Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(4)

18756J.Phys.Chem.C,Vol.114,No.44,2010Figure4.(a)Adsorption-desorptionisothermsofmesoporousR-Fe2O3and(b)poresizedistributioncalculatedfromthedesorptionisotherm.

Figure5.Temperaturedependenceofmagneticmomentofmesopo-rousR-Fe2O3forzero- eld-cooledmeasurementsuponwarming(ZFCW), eld-cooledmeasurementsuponcooling(FCC),and eld-cooledmeasurementsuponwarming(FCW).

measurements,however,thereisnoincreaseofthemomentwithtemperatureforT<50K.Instead,themagneticmomentslightlydecreaseswithtemperatureupto50Kfollowedbyafurtherdecreasewithadifferentrate.ThereisnooverlapofZFCW,FCC,orFCWbranchesinanymeasuredtemperaturerange.TheMorintransitionoccursatlowertemperaturefortheFCWandFCCmeasurementsthanfortheZFCWones.ThewidthoftheMorintransitionof～40Kindicatesawidedistributionofthecrystalpropertiesinthemesoporoussamples.TheMorintransitionoccursatMorintemperatureTMbecauseofcompetitionbetweendifferentanisotropytermsinR-Fe2O3asdescribedbyArtmanetal.39AntiferromagneticallyorderedspinsinmacroscopicR-Fe2O3crystalsareorientedalongtrigonal

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[111]axisbelowTM.Thesamplehasnonetmagneticmomentatthesetemperatures.Thespinsare ippedintothebasal(111)planeaboveTMandarealsoantiferromagneticallyordered.However,theyareslightlycantedoutoftheperfectantiferro-magneticalignment.Thiscantedcomponentofthespinscouplesferromagnetically,whichgivesrisetoasmallnetmagneticmomentofR-Fe2O3aboveTM.

Thetransitionbetweentheferromagnetic(FM)andantifer-romagnetic(AFM)stateisthe rst-ordermagnetictransition.AlthoughthebulkfreeenergyoftheAFMandFMstatesarethesameatTM,thereisanenergybarrierassociatedwiththetransitionbetweenthetwostates.ChowandKeffershowedthatsurfaceanisotropiesandtheirinteractionwiththebulkstateshavetobeconsideredtoexplainthesetransitions.40Thesurfacestatesdonotcontributeappreciablytothenetmagneticmomentbecausetheyarecon nedtoathinsurfacelayer.However,theyde nethewaytheenergybarrierisovercomeuponthetransitionbetweenthetwostates.Inthecaseofweaksurfaceanisotropy,thetransitionfromAFMtoFMstateduringwarmingoccursatTMofthebulkspins.However,FMtoAFMtransitionduringcoolingoccursatalowertemperaturethanTM.40Forstrongsurfaceanisotropy,itistheopposite:FMtoAFMtransitionoccursatbulkTMuponcoolingandAFMtoFMtransitionoccursataT>TMuponwarming.

OurmeasurementsareconsistentwiththemodelofChowandKeffer.40ThetransitionsbetweenAFMandFMstatesuponwarmingoccurathighertemperaturesthanuponcooling(Figure5).ThedifferencebetweenZFCWandFCWcouldbeassociatedwithirreversibilitiesofsurfacestates.Magneticanisotropyisde nedbycouplingofspinstothecrystallatticeorder.Intheabsenceofastrongcrystalorder,magneticanisotropydecreases.Becausesurfaceofournanoparticlesisassociatedwithacrystaldisorderthatresultsinunpairedspins,itisreasonabletoassumethatmagneticanisotropyonthesurfaceisweakerthandeeperinsidethenanoparticles.AccordingtothemodelofChowandKeffer,TMisthenobtainedfromthetransitioninZFCWmeasurements.

ThevalueofTMformacroscopicR-Fe2O3is263K,anditdependsonthecrystalsizewhenthecrystalsaresmallerthan～ingthedatareportedbyZysleretal.,41thevalueofTMof209Kforoursamplessuggeststheaveragecrystalsizeofabout20-40nm,whichisconsistentwiththeresultofXRDanalysis.ThesecrystallitesagglomerateintolumpsofafewhundrednanometersinsizeasobservedbyTEMandHRTEManalysis.ThisissimilartothereportonR-Fe2O3particlesobtainedbyreactionof2×10-2MFeCl3and3×10-4MNaH2PO4.41

ThetransitiontemperaturebetweenAFMandFMstatesis eld-dependent,whichwasalsoreportedbyQianetal.42ThetransitiontemperatureinourFCCmeasurementsdecreaseswiththe eldatarateof9K/Tfromitsvalueof191Kat0.01T.This elddependenceoftransitiontemperaturepointstomagneticirreversibilitiesassociatedwithferromagneticorderinginR-Fe2O3thatwasnotdestroyedbywarmingthesampleto305K.ThisisduetoexistenceoftheintrinsicferromagneticorderingaboveTMofArtman-typebutisalsoduetotheunpairedspinsonthesurfaceofR-Fe2O3crystals.Thereisalsointeractionbetweennanoparticlesofagglomerates,whichwilldependonthermalhistoryduetoreorientationofthenanoparticleswithintheagglomerates.

Theotherfeaturesoftheobservedtemperaturedependenceofthemagneticmomentcanbeexplainedintermsofferro-magneticorderingofunpairedsurfacespinsonthetopoftheantiferromagneticmomentofthebulkhematite.Themagnetic