Prajna P. Mohapatra , Suresh Pittala , Pamu Dobbidi for microwave devices and electrical studies on Li Mg Fe O Temperature dependent broadband dielectric,magnetic Original Article

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w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original Article

Temperature dependent broadband dielectric,

magnetic and electrical studies on Li _1- _x Mg ₂ _x Fe _5- _x O ₈ for microwave devices

Prajna P. Mohapatra

^a

, Suresh Pittala

^b

, Pamu Dobbidi

^a^,∗

aDepartmentofPhysics,IndianInstituteofTechnologyGuwahati,Guwahati781039,India

bDepartmentofPhysics,IndianInstituteofScience,Bangalore,Karnataka560012,India

a r t i c l e i n f o

Articlehistory:

Received26August2019 Accepted13January2020 Availableonline27January2020

Keywords:

Ferrimagnetism Lithiumferrite Dielectricconstant Microwavedevices

a bs t r a c t

PolycrystallinesamplesofMgsubstitutedlithiumferrites(Li_1-xMg_2xFe_5-xO₈ (LMFO)(x=0, 0.003,0.005,0.007and0.01)havebeenpreparedbyconventionalsolid-statereaction.X- raydiffractionpatternsrevealedtheformationofasingle-phasecubicstructurewithP4132 spacegroup.ThedielectricconstantoflithiumferriteincreasedwithMgsubstitutionattain- inga maximumvalueforx=0.005(εr=3034,tanı=0.001at1MHz)atroomtemperature.

Temperature-dependentdielectricresponse inthe high frequencyrange (1MHz–1GHz) showedtherelaxorbehaviourofthedipoles,evidentfromtheshiftinthelosstangent with frequency. The activation energy of LMFO is foundto be in the range of 1.39- 0.35eV.Anenhancedpermeabilityisfoundtobe∼29forx=0.007at1MHz,RT.Thevari- ationinpermeabilityisattributedtothevariationinthicknessofthedomainwallsand magneto-crystallineanisotropy.Thetemperature-dependentmagnetizationcurvesreveal theferrimagnetictransitionofLFOat600^◦C,whichreducedwithMgsubstitution.Magnetic hysteresisloopsshowedthehighestsaturationmagnetization(Ms=54.6emu/g)forx=0.007 amongallthesamples.ThemagneticpropertiesareexplainedbyconsideringNeel’stwo sub-latticemodel.Thus,LMFOsamples,(especiallywithx=0.005)areverypromisingfor circulatorandphaseshifterapplications.

1. Introduction

Spinelferritesarematerialsthathaveattractedmuchatten- tionforalongtimebyvirtueoftheiruniqueoptical,electric, andmagneticproperties[1–3].Therefore,thesematerialsare verypromisingforvariousapplicationsinnoveltechnologi- caldevicessuchascirculator,gyrator,phaseshifter,etc.[4,5].

∗ Correspondingauthor.

E-mail:[email protected](P.Dobbidi).

Amongthespinelferrites,lithiumferrite(LiFe5O8/Li0.5Fe2.5O4) (LFO)hasbeenextensivelystudiedowingtoitschemicalsta- bility,exceptionalmagneto-dielectricproperties.LFOexhibits highCurietemperature(T_c),squarehysteresisloop,highsat- uration magnetization (M_s), excellent dielectric properties, and high resistivity, etc. [6,7]. Because of its low cost, it can be a better substitution for extensively used yttrium iron garnet (YIG)formicrowave applications [8,9]. Further- more, theyare alsousedinlithium-ion batteries,memory, andmagneticrecordingdevices,magneticresonanceimaging

https://doi.org/10.1016/j.jmrt.2020.01.050

creativecommons.org/licenses/by-nc-nd/4.0/).

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suchas sol-gel [19], solvothermal [20], microemulsion[21], spraypyrolysis[22],coprecipitation[23],microwave-induced combustion[24],etc.Moreover,dielectricandmagneticprop- erties of lithium ferrite can be tailored and optimized to device-speciﬁc by choosing appropriate chemical substitu- tionswithnonmagneticions.Soibametal.[25]reportedthat thereplacementofCo²⁺doesnotaffectquadrupolesplitting andisomershift.Itisreportedthatthealuminium(Al)sub- stitutedlithiumferritenanoparticles preparedusingcitrate gelautocombustionmethodshowareductioninthesatu- rationmagnetization[26].WhereasthesubstitutionofCaon Li-Znferrite isfoundtoenhanceporosityanddegradesthe permeability.Sattaretal.[27]foundthatthereplacementof CainplaceofZnisabetteroptionforimprovingelectrical andmagneticpropertiesratherthanforFeandLi.Rathod[28]

etal.explainedthecorrelatedvibrationsoftheoctahedraland tetrahedralcomplexesandtheabsorptionbandsinFTIRspec- traofLi-Znferrites.Hillietal.[29]studiedtheelectro-magnetic propertiesofsamarium (Sm)doped LFOnanoparticles.The resistivityofSm doped LFO increased withSm concentration.Bammeetal.[30]reportedthe dielectricpropertiesof CdsubstitutedLFOusingthemicrowave-inducedcombustion method. The dielectric constant degraded with Cd substi- tutionandthequality factorshowscusps inthe frequency rangeof80–400kHz.Thecitrateprecursormethodwasusedto preparenanosizedzirconium(Zr),andmanganese(Mn)sub- stitutedLFO[31].WithanincreaseofZrandMncontent,DC electricalresistivityenhanced,whichfollowsthepolaronhop- pingmechanism.

Similarly,withtheincreaseinZr–Coconcentration,coer- civityincreases,butsaturationmagnetizationdecreases[32].

Nutanet al. [4] studiedthe complex permittivity, and per- meabilityofCosubstitutedLFOformicrowaveapplications.

Argentinaetal.[33]reportedthatLFOisbetterthanthemag- nesium ferrites and nickel ferrites used at the microwave frequencyregionduetoitsrelativeeffectiveness.El-Shaarawy et al. [10] studied structural, dielectric behavior, and ac conductivity in the lower frequency range (20Hz–10MHz) ofMg-dopedLFO powders and foundthat ACconductivity increasedwithMgconcentration.DuetothehighCurietem- perature,highelectricalresistivity,andlowproductioncost, magnesiumsubstitutedLFOhasbeenusedinmanyelectri- caldevicesforhigh-frequencyapplications[34].Nevertheless, mostofthestudieswiththismaterialfocusedeitherpurely onthemagneticpropertiesorelectricalanddielectricprop- ertiesatlowerfrequencyregions,whereastheferritegrains were moreeffectiveinhigh frequencyregime.However, to thebestofourknowledge,thereisnostudyavailableonthe temperature-dependentbroadbanddielectricpropertiesand

ferrite. In the present study, the objective of substituting Mg is to lower the loss tangent, as lithiumferrite exhibit higher loss tangent, and the minimization of losstangent isessentialformicrowaveapplications. Thesubstitutionof divalentions(Mg²⁺)intheplace ofmonovalent (Li⁺)and a trivalentcation(Fe³⁺)alsoaffects bondlength(Fe O,Li O) and bond angle Fe OF e, which eventually asserts inﬂu- ence onthe electricandmagneticproperties.Theeffectof Mg-substitutiononstructural,microstructural,andmagnetic responseisstudied;dielectricandpermeabilityresponsesare investigatedinbroadfrequencyrangesatdifferenttemper- atures (−140–250^◦C). Further,the electricalconductivity of thesesamplesisalsostudied.

2. Experimental procedure

PolycrystallineLi_1-xMg_2xFe_5-xO8;x=0,0.003,0.005,0.007and 0.01 (LMFO) were prepared bythe conventional solid-state reactionmethodusingcommerciallyavailableconstituents.

Thestarting reagents were Fe₂O₃ (purity 99%,M/s JiangXi- HaiTeAdvanceMaterial,China),MgO(purity99%,M/sSigma Aldrich)andLi2CO3(purity99%,M/sSigmaAldrich)wereused as instoichiometricratio.Thestartingmaterialswere pul- verizedusingaplanetaryball mill(M/sFritsch,Pulverisette 6),andtheobtainedpowderswerecalcinedat800^◦Cfor3h.

Thecalcinedpowderswerere-milledagainfor5h(toreduce particle sizeand enhancethe density)and compactedinto two different shapes disc (diameter – 10mm, thickness – 1mm)forelectricalandtoroidalshapesforpermeabilitymea- surements.Toobtainthemaximumrelativedensity,initially, thesamplesweresinteredatdifferenttemperatures,and it wasfoundthatthesamplesinteredat1050^◦Cexhibitedthe maximum relative density. Therefore, furtherstudies were carriedoutwiththesesamples.TheArchimedesmethodwas usedtodeterminetherelativedensityofthesinteredsam- ples.

Thestructuralcharacterizationsofthepreparedsamples werestudiedusingX-raydiffractometer(M/sRigakuTTRAX- III,18kW,Cu-K_␣ radiation␭=1.5405Å)inthe2␪rangeof20^◦ to70^◦.Fieldemissionscanningelectronmicroscopy(FE-SEM) (M/sZeiss,Sigma)wasusedtorecordthesurfacemorphology.

The temperature and frequency-dependent dielectric constant (εr), dielectric loss (tan ı) and permeability (r)were measured from −140^◦C to 250^◦C in the frequency range of 1MHz to 1GHz using impedance and material analyzer (M/s Agilent Technologies, 4991A) witha basicaccuracyof

±0.8%attachedtoanautomatictemperaturecontroller(M/s Novocontrol,BDS2300).Themagneticcell(M/sAgilentTech-

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nologies,16454A)wasusedformeasuringthepermeabilityof thepreparedsamples.VibratingSampleMagnetometry(VSM) (M/sLakeshore,7410series)withﬁeldaccuracy±0.05%was usedtocarryouthigh-temperaturemagneticmeasurements.

Magnetizationwasalsomeasuredatlow-temperatureranges (10K–300K)byusingthephysicalpropertymeasurementsys- tem(M/sQuantumDesign,DynaCool)withnoiselevelsless than6E-7emu.

3. Result and discussion

3.1. Structuralanalysis

TheX-raydiffractionpatterns(XRD)ofLMFOceramics sin- teredat1050^◦CareshowninFig.1(a).LFOisknowntoexist intwodifferentcrystallinephases:␣-phaseand␤-phase.␣- phase(orderedphase)exhibitsFCCinversespinelstructure (spacegroup-P4132),while␤-phaseexhibitsdisorderedphase (spacegroup-Fd3−m).TheobservedXRDrevealstheforma- tionofcubicferrite(JCPDSCardNo.82-1436),conﬁrmingthat Mg-substitutedLFOexistsonlyinthe␣-phase.Thepresenceof

␤-phaseoranyothersecondaryphasesduetothesubstitution ofMgisnotobservedinXRDpatterns,conﬁrmingthephase purityofthepreparedsamples.Theenlargedviewofthepre- dominantdiffractionpeak(311)around2␪∼35^◦ isdepicted in Fig. 1(b). As the concentration of Mg increased, Bragg’s reﬂectionsshifted towardslower angles,which resulted in anincrease inthe latticeparameters and cell volume(see Table1)associatedwiththestructuraldistortionduetodis- similarionicradiioftheMg²⁺(0.71Å)withLi⁺(0.76Å)andFe³⁺

(0.64Å).

FurtheranalysiswascarriedoutusingtheRietveldrefine- ment method to estimate the crystal phase, space group, andatomicpositionsbyusingFullProfSuite.Fig.1(c)shows the representative refined XRD pattern forx=0.005. Itwas notedthatMgsubstitutedlithiumferritewasinasingle-phase cubic structure with (space group - P4132) [10]. The good- nessofthefitting wasestimatedbytheR-values,aslisted inTable1.ToobservethecelldistortionduetoMgsubstitu- tioninLFO,theillustrativeunitcellswere producedbythe Vestasoftware,asshowninFig.2.Itisobservedthatthetrig- onalbipyramidsof ironand lithiumshow atiltas the Mg contentincreases.Thesetiltseventuallyaffectthemagnetic aswell aselectricproperties,discussedinlaterpartofthis paper.

ThelatticestrainandcrystallitesizeintheLMFOceramics wereestimatedbyusingtheWilliamson-Hallplotmethod:

ˇcos=k

D +4εsin (1)

Wherek is the shapefactor, ˇthe full-widthhalf maxima (FWHM)ofthe diffractionpeak, theincident X-raywave- length,istheBraggangle,Disthecrystallitesize,andεis thestrain[35].Fig.3showstheplotsbetweenˇcosand4sin.

Strainandcrystallitesizewascalculatedfromtheﬁttedline listedinTable1.Thecrystallitesizeandstrainwerefoundto beintherangeof70–95nmand0.09%–0.15% forx=0–0.01, respectively.Thecrystallitesizeandstrainbothwerefoundto beenhancedwithanincreaseinMgcontent.

3.2. Surfacemorphologyanalysis

ThesurfacemorphologiesoftheLMFOceramicsareshownin Fig.4.ItisobservedthatthepureandMg-dopedLFOsamples exhibituniformgraindistributionwithdensemicrostructure.

ThemicrostructureandgrainsizearestronglyaffectedbyMg substitution. Thedensity ofthe LMFOis foundtoincrease withMgconcentration.Theaveragegrainsizeismeasuredby ImageJsoftware(linearinterceptmethod),andtheobtained grainsizesareintherangeof1.265–0.978␮m.Thevariationsin thegrainsizecanbeexplainedbydifferentparameters,such astheconcentrationofvariousionsanddiffusioncoefficient [36].Thegrainsizeoftheferritesishighlyinfluencedbythe domainwallcontributionsinthelow-frequencyregime[37], whichcanalsobeseeninpermeability.Theflexibilityofthe grainboundarycausesgraingrowth.Further,graingrowthand re-crystallisationsarerelatedtothemobilityofgrainbound- aries[36,38].

3.3. Dielectricproperties

3.3.1. Dielectricdispersionwithfrequency

Thefrequency-dependentdielectricconstantmeasuredinthe temperature range of −140^◦C to 230^◦C is shownin Fig. 5.

The dielectric constant of the LMFO samples is found to decreasewithappliedfrequency,whichisatypicalresponseof alineardielectric.Thecontributiontothedielectricconstant comesfromdifferentpolarizationmechanisms,suchasionic, electronic,interface,anddipolepolarization,whichinﬂuence the frequency-dependent dielectric response. In the high- frequencyregime,thedielectricresponsemainlygetsaffected byadipole,ionic,andelectronicpolarization;thefrequency- dependentdielectric constantmay alsobeexplainedbased on theelectron hoppingmechanism anddipole relaxation.

Thereductionofdielectricconstanttill100MHzcanbedue totherelaxationofdipoles.Onanapplicationoftheexter- nalelectricﬁeld,theinterchangeofelectronsinbetweenFe³⁺

↔ Fe²⁺ buildsuplocaldisplacementofchargesthatcauses polarization. Polarization reduceswith anenhancementin frequency.Thehoppingofelectronsissigneduptoaparticu- larfrequency,beyondwhichitgetsrelaxed.Hence,␧rbecomes constant[39–41].Whereas,thelargervalueofdielectriccon- stant atlower frequency regimemay be ascribed to grain boundarydefectsand predominanceofFe²⁺ ions.Asimilar kindofresponseisreportedbyIqbal[31]andSurzhikov[40].It isnotedthatthedielectricconstantofthesamplesenhanced withMgsubstitutionuptox=0.005compositionsandreduced furtherwiththeadditionofMg.Thiscanbeattributedtothe higherpolarizabilityofMg,uniformgrainsize,andmaximum relativedensity.

Dielectricconstantimproveswithtemperature,ascanbe seen from Fig.5. Athigher temperatures, thedipoles align easily,leadingtoahigherdielectricresponse.Thedielectric responseisenhancedwithMgsubstitution.Fromtheearlier studies,itisfoundthatLi⁺ionsresideonlyattetrahedralsites, whereasMg²⁺ prefersbothoctahedralandtetrahedralsites.

Hence,itreplacestheFe³⁺ionstoB-sitefromA-site.Conse- quently,moreFe²⁺-Fe³⁺ionpairsgetaccumulatedatB-site, thus increasing the hoppingmechanism [10]. Interestingly, x=0.005samplesexhibitedthebestdielectricresponseatthe

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Fig.1–(a)X-raydiffractionpatternofLi_1-xMg_2xFe_5-xO₈samples.(b)ZoomedportionoftheXRDaround35^◦.(c)Reﬁnement patternofLMFO(x=0.005).

Table1–UnitcellparameterscalculatedfromtheXRDpatternoftheLMFOceramics.

Composition x=0 x=0.003 x=0.005 x=0.007 x=0.01

a=b=c(Å) 8.3259(6) 8.3344(4) 8.3293(2) 8.3358(4) 8.3506(7)

V(Å³) 577.15 578.92 577.87 579.22 582.31

D(nm) 70.11 82.16 70.31 90.14 95.11

Strain 0.009 0.0011 0.0014 0.0014 0.0015

² 1.76 2.89 2.53 2.92 2.62

Fig.2–SchematicrepresentationofunitcellsofLMFO.ThegreencolourandthebluecolourshowtheLi/Mgtrigonal bipyramidsandFe/Mgtrigonalbipyramidsrespectively.

wholemeasured temperaturerange, and all other samples displayedbetterdielectricresponseascomparedtopureLFO.

Thevaluesofdielectriclossanddielectricconstantobtained atsomefrequenciesandtemperaturesarelistedinTable2.

VerylowdielectriclossisachievedwithMgsubstitutionwhen comparedtopreviousreports[26,30,31].

3.3.2. Variationofthedielectricpropertywithtemperature Thetemperature(-140to250^◦C)dependentdielectricconstant (εr)anddielectricloss(tanı)obtainedatdifferentfrequencies areshowninFigs.6and7,respectively.Asthetemperature increases from −140^◦C,the dielectric constant of LFO initially increases and exhibitsa hump ataround 170^◦C and thendecreasesforafurtherriseintemperature.Thistrend isalsotruefortheother compositions;x=0.003,0.005,and 0.007.However,thehumpindielectricconstantshiftstoward highertemperatureswithanincreaseintheMgconcentration, tillX=0.007.In contrast,forx=0.01 samples,the dielectric constant increases continuouslywith temperature without

any hump or reductioninthe values. Thevalueof dielec- tricconstantincreasesfrom619to3034whilethedielectric lossreducesremarkablyfrom2.87to0.001astheMgconcen- trationincreasesfromX=0toX=0.005atRT.ForLFO,asthe temperatureincreases,tanırisesinitiallyexhibitingacuspat

−40^◦Candagainincreasessharplyabove170^◦Cat1MHz.The sametrendisfollowedforallfrequencies.Thesampleswith x=0.003,0.005,and0.007alsofollowedthesametrend,butthe humpshiftstowardalowertemperaturerange.Theshiftinthe humpisattributedtotherelaxationofdipoles.Forx=0.01,the valuesoftanıincreasesmoothlywithtemperature,accom- panied bya sharp rise above 180^◦C. Therise in dielectric responseisattributedtothehigherrelativedensity,andthe latticedistortionoftheMgsubstitutedcompounds.Further, itisfoundthatthedielectricconstantanddielectriclossare improvedwithanincreaseintemperatureduetothedisplace- mentsoflocalizedchargecarriers.However,theenhancement intheconductivityofthesamplesmaybeattributedtothe increasingmobilityofchargecarriersasaresultofapplied

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Fig.3–W-HplotofLMFOceramics(a)x=0,(b)x=0.003,(c)x=0.005,(d)x=0.01.

Fig.4–FE-SEMmicrographoftheLMFOsamplesfor(a)x=0,(b)x=0.003,(c)x=0.005,(d)x=0.01.

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Fig.5–FrequencydependentdielectricconstantofLMFOceramics,measuredatdifferenttemperatures(−140–230^◦C)(a) x=0,(b)x=0.003,(c)x=0.005,(d)x=0.01.

Table2–TheobtainedpermittivityanddielectriclosstangentvaluesofLMFOceramics.

Composition x=0.00 x=0.003 x=0.005 x=0.007 x=0.01

Temp (^◦C)

Frequency ␧r Tan␦ ␧r Tan␦(10⁻²) ␧r Tan␦(10⁻²) ␧r Tan␦(10⁻²) ␧r Tan␦(10⁻²)

−140 1MHz 26 0.63 134 0.3 181 0.42 121 0.27 80 0.18

10MHz 13 0.40 40 0.29 45 0.31 36 0.28 35 0.16

100MHz 10 0.14 20 0.12 21 0.15 20 0.10 20 0.11

1GHz 11 0.09 17 0.05 18 0.06 18 0.05 16 0.06

30 1MHz 619 2.87 1759 0.15 3034 0.16 1670 0.16 626 0.20

10MHz 74 3.22 756 0.26 1201 0.28 723 0.23 273 0.26

100MHz 36 0.91 139 0.56 143 0.88 157 0.51 96 0.31

1GHz 16 0.70 17 0.98 12 0.93 21 1.03 19 0.55

100 1MHz 1733 3.51 2941 0.17 4859 0.16 3188 0.16 1226 0.26

10MHz 185 4.15 1333 0.22 2113 0.24 1350 0.23 463 0.25

100MHz 48 1.81 239 0.66 174 1.03 250 0.67 155 0.36

1GHz 24 1.17 7.35 3.9 7 1.9 24 3.9 13 1.02

170 1MHz 2614 4.64 4796 0.24 7227 0.23 5459 0.16 2449 0.32

10MHz 412 4.24 2135 0.23 3251 0.23 2306 0.24 765 0.33

100MHz 53 4.00 211 1.12 61 1.4 205 1.6 192 0.53

1GHz 22 1.82 4 3.8 8 1.7 24 2.5 14 1.3

240 1MHz 143 22.8 4857 1.85 6056 1.89 6738 0.90 2830 1.31

10MHz 315 9.75 2290 0.72 3129 0.71 3306 0.38 1060 0.60

100MHz 37 8.39 121 1.62 66 1.5 29 2.1 128 1.2

1GHz 19 2.62 3 1.6 14 1.6 10 2.32 20 1.61

thermalenergy. Theobserved high dielectric constant and lowloss(orderof10⁻³)inthehigh-frequencyregimemakes LMFOasabettercandidateformicrowaveapplications[33,42].

Atlowertemperatures,boththedielectricconstantandloss tangentsarelowered,anditisduetothelowerthermalvibra- tionsandthefreezingofdipoles.Further,thesubstitutionof

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Fig.6–TemperaturevariationdielectricconstantofLMFOceramics,measuredatdifferentfrequenciesfor(a)x=0(b) x=0.003(c)x=0.005(d)x=0.01.

Fig.7–TemperaturevariationdielectriclosstangentofLMFOceramics,measuredatdifferentfrequenciesfor(a)x=0(b) x=0.003(c)x=0.005(d)x=0.01.

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Where,0,T,k_B isthepre-exponentialfactor,absolutetem- perature,andBoltzmannconstant,respectively.Theln(ac) vs 1000/T isplotted at 1MHzfor LMFO (x=0, 0.003, 0.005, 0.007,0.01),andisshowninFig.8(a).ACconductivityisfound to increase with temperature and Mg²⁺ ion concentration.

Theprinciple of the electrical conduction mechanism can beunderstoodbythedielectricpolarizationmechanism.The increaseintheconductivitywithaconcomitantincreasein temperatureisbecauseofthermallyactivatedinterchangeof thechargecarriersamongFe²⁺andFe³⁺.Further,theincrease intheconductivityduetoMg²⁺ionisattributedtothecon- centrationratioofFe³⁺/Fe²⁺ionsontheBsite.Theactivation energyisextractedfrom the slopeofthe ﬁttedline and is foundtobe1.39eVforx=0,whichisdecreasedto0.35eVfor x=0.005.However,itshootsupagainwithanincreaseinMg concentration.

Thefrequencyvariantacconductivitycanbeexplainedby usingthepowerequation.

ac(w)=Awⁿ (3)

where A is the temperature-dependent parameter, w the angular frequency, and n is dimensionless temperature- dependentparameterwhosevalueliesbetween0and1.For n=0,theacconductivityisindependentoffrequency,andfor n≤ 1,the acconductivity isdependent onfrequency. Fur- ther,thisregionisdividedintotwoparts,for0.5≤n≤1,the exchangeofelectronoccursamongFe²⁺andFe³⁺ions,andfor n≤0.5,thedominantcontributionisfromionicconductivity.

Fig.8(b)showsthelogarithmicvariationofroomtemperature acvsfrequency.Theacconductivityincreaseswithfrequency, whichisatypicalcharacteristicofsemiconductingmaterials.

Thisbehaviorcanalsobeexplainedusingthepumpingforce correspondingtoappliedfrequency,whichisresponsiblefor thehoppingofchargecarriers[26,44].Thevalueofnwascal- culatedfromtheslopeoftheﬁttedcurve.Theobtainedvalue forLFOis0.138,whichenhanceswithanincreaseinMgcon- centrationand thevaluesarefoundintherangeof0.689- 0.761,indicatingtheexchangeofelectronsbetweenFe²⁺and Fe³⁺ions.

3.5. Magneticproperties

Fig.9showsthevariationofmagnetizationcurvesasafunc- tionoftemperature(M-T)forLMFOsamples.Magnetization decreaseswithanincreaseintemperature,whichcanbedue tothefactthattheironspinsatoctahedralandtetrahedral sitesaresealedinthedirectionoftheappliedmagneticﬁeld atlowtemperatures. Witha rise intemperature, spinsget

The room temperature M-H loop ofLMFO ceramics are shown in Fig. 10(a). The values of coercivity (H_C), saturation magnetization(M_S), and remanentmagnetization (M_r) are listed in Table3and plotted withMg concentrationin Fig.10(b).Itisobservedthatsaturationmagnetizationreduces asafunctionofMgconcentration.Nevertheless,thesample withx=0.007exhibitsthehighestsaturationmagnetization amongalltheothersubstitutedsamples.Theobservedmag- netizationcouldbeduetotheimpactofseveralfactorssuch asA-Bexchangeinteraction,cationdistribution,anisotropy, density,andgrainsize[26,45–48].Thiscanbebetterunder- stoodbyNeel’stwo-sublatticemodel.AccordingtoNeel,three typesofinteractionsexistA-A,B-B,andA-B.Amongthese,the superexchangeA-Binteractionisthestrongestone.Theresul- tantmagnetizationarisesasaresultofavectorsumoftwo sublatticesAandB[49].InordertoaccommodatetheMg²⁺

ionsattheA-site,Fe³⁺ ionsgettransferredfromAsitetoB site,whichcausestheinitialdecreaseinsaturationmagneti- zation(M_s).Hence,Fe³⁺ioncontentincreasesatB-site,causing theantiparallelspincoupling,whicheventuallyresultsinthe reductionofA-B exchangeinteractionstrength,andconse- quently,decreasesthemagnetization.TheincreaseinM_sfor x=0.007maybeduetoMg²⁺ ions,whicharesubstitutedfor Li⁺andoccupythetetrahedralAsitewithananalogoustrans- ferofFe³⁺fromAtoBsite.Thisenhancesthemagnetization intheBsite.TherandombehaviorofH_Cisattributedtothe variationofparticlesizeandmagneticanisotropy[50,51].The MgsubstitutiononLFOleadstothestructuraldistortion,as depictedinFig.3,whichmodiﬁesFe OF ebondanglesatboth thetetrahedralandoctahedralpositionsandtheFe Obond length(Table3).Further,themagnetocrystalline anisotropy wasdeterminedbyanalyzingtheinitialM-Hcurvewiththe lawofapproach(LAP)tosaturation[52]:

M=Ms

1− 8

105²₀M²_s

_K

1

H

2

+H (4)

Wherethe coefﬁcient8/105isthe polycrystallinespecimen with cubicanisotropy,K1, and Hare the cubic anisotropy constantandforcedmagnetizationduetotheincreaseofsat- urationmagnetization,respectively.Fig.10(c)showstheﬁtted curveofLAPwiththeexperimentalinitialmagnetizationdata.

TheanisotropyconstantisfoundtodecreasewithMgcontent, exceptforx=0.003.Theobtainedvaluesare1.17E5and1.03E5 erg/cm³forx=0.00and0.007,respectively.

Fig. 11(aand b)shows the variationofzero-field cooled (ZFC)andfieldcooled(FC)magnetizationasafunctionoftem- perature(10K–300K)forx=0and0.007samples.TheZFCcurve isobtainedbycoolingthesampleinzerofieldfromroomtem-

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Fig.8–Logarithmicvariationof␴acasafunctionof(a)1000/Tand(b)ln(frequency)ofLMFOceramicsx=0,0.003,0.005, 0.007and0.01.

Fig.9–TemperaturedependentmagnetizationcurveofLMFOsamples.

Table3–MagneticparametersofLMFOceramics.

Parameters x=0 x=0.003 x=0.005 x=0.007 x=0.01

Ms(emu/g) 59 48 50 55 53

Mr(emu/g) 5.44 3.51 3.32 3.75 4.78

Hc(Oe) 80.38 88.09 81.25 86.47 105.92

Tc(^◦C) 600 556 598 605 596

Fe O(Å) 1.9236 1.9199 1.9188 1.9202 1.9237

Fe OF e(^◦) 121.20 120.15 123.48 127.56 132.39

peratureto10K,thenthe magneticﬁeld isappliedat10K, andsimultaneouslymagnetizationisrecordedwhileincreas- ingthetemperature.Similarly,FCmagnetizationisobtained bycoolingthesamplesat1kOe.ForLFO,ZFCandFCcurves showadecreaseinmagnetizationwithanincreaseintemper- ature,andbothcurvesmergedat203K.However,forx=0.007, ZFCcurveinitiallyincreaseswithanincreaseintemperature

upto30K,thendecreases,andmergedwithFCcurveat210K.

SimilarkindofresponseisalsoobservedbyShirsath[53]and Dar [54].Theslowrelaxationofthe particlesand the exis- tenceoftheenergybarrierofmagneticanisotropymaybethe maincauseforbifurcationofZFCandFCcurve[21,55].These particles havealow anisotropyenergybarriertoovercome theirenergybarrierbythethermalactivationenergy.TheM-

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Fig.10–(a)RoomtemperatureM-HcurvesoftheLMFO.Insetshowstheenlargedviewneartheorigin.(b)Thevariationof magneticparameterswithMgcomposition.

Fig.11–M-Tcurvesbelowroomtemperature(10K–300K)for(a)LFOand(b)x=0.007.

Fig.12–M-Hloopsbelowroomtemperature(20Kand280K)for(a)LFOand(b)x=0.007.

Hloopat20Kand280Kwererecordedforx=0.00and0.007, asshowninFig.12(aandb),respectively.Theobtainedsatu- rationmagnetizationforLFOis66.52and62.07emu/gat20K and280K,respectively.Thex=0.007exhibitsenhancedsatu- rationmagnetization,73.04and57.85emu/gat20Kand280K, respectively.Thecoercivityand remanentmagnetization is improvedwithadecreaseintemperature.

3.6. Permeabilityanalysis

The frequency (1MHz to 1GHz) and temperature (-140 to 250^◦C)dependentrealpartofpermeabilityisplottedforLMFO ceramics,asshowninFig.13(a)and(b).Theobtainedperme- abilityforpureLFOisr=27.88at1MHzfrequency,whichis acomparablevaluewithpreviouslyreporteddata[56].Perme- abilitystronglyinﬂuencedbyMgconcentration.Forx=0.003, thepermeabilitydecreasesto23.94.However,forx=0.005and x=0.007,thepermeabilityraisesto28.56and29,accordingly.

Thevariationinpermeabilitymainlydependsonthethick- nessofthedomainwalls(␦),magneto-crystallineanisotropy (K₁),theaveragegrainsize(D_m),andinnerstress(),whichcan bewellunderstoodbythisrelation[57].

∝0M²_SDm/[ 1+(3/2)S]ˇ¹

⁄

³_ı ₍₅₎

where,M_s, s,andˇarethesaturationmagnetization,satu- rationmagnetostrictionconstant,andvolumeconcentration ofimpurity,correspondingly.Accordingtothepreviousanal- ysis (initial magnetization curve with Law of approach to saturation),K₁decreasedwithMgconcentration,whichcan be ascribed to the enhanced permeability. A transition in the rangeof70 to90^◦C isobservedfromthe temperature- dependentpermeabilityforalltheLMFOceramics.Fig.13(c) shows the variation ofmagnetic loss asa function offre- quencyforLMFOceramics.ForLFO,themagneticlossinitially decreases with arise infrequencyup to 10MHz and then

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Fig.13–(a)FrequencyvariantpermeabilityforLMFOceramicsmeasuredat30^◦C,(b)Temperaturevariantpermeabilityfor LMFOceramicsat1GHzand(c)FrequencydependentmagneticlossforLMFOceramicsatRT.

increasesexhibitingabroadpeakataround100MHz.Allother ceramics follow the same trendwhereas, they exhibited a sharp peakwitha shifttowardshigherfrequency withMg concentration.

According to the surface morphology analysis, Mg sub- stitutedLFO exhibitedsmallergrainsize,whichisascribed toinhibitionofgraingrowthduetoMg.Fromthedielectric response,itisfoundthatwithMgsubstitution,thedielectric constantenhancedalongwithreducedlosstangent.Thisvari- ationisexplainedbasedontheelectronhoppingmechanism.

Again,magnetizationdecreasedwithanincreaseinMgcon- centration,whichisduetothechangeincationicdistribution andexchangeinteraction.Further,permeabilityresponseis correlatedwithmagneticandsurfacemorphologyresults.

4. Conclusions

LMFO ceramics have been successfully synthesized bythe conventional solid-state reaction method. XRD analysis and Rietveld reﬁnement revealed the formation of a pure phasecubicspinelstructure.Alltheceramicsexhibitdense microstructure.Enhanceddielectric(εr =3034,tanı=0.001at RT,1MHz)isobservedfortheMgcomposition,x=0.005.AC conductivityincreaseswithanincreaseintemperatureaswell asMg²⁺concentrationduetotheexchangeofelectronsamong Fe²⁺andFe³⁺ions.TheLMFOwithx=0.007exhibitsbestper- meability(r=29)andmagneticproperties(M_s=55.64emu/g) atroomtemperature. Thechange inmagneticbehaviouris explainedbysuperexchangeinteractionsandNeel’stwosub- latticemodels.Acombinationofhighdielectricconstant,low losstangent(orderof10⁻³),highpermeability,andmagnetic propertiesofLMFOmakex=0.005specimensagoodcandi- dateformicrowaveapplicationssuchasphaseshifterandthe circulator.

Conﬂict of interest

Inthispaper,alltheauthorsdeclarethatthereisnoconﬂict ofinterest.

Acknowledgements

TheauthorsaregratefultoCentralInstrumentsFacilityand the Center for Nanotechnology, Indian Institute of Tech- nology Guwahati, India, for providing the characterization facilities. Theauthors would also like to acknowledge the Department of Physics, IIT Guwahati for XRD measure- ment. Also,the authors are highlythankful to DST FIST-II (SR/FST/PSII-037/2016)foravailinglow-temperaturemagnetic measurements.

references

[1]JoshiS,KumarM,ChhokerS,SrivastavaG,JewariyaM,Singh VN.Structural,magnetic,dielectricandopticalpropertiesof nickelferritenanoparticlessynthesizedbyco-precipitation method.JMolStruct2014;1076:55–62,

http://dx.doi.org/10.1016/j.molstruc.2014.07.048.

[2]SinghalS,JauharS,SinghJ,ChandraK,BansalS.

Investigationofstructural,magnetic,electricalandoptical propertiesofchromiumsubstitutedcobaltferrites (CoCr_xFe_2-xO₄,0≤×≤1)synthesizedusingsolgelauto combustionmethod.JMolStruct2012;1012:182–8, http://dx.doi.org/10.1016/j.molstruc.2011.12.035.

[3]RaoTD,KarthikT,AsthanaS.Investigationofstructural, magneticandopticalpropertiesofrareearthsubstituted bismuthferrite.JRareEarths2013;31:370–5,

http://dx.doi.org/10.1016/S1002-0721(12)60288-9.

[4]GuptaN,DimriMC,KashyapSC,DubeDC.Processingand propertiesofcobalt-substitutedlithiumferriteintheGHz frequencyrange.CeramInt2005;31:171–6,

http://dx.doi.org/10.1016/j.ceramint.2004.04.004.

[5]SchloemannE.Advancesinferritemicrowavematerialsand devices.JMagnMagnMater2000;209:15–20,

http://dx.doi.org/10.1016/S0304-8853(99)00635-6.

[6]ThomasN,ShimnaT,JithinPV,SudheeshVD,Choudhary HK,SahooB,etal.Comparativestudyofthestructuraland magneticpropertiesofalphaandbetaphasesoflithium ferritenanoparticlessynthesizedbysolutioncombustion method.JMagnMagnMater2018;462:136–43,

http://dx.doi.org/10.1016/j.jmmm.2018.05.010.

[7]DasariM,GajulaG,HanumanthaR,ChintabathiniA, KurimellaS,SomayajulaB.Lithiumferrite:thestudyon magneticandcomplexpermittivitycharacteristics.Process

(12)

http://dx.doi.org/10.1007/s10854-015-3181-2.

[11]DiptiKumarP,JunejaJK,SinghS,RainaKK,PrakashC.

Improveddielectricandmagneticpropertiesinmodiﬁed lithium-ferrites.CeramInt2015;41:3293–7,

[12]SrivastavaM,LayekS,SinghJ,DasAK,VermaHC,OjhaAK, etal.Synthesis,magneticandMössbauerspectroscopic studiesofCrdopedlithiumferritenanoparticles.JAlloys Compd2014;591:174–80,

http://dx.doi.org/10.1016/j.jallcom.2013.12.180.

[13]MazenSA,Abu-ElsaadNI.Structural,magneticandelectrical propertiesofthelithiumferriteobtainedbyballmillingand heattreatment.ApplNanosci2015;5:105–14.

[14]ZengH,TaoT,WuY,QiW,KuangC,ZhouS,etal.Lithium ferrite(Li_0.5Fe_2.5O4)nanoparticlesasanodesforlithiumion batteries.RSCAdv2014;4:23145–8,

http://dx.doi.org/10.1039/c4ra02957g.

[15]SurzhikovAP,LysenkoEN,MalyshevAV,NikolaevEV, ZhuravkovSP,VlasovVA.Investigationofthecomposition andelectromagneticpropertiesoflithiumferriteLiFe5O8

ceramicssynthesizedfromultradisperseironoxide.Russ PhysJ2015;57:1342–7,

http://dx.doi.org/10.1007/s11182-015-0387-y.

[16]RezlescuN,DorofteiC,RezlescuE,PopaPD.Lithiumferrite forgassensingapplications.SensorsActuatorsBChem 2008;133:420–5,http://dx.doi.org/10.1016/j.snb.2008.02.047.

[17]ThakurP,SharmaP,MatteiJL,QueffelecP,TrukhanovAV, TrukhanovSV,etal.Inﬂuenceofcobaltsubstitutionon structural,optical,electricalandmagneticpropertiesof nanosizedlithiumferrite.JMaterSciMaterElectron 2018;29:16507–15,

http://dx.doi.org/10.1007/s10854-018-9744-2.

[18]PachauriN,KhodadadiB,AlthammerM,SinghAV,LoukyaB, DattaR,etal.Studyofstructuralandferromagnetic resonancepropertiesofspinellithiumferrite(LiFe5O8) singlecrystals.JApplPhys2015;117:233907,

http://dx.doi.org/10.1063/1.4922778.

[19]VijayaBhaskerReddyP,RameshB,GopalReddyC.Electrical conductivityanddielectricpropertiesofzincsubstituted lithiumferritespreparedbysol-gelmethod.PhysBCondens Matter2010;405:1852–6,

http://dx.doi.org/10.1016/j.physb.2010.01.062.

[20]LiB,XieY,SuH,QianY,LiuX.Synthesisofthe

nanocrystalline␣-LiFe5O8inasolvothermalprocess.Solid StateIon1999;120:251–4,

http://dx.doi.org/10.1016/S0167-2738(98)00556-6.

[21]DarMA,ShahJ,SiddiquiWA,KotnalaRK.Inﬂuenceof synthesisapproachonstructuralandmagneticpropertiesof lithiumferritenanoparticles.JAlloysCompd2012;523:36–42, http://dx.doi.org/10.1016/j.jallcom.2012.01.083.

[22]ErnstFO,KammlerHK,RoesslerA,PratsinisSE,StarkWJ, UfheilJ,etal.Electrochemicallyactiveﬂame-made nanosizedspinels:LiMn2O4,Li4Ti5O12andLiFe5O8.Mater ChemPhys2007;101:372–8,

http://dx.doi.org/10.1016/j.matchemphys.2006.06.014.

[26]DarMA,BatooKM,VermaV,SiddiquiWA,KotnalaRK.

Synthesisandcharacterizationofnano-sizedpureand Al-dopedlithiumferritehavinghighvalueofdielectric constant.JAlloysCompd2010;493:553–60,

[27]SattarAA,AgamiWR.Studyofthephysicalandmagnetic propertiesofLi0.3Zn0.4-xCaxFe2.3O4ferrite.JAlloysCompd 2010;496:341–4,

[28]RathodV,AnupamaAV,VijayaKumarR,JaliVM,SahooB.

Correlatedvibrationsofthetetrahedralandoctahedral complexesandsplittingoftheabsorptionbandsinFTIR spectraofLi-Znferrite.VibSpectrosc2017;92:267–72, http://dx.doi.org/10.1016/j.vibspec.2017.08.008.

[29]Al-HilliMF,LiS,KassimKS.Structuralanalysis,magneticand electricalpropertiesofsamariumsubstitutedlithiumnickel mixedferrites.JMagnMagnMater2012;324:873–9, http://dx.doi.org/10.1016/j.jmmm.2011.10.005.

[30]WataweSC,BamneUA,GonbareSP,TangsaliRB.Preparation anddielectricpropertiesofcadmiumsubstitutedlithium ferriteusingmicrowave-inducedcombustion.MaterChem Phys2007;103:323–8,

[31]IqbalMJ,HaiderMI.Impactoftransitionmetaldopingon Mössbauer,electricalanddielectricparametersof structurallymodiﬁedlithiumferritenanomaterials.Mater ChemPhys2013;140:42–8,

[32]GuravSK,ShirsathSE,KadamRH,ManeDR.Low

temperaturesynthesisofLi0.5ZrxCoxFe2.5-2xO4powderand theircharacterizations.PowderTechnol2013;235:485–92, http://dx.doi.org/10.1016/j.powtec.2012.11.009.

[33]ArgentinaGM,BabaPD.MicrowaveLithiumferrites:an overview.IEEETransMicrowTheoryTech1974;22:652–8, http://dx.doi.org/10.1109/TMTT.1974.1128308.

[34]RavinderD,ReddyPVB.Thermoelectricpowerstudiesof polycrystallinemagnesiumsubstitutedlithiumferrites.J MagnMagnMater2003;263:127–33,

http://dx.doi.org/10.1016/S0304-8853(02)01545-7.

[35]PrabhuYT,RaoKV.X-rayanalysisbyWilliamson-Halland size-strainplotmethodsofZnOnanoparticleswithfuel variation.WJNSE2014:21–8,

http://dx.doi.org/10.4236/wjnse.2014.41004.

[36]MazenSA,Abu-ElsaadNI.Characterizationandmagnetic investigationsofgermanium-dopedlithiumferrite.Ceram Int2014;40:11229–37,

[37]CaltunOF,SpinuL.Structureandmagneticpropertiesof Ni-Zn-Cuferritessinteredatdifferenttemperatures.J OptoelectronAdvM2002;4:337–40.

[38]ManjurulHaqueM,HuqM,HakimMA.InﬂuenceofCuOand sinteringtemperatureonthemicrostructureandmagnetic propertiesofMg-Cu-Znferrites.JMagnMagnMater 2008;320:2792–9,

(13)

[39]IqbalMA,IslamMU,AliI,AzharM,SadiqI,AliI.High frequencydielectricpropertiesofEu⁺³-substitutedLi–Mg ferritessynthesizedbysol–gelauto-combustionmethod.J AlloysCompd2014;586:404–10,

[40]SurzhikovAP,MalyshevAV,LysenkoEN,VlasovVA, SokolovskiyAN.Structural,electromagnetic,anddielectric propertiesoflithium-zincferriteceramicssinteredbypulsed electronbeamheating.CeramInt2017;43:9778–82,

[41]NasirS,SaleemiAS,Fatima-tuz-Zahra,Anis-ur-RehmanM.

EnhancementindielectricandmagneticpropertiesofNi–Zn ferritespreparedbysol–gelmethod.JAlloysCompd 2013;572:170–4,

[42]CollinsT,BrownAE.LowLossLithiumferritesformicrowave latchingapplicationslow-lossLithiumferritesformicrowave latchingapplications.JApplPhys1971;42,

http://dx.doi.org/10.1063/1.1660752.

[43]HomesCC,HomesCC,VogtT,ShapiroSM,WakimotoS.

Opticalresponseofperovskite-relatedoxide.Science 2012;673,http://dx.doi.org/10.1126/science.1061655.

[44]HitiME.ACelectricalconductivityofNi-Mgferrites.JPhysD ApplPhys1996;29:501,

http://dx.doi.org/10.1088/0022-3727/29/3/002.

[45]KadamAA,ShindeSS,YadavSP,PatilPS,RajpureKY.

Structural,morphological,electricalandmagneticproperties ofDydopedNi–Cosubstitutionalspinelferrite.JMagnMagn Mater2013;329:59–64,

[46]CherukuR,GovindarajG,VijayanL.Super-linearfrequency dependenceofacconductivityinnanocrystallinelithium ferrite.MaterChemPhys2014;146:389–98,

[47]TeixeiraSS,GracaMPF,CostaLC,ValenteMA.Studyofthe inﬂuenceofthermaltreatmentonthemagneticproperties oflithiumferritepreparedbywetball-millingusingnitrates asrawmaterial.MaterSciEng2014;186:83–8,

http://dx.doi.org/10.1016/j.mseb.2014.03.008.

[48]RathodV,AnupamaAV,JaliVM,HiremathVA,SahooB.

Combustionsynthesis,structureandmagneticpropertiesof

Li-Znferriteceramicpowders.CeramInt2017;43:14431–40, http://dx.doi.org/10.1016/j.ceramint.2017.07.213.

[49]NeelL.Magnetismandlocalmolecularﬁeld.Science 1971;174:985–92,

http://dx.doi.org/10.1126/science.174.4013.985(80-).

[50]HusseinSI,ElkadyAS,RashadMM,MostafaAG,MegahidRM.

Structuralandmagneticpropertiesofmagnesiumferrite nanoparticlespreparedviaEDTA-basedsol-gelreaction.J MagnMagnMater2015;379:9–15,

[51]RashadMM,NasrMI.Controllingthemicrostructureand magneticpropertiesofMn-Znferritesnanopowders synthesizedbyco-precipitationmethod.ElectronMaterLett 2012;8:325–9,http://dx.doi.org/10.1007/s13391-012-1104-4.

[52]MelikhovY,SnyderJE,JilesDC,RingAP,PaulsenJA,LoCCH, etal.Temperaturedependenceofmagneticanisotropyin Mn-substitutedcobaltferrite.JApplPhys2006;99:08R102, http://dx.doi.org/10.1063/1.2151793.

[53]ShirsathSE,KadamRH,GaikwadAS,GhasemiA,Morisako A.Effectofsinteringtemperatureandtheparticlesizeon thestructuralandmagneticpropertiesofnanocrystalline Li0.5Fe2.5O4.JMagnMagnMater2011;323:3104–8,

[54]DarMA,ShahJ,SiddiquiWA,KotnalaRK.Inﬂuenceof synthesisapproachonstructuralandmagneticpropertiesof lithiumferritenanoparticles.JAlloysCompd2012;523:36–42, http://dx.doi.org/10.1016/j.jallcom.2012.01.083.

[55]VermaS,JoyPA.Lowtemperaturesynthesisof nanocrystallinelithiumferritebyamodiﬁedcitrategel precursormethod.MaterResBull2008;43:3447–56, http://dx.doi.org/10.1016/j.materresbull.2008.01.023.

[56]VermaV,GairolaSP,PandeyV,TawaleJS,SuH,KotanalaRK.

HighpermeabilityandlowpowerlossofTiandZn substitutionlithiumferriteinhighfrequencyrange.JMagn MagnMater2009;321:3808–12,

[57]VermaV,SharmaP,SinghA.Effectofironcontenton permeabilityandpowerlosscharacteristicsof Li0·35Cd0·3Fe2·35O4andLi0·35Zn0·3Fe2·35O4.BullMaterSci 2014;37:855–9,http://dx.doi.org/10.1007/s12034-014-0017-2.