Magnetic and Dielectric Property Studiesin Fe- and NiFe-Based Polymer Nanocomposites

HIMANI SHARMA,1 SHUBHAM JAIN,1,2

PULUGURTHA MARKONDEYA RAJ,1,4 K.P. MURALI,1,3

and RAO TUMMALA1

1.—Packaging Research Center, Georgia Institute of Technology, 813 Ferst Dr NW, Atlanta,GA 30332, USA. 2.—Department of Ceramic Engineering, IIT (BHU), Varanasi, India. 3.—Centerfor Materials for Electronics Technology (C-MET), Thrissur, Kerala, India. 4.—e-mail: raj@ece.gatech.edu

Metal–polymer composites were investigated for their microwave propertiesin the frequency range of 30–1000 MHz to assess their application as inductorcores and electromagnetic isolation shield structures. NiFe and Fe nanopar-ticles were dispersed in epoxy as nanocomposites, in different volume frac-tions. The permittivity, permeability, and loss tangents of the composites weremeasured with an impedance analyzer and correlated with the magneticproperties of the particle such as saturation magnetization and fieldanisotropy. Fe–epoxy showed lower magnetic permeability but improved fre-quency stability, compared to the NiFe–epoxy composites of the same volumeloading. This is attributed to the differences in nanoparticle’s structure suchas effective metal core size and particle-porosity distribution in the polymermatrix. The dielectric properties of the nanocomposites were also character-ized from 30 MHz to 1000 MHz. The instabilities in the dielectric constant andloss tangent were related to the interfacial polarization relaxation of theparticles and the dielectric relaxation of the surface oxides.

Key words: Magnetic nanocomposites, microwave properties, frequencystability, inductors, magnetic permeability, dielectricpermittivity, magnetic loss

INTRODUCTION

Metal–organic hybrids or metal–polymer nanocom-posites have gained popularity in diverse potentialapplications such as microwave dielectrics, inductorcores, inductive links, energy storage devices, mag-netic sensors, microwave absorbers, etc.,1,2 owing totheir unique combination of dielectric and magneto-dielectric properties, and their frequency depen-dence. The polymers have been serving as an at-tractive host due to their excellent properties such ashigh resistivity, flexibility, easy and low-temperatureprocessability, light weight, and ability to engineerthe dielectric behavior.3 Embedded magnetic metal

nanoparticles in polymer matrices with tailoredmagneto-dielectric properties are especially of highinterest in microwave communications and radarsystems applications.4 The demands for the reducedsize but increase in the operating frequency of theconsumable electronics such as smart phones orportable computers are driving the need for develop-ing a new class of materials with both high permit-tivity and permeability.

Ferrites, due to their high resistivity, are gener-ally used for moderate-frequency application rangeof 0.1–1 MHz.5 However, their physical propertiessuch as permeability and permittivity show strongdependences on the frequency leading to ferromag-netic resonance (FMR) and higher losses at radiofrequencies. Moreover, ferrites also suffer fromlower saturation magnetization and high processing

(Received February 3, 2015; accepted April 15, 2015;published online June 10, 2015)

Journal of ELECTRONIC MATERIALS, Vol. 44, No. 10, 2015

DOI: 10.1007/s11664-015-3801-x� 2015 The Minerals, Metals & Materials Society

3819

temperatures that make them incompatible withsilicon and organic substrates.

Several studies on polymer composites containingferromagnetic materials such as iron, cobalt, nickel,and also ferrites have been reported to overcome thelimitations of the ferrites at higher frequencies.6,7

Numerous organic polymers such as acrylates,8

styrenes,9 polyvinyl alcohols,10 epoxies,11 and inor-ganic matrices such as silica (SiO2),12 alumina(Al2O3), and titania (TiO2)13 have been used to fab-ricate nanocomposites. However, most of the com-posite studies employ single metal componentswhile a few others also reported magnetic alloyssuch as Ni-Fe, Co-Ni, Fe-Si, Fe-Ni-Co with muchsofter properties.14 However, their low resistivitycharacteristics generate high eddy currents andthus limit their applications to higher frequencies.Metal nanoparticles in oxide matrices suppresseddy current losses and also infuse exchange cou-pling between the neighboring metal particles,leading to magnetic ordering and improved andstable properties. Currently, this is only achieved byco-sputtering to fabricate thin films of soft magneticmaterials. For most applications, where a thickmagnetic film with easy and cost-effective process-ing is required, metal–polymer composites are themost viable option. However, their properties areinferior to those of the sputtered thin films.

Polymer nanocomposites loaded with metalnanoparticles also show enhancement in permit-tivity with higher metal-filler content. The addi-tional polarization, which is strongly frequencydependent, occurs because of the charge redistri-bution at the metal–polymer interfaces, which cre-ates electric dipoles. Permittivities of �10 aretypically seen in metal–polymer nanocomposites inthe 1–5 GHz range with oxide-passivated nanoscaleparticles. Several models were proposed in the lit-erature to explain the dielectric properties of metal–polymer nanocomposites. The models suggest thatthe particle conductivity, size, and permittivity ofthe medium surrounding the nanoparticles influ-ence the dielectric relaxation frequency. Beyond therelaxation frequency, the permittivity of thenanocomposite approaches that of the matrix.

This paper investigates the effects of nanoparticlefiller chemistry, structure, and loading in the met-al–polymer nanocomposite on the magneto-dielec-tric properties at the microwave frequencies(1–1000 MHz). A comparative study of the Fe–epoxywith NiFe–epoxy nanocomposite system was carriedout to elucidate the frequency stability of thenanocomposites.

EXPERIMENTAL

Magnetic nanocomposites were fabricated bymechanically mixing the metal nanoparticles inpolymer matrices. Iron and NiFe nanoparticles werecommercially obtained from the US ResearchNanomaterials Inc., (Houston, Texas) as powders

with particle sizes of 25 nm for Fe and 40–100 nmfor NiFe. The as-received nanopowders consisted ofhard aggregates that required aggressivemechanical milling in solvent-dispersant-millingmedia suspensions to break the aggregates to theirprimary particle sizes. This was performed inethanol media with L-cysteine ethyl esterhydrochloride as the dispersant and fine zirconiaballs for milling. The as-received metal nanoparti-cles were partially passivated with a thin layer ofnatural oxide as a shell. A thermoset polymer withstrong mechanical properties and high temperatureand chemical resistance, Epoxy, was chosen as ahost matrix for microwave (MW) frequency appli-cation of the nanocomposites. The polymer resin,epon-828 and its hardner, epikure-3300 wereacquired from Momentive Inc.

Several nanocomposites with varying metal-to-polymer volume ratios were fabricated to study theeffects of metal content on the magnetic anddielectric properties. The metal–polymer suspen-sions after ball milling for about 12 h were dried ina nitrogen oven at 80�C for 30 min to form driedcomposite powder. The finely crushed powder waslater dry pressed to form toroids and pellets using ahydraulic press—hydraulic unit # 3925, Carver Inc.The pressed metal–polymer composites were ther-mally cured in two steps with soft curing at 80�C for90 min, followed by hard cure at 150�C for 90 minunder inert atmosphere to prevent any furtheroxidation of the metal nanoparticles. A pictorialdepiction of the cross sections of the finalnanocomposites is shown in Fig. 1a and b.

The structural and morphological characteriza-tions of the nanocomposite were carried out throughXRD (x-ray diffraction) (Philips 1813 diffractometer,Westborough, MA) and field emission SEM (LEO1530). The chemical composition was analyzedthrough EDS (Energy Dispersive Spectroscopy).B–H loops were obtained using a vibration samplemagnetometer (VSM, Lakeshore 736 series,Westerville, OH). Measurements up to 1 GHz wereperformed with an impedance analyzer (Agilent4291B impedance analyzer).

RESULTS AND DISCUSSION

Structural Characterization

The crystallinity in the metal–polymer nanocom-posite structure was determined using XRD, asshown in Fig. 2. The metal–polymer volume ratiosfor both the NiFe and Fe composites were 1:1. TheXRD pattern of iron-embedded epoxy nanocompos-ites (Fig. 2a) showed peaks corresponding to amixed iron-oxide, Fe3O4, indicating substantialoxidation of the nanoparticles, in addition to thepeaks corresponding to the metallic iron core. Thepeaks are indexed as (311), (511), and (440) for theoxides (Fe3O4- JCPDS-6-0424) and (110) and (211)for pure iron (Fe- JCPDS-3-7116). The XRD of NiFecorresponds to Ni0.5Fe0.5 metallic alloy (JCPDS #

Sharma, Jain, Raj, Murali, and Tummala3820

3-4102) as shown in Fig. 2b. In comparison with theiron–epoxy nanocomposites that showed significantoxide formation, no detectable peaks of oxide ofeither Fe or Ni metal were observed for the NiFealloy. It is important to note that the absence of anyoxide peak in XRD does not preclude the presence ofany amorphous oxides. The peaks of pure metals ofNi or Fe were also not detected, indicating no phaseseparation in the bimetallic alloy. The sharp intenseNiFe peaks show the polycrystallinity and thelarger particle size of the alloy, in contrast to thebroad and weak peaks of iron and its oxide in finersized iron–epoxy nanocomposite.

Representative SEM micrographs of Fe and NiFenanocomposites in epoxy matrix are shown inFig. 3a and b. The metal loading in both the com-pacts is 70 vol.%. Fe–epoxy nanocomposite (Fig. 3a)with narrower particle size distribution (averageparticle size, 25 nm) compared with NiFe(40–100 nm) showed higher pore fraction. This wasalso reflected in the compact densities, as shown inTable I.

The higher measured densities in the NiFenanocomposite can thus be attributed to theimproved particle packing density due to wider par-ticle size distribution than the Fe–epoxy nanocom-posites. The composite density increased significantlywith the increasing filler fraction at lower loadingwhile exhibiting only marginal improvement in den-sity at higher metal fraction.

Magnetic Characterization

The magnetic properties of the Fe and NiFe–epoxy nanocomposites were measured using VSMat room temperature to extract their saturationmagnetization, coercivity, and field anisotropy. Thehysteresis loops of 30 vol.%, 50 vol.%, and 70 vol.%Fe–epoxy and NiFe–epoxy nanocomposites areshown in Fig. 4. The saturation magnetization (Ms)of the oxide-passivated iron–epoxy nanocompositeswith 1:1 volume ratio was measured as 57.5 emu/g,while the same volume ratio of NiFe–epoxy

nanocomposite (NiFe wt.% of 60) showed higher Ms

of 63.8 emu/g. The bulk Ms for iron is 222 emu/g15

while that of NiFe is known to be �110 emu/g.Using Eq. 1, the effective metal fraction calculatedfor NiFe composite with composite density of2.95 g/cc (Table I) and Ms of 63.8 emu/g was foundto be 0.21, while that of Fe composite (Ms of57.5 emu/g and density of 2.63 g/cc) was calculatedto be only 0.075.

Fig. 1. Pictorial illustration of the nanocomposites’ cross sections in epoxy with (a) Iron (b) NiFe nanoparticles.

Fig. 2. XRD spectra of epoxy nanocomposites emdedded with (a)Fe; (b) NiFe nanoparticles.

Magnetic and Dielectric Property Studies in Fe- and NiFe-Based Polymer Nanocomposites 3821

Effective metal fraction

¼MsðcompositeÞ � DensityðqÞðcompositeÞ

MsðbulkmetalÞ � Density qð ÞðbulkmetalÞ

(1)

This dramatic difference in the effective metalfractions of NiFe–epoxy and Fe–epoxy, when thesame amount of metal was added during the com-posite synthesis, can be attributed to the thickeroxide shell on iron nanoparticles compared withthinner shells on NiFe particles The smaller size ofFe particles (25 nm) makes them more susceptiblefor oxidation due to higher surface energy leading tothicker nonmagnetic oxide shell compared with40–100 NiFe nanoparticles. Increasing the metalloading in both Fe and NiFe composites, from30 vol.% to 70 vol.%, showed increased Ms in bothcomposites, as expected. However, the Ms for ironcomposite with 30 vol.% and 50 vol.% remainedunaltered (�56 emu/g) suggesting larger oxide con-tribution in 50 vol.% composite with effective metalweight fraction of 0.075.

The lower coercivity (Hc) value of the NiFe (120Oe) indicates softer magnetic properties comparedwith iron–epoxy composite with coercivity of 400 Oe.This is primarily attributed to the coarser particlesof NiFe compared with iron. This ferromagneticresponse with higher value of coercivity (400 Oe) foriron–epoxy composite indicates the particle struc-ture approaching that of single domain with

geometric and size confinement effects, and thepresence of natural oxides on metal particle sur-faces.16 It is also conventionally known thatdecreasing the iron component in an iron-basedbinary or ternary alloy decreases the coercivitymaking it magnetically softer.17 However, thecoercivity of either nanocomposites did not varywith the filler loading.

Microwave Characterization of MagneticProperties

In order to evaluate the VHF–UHF (very highfrequency–ultra high frequency) magneto-dielectricproperties of the metal–polymer nanocomposites,complex permittivity (e) and permeability (l) weremeasured in the frequency range of 30–1000 MHzusing an impedance analyzer. Representative fre-quency-dependence curves of the real and imagin-ary parts of the relative effective permeabilities forFe and NiFe composites are shown in Fig. 5.Clearly, both types of composites showed improvedpermeabilities with the increasing metal content.The iron composite, presumably with a thick anti-ferromagnetic oxide, showed the highest perme-ability of 1.8 with effective metal loading of 0.099(Table I). The rapid increase in permeability for ironcomposites at high frequency (beyond 500 MHz), asshown in Fig. 5a, is indicative of the onset ofresonance.18 When the frequency of the appliedmagnetic field matches with the electron spins,resonance is observed, and energy is transferred

Fig. 3. SEM micrographs of nanocomposite with 70 vol.% metal loading; (a) Fe–epoxy; (b) NiFe–epoxy.

Table I. Measured densities and magneto-dielectric properties of NiFe–epoxy and Fe–epoxy nanocompositesat 100 MHZ

Filler

Vol.loading

(%)Density(g/cc)

Ms

(emu/g)

Effectivemetal

fractionl at

100 MHz

MagneticLoss(l¢¢/l¢)

e at100 MHz

NiFe 30 2.32 60.4 0.14 3.7 0.04 12.850 2.95 63.8 0.21 4.9 0.089 36.570 3.32 68.5 0.23 5.8 0.10 41.8

Fe 30 1.86 56.7 0.052 1.5 0.032 11.150 2.63 57.5 0.075 1.7 0.021 23.170 2.82 70.2 0.099 1.8 0.034 23.8

Sharma, Jain, Raj, Murali, and Tummala3822

from the applied field to the particles for orienting themagnetic dipoles.19 This resonance, often referred toas FMR for iron nanoparticles, is in higher GHz rangewhich is beyond the measuring capability of the im-pedance analyzer used in this study, and is thus notcompletely reproduced in Fig. 5a.

On the other hand, the permalloy compositesshowed the highest permeability of 5.75 at 100 MHzfor 70 volume % fraction filler loading, since theeffective metal (NiFe) fraction of 0.23 was muchhigher than that of its Fe composite (0.099) coun-terparts (Table I). The larger NiFe particles withmultiple magnetic domains exhibit softer proper-ties20 and thus enhance permeability in largerparticles. However, the permeability showed muchlower frequency stability (100 MHz, Fig. 5b) incomparison with iron–epoxy composites. This fre-quency dispersion of l for permalloy composite21 isattributed to magnetic resonances from spin rota-tions and domain wall motions that dominatebeyond 100 MHz. It is also interesting to note thatthe composites with higher metal loading starts toshow rapid decrease in permeability at lower fre-quency compared with composites with lower metalloading (like 30 vol.%).

The magnetic loss spectra for the metal–polymercomposites are shown in Fig. 5c and d for Fe–epoxyand NiFe–epoxy composites respectively. In bothcases of metal–polymer nanocomposites, the

imaginary part of permeability l¢¢ is seen to increasewith metal loading. In particular, for 70 vol.%Fe–epoxy composites, the high loading of metalshows extremely high losses dominated by eddycurrents beyond 250 MHz. These losses are relatedto electrical resistivity and increase with poor insu-lation between the filler particles, as seen in highlyloaded composites. On the other hand, l¢¢ curves ofNiFe–epoxy composites (Fig. 5d) showed a broadpeak at 200 MHz, indicating a magnetic resonancewhich is in accordance with the drop in permeabilityalso seen at same frequency in Fig. 5b. The furtheronset of a peak beyond 1 GHz in case of NiFenanocomposites may be attributed to the dielectricresonance which is dominant in the multi-GHz fre-quency range as discussed in the next section.

To further understand and quantify the magneticbehavior of these nanocomposites, the effectivecomposite permeability variation as a function ofmetal loading was fitted with the Bruggeman’s ef-fective medium theory model (EMT)22 and is shownin Fig. 6. The EMT equation is represented as

cala � leff

la þ 2leff

þ cblb � leff

lb þ 2leff

¼ 0 (2)

where la and lb refer to the permeabilities of thefiller and matrix, ca and cb refer to the volumefractions of the filler and matrix, and leff is theeffective nanocomposite permeability. A set ofnanocomposite permeability versus effective metalloading curves was generated assuming differentparticle permeabilities. The effective particle per-meability for each composite (metal core + metaloxide) was extracted by mapping the experimentalmeasurements with permeability plots. Best fit forFe–epoxy nanocomposite was obtained when theparticle (iron) permeability is five from the EMTmodel, as shown by the superimposed plot in Fig. 6.However, the NiFe’s experimental data do not quitematch with the calculated particle permeabilityplots for lower metal volume fraction but show aparticle permeability of �50 with metal loading of>20 vol.%. The effective metal volume fraction ineach composite case is estimated using density andthe Ms (estimated using VSM) of the compacts.

The FMR frequency and permeability are writtenin terms of field anisotropy (Heff), saturation mag-netization (Ms), and anisotropy energy density (K)and are represented as22,23:

FFMR ¼ c2p

HEff (3)

FFMR ffi 4K

3loMs(4)

lr ffiMs

Hkþ 1

Fig. 4. B–H cuves of 50 vol.% metal loading; (a) Fe–epoxy;(b) NiFe–epoxy.

Magnetic and Dielectric Property Studies in Fe- and NiFe-Based Polymer Nanocomposites 3823

where c is the gyromagnetic ratio, Heff is the effec-tive field anisotropy, and K is the effective aniso-tropy energy density. Based on the predictedparticle permeability of �5 in Fe–epoxy, the esti-mated field anisotropy is �5000 Oe using Eq. 3.

Therefore, the estimated FMR from iron nanopar-ticles is around �14 GHz, much beyond the mea-surement capability of the impedance analyzer.Based on the measurement for peak magnetic loss,FMR estimated for the NiFe system is �200 MHzfor the (Fig. 5d). From Eq. 4, this gives K a value of4.79 9 105 J/m3 corresponding to field anisotropy of71 Oe which is significantly higher than that of thebulk NiFe (10–20 Oe). The estimated NiFe particle’spermeability, based on this field anisotropy, is �126.The effective field anisotropy is enhanced innanoparticles because of surface anisotropy andferromagnetic/antiferromagnetic coupling at themetal/metal oxide (such as Ni/NiO) interface. Thefiner particle size also stabilizes single domainwithin the particle which contributes to high fieldanisotropy and suppressed permeability in mag-netic nanocomposites as seen in iron and nickel-ironcomposites.

Microwave Characterization of DielectricProperties

Variation of dielectric permittivity with frequencyfor Fe–epoxy and NiFe–epoxy composites are shown

Fig. 5. Frequency dependence of relative permeability variations of iron–epoxy and NiFe–epoxy nanocomposites for a given value of metalvolume fraction; (a, b) Real part l¢; (c, d) Imaginary part (l¢¢) of the permeability.

Fig. 6. Nanocomposite permeability vs. effective volume loading,assuming different particle permeabilities (in the brackets) usingeffective medium theory (EMT).

Sharma, Jain, Raj, Murali, and Tummala3824

in Fig. 7a and b, respectively. The dielectric per-mittivity decreases rapidly at low frequencies forFe–epoxy composites with higher loading and tendsto stabilize at higher frequencies (>600 MHz),while at lower loadings, the permittivity isrelatively independent of frequency till �900 MHz.This is because, in case of the Fe–nanocomposites athigher loading, the permittivity is dominated by theinterfacial oxide on iron particles as shown by thefollowing analysis.

In general, the dielectric properties of insulatingresin filled with conductive particles depend on thepermittivities and conductivities of the matrix andfillers, the volume fraction of the filler, the con-figuration and the internal structure of the com-posites, and the frequency of operation.24 Theconducting metal particles in an insulator matrixbecome polarized when an electric field is applied,enabling the interfacial-polarization process (alsoreferred to as Maxwell–Wagner–Sillars polariza-tion) at the metal–insulator interfaces. The fre-quency at which this polarization relaxes isdependent on the conductivity of the particle. Par-ticles with higher conductivity stabilize the polar-ization to higher frequencies. The relaxationfrequency (F) and permittivity are estimated as25:

F ¼ 1

3

ð1 � f Þrp

2pemeo(5)

ec ¼em

ð1 � f Þ3(6)

where rp is the conductivity of the particle; em andeo refer to the permittivity of matrix and vacuum,respectively; and f is the volume fraction of theconducting particles. Assuming a reasonable

particle conductivity of at least 100 S/m for ironand nickel-iron composites, the equation predictsthat the relaxation frequency is above 10 GHz.Similarly, the theoretical estimation of matrixpermittivity was made using Eq. 6 for bothFe–epoxy and NiFe–epoxy composites. Using com-posite permittivity, ec of 25, obtained from Fig. 7band volume fraction (f) of 0.21 for NiFe–epoxycomposite, the estimated matrix permittivity iscalculated as 11.83 (Eq. 6). Similarly, in case ofFe–epoxy, with the composite permittivity (ec) of 47at lower frequencies (from Fig. 7a) and volumefraction of 0.085, the matrix permittivity is esti-mated as 36. This additional permittivity contri-bution in Fe composites at lower frequencies isattributed to the contribution from iron oxideshells. The strong frequency dependence in Fe–epoxy composite is also attributed to the dominanceof thick iron oxide on the matrix permittivity.

The dielectric dispersion at lower frequency inFe–epoxy composites is attributed to the interfacialpolarization due to accumulated charges at theinterface of conducting (metallic iron core) andpoorly conducting interfaces, which are dominatedby iron oxide shells.23 The change in permittivity atfrequencies in the range of 100–500 MHz is, there-fore, attributed to the relaxation of polarization thatoccurs between the particle contacts. The presenceof Fe3+ and Fe2+ in the metal oxide shell (Fe3O4) ofFe nanoparticles and the electron hopping betweenthe two ionic states give rise to dielectric loss fromorientational polarization relaxation which alsoleads to frequency instability.

The dielectric characteristics of NiFe–epoxynanocomposites are, however, different compared tothose of Fe–epoxy nanocomposites because of theirrelatively larger particle size and a surface oxide thatis more insulating (nickel oxide>102 X cm) comparedwith iron oxide (10�2 X cm).23 The larger particle size

Fig. 7. Dielectric permittivity of (a) Fe–epoxy; (b) NiFe–epoxy nanocomposite as a function of frequency.

Magnetic and Dielectric Property Studies in Fe- and NiFe-Based Polymer Nanocomposites 3825

of NiFe also results in larger separation between theparticles and suppresses the carrier hopping betweenthe particles. The nanocomposites with higher loadingshowed relatively greater dominant behavior from thesurface multicomponent oxides from iron and nickel,respectively, and hence resulted in higher permit-tivity. However, the permittivity is more stable withfrequency in this case because of the relativelystronger influence of the polymer matrix andimproved insulating characteristics of these surfaceoxides compared to those of the iron oxides.

CONCLUSIONS

Epoxy resin nanocomposites with oxide-passivatedFe and NiFe nanoparticles, in various volume frac-tions, were prepared and investigated for theirmicrowave properties in the frequency range of100–1000 MHz. The nanocomposites were characterizedfor structural, magnetic, and electric properties overmicrowave the frequency range. The permittivity,permeability, and loss tangents for both thenanocomposites showed strong dependence on fre-quency and metal particle loading. NiFe nanocom-posites showed higher compact density, due to theirwider distribution of particle size compared withFe–epoxy nanocomposites. The increased effectivemetal loading in NiFe nanocomposites resulted inhigher permeability (l = 5.75 at 100 MHz) anddielectric permittivity in NiFe–epoxy nanocomposites.Analytic models for frequency stability and nanocom-posite’s permeability were correlated to the measure-ments in order to estimate the particle permeabilityand field anisotropy for NiFe and Fe–epoxy compos-ites. The higher instability in the dielectric permit-tivity and loss tangents in Fe–epoxy composites withhigher metal loading are related to the orientationaland interfacial polarization relaxation of the ironparticle’s core and ferrimagnetic iron oxide’s shell.

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