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
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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.
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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.
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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!.
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