Abnormal magnetic behaviors and large magnetocaloric effect in MnPS3 nanoparticles

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Abnormal magnetic behaviors and large magnetocaloric effect in MnPS3 nanoparticles

R. Zeng, S. Q. Wang, G. D. Du, J. L. Wang, J. C. Debnath, P. Shamba, Z. Y. Fang, and S. X. Dou

Citation: Journal of Applied Physics 111, 07E144 (2012); doi: 10.1063/1.3679409 View online: http://wendang.chazidian.com/10.1063/1.3679409

View Table of Contents: http://wendang.chazidian.com/content/aip/journal/jap/111/7?ver=pdfcov Published by the AIP Publishing

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JOURNALOFAPPLIEDPHYSICS111,07E144(2012)

AbnormalmagneticbehaviorsandlargemagnetocaloriceffectinMnPS3nanoparticles

R.Zeng,1,a)S.Q.Wang,2G.D.Du,1J.L.Wang,1J.C.Debnath,1P.Shamba,1Z.Y.Fang,1andS.X.Dou1

12

InstituteforSuperconductingandElectronicMaterials,UniversityofWollongong,NSW2522,AustraliaSchoolofChemicalEngineering,UniversityofHubei,Wuhan430036,People’sRepublicofChina

(Presented1November2011;received11October2011;accepted7December2011;publishedonline12March2012)

AnanostructuredhoneycomblatticeconsistingofMnPS3nanoparticlessynthesizedviatheion-exchangetechniquewasfoundtohaverestackedmolecularlayersstabilizedbyH2Oinsertionbetweenthelayers.Susceptibility(v)andheatcapacitymeasurementsshowedtheabsenceoflongrangemagneticordering,atleastdownto2K.However,thevdatashowedthatthesystempossessesahigheffectiveCurietemperature,suggestingthatthesystemisinahighspinlatticedisorderedstate.Evaluationofthemagnetocaloriceffectindicatesthatthesystemhasalargereversiblemagnetic-entropychange(ÀDSm)of6.8and12.8J/kgKandanadiabatictemperaturechange(DTad)of3.8Kand8Kat2.85Kformagnetic eldchangesof3Tand9T,

C2012AmericanInstituteofPhysics.[doi:10.1063/1.3679409]respectively.V

MPX3(M¼Mn,Fe,Co,Ni,etc.transitionmetals;

X¼S,Se)compoundshaveattractedmuchattention1–13tothisfamilyduetotheirlayeredstructurecombinedwithhighanisotropy;theirspecial,butstilluncertainandinteresting,magneticorderings1,2;andtheirpotentialapplicationsascath-odematerialforsecondarybatteries,3ion-exchangeapplica-tions,4ferroelectricmaterials,5andnon-linearopticallyactivematerials,6,7aswellastheirveryinterestingpotentialtoyieldmolecularmagnetsviatheintercalationofexoticpolymerlayers.9–12BulkMnPS3isalayeredhoneycomblatticequasi-2-dimensionalantiferromagnet,1andstudiesofitsmagneticpropertiesandneutronspectrahavereportedpowderandcrys-talmaterialswithaNe´elorderingtemperatureTN¼78Kandalargespinpotential(S¼5/2)oftheMnions,buttherehavebeennoreportsonthemagneticpropertiesandmagneto-caloriceffectofitsspecialnanostructureformasyet.

ApolycrystallinenanoparticlesampleofMnPS3waspreparedviatheion-exchangesolvothermalmethodaccord-ingtotheliterature.13

Figure1(a)displaystheXRDpatternoftheMnPS3nano-particles.TheindexingofdiffractionpeaksoftheMnPS3nanoparticlesismarked,withthelinesofthecorrespondingbulkmaterialstandard(JCPDSNo.78-0495)includedaswell.Itshouldbenotedthatalldiffractionpeakscanbeindexedtoamonocliniccellwithlatticeconstants

,b¼10.524A ,c¼6.769A ,andb¼107.35 ,a¼6.077A

indicatingthatthecrystalstructureoftheMnPS3nanopar-ticlesbelongstospacegroupC2/m.Thepeaksofnanopar-ticlesaremuchbroaderthanthoseofthebulkmaterialsduetothesmallerparticlesizeandthelatticedistortionduetothenanosize.Only{00l}Braggpeaksaredetectable,andtheindividualpeaksshowwatertails,i.e.,Warrentails,14suggest-ingthatthesystemisamonolayerorconsistsofrestackedmonolayerswithawaterstabilizedstructure.Frindtetal.have

a)

Authortowhomcorrespondenceshouldbeaddressed.Electronicmail:rzeng@uow.edu.au.

exfoliatedthelayeredcompoundsMnPS3andCdPS3toformsinglemolecularlayersinsuspensioninwaterusingthesameionexchangemethod.14OurXRDresultsarethesameasintheirreports,indicatingthatournanoparticlesarerestackedsinglemolecularlayersseparatedbycrystallisedH2O.TheH2Oseparatestheindividualmolecularlayersandstabilizestherestackedstructure.TheXRDdataalsoshowanearlypuresinglephase,asthereisnotanobviousimpuritypeakinFig.1(a).ThemorphologyoftheMnPS3nanoparticleswasinvestigatedviaSEM,asshowninFig.2(b).TheMnPS3isshowntohaveplateletshapedparticles10to20nmthickwithadiameterdistributionrangeof20to50nm.

Thetemperaturedependenceofthesusceptibility(v)underzero eldcoolingand eldcoolingina eldH¼100OeisplottedintheinsetofFig.2(a).Thetemperaturedependenceoftheinversesusceptibility(1/v)andtheCurieconstantCCW(T):vTarealsoshowninFig.2(a).Thereisnosignofanylong-rangeorderingtemperaturefortheseMnPS3nanopar-ticlesfromthev–T,1/v–T,andvT–Tcurves.

The1/v–Tcurvesfortemperatures>150Khavebeen t-tedbyusingtheCurie-Weissformula(vC¼CCW/(TþHCW)).The ttingandcalculationresultsshowunusualfeatures:averylargeCCW¼23.3emu/mol,leff¼13.6lB,andHCW¼À519Kunder eldH¼100Oe.Selectedinversesusceptibility(1/v)–Tcurvesunderdifferent eldsareshowninFig.2(a),andthe t-tingresultsareplottedinFig.2(b).

The ttingandcalculationsindicatethatthissystempresentsunusualelectronicandspinfeatures,includingalargeeffectivemagneticmoment(leff¼13.6lB)undera eldof100Oe,whichisdoublethemolecular eldtheoreti-Th

¼[4S(Sþ1)]1/2calMn2þionlocalspinmomentleff

¼5.9lB(S¼5/2forMn2þion).However,theeffectivemagneticmoment(leff)presentsstrong elddependence;forexample,leffdecreasesfrom13.6lBundera eldof100Oeto5.9lBunder elds>20kOe,asshownintheinsetofFig.2(b),whichpresentsthenon-linear eld-polarizationcharacterizationofthe2Delectronsystem.17Also,

C2012AmericanInstituteofPhysicsV

0021-8979/2012/111(7)/07E144/3/$30.00111,07E144-1

07E144-2Zengetal.J.Appl.Phys.111,07E144(2012)

layersthroughtheinsertionofH2OandthebreakingofS-Sbonds.Theabsenceofthermalanomaliesisconsistentwiththeabovementionedmagneticsusceptibilitymeasurements.Con-sideringtheupturninCP,inordertomoreaccuratelyevaluatetheconductionelectronspeci cheatcoef cientc,wedirectly ttedthelow-temperatureCPdatatotheformula

Cp¼A=T2þcTþbT3;

FIG.1.(Coloronline)(a)X-raydiffractionpatternwithlinesfromthecor-respondingbulkstandardincludedand(b)SEMimageoftheMnPS3

nanoparticles.

(1)

HCW¼À519K,andthepossiblelongrangeorderingtem-perature<2K,sothesystem’sfrustrationfactorf¼HCW/TC(TN)>259.5,whereahighvalueoffindicatesthatthesystemhasfallenintostrongdisorderedelectronandspincompetinginteraction.

Ouranalysisofv,theheatcapacity(CP)measurements,andWilson’sratio(RW,alsoknownastheSommerfeldratio)supporttheconjecturethatthepossiblespin-liquid-likeabnormalspinbehaviormightemergefromthissystem.AsshowninFig.2(a),whenthetemperature<8K(100Oe),vhasalinearrelationshipwithÀlog(T),andallstraightlinesunderdifferentapplied eldsarenearlyparalleltoeachother,withthe eldonlyslightlychangingtheslopeoftheline.Thelog-logplotsoftheCP/T–Tcurvesunder eldsfrom0to9TareshowninFig.2(c),andthesemi-logplotsarepresentedintheinset.Thecurvesdonotshowanyanomalouspeaks,buttheydoshowaslightupturninthelowertemperaturerange.Theseresultsagainindicatetheabsenceoflong-rangeorderingovertheentiremeasuredtemperaturerange.Theupturnappearingatthelowesttemperaturesmaybeattributedtoanu-clearSchottkycontributionoftheMncationsinthecounter-anion,mostlylikelytheSanions,becausethesystemconsistsofmulti-layeredparticlesbytherestackingofmonomolecular

wherethevaluesforthecoef cientsareA¼0.25mJKmolÀ1,c¼156mJKÀ2molÀ1,andb¼2.3mJKÀ4molÀ1forH¼0T.Similarly,wedirectly ttedtheCPtemperaturedependencecurvesunder eldsfrom0to90kOe,andtheresultsareshowninFig.2(d).Wilson’sratioisde nedasthedimensionlessquantity

2

vð0Þ=3ðglBÞ2c;RW 4p2kB

(2)

FIG.2.(Coloronline)(a)Semi-logplotsof1/v–Tcurvesundertheindi-cated elds;theinsetisthetemperaturedependenceofthemagneticsuscep-tibility(v)andinversesusceptibility(1/v)underzero eldcooling(black

symbolline)and eldcooling(redsymbolline)inanexternalmagnetic eldH¼100Oe(v–Tandinverse1/v–Tcurve).(b)Susceptibility ttingresultsathightemperaturerange(T>200K).(c)Log-logplotsofCp/TvsTinthelow-temperatureregionunderdifferent elds;semi-logplotsareincludedintheinsets.(d)v(0)%v0(T!0K), niteelectronicheatcapacitycoef cientc,andWilsonratioRWdeterminedbyusinglowtemperaturedatafrom2.1Kto10K(seetext)asafunctionof eld.

wheregisthegyromagneticratiointheabsenceofinterac-tions,kBBoltzmann’sconstant,lBistheBohrmagneton,andv(0)istheresidualmagneticsusceptibilityattempera-tureT!0K,e.g.,v(0)¼v(T!0).Foranon-interactingFermigas,RW¼1.WilsonshowedthatfortheKondomodel,theimpuritycontributionstov(0)andcgiveauniversalvalueofRW¼2,independentofthestrengthoftheinterac-tions.18,19ExtrapolationofthelineartemperaturedependenceoftheinversesusceptibilitydowntoT¼0Kvialine ttingsimplygivesaresidualmagneticsusceptibilityof1/v(0).Thev(0)valuesarecalculatedfromtheabovedataon1/v(0),andtheresultsareshowninFig.2(a).TheWilsonratioRWcanbedeterminedaccordingtoEq.(2),andtheresultsareshowninFig.2(d).AsFig.2(d)shows,cisnotseriouslyaffectedbymagnetic eld,butitslightlyincreasesasthe eldincreasesupto3T,andthenitslightlydecreases.RWhasitsminimumvalueatthe eldH¼3.0T,indicatingadecreasingelectron-electroncorrelation,becausethelargetemperatureindependentmagneticsusceptibilityv0isintherangeofthefreeelectronsystem.ThesevaluesofRW(0.6to2.16)implyaproperscal-ingofv(0)andsuggestthatcexistsintheFermiliquidstate.

Thedegeneracyoftheenergystatesofdisorderedabnor-malspinsshouldgiverisetogaplessexcitationsorcontinu-ouslygappedexcitations,whichareverybene cialforthemagnetocaloriceffect(MCE)because(i)theymakethespinsinthesystemeasiertoalignunderexternalmagnetic eld,andthelowlyingenergyandevengaplessspectrumcreateahighentropygroundstate,andthisplusthelargespinpoten-tial(S¼5/2)oftheMn2þionsmakeitpossibletoachieveveryhighMCE;and(ii)thetheoreticalmomentis5.9lB/f.u.fortheMnPS3system,basedonthelocalmagneticmomentmodel,andthegreatestpossiblemagneticentropychangeÀDSMwouldbeÀDSM¼NkBln(2Jþ1)%83.15J/kgK,whereNisthenumberofspinsandJisthequantumnumberofthespin.ThevalueofÀDSMindicatesthatthesystemmayachievealargeMCEinthelowtemperaturerange.

TheisothermalmagnetizationcurvesfortheMnPS3nanoparticlesinthetemperaturerangebetween2.7and24Kunder eldsofupto9TareshowninFig.3(a).Arrottplot(notshownhere)analysisindicatedthatthesystemdidnothaveanytransitions,onlyspin uctuations,which

con rmed

07E144-3Zengetal.J.Appl.Phys.111,07E144(2012)

FIG.3.(Coloronline)(a)M-HcurvesforMnPS3nanoparticlesattempera-turesfrom2.7Kto24K.(b)ÀDSMattemperaturesfrom2.85Kto23Kformagnetic eldchangesfrom0–1Tupto0–9T.(c)ÀDSMattemperaturesfrom2.85Kto23Kformagnetic eldchangesof0–1,1–2,2–3,3–4,4–5,5–6,6–7,7–8,and8–9T.(d)DTadattemperaturesfrom2.85Kto23Kformagnetic eldchangesfrom0–1to0–9T,withafullM-Hloopat2.7Kpre-sentedintheinset.

thevandCPresults.AccordingtoRef.[20],themagneticentropychange(ÀDSM)canbecalculatedasafunctionoftemperatureformagnetic eldchangesfrom0–1to0–9T,andtheÀDSMresultsaredisplayedinFig.3(b).Thecurvespresentacharacteristicshapewithnopeakovertheentiretemperaturerange,indicatingthatthereisnotransitionandnolong-rangeorderingofthiselectron/spinsystem.Themagni-tudeofÀDSM,atitsmaximum,increaseswithincreasingmagnetic eldupto4.5,9.8,and12.8J/kgKat2.85Kunder eldsof2,5,and9T,respectively.Themagnetizationhyster-esisloopat2.7K(inFig.3(a))indicatesthatthereisnohys-teresislossandalsoindicatesfullyreversiblebehavior.

Becausetheheatcapacitystronglydependsontheexternalmagnetic eld,especiallyinthelowtemperaturerange,20inordertorelativelypreciselycalculateDTad(T,H),were-calculatedtheÀDSM(T,DH)(DH¼0to1,1to2,…,8to9T)foreverymagnetic eldchangeof1T,andtheresultsareshowninFig.3(c).WethencalculatedtheDTad(T,DH)accord-ingtoRef.[20],usingthemeasuredCPdataunderrelativelydifferentexternal elds.TheDTad(T,H)dataunderexternal eldchangesof0toHcanthenbeobtained,asshowninFig.3(d).Themaximumadiabatictemperaturechange(DTad)¼2.8,5.3,and8Kat2.85Kfor eldchangesof2,5,and9T,respec-tively.TheMnPS3nanoparticlesshowveryhighDTad,whichsatis esoneoftheimportantcriteriaforselectingmagneticrefrigerants(i.e.,alargeadiabatictemperaturechange).

ThevaluesofÀDSMobtainedfortheMnPS3nanoparticlesystemareverylarge,withnomagnetic/heathysteresis,andtheM-Hloopat2.7K(asshownintheinsetofFig.3(d))indi-catesfullreversibility.ÀDSMiscomparablewiththevaluesforanyreportedparamagneticsaltsornanosizedgarnets,21andforwelldesignedmolecularnanomagnets22,23forultra-lowtemperatureapplication,whichexhibitsecondorderor rstorderparamagnetictoferromagnetic(orantiferromag-netic)phasetransitionsatafewdegreesKelvin.Furthermore,theÀDSMandDTadofthepresentmaterialarehigherthanforthosematerialsoverthewholetemperaturerange(2to20K).

ThesepropertiesmakeMnPS3nanoparticlesapromisingcandidateasarefrigerantatultra-lowtemperatures.

Thecrystalstructureofsinglecrystalhasbeencharacter-izedverywell1;themagneticlatticehasbeenmodeledasMn2þhoneycombarrangedantiferromagneticinteractionintra-planarandferromagneticcouplinginter-planarwithanangleof107.35 .2Thehoneycomblatticeitselfmightexhibitahighlyfrustratedstateandevenanemergentspin-liquidstate.16,24Therestackedsinglelayerscausedistortionsinthecrystalstructure15thatmightvarythespinvectorsinthehoneycombspinlattice;giverisetohighlyfrustratedspinstates;and/orgiverisetodelocalizedspins,freeelectrons,andcompetitionduetointeractionsbetweentheseexoticstates,leadingtotheemergenceofabnormalmagneticproperties.

Insummary,microstructureanalysisindicatedthatthenanoparticleswereformedbytherestackingofmolecularlayersthatwerestabilizedviaH2Oinsertionbetweenthelayers.Long-rangespinordering,whichexistsinthebulkmaterial,iscanceledwithintherangeof2Kto300K.Sus-ceptibility(v)measurementsandheatcapacityresultssug-gesttheemergenceofalow-lyingenergyspinstateinthissystem.TheactualMCEevaluationindicatedthatthesystemhasalargereversibleÀDSMof6.8and12.8J/kgKandDTadof3.8Kand8Kat2.85Kfor eldchangesof3Tand9T,respectively,andÀDSmmonotonicallyincreaseswithdecreasingtemperature,whichindicatesthatMnPS3innano-particleformisapotentialcandidateforapplicationinmag-neticrefrigerationintheultra-low-temperaturerange.TheauthorsthankDr.T.Silverforherhelpandusefuldis-cussions.ThisworkissupportedbytheAustralianResearchCouncilthroughaDiscoveryproject(projectID:DP0879070).

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