo2–R400represents WCo2hexaferrite reduced at 400°C.Phase identification of the powders was investigated by x-ray diffraction͑XRD͒using a Philips PW1050diffrac-tometer equipped with Cu K␣radiation.Metal content in the composite is evaluated by wet chemical analyses5as well by comparing the relative intensity of x-ray reflections of hexa-ferrite and iron from standard mixtures.Further,the compo-sitional variations are evaluated by x-ray photoelectron spec-tra͑XPS͒of the samples with a VG ESCA3MK II instrument using Al K␣x rays.Transmission electron mi-croscopy observations were performed on the nanocompos-ites for morphological and lattice imaging studies,with a JEOL,JEM200CX microscope.Magnetization measure-ments were carried out betweenϪ9andϩ9kOe at room temperature,by means of a vibrating sample magnetometer ͑VSM͒͑Lakeshore7300͒.Complex permittivities(␧Ј–j␧Љ) and complex permeabilities(␮Ј–j␮Љ)were calculated from the measured reflection coefficients using the vector network analyzer͑Agilent8722ES͒in the frequency range of4–18 GHz,wherein the samples are loaded in coaxial cells.III.RESULTS AND DISCUSSIONFigure1shows the XRD patterns of WCo2and YCo2 hexaferrites heat treated at1250°C.The relative intensities of WCo2x-ray reflections show oriented grain growth along certain crystallographic planes,predominantly(00l)reflec-a͒Deceased on16th July2003.b͒Author to whom correspondence should be addressed;electronic mail:kutty@mrc.iisc.ernet.inJOURNAL OF APPLIED PHYSICS VOLUME94,NUMBER91NOVEMBER200360300021-8979/2003/94(9)/6030/4/$20.00©2003American Institute of Physicstions and (hkl )with larger l (Ͼh ϩk )as strong peaks,indi-cating that the particles are grown as platelets with the c axis nearly perpendicular to the plane of observation.The average grain size of the particles from scanning electron microscopy is ϳ20␮m.The XRD patterns of YCo 2also show narrow linewidths indicative of high crystallinity.On reducing these crystallites chemically at 350–450°C in hydrogen atmo-sphere,reflections of Fe–Co alloy ͑bcc ͒show up in the XRD patterns along with those of hexaferrite.However,reflections of the latter become line broadened and the intensities de-crease drastically with the extent of reduction.At T R у450°C,the intensities of Fe–Co reflections increase.The characteristic temperature at which discernible reduction takes place,as observed from XRD and chemical analyses,differs for WCo 2and YCo 2.WCo 2has ϳ5%FeCo forma-tion at T R Х400°C (WCo 2–R400)for 1h,whereas YCo 2has the same percent of reduction at T R ϳ425°C (YCo 2–R425).Above these temperatures,the percentage of Fe–Co increases exponentially.The estimated ͑Fe–Co ͒con-tent for samples heat treated at different temperatures in hy-drogen atmosphere is given in Table I.The atomic ratio of Fe and Co estimated from XPS analyses in the reduced samples of WCo 2is in the range of 6–8,whereas in YCo 2it is around 4–6.Figure 2͑a ͒shows the high resolution electron micro-scope image of the oriented hexaferrite particles (WCo 2)reduced at 425°C.Uniform lattice fringes are discernible with regular coherent growth of the basic structural blocks in WCo 2,which are disturbed with the extent of chemical re-duction.Just near the characteristic reaction temperature,T R ϳ400°C,the lattice images reveal embedded metal par-ticles coherently growing along the a –b planes.The metallic region expands with higher T R ͑у425°C ͒.The lattice gets distorted as can be deciphered from the wrinkling of the lattice fringes due to stacking faults.This leads to further disintegration of the monocrystalline particles at T R у450°C to composite multicrystallites of separate phases ͑Fe–Co alloy,BaO,and BaFe 2O 4).Interestingly,the reduced regions grow perpendicular to the c axis ͑along a –b plane ͒of hexagonal magnetoplumbite structure.The high resolution image of a particle taken with the beam along the c axis reveals the internal structure of hexaferrite platelet with the embedded metal particle as thin nanoflakes ͓Fig.2͑b ͔͒.The particles are well separated at T R р425°C.The selectedareaFIG.1.XRD patterns of hexaferrites (WCo 2and YCo 2)before and afterreduction.FIG.2.High resolution electron micrographs of the WCo 2–R425nanocom-posite with beam direction along ͑a ͓͒110͔and ͑b ͓͒001͔.Corresponding SAED patterns are shown in the insets.TABLE I.Estimated %of FeCo at different T R and magnetic properties of WCo 2and YCo 2hexaferrites.SampleIdentification T R ͑°C ͒%of FeCo ͑Fe/Co from XPS analyses ͒M S͑emu/g ͒H C ͑Oe ͒WCo 2–R375375-----69.7200WCo 2–R400400 4.962247WCo 2–R42542512.6͑7.5͒53496WCo 2–R45045017.8͑7.1͒50.5581WCo 2–R50050035͑6.5͒69645WCo 2–R70070075͑6.6͒169.752YCo 2–R400400-----35205YCo 2–R425425 5.232.4275YCo 2–R45045014͑5.2͒30.8437YCo 2–R50050025͑4.6͒38.6694YCo 2–R70070054͑4.6͒122.672electron diffraction ͑SAED ͒of such composites show very feeble reflections from the embedded metal nanoparticles.Similar observations were made with Y–Co 2hexaferrite as well.However,the metal particles grow and coalesce,fol-lowed by the disintegration of the matrix at higher T R .The particles with composite characteristics,namely the hexaferrite matrix still remaining intact with minimum de-fects and the coherent flaky metallic phase ͑FeCo alloy ͒pre-cipitated in situ ,give rise to modified magnetic properties as well as microwave absorption in contrast to the correspond-ing bulk phases.The variation of magnetization as a function of applied magnetic field for differently reduced samples is shown in Fig.3.For W–Co 2hexaferrite,the as-prepared particles have specific magnetization,M S ϭ69.7emu/g and coercivity,H C ϭ190Oe,whereas samples reduced at 400°C in H 2have M S Ϸ62emu/g and H C Ϸ250Oe.For WCo 2–R425M S decreases further to ϳ53emu/g,whereas H C increases to ϳ500Oe.The decrease in M S can be un-derstood from the formation of defective hexaferrite matrix with nanocrystalline Fe–Co particles.On the other hand,the coercivity of the particles increased due to the pinning of the domains by the presence of defects.Thus,the composites exhibit combination dependent magnetic property rather than contributions from the individual components.For samples reduced at 500°C,M S increases to 69emu/g with H C further increasing to 645Oe.The increase in M S results from the larger amounts of coarser Fe–Co alloy formation.The in-creasing trend of coercivity indicates that the defects still play a role in pinning down the domain reversals in the fer-rimagnetic components.The trend of changes in M S and H C with the T R is shown clearly in the inset of Fig.3.For deeply reduced samples ͑at ϳ700°C ͒the magnetization increasesenormously ͑ϳ169emu/g ͒,indicating ϳ75%͑by weight ͒constitute the metallic FeCo particle.Y–Co 2hexaferrites re-duced in H 2display nearly similar changes ͑Fig.3͒.The magnetic properties of reduced samples of YCo 2hexaferrites are listed in Table I.The as-prepared samples of Y–Co 2have M S ϭ35emu/g and H C ϭ205Oe.The coercivity initially in-creases with the T R ͑Ͻ500°C ͒and then decreases with the precipitation of more Fe–Co in the composite.Magnetiza-tion decreases when T R р450°C,with a reversing trend when the samples are reduced at higher temperatures.The complex dielectric as well as magnetic properties of the ferrite powders are measured in the region of 4–18GHz.The dielectric properties of the samples do not show signifi-cant changes in comparison to the bulk at these frequency ranges ͓Figs.4͑a ͒and 4͑b ͔͒.However,the properties differ for the hexaferrites after reduction.The as-prepared powders have real part of complex permittivity (␧Ј)around 7–8and the imaginary parts (␧Љ)of 0.1–0.3for WCo 2and YCo 2.The ␧Јincreases with the extent of reduction.For WCo 2–R400,␧Јincreases to ϳ12and further to ϳ50for WCo 2–R425.␧Јdecreases with increasing frequency for WCo 2–R425and YCo 2–R425.The imaginary part of per-mittivity (␧Љ)also shows an increase to ϳ30for these samples.For the as-prepared samples,the dielectric loss tan-gent (tan ␦ϭ␧Љ/␧Ј)is very small with near zero values,whereas,tan ␦is high for reduced samples.The real (␮Ј)and imaginary (␮Љ)parts of relative complex permeability are shown in Figs.4͑c ͒and 4͑d ͒,wherein ␮Јdecreases with frequency in all samples owing to the broadening of ferri-magnetic resonance.The magnetic resonance behavior is bet-ter discernible in oriented WCo 2and YCo 2grains.The natu-ral resonance frequency due to spin rotation is around 12–13GHz for WCo 2and ϳ18GHz for YCo 2,corresponding to the maximum observed in ␮Љfrequency curves.These samples have larger values of magnetic loss tangent (tan ␦ϭ␮Љ/␮Ј)in the vicinity of the resonance frequency,whereas for unoriented crystals,␮Љis low ͑ϳ0.3͒with shallow mag-netic loss and the resonance is discernible around 8–10GHz.The magnetic loss spectra have similar changes with ␮Љand have the highest absorption loss just above the resonance frequency,where the loss tangent is maximum.Themaxi-FIG.3.M S –H curves of WCo 2and YCo 2samples reduced at different T R .Inset shows the variation of M S and H C with T R for reduced WCo 2hexa-ferrites.FIG.4.Frequency dependence of ͑a ͒␧Јand ␧Љ,͑b ͒tan ␦(␧Љ/␧Ј),͑c ͒␮Јand ␮Љ,and ͑d ͒tan ␦(␮Љ/␮Ј)of WCo 2and YCo 2before and after reduction.mum magnetic loss is observed for oriented W–Co2hexafer-rite atϳ15GHz.For reduced samples,permeability in-creases slightly with the reduction temperature.The largerpermeability in the high frequency region for the compositeparticles in which theflaky thin metal nanoparticles are ori-ented in the a–b planes of the hexagonal platelets,resultsfrom the anisotropic morphology of the constituent particles.Thus,the hexaferrite metal nanoparticle composites showconsiderably smaller reluctance in the oriented directionsthan the unoriented particle system.The high permeability ofthe magnetic composite over a broad range of frequency ͑4–18GHz͒makes it possible to produce thinner electro-magnetic wave absorbers.The magnetic structure at themetal/hexaferrite interface in the composite,with magnetiza-tion direction dictated by the combined anisotropyfields ofthe ferrimagnetic components,can lead to its own character-istic relaxation processes due to complex spin reorientationat high frequencies.Thus the magnetic loss spectra showbroad absorption from4to18GHz.IV.CONCLUSIONIn conclusion,composites of hexaferrites containing Fe–Co alloy͑iron rich͒nanoparticles precipitated in situ are good candidates for wide bandwidth electromagnetic micro-wave absorption in broad frequency ranges.Such nanocom-posites are easier to prepare when compared to iron particle dispersed in oxide matrix by mechanochemical milling. ACKNOWLEDGMENTThe Board for Smart Materials Research and Technology of NPSM,Government of India,is thanked for the research funding.1S.-H.Yu and M.Yoshimura,Adv.Funct.Mater.12,9͑2002͒.2T.Inui,K.Konishi,and K.Oda,IEEE Trans.Magn.35,3148͑1999͒.3M.Pardavi-Horvath,J.Magn.Magn.Mater.215–216,171͑2000͒.4C.Sudakar,G.N.Subbanna,and T.R.N.Kutty,J.Magn.Magn.Mater. 263,253͑2003͒.5A.I.V ogel,Textbook of Quantitative Chemical Analysis,5th ed.͑Long-man,Singapore,1991͒,p.376.。