j. sort et al- magnetic hardening induced by exchange coupling in mechanically milled...

8/3/2019 J. Sort et al- Magnetic Hardening Induced by Exchange Coupling in Mechanically Milled Antiferromagnetic-Ferromag

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.MATERIALS RESEARCHSOCIETYSVMPOSIUM PROCEEDINGSVOLUME 581

N anophase andN anocompositeMaterials IIISymposiumheld November 29-December 2, 1999,Boston, Massachusetts. .s.A.

Sridbar KomarneniThe PennsylvaniaState UniversityUniversity Par\,Pennsylvania,U.S.A.Jobo C. ParkerCirQon TechnologiesCorporationQumce, llinois, U.SA

Rorst RaboDannstadt University ofTechnolqyDannstadt, Germany

[M1RlliIResearchWarrendale, PennsylvaniaMaterlals

EDITORS:

Soclety


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Manula'hlred In Ihe ll'llted S.".es ,,1 An":,..~OO.O~I7S0


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~ ,,~~!! j ~MAGNETIC HARDENING INDUCED BY EXCHANGE COUPLlNG INMECHANICALL Y MILLED ANTIFERROMAGNETIC - FERROMAGNETlCCOMPOSITES

J. SORT, J. NOGUS. X. AMILS. S. SURIIi)ACH. J.S. MU1iloz,~. BARDcpartament de Fisica, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainABSTRACT

Mcchanical milling has been uscd to synthesizc fcrromagnctic (FM. Co) .antiferromagnetic (AFM. NiO) compositcs. The coercivity, He, and energy producto BHMa. ofthese compositcs can be enhanced at room temperature alter appropriate heat treatments abovefue Nel temperature of the AFM, T N. Although fue maximum He is achieved rOl fue(NiO)\: \ (Co) weight ratio, BHwu is further enhanced rOl fue (NiO)2:3(Co) ratio, where highersaturation magnetization is obtained due to fue larger amount of FM. Exchange coupling,rcsponsible rOl fuese effccts, decreasesas fue temperature s increased and vanisbcs clase to T N.The thermal stability of fue coercivity enhancement emains rather insensitive to fue somewhatbroad distribution ofblocking temperaturesofthis system.INTRODUCfION

Sincemecbanical lloying (MA) was developedn fue ate 196081], Ibis tecbnique asbeenusedas a processing oute for fue syntbesis f a large variety of equilibrium and non-equilibriumphases nd pbascmixtures 2]. This tecbnique asbeenshown o be successfulnreducingcrystallite szel to fue nanometric ange. Thesesmall particles can exhibit ratherdifferentpbysicaJ ropertiesrom tboseoftbeir bulk counterpart3].Mechanical alloying has aIso been used lo produce direct or indirect magneticinterxtions. tike in excbange pring magnets milling 8Oft-barderromagnets)405]or jantmagnetoresistanceaterials milling magnetic:-nonmagncticbascs) 6]. However, studiesofexchangenterxtion between erromagnctic nd antiferromagncticmaterials nducedby ballmilling are scarce.Nevertbelesa,ecently ue possibility of incrcasing ue room temperaturecoercivityby mecbanicaJlloying ransitionmetals Ni, Co, Fe) and bar own antiferromagncticoxides NiO or CoO) or sulfides FeS) has beenproven 7]. Moreover, t is weJl known tbatoxidized erromagncticine particlescan al80 exhibit coercivity enbancements8]. However,ibis enbancements mainly observed ar below room temperature, hich maleesbis propertynot useui for applications.t is notewortby hat coercivity ncreases ue o AFM-FM exchangecouplingarealBO eingstudied n tbin film systems9,10].In Ibis papelwe show be possibility of increasingbe room emperature oercivity andenergyproductby ball milling FM and AFM composed f different ransitionmetals.e.g. Coand NiO. after adequatemagnetic ield heat treatments. he bardeningof the FM phase soptimized y varying be AFM:FM rabo.EXPERIMENTAL

Different AFM:FM weight ratios (0:1, 3:7,2:3, 1:1 and 3:2) of gas - atomized powdersofNiO (99%.


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times (0.1-30 hours), at SOO pm, using a planetary mili. To induce excbange coupling, the asmilled powclers were annealed at difTerent temperaturea,T ANN 300 - aso K, rOl difTerent times(Q-S hours) under VICUum, in (be presente of difTerent magnetic fielda (O - 10 kOe), and fieldcooled to room temperature.The microstructure f tbe powderswas studiedusinl standard -ray difTrlction (XRD)(Cu-K. radiation). The x-ray difJ'rlction peaks were decoavoluted lDd fitted usin& a pICUdo-Voigt function. The cryatallite size, , wu evaluated usinl a single peak method fi'om itsintegral breadth (Cauchy component), following Scherrer's approximation. Their morphololicalcbaracterization wu performed by using scanning electroa microscopy (SEM), equipped withenergy dispersivc x-ray anaIysis (EDX). Mapetic bystcresis loops, up 10 10 kOe, weremeasurcd n loosely paclted powdcrs, at temperaturesbetwcen room temperature and 7S0 K, bymcans of vibratias samplc magnctomctry (VSM).RESULTSANO DISCUSSION

Shown in fig. l(a) ia a SEM image (secondary electron) Cor he NiO-Co powdera ballmillcd Cor20 boura in a weigbt rano oC 1: . SaU milling indUCCI broad range oCparticle sizes,fiom 1 .un o 30 .un 11). Tbe larger particles (an mlargement oCODe CtbCIepar1ides is shownin fi.. 1) are composed oCabout 1 .un thick lamellae, lUJ1'OUDdcdy small particles. M can beseen n fig. 1(b) the Co x-ray mapping indicates that the lamellae (brigbt in fig. l(a correspond10 Co wbile the par1icles (gray in fig. 1(1 correspond lo NiO. This microslnJdure, typicaJ oCball milled ceramic-metal mixtures, il due lo the ductile md brittle character oC Co and NiOrespecveJy. Moreovcr, tbil miCl"Oltructure allOWI the exiltence oC a large interface arcabetween he FM md tbe AFM pbues.-- - - - --------

(a)FIC. 1 (a) SEM secondary c1cctron mage lIId (b) Co EDX mappin for I(NiO)l:l(Co) bal1 mi11ed or 20 h.

X-ray difTr8ction (XRD) pattems baye becn obtaiDcd for tbc IIDIIIJedCo and NiO andfor tbe 20 h ball milled NiO-Co in a weight rabo of 1:1, before 8Dd aftcr field IllDaling them ina S kOe magnetic fie1d at T ANN 600 K. The XRD pattcms of the unmilled Co revea1 hat itcorrespondI lo a mixturc of bcp aod fee Co, wbile the unmilled NiO powcIer difl'ractjon peaksconapond to I fee phaae. The cryataIlite &iza. evaluated for the Co bexaonal (002) end theNiO (111) peab, are Co,Ha 42 :t 4 nm 8Ddt.o > 100 nuI, respectiYe1y11 ).Wben Co and NiO powden are balI milled topther in al: 1 weight rano the intensity ofthe peaks corresponding lo the fee Co phue rapidly decreues. Tm. allotropic IrInsfonnationhas been previoualy reponed for ball milled Co powden [12). The peak widths oribe differentphues aftcr ball mining sbow en important inc:reue. Tbis iI re1ated o tbe c:rystaIlite lilOreduction. the introduc:tion of p1anardefectl and the microslrain increue. The crysta1lite &izes,after 20 bours of milling, estimated from the (002>c..H.. 8Dd the (111)..10 reflcctions, are:c.,Ha - lO :t 2 nm and N;o - 14:t 2 Dm, respectiyely. ID Iddition. the poIitions of the

~2J.U11b)

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NiO peaIa do not changeaftcr ball-mimng, indicating no importantvanabon n its !articeparameter. his suggestshat ue ditTusion fCo joto fue NiO uU is vay small 11].Whcn the hall millcd NiO-Co powdcrs are annealedat TANN- 00 K for O.S ours he x-ray diffractionpattem s vay similar o fhat of the as millcd powden,whcre ue NiO pealcs renot displacedand only a minar decreasen the pcalt width is obtained.Thc c:rystal1iteiucalculatcd from the (11 . 'O reOection is wiO = 18 :t: 2 lU11. Tbe Co-hcp phase shows asimilar behavior, with co.He. 18:t: 2 nm, calculatcd from thc (002>c peak. Moreovcr, atthis anncaling tcmperature, a slight increasc in the intcnsity of the (200)c.,cubC eak can aIrcadybe obscrvcd, indicating the starting point for an allotropic traIIsfonnation from bcp to hcp+fee[12). Howcvcr, aftcr anncaIing at TANN 740 K for O.S hours, the crysta11itcsizcs. ca1culatcdfrom (IOO)co,Ha nd (1 I 1}N;opca1cs,ncrcase significantly to co,Ha- 44:t: 4 lU11 and wiO... 40 :t 4 nm and the intcnsity of the pcaIts corrcsponding to the cubic phase incrcascs. Tbus,anncaling at T ANN 740 K induces an important crystal growth and the allotropic phasctransformation from hcp to fee Co [1 1J.The room-tcmpcrature cocrcivity, He, has becn mcasurcd as a fimc:tion of tbe millingtime for the diffcrcnt NiO:Co wcight rabos for botb as-millcd and annealcd (T ANN .. 600 K)powden. In pure Co, He incrcascs ftcr a sbort mimng time (1 hour) from He - 170 Oc (asobtaincd) to 290 Oc (1 hour hall millcd) [13]. This increase s attributcd 10 he aIlotropic phasetransformaban from fee+bcp Co 10 bcp Co, where hcp Co is known 10 bave largcr He than eeCo [14). Furthcr mil1ing dec:rcases e (e.g. He ... 125 Oc after milling 30 hours), whic:b isprobably linlccd witb thc disordering of the hexagonal structure. as has been reportcd for long-terro milling [12). A maximum in He is liso observcd for bal1 millcd NiO:Co. Howcvcr, forcach AFM:FM rabo, tbc largest He is obtaincd or diffcrent milling times.which increase s ueNiO content incrcases, ndicating fhat NiO probably delaya hc Co atructura1 hanga.ADneaJjng bc hall miUcd pure Co powders at T ANN 600 K (i.c. T N < T ANN T c) for O.Shours in H ... S ItOc resulta in a slight decrease in He. Howcvcr, a significant enhanc:ement of Heis observcd aftcr annea1ing ue hall millcd NiO-Co powdcrs at thc ame tcmpcraturc, T ANN600 K, and field cooling (H ... S tOc) to room tcmpcrature [13). Shown in fig. 2 is thcdependenu of fue cocrcivity enbancement (defincd as the differenc:e of He aftcr and beforeannealing at TANN,MIc) on tbc annea1ing emperature for (NiO)I:I(Co) hall millcd 20 h. As canbe leen in fue figure, thc room-tcmpcrature He inc:reases nly when the anncaling tempcrature sIargcr than fue NiO Nel tcmpcrature (i.e. T N - S9O K), as expectcd for an exc:bange couplcdFM-AFM system [9]. The decreasc of AMe aftcr annealing at excecdingly high tempcratures(T ANN> 700 K) is duc to the aIlotropic pbase traIIsformation from fcc+bcp Co to fee Co, as80

60~ 40....U~ 20O-20

300 400 Soo 600 700T ANN [K]Fle- 2 Dependenceof fue cocrcivity enbancement,MIc, on fue annealingtempcrature for a (NiO)I:I(Co) hall millcd for 20 h.

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ooscrveorom XJU). Loop stults In tbe field axis, HE, towards tbe neplive field axis are aJsopreaent n the hystcresis loop. orIbe hall mllcd NiQ-Co limpies for all AFM:FM rabos, exceptin pUle Co. The loop shifts depend on tbe anncalinl tempcraturc witb a similar dcpendence uMIc, confirming tbe assumption that tbe exchange coupling betwecn Ihe AFM and tbe FM isresponsible or botb efTccta,6Hc andHE.Exchange couplinl produces not only an cnhanccmcnt of He but liso an enhancementofthe squarencss,Ma/Ms [13], and consequcntly an increue of the energy product, Bn Thedependencc of HE, Mfc and .mH,..u (defincd u !be difTerence of BHMu after and beforeannca1ing al T ANN - 600 K) OD be Co content (NiO:Co ratio) is shown in fil. 3. The valucsshown n the figure corrolpOndo tbe milling timeswhich give the maximumvalueaof He foreachNiO:Co raUo. A. can be scen n thc figure, tbe 1arestexcbange oupling efTec:ts,.e.maximumcnhanc:ementIf He and BH,..u and maximum oop shifts, are exhibitcd for fue(NiO)I:l(Co) rab~. However, hc maximumvalueofBn after annealing t TANN 600 K isnot obtained or (NiO)I:l(Co) (8H, - 62000 G.Oc) but for (NiO)2:3(Co) 8HMu - 83000G.Oc).This s due o thc interplaybetwc:cn e andMs ODBn The ddition of ID AFM andthe Ubsequent cat reatmentl neRMeHe, howcver he AFM alsopromotes rcduction n theovcra11 s of the composite.Henee. he cnh.ancemcntf Bn is a compromise etwecn otheffccts. 30~ 0 ~ a)0:( O

60~ 40 (b)....i2:-20J~ ~ (e)~ ~ ~~ -4 40 50 60 70

3:2 1:1 2:3 3:7%CoFIl. J DependencerIbe (a) exchange ias.H, (b) c:oercivity nhancement, Ic.

and e) euhancemeutfthe energy roduct,6(BH)w., ODhe Co percentage(NiO:Co atio).Shown n filo 4 is fue depcndcnce f fue loop lhift on !he meuuring temperat\u'e.expcctcd rom exchange oupledAFM-FM 5)'ItemI. He dec:reuesM measuringemperatureincreasea, ecoming zero at T . 600 K. i.e. at !he N6el temperature 'f the AFM (T N (NiO) - S90K). A similar behavior is obaervcd for MIc and 6(BH)w... The dccreascof fuese propertiea withincreasing mcasuring temperature s the rcsult of a reduc:tionof tbe AFMIFM interface coupling,

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as a conscquenceof eithcr the 1011 f the magnetic arder in the AFM andIor the rcduction of theAFM magnetocrystalline anisotropy. The critical tempcrature at which exchange couplingeffects completely vanish is usually denotcd u the blocking tempcrature, T 8 [9].To study the thermal stability of fue hall millcd NiO:Co systems due to fue concomitantdistribution of blocking tempcratures [15) (brought about by the distribution of crystallite sizcsand defccts), fue NiO:Co sIMples wcrc warmcd to a tempcrature T o. 300 K < T o < 700 K, andsubscqucntly coolcd to room tempcrature in a field H - . 5 kOe, Le. opposite to the one uscd toinduce the original coupling (H - 5 ItOc).

~....:lFIl:. 4 Temperature dependenceafilie exchangebias, HE (8), on fue measuringtemperature, and HE (O) after field cooling in H - -S kOe from ditTerent

temperatures.Following Ibis proccdure, if tbe local bloclciDg cmpcrature of a crystallite (T 8i) is belowT o. tbc crystallitc becomes paramagndC:, bus losing tbe informaban of tbc origiDal c:oupling.Cooling suc:hcrystallitc in H - - 5 ltOc induces a c:oupling oppositc lo thc original onc, henc:cshifting tbc loop towards positw fields. However, crystallites witb T 81> T o remain IlDChInged.Consequcntly, tbc HE obtained after warming lo cach T o ives a measurc of tbe numbcr ofcrystallitcs witb Tli < To [15]. M can be secn n fig. 4, alrcady at To - 350 K a reduc:tion n HEcan be observed. Moreover, CorTo - 500 K HE is reduc:ed o zero (i.c. SO% oftbe c:rystalliteshavc T 8i < 500 K). Thesc resulta iDdic:atca broad distributiOD oCbloclcing tcmperatures in Ibissystcm. However, altbough tbe tbermal stability oCHE is stroDgly iDflucnc:edby tbe distributionof TI, wc observed that He is ratber nscnsitivco it. This is probably bec:auscaltbough tbeovera11 HE dependa on tbc strength and sign of tbe c:oupling for eac:h crystallite, tbcenhanc:cmcnt f He dcpcnds only on tbe strength of tbe c:oupling.

CONCLUSIONSIn conclusion, we haye shown that mechanically miUed AFM(NiO) - FM(Co)

composites display significant enhancements of the room temperature coercivity and energyproduct due to the exchange coupling induced after heat treating the samples at T ANN T N undermagnetic fields. The as-milled powders exhibit a peculiar microstructure (Co lameUaesurrounded by refined NiO particles), which allows the existence of a large interface arcabetween the FM and the AFM. The maximum magnetic hardening is shown to be a trade offbetween the coercivity enhancement produced by exchange coup1ing and the oyerall reductionof saturation magnetization due lo the antiferromagnet. All the efi'ects (He, HE and BH )decrease with increasing temperature and the NiO-Co samples become uncoupled when the

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measuringemperaturea larger than the antiferromagnetic el temperature. lthougb HE sstrongly nf1uenccd y the distributionofT.. He remains ather nsensitiveo it.ACKNOWLEDGEMENTS IThis work wu supported n put by DGESBICunder contraetMA T98-0730and byCICYT undercontrlct PBTRI9S-031l-0P. .S. thanb the DGU for bis fellowship. .N. IJIdX.A. ICknowledgehe SpanishGovemmentor ita financia! upport.

REFERENCES1.1. S. Benjamin,Metan.Tnns. 1,2943 1970);Mater.Sci. Forum88-90,1 1992).2. For a rcvicw, ce B. S. Murty andS. Ranganathan,ntcr. Matcr.Rcv.43, 101 1998).3. For a rcvicw, CCR. W. Siegel,Nanostruc:lMatcr.3, 1 (1993).4. P. G. McConnic:k.W. R. Miao, P. A. l. Smith, J. Dio. R. Street.1. Appl. Phya.83, 6256(1998).5. K. Ravipruad,M. RuuJeoabi ndM. Umemoto, . Appl Phys.83, 921 1998).6. A. J. Farsan,M. Vire( andJ. M. D. Coey,J. Phys:Cond.Maner.7, 8953 1998).7. D. S. Gcogbeglll,P. G. McConnidt andR. Stred, Mat. Sci. Fonun179-181,6291995).8.W.H. MeildejohnandC. P. Bean,Pbys.Rev.101,1413 1956);105,904 1957).9. For. review, ee J. Nop and van K. Scbuller, . Map. Map. Mater.192.203 1999).10. A. S. Edclstcin, R. H. Kodama, M. Millcr. V. Browning. P. Lubitz, S. F. Cbeng IIId H.Sicbcr, Appl. Phya. Lctt. 74, 3872 (1999); Y. J. Taang. B. Roos, T. Mewes, S. O. Dcmokritov,B. Hillcbranda and Y. J. Wang, Appl. Phya. Lctt. 75, 700 (1999).11. J. 80Ft, J. Nogu~, X. Amils. S. Suriach, J. S. Muftoz lJ1d M. D. Bu, Mat. Sci. Forum(2000) in prea.12.1. Y. Huana. Y. K. Wu lIIId H. Q. Ye. Appl. Pbys. Lett. 66, 308 (1995); F. Cardcllini lIIId G.MuzoDC,Pbilos.Mag. A 67,1289 (1993).13.1. Sort. J. Nogu~ X. Amils, S. Suriftach, J. S. Muftoz and M. D. BU',Appl. Pbys. Lett. 75,3177(1999).14. H. Sato, O. Kitakami, T. Sakurai, Y. Shinwla, Y. Otani and K. Fubmichi, J. Appl. Pbys.81. 18S8 1997).15. S. Soeya,T. Imasawa,K. MitluOD and S. Narishiac, . Appl. Phys.76,5356 (1994); C.TSIIIaandKennethLee,J. Appl. Phys.53. 2605 1982).

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j. sort et al- magnetic hardening induced by exchange coupling in mechanically milled...

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