Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

E idence o spin diso de a he su ace–co e in e ace o oxygen passi a ed Fe nanopa icles L. Del Bianco,a) A. He nando, M. Mul igne , and C. P ados Ins i u o de Magne ismo Aplicado, UCM-RENFE, P.O. Box 155, Las Rozas, Mad id 28230, Spain J. C. Sa ´nchez-Lo ´pez and A. Fe na ´ndez Depa amen o de Quı ´mica Ino ga ´nica, Ins i u o de Ciencia de Ma e iales de Se illa, Cen o de In es igaciones Cien i ı ´cas Isla de la Ca uja, A da. A. Vespuccio, s/n. Se illa 41092, Spain C. F. Conde and A. Conde Depa amen o de Fı ´sica de la Ma e ia Condensada, Uni e sidad de Se illa-Ins i u o de Ciencia de Ma e iales de Se illa, P.O. Box 1065, Se illa 41080, Spain ~Recei ed 10 Feb ua y 1998; accep ed o publica ion 1 May 1998! Hys e esis, he mal dependence o magne iza ion, and coe ci i y o oxide coa ed ul a ine Fe pa icles p epa ed by ine gas condensa ion and oxygen passi a ion ha e been s udied in he 5–300 K ange. The esul s a e ound o be consis en wi h a spin-glasslike s a e o he oxide laye inducing, h ough exchange in e ac ion wi h he e omagne ic co e, a shi o he ield cooled hys e esis loops a empe a u es below he eezing a app oxima ely 50 K. © 1998 Ame ican Ins i u e o Physics. @S0021-8979~98!07315-0# INTRODUCTION The s udy o he magne ic p ope ies o ine pa icles has been adi ionally u ged by bo h a echnological and heo e - ical in e es connec ed wi h he possibili y o de eloping a be e unde s anding o magne ic phenomena ela ed o size e ec s. In ecen imes, new me hods o syn hesis ~ine gas condensa ion, laye deposi ion, ul a apid quenching, me- chanical a i ion, ae osol!ha e been used o ab ica e mag- ne ic sys ems wi h cha ac e is ic dimensions on a nanome e scale. Inno a i e nanos uc u ed sys ems wi h excellen so o ha d magne ic p ope ies1,2 o wi h gian magne o esis- ance esponse3ha e been c ea ed, which a p esen play an impo an ole in mode n echnology and esea ch. Tha was also he occasion o a enewed in e es in he ield o magne ism o ul a ine pa icles, p obably in iew o hei po en ial applica ions in high densi y magne ic eco d- ing o as e o luids, which has in a iably led o he disco - e y o unexpec ed magne ic beha io s ic ly depending on he educed dimensional egime.4 A numbe o esea ch wo ks has been published du ing he las ew yea s dealing wi h he magne ic beha io o nanome ic Fe, Co, and Ni nanoc ys alline pa icles, mainly p epa ed by he ine gas condensa ion me hod, and p esen - ing di e en deg ees o su ace oxida ion.5–9 In his a icle, once mo e a en ion has been ocused on he low empe a u e magne ic beha io o oxide coa ed ul a ine Fe pa icles and a di e en explana ion, based on he spin-glasslike s a e o he oxide laye , is p oposed o accoun o he obse ed e- sul s. EXPERIMENT Nanoc ys alline Fe pa icles ha e been p epa ed by e apo a ion o i on in a ungs en boa a 1773 K. The p es- su e o gas phase du ing e apo a ion was 133 Pa ~1 To ! helium. Th ough in e a omic collision wi h he ine gas a oms, he e apo a ed a oms lose kine ic ene gy and condense as an ul a ine powde ha accumula es on a cold inge .10 The passi a ion was achie ed by dosing oxygen ~266 Pa o 10 min!be o e opening he chambe o ai . The loose powde was smoo hly s ipped o and s o ed in ai . Figu e 1~a!shows a ansmission elec on mic oscope ~TEM!mic og aph o he passi a ed ul a ine powde , which has been ca ied ou h ough a Philips CM200 mic oscope wo king a 200 kV. Fo he TEM examina ion he powde was p e iously dispe sed in e hanol by sonica ion and d opped on a con en ional ca bon coa ed coppe g id. The ma e ial consis s o nanome e sized pa icles nea ly sphe i- cal ~15–40 nm in diame e !p esen ing a co e–shell s uc- u e. The da k inne co e, isible in he mic og aph, co e- sponds o me allic i on, whe eas he su ounding ligh laye is expec ed o be he oxide phase. X- ay di ac ion ~XRD!analysis o he as-p epa ed ul- a ine powde @Fig. 1~b!# e eals he p esence o pu e a -Fe and some small and e y b oad ea u es a he posi ion o he peaks o g -Fe2O3and Fe3O4~i is no possible o di e en- ia e be ween hese wo phases by XRD as hei la ice pa- ame e s a e e y simila !. These small peaks can be due o a poo c ys alliza ion o hese compounds in he hin passi a- ion coa ing. The p esence o oxide species a he su ace o he passi a ed nanoc ys alline powde s was also ully con- i med by x- ay pho oelec on spec oscopy.11 The magne ic beha io o he ul a ine powde has been in es iga ed h ough a comme cial semiconduc ing quan um in e e ence de ice ~SQUID!magne ome e supplying a a!Au ho o whom co espondence should be add essed; on lea e om: Di- pa imen o di Fisica, Uni e si a `di Bologna and INFM, Viale Be i Picha 6/2, 40126 Bologna, I aly. JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 4 15 AUGUST 1998 21890021-8979/98/84(4)/2189/4/$15.00 © 1998 Ame ican Ins i u e o Physics Downloaded 20 Feb 2003 o 192.167.227.94. Redis ibu ion subjec o AIP license o copy igh , see h p://ojps.aip.o g/japo/japc .jsp magne ic ield up o 55 kOe and ope a ing in he 5–300 K empe a u e ange. RESULTS AND DISCUSSION In Fig. 2, he empe a u e dependence o he magne iza- ion, measu ed a an applied ield H550 kOe is shown. Full squa e cu es ha e been measu ed a e ze o- ield cooling and open ci cles co esponds o he ield cooling ~a 50 kOe! p ocedu e. Up o app oxima ely 30–50 K, he magne iza ion in he ze o- ield cooling cu e is seen o ise and hen de- c eases egula ly up o oom empe a u e. In Figs. 3~a!and 3~b!, hys e esis loops measu ed a a empe a u e o 5 K acco ding o he con en ional ze o- ield cooled ~ZFC!and ield cooled ~FC!me hods a e displayed. The ield applied a 300 K o he ield cooling p ocedu e was o 20 kOe. I has been obse ed ha he ZFC loop is symme ic abou he o igin, whe eas a e ield cooling, a shi o he loop is expe ienced. The measu emen s ha e been epea ed a di e en measu ing empe a u es Tm, ol- lowing he same p ocedu e. In Fig. 4, he coe ci i y (Hc)o he FC loops ~cu e a!and he shi ~cu e b!be ween ZFC and FC loops is epo ed as a unc ion o Tm. I is no ewo - hy ha he loop shi dec eases wi h inc easing Tmand an- ishes nea 50 K, namely whe e he maximum in he magne- iza ion agains he empe a u e cu e is obse ed ~Fig. 2!. The coe ci i y also dec eases ab up ly wi h empe a u e go- ing om abou 1500 Oe a 5K o200Oea oom empe a- u e and ollowing a empe a u e dependence simila o he loop shi . I is well known ha a shi o a FC hys e esis loop may be o igina ed by an exchange aniso opy e ec de e mined by an exchange coupling be ween a e omagne ic and an an i e omagne ic ~o e imagne ic!phase in in ima e con- ac so ha he exchange in e ac ion can p opaga e h ough he in e ace.12 A necessa y equi emen o he es ablish- men o exchange aniso opy is ha he ield cooling is pe - FIG. 1. TEM mic og aph ~a!and x- ay di ac ion spec um ~b!o he oxy- gen passi a ed ul a ine Fe powde . FIG. 2. Ze o- ield cooled ~ ull squa es!and ield cooled ~open ci cles! em- pe a u e dependence o magne iza ion o he ul a ine Fe powde ~applied ield H550 kOe!. FIG. 3. Hys e esis loops measu ed a Tm55 K a e cooling in ze o ield ~a! and a e cooling om oom empe a u e in a ield H520 kOe ~b!.In he inse : cen al pa o he loop. 2190 J. Appl. Phys., Vol. 84, No. 4, 15 Augus 1998 Del Bianco e al. Downloaded 20 Feb 2003 o 192.167.227.94. Redis ibu ion subjec o AIP license o copy igh , see h p://ojps.aip.o g/japo/japc .jsp o med ac oss he Ne ´el empe a u e o he an i e omagne ic. Owing o he pa icula s uc u al con igu a ion o he Fe pa icles, an exchange aniso opy e ec was expec ed. How- e e , he magne ic o de –diso de ansi ion empe a u es o g -Fe2O3(;1020 K) and Fe3O4~858 K!, namely, he Fe ox- ides iden i ied by XRD, and also o FeO ~198 K!a e consid- e ably highe han he empe a u e abo e which he loop shi disappea s. On he o he hand, u he e idence o he only neces- si y, in o de o obse e he shi , o go h ough he c i ical empe a u e o 50 K du ing he ield cooling p ocedu e, has been ob ained by pe o ming a FC loop a 5 K, aking ca e o apply he ield a 60 K. In his case oo, a shi o he FC loop o he same en i y as in Fig. 3~b!has been induced. We p opose ha he shi o he hys e esis loop is de e - mined by an exchange in e ac ion be ween he e omagne ic co e o he ul a ine pa icles and he oxide su ace wi h spin- glasslike magne ic beha io and eezing empe a u e T 550 K. In his hypo hesis, he applica ion o he ield a T highe han T o ces he magne iza ion o he pa icle co e o lie along he ield di ec ion. Due o he exchange coupling wi h he co e spins, he spins a he oxide laye assume a p e e en ial o ien a ion which becomes ozen below T .In u n, below T , he p esence o he su ace ozen spins a- o s he e omagne ic co e o he pa icle being magne ized in he ield cooling di ec ion, esul ing in a shi o he hys- e esis loops.13 One cha ac e is ic o a spin glass is o possess mul iple s able con igu a ions o he ozen s a e. The applica ion o a s ong ield, a a empe a u e lowe han eezing, can o ce he spins o align along he ield di ec ion. As a ma e o ac , he inc ease o he magne iza ion wi h empe a u e up o abou 50 K ~Fig. 2!can be accoun ed o by in oking he - mally ac i a ed ansi ions owa d he spin con igu a ion ~ a- o ed by he p esence o he magne ic ield!in he oxide coa ing o he Fe pa icles. Acco dingly, in o de o suppo he hypo hesis o a spin-glasslike cha ac e o he oxide laye , an hys e esis loop has been measu ed a 5 K, a e ield cooling om 20 K. In ac , o he abo e conside a ions, i was expec ed ha he ield cooling p ocedu e, e en i ca ied ou s a ing om a empe a u e lowe han T , could selec a spin con igu a ion and lead he e o e o a displacemen o he loop a 5 K. A mino ~wi h espec o he case o ield cooling om 300 K! bu conside able shi o abou 150 Oe has been indeed ob- se ed. Wi h ega d o he empe a u e dependence o he coe - ci i y @Fig. 4~a!#, a simila end ~e en i ela i e o ZFC loops! o Fe pa icles p epa ed in he same way, was al eady obse ed by Gangopadhyay e al.14 and explained h ough a ‘‘shell–co e’’ model based on he p esence, a he pa icle su ace, o an oxide coa ing in he o m o e y small c ys- alli es, supe pa amagne ic abo e a blocking empe a u e o 30–40 K. Howe e , he high coe ci i y alue a 5 K and he s ong dec ease wi h inc easing empe a u e @Fig. 4~a!# may be equally well accoun ed o conside ing ha he p esence o he ozen su ace cons i u es a s ong hind ance o he e e - sal o he e omagne ic phase. Mo eo e , i mus be consid- e ed ha , owing o he small size o he Fe co e, some pa - icles may be supe pa amagne ic a oom empe a u e, which would esul in a u he dec ease o Hc. This las hypo hesis has been indeed con i med by pe o ming an usual ZFC-FC magne iza ion agains empe a u e measu emen a an ap- plied ield H5800 Oe ~Fig. 5!. No e ha his weak ield, in compa ison wi h ha used in Fig. 2, allows us o ob ain in o ma ion abou he possible supe pa amagne ic beha io o he Fe co es, whe eas he esul o Fig. 2 can be co ela ed wi h he magne ic beha io o he oxide coa ing. I is no e- wo hy ha he ZFC magne iza ion in Fig. 4 does no show a de ined maximum and ha he ZFC and FC b anches emain sepa a ed up o p ac ically oom empe a u e. This beha io is compa ible wi h a wide size dis ibu ion o he pa icle co es which, he e o e, en e he supe pa amagne ic egime a di e en empe a u es. Mo ¨ssbaue spec oscopy s udies by Haneda and Mo ish15 and by Linde o h e al.16 on su ace oxidized Fe nanopa icles e ealed ha he su ace laye consis ed o e y small c ys alli es wi h spinel s uc u e and ha a la ge spin can ing cha ac e ized he oxide phase. A can ing o he spins was also obse ed by Kodama e al.17 a he su ace o e i e nanopa icles. In ha case oo, i was ound ha he p esence o he can ed spins, a ising FIG. 4. Coe ci i y ~cu e a!and shi ~cu e b!o he hys e esis loops, measu ed a e cooling in a ield H520 kOe, as a unc ion o he measu ing empe a u e. FIG. 5. Ze o- ield cooled ~ci cles!and ield cooled ~ iangles!magne iza ion as a unc ion o empe a u e a an applied ield H5800 Oe. 2191J. Appl. Phys., Vol. 84, No. 4, 15 Augus 1998 Del Bianco e al. Downloaded 20 Feb 2003 o 192.167.227.94. Redis ibu ion subjec o AIP license o copy igh , see h p://ojps.aip.o g/japo/japc .jsp om educed coo dina ion and b oken exchange bonds be- ween su ace spins, was compa ible wi h a spin-glasslike beha iou o he su ace i sel . I should be ema ked ha he beha io o he su ace becomes ele an due o he e y educed dimensions o he pa icles so ha he numbe o a oms a he su ace is compa able o he numbe o a oms a he co e. Recen ly, compu e modeling o he in luence o su ace diso de and spin uncompensa ion on he magne ic p ope ies o nanopa icles and on he exchange aniso opy o hin ilms has been epo ed.18 In ou case, assuming ha he oxide laye is made up o small c ys alli es, i ollows ha a high pe cen age o he o al numbe o spins o he oxide is loca ed a he c ys alli es su ace, mo e p ope ly indica ed as he in e acial o g ain bounda y egion,19 as well as a he oxide–Fe in e ace which would he e o e be ~in ou opinion! he ul ima e e- sponsibili y o he obse ed exchange aniso opy e ec . CONCLUSION In conclusion, an app oach has been p oposed in o de o explain he low empe a u e magne ic beha io ~ empe a u e dependence o magne iza ion and coe ci i y!o oxide coa ed ul a ine Fe pa icles, based on he spin-glasslike na u e o he oxide laye . The occu ence o an exchange aniso opy e ec be ween he e omagne ic co e and he su ace esul s in a shi o he FC hys e esis loops below 50 K, co espond- ing o he eezing empe a u e o he spin-glass phase. I is wo h no ing ha he possibili y o obse ing he abo e desc ibed e ec s is s ic ly connec ed wi h ob aining sys ems modula ed on a leng h scale in he nanome e e- gime. In ac , i has been p oposed ha he spin-glasslike s a e o he oxide su ace is a o ed by he e y educed dimensions o i s cons i u ing c ys alli es and, mo eo e , i is clea ha he exchange in e ac ion would no be s ong enough o a ec conside ably he magne ic beha io i he pa icle co e and he oxide laye we e no ex ended on a sho and compa able scale leng h. ACKNOWLEDGMENTS The au ho s acknowledge P ojec Nos. CICYT MAT95- 1042-C02-02 and PGC PB96-0863-C02-02. LDB acknowl- edges he Eu opean Commission o inancial suppo ~G an No. ERBFMBI-CT95-0534!. 1Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl. Phys. 64, 6044 ~1988!. 2E. F. 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