So magne ic p ope ies o high- empe a u e nanoc ys alline alloys: Pe meabili y and
magne oimpedance
J. S. Blázquez, V. F anco, A. Conde, and L. F. Kiss
Ci a ion: Jou nal o Applied Physics 93, 2172 (2003); doi: 10.1063/1.1539294
View online: h p://dx.doi.o g/10.1063/1.1539294
View Table o Con en s: h p://sci a ion.aip.o g/con en /aip/jou nal/jap/93/4? e =pd co
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So magne ic p ope ies o high- empe a u e nanoc ys alline alloys:
Pe meabili y and magne oimpedance
J. S. Bla
´zquez, V. F anco, and A. Condea)
Depa amen o de Fı
´sica de la Ma e ia Condensada, ICMSE-CSIC, Uni e sidad de Se illa, P. O. Box 1065,
41080 Se illa, Spain
L. F. Kiss
Resea ch Ins i u e o Solid S a e Physics and Op ics, Hunga ian Academy o Sciences, P. O. Box 49, 1525
Budapes , Hunga y
共Recei ed 5 June 2002; accep ed 21 No embe 2002兲
The echnological applicabili y o FeCoNbBCu alloys is sugges ed in e ms o measu emen s o
oom empe a u e magne oimpedance and empe a u e dependence o magne ic pe meabili y
␮
.
Resul s o he Fe78-xCoxNb6B15Cu1alloy se ies show ha oom empe a u e so magne ic
p ope ies a e enhanced in he lowes Co con aining alloy (
␮
⬃10 500 and magne oimpedance a io
⬃60% a 1 MHz兲. Howe e , pe meabili y exhibi s a smoo he he mal dependence in he alloys wi h
medium and high Co con en . A adeo be ween magne ic so ness and i s he mal s abili y e eals
he alloy wi h 39 a . % Co as he mos sui able composi ion among hose s udied, cha ac e ized by
a empe a u e coe icien o ⬃0.02%/K om oom empe a u e up o 900 K. This alue is 1 o de
o magni ude smalle han hose obse ed o FeSiBCuNb 共FINEMET- ype兲alloys and Mn e i es
and ex ended o e a much wide empe a u e ange han in hese ma e ials. © 2003 Ame ican
Ins i u e o Physics. 关DOI: 10.1063/1.1539294兴
I. INTRODUCTION
So magne ic nanoc ys alline alloys ha e been p o usely
s udied since he de elopmen o FINEMET 共FeSiBNbCu兲1
and NANOPERM alloys 共FeMB and FeMBCu, M⫽Nb, Z ,
H ...兲.2These ma e ials p esen a wo-phase mic os uc u e,
in which e omagne ic nanoc ys als 共
␣
-FeSi o
␣
-Fe, espec-
i ely兲a e embedded in a esidual e omagne ic amo phous
phase. Thei ou s anding so -magne ic p ope ies a oom
empe a u e a e widely co e ed in he li e a u e.3Howe e ,
as he empe a u e inc eases, so magne ic p ope ies a e los
due o he ansi ion o he amo phous phase o he pa amag-
ne ic s a e,4which limi s he applicabili y o hese ma e ials
a high empe a u es. Sea ching o so magne ic nanoc ys-
alline alloys wi h high empe a u e applicabili y led o he
de elopmen o HITPERM- ype alloys 共FeCoMBCu兲,5in
which pa ial subs i u ion o Fe by Co was shown o enhance
he Cu ie empe a u e o he amo phous phase.
The highe coe ci i y exhibi ed by HITPERM alloys,
compa ed o FINEMET and NANOPERM, has been ex-
plained in e ms o he di e en e olu ion o he sa u a ion
magne os ic ion cons an ␭Sdu ing nanoc ys alliza ion.
Whe eas in FINEMET and NANOPERM alloys he con i-
bu ions o ␭Scoming om he c ys alline and esidual amo -
phous phases a e o di e en sign, gi ing an o e all alue
close o ze o, in HITPERM alloys bo h con ibu ions a e
posi i e.3In his case, he so ness o he ma e ial is con-
olled by he ela i e a ia ion o he magne os ic ion om
he amo phous phase o he c ys alline one.6
The e ha e been nume ous s udies conce ning he mi-
c os uc u al cha ac e iza ion and magne ic phase ansi ion
in HITPERM- ype alloys.3,5,7–10 Howe e , he e a e s ill ew
esul s on magne ic p ope ies ele an o possible applica-
ions o hese ma e ials, such as magne ic pe meabili y
␮
and gian magne oimpedance e ec 共GMI兲. The la e has
been a ield o in ensi e esea ch in ecen yea s;11–13 o da e
i is a new way o he applica ion o so magne ic
ma e ials.14–17 The e a e nume ous s udies o he di e en
ac o s a ec ing he magne oimpedance beha io o so
magne ic nanoc ys alline alloys as FINEMET and NANOP-
ERM, analyzing, o example, he in luence o he he mal
ea men s,18–25 composi ional e ec s,18–20 he in luence o
domain s uc u e and induced aniso opies,21–23 he e ec o
he measu ing equency,19–22,24 and empe a u e,24,25 e c.
Howe e , he e is a lack o da a in he li e a u e conce ning
he HITPERM amily o alloys.
In his wo k he in luence o he Co o Fe a io on he
oom empe a u e magne oimpedance and he mal depen-
dence o magne ic pe meabili y o HITPERM- ype alloys
has been s udied. I will be shown ha al hough Co addi ion
de e io a es he oom empe a u e so magne ic p ope ies o
he alloy, a wide empe a u e ange o s able magne ic p op-
e ies is ob ained.
II. EXPERIMENT
Amo phous ibbons, ⬃20
␮
m hick and ⬃5 mm wide, o
nominal composi ions Fe78⫺xCoxNb6B15Cu1共x⫽18, 39,
60兲we e p oduced a Wa saw Uni e si y o Technology by
he single olle mel -spinning echnique. The de i i ica ion
a兲Au ho o whom co espondence should be add essed; elec onic mail:
[email p o ec ed]
JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 4 15 FEBRUARY 2003
21720021-8979/2003/93(4)/2172/6/$20.00 © 2003 Ame ican Ins i u e o Physics
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p ocess o he alloys has been s udied by di e en ial scan-
ning calo ime y 共DSC兲, ansmission elec on mic oscopy
and x- ay di ac ion echniques.
Pe meabili y measu emen s we e pe o med on o oidal
samples a a equency o 6 kHz and an applied ield low
enough o assu e he measu emen o ini ial pe meabili y 共⬃
0.5 A/m兲. The expe imen consis ed o se e al hea ing–
cooling cycles and bo h he measu emen and hea ea men
o he samples we e pe o med a he same ime. Fo hese
measu emen s wo coils we e wounded a ound he sample.
Wi h he i s coil an ac ield is applied o he sample,
whe eas he signal induced in he second coil is de ec ed
using a lock-in echnique. An impedance analyze 共Hewle -
Packa d 4192A兲was used o calib a e he alues o pe me-
abili y a oom empe a u e.
Fo magne oimpedance measu emen s he impedance
analyze was used. Two elec ical con ac s we e spo welded
o he samples a a dis ance o 5 cm. DSC expe imen s con-
i med ha his p ocedu e did no a ec he de i i ica ion
p ocess o he alloys s udied. Samples we e annealed in a
halogen-lamp u nace in acuum a di e en s ages o he
de i i ica ion p ocess p esen ed in Table I. Fo measu ing
he ield dependence o he impedance, an ac in ensi y o 10
mA a equencies anging om 0.5 o 5 MHz was applied
h ough he sample. The maximum applied ield was 7.2
kA/m.
III. RESULTS
A. De i i ica ion p ocess
The de i i ica ion p ocess o he as-cas samples occu s
in wo main s ages.26 Du ing he i s one, a nanoc ys alline
mic os uc u e is o med, in which
␣
-FeCo nanoc ys als wi h
an a e age g ain size o ⬃5 nm a e embedded in a esidual
amo phous ma ix. The c ys alline olume ac ion a he end
o he nanoc ys alliza ion p ocess is ⬃55% o he alloys
wi h 18 and 39 a . % Co, bu i is lowe in he alloy wi h 60
a . % Co 共⬃ 45%兲.26 A highe empe a u es 共⬎900 K兲,
ec ys alliza ion o a ac ion o he
␣
-FeCo c ys alli es and
he esidual amo phous ma ix p oduces a ully c ys alline
alloy, wi h he appea ance o bo ide phases, mainly 共FeCo兲
共FeCo兲23B6.27
B. Ini ial pe meabili y
Figu e 1共a兲shows he e olu ion o he ini ial pe meabil-
i y wi h empe a u e o Fe60Co18Nb6B15Cu1as-cas sample
submi ed o con inuous hea ing and cooling cycles. The
cha ac e is ic ea u es ha e been numbe ed in he igu e. Ini-
ially 共1兲 he elaxa ion o he in e nal s esses p o okes a
p og essi e inc ease o he oom empe a u e pe meabili y;
cooling cycles 共2兲show ha pe meabili y emains cons an
o empe a u es lowe han 550 K a his s age. A cha ac e -
is ic Hopkinson peak is obse ed a he Cu ie empe a u e o
he amo phous phase, which is due o a as e dec ease o
magne ic aniso opy han ha o magne iza ion wi h inc eas-
ing empe a u e, and hus he pe meabili y di e ges a he
Cu ie poin .28 Abo e he Cu ie poin o he amo phous
phase, an ab up all in he pe meabili y is obse ed 共3兲, due
o he ansi ion o he sys em o he pa amagne ic s a e. A
highe empe a u es, an inc ease o pe meabili y a he onse
o nanoc ys alliza ion occu s 共4兲, which is connec ed wi h
he appea ance o he e omagne ic nanoc ys als o he
␣
-FeCo phase.29 When his mic os uc u e is achie ed, he
sys em shows he highes oom empe a u e pe meabili y.
Howe e , no Hopkinson peak can be de ec ed a he Cu ie
poin o he esidual amo phous phase (TC
am), which can be
due o he non-ze o alue o he e ec i e magne ic aniso -
opy o he whole sys em: he nanoc ys als p esen a much
highe Cu ie empe a u e han he amo phous ma ix and
he e o e,
␮
does no di e ge a TC
am . Also, he all in
␮
a
TC
am is smoo he o he nanoc ys allized sample han o he
ully amo phous one. In ac , he esidual amo phous ma ix
is no homogeneous30 and hus he Cu ie empe a u e is no
well de ined.31 Fo his alloy, oom empe a u e pe meabili y
shows high alues a his s age, bu i depends s ongly on
empe a u e, which is a nondesi able e ec o high empe a-
FIG. 1. Ini ial pe meabili y s empe a u e in successi e hea ing–cooling
cycles o : 共a兲Fe60Co18Nb6B15Cu1and 共b兲Fe39Co39Nb6B15Cu1alloys.
Numbe s indica e he sequence o hea ing-cooling cycles.
TABLE I. The mal ea men s o samples submi ed o magne oimpedance
expe imen s. Ini ial nanoc ys alline co esponds o samples wi h low c ys-
alline olume ac ion, while inal nanoc ys alline co esponds o samples
annealed up o he end o he i s c ys alliza ion s age.
Alloy Sample Hea ea men
Fe60Co18 Nb6B15 Cu1
S uc u ally elaxed 15 min a 600 K
Ini ial nanoc ys alline 15 min a 725 K
Final nanoc ys alline 15 min a 800 K
Fe39Co39 Nb6B15 Cu1
S uc u ally elaxed 15 min a 600 K
Ini ial nanoc ys alline 15 min a 725 K
Final nanoc ys alline 15 min a 850 K
S uc u ally elaxed 15 min a 600 K
Fe18Co60 Nb6B15 Cu1Ini ial nanoc ys alline 15 min a 700 K
Final nanoc ys alline 15 min a 800 K
2173J. Appl. Phys., Vol. 93, No. 4, 15 Feb ua y 2003 Blazquez
e al.
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u e applica ions o he ma e ial. As nanoc ys alliza ion
p og esses, pe meabili y inc eases 共5兲, bu o empe a u es
abo e he second ans o ma ion s age pe meabili y de-
c eases 共6兲, and he so magne ic p ope ies o he ma e ial
a e los in all he explo ed empe a u e ange 共7兲due o he
p esence o bo ide- ype phases. The sligh inc ease in
␮
as
empe a u e dec eases can be explained in e ms o he Cu ie
empe a u es o he bo ide- ype phases o med, especially he
共FeCo兲23B6phase, wi h TC⫽784 K.32
The beha io o he o he alloys s udied shows simila
ends, howe e a wide empe a u e ange o cons an pe -
meabili y can be obse ed o he nanoc ys alline samples o
he alloys wi h highe Co con en 关Fig. 1共b兲兴: In con as wi h
he 18 a . % Co alloy, whe e
␮
dec eases a empe a u es
abo e 500 K, he alloys wi h 39% and 60% ha e a cons an
alue o pe meabili y up o 950 and 900 K, espec i ely. This
can be asc ibed o he highe Cu ie empe a u e o he amo -
phous ma ix, which migh allow he magne ic coupling be-
ween he nanoc ys als h ough he ma ix up o highe
empe a u es.4
C. Magne oimpedance
Figu e 2 shows he modulus o impedance 共兩Z兩兲 e sus dc
applied magne ic ield 共H兲, ob ained a oom empe a u e a a
equency o 1 MHz, o samples o he 18 a . % Co alloy in
he as-quenched s a e and a e annealing. The alue o 兩Z兩a
he maximum applied ield dec eases as he c ys alline ac-
ion inc eases, being he same o as-cas and elaxed
samples, in ag eemen wi h he educ ion o dc esis i i y
wi h he p og ess o c ys alliza ion.
Usually he GMI e ec is desc ibed in e ms o he mag-
ne oimpedance a io 共MIR 兲, calcula ed as:
MIR ⫽100•
兩
Z共H兲
兩
⫺
兩
Z共Hmax兲
兩
兩
Z共Hmax兲
兩
,
whe e
兩
Z(Hmax)
兩
is he impedance modulus measu ed a
maximum applied ield 共7200 A/m兲. Figu e 3 shows MIR
alues e sus H, o all he s udied alloys in he as-cas ,
s uc u ally elaxed, ini ial-nanoc ys alline and inal-
nanoc ys alline s a es. The maximum alue o MIR e sus
annealing empe a u e is ep esen ed in Fig. 4. Fo he alloy
wi h 18 a . % Co he GMI e ec inc eases o he elaxed
sample wi h espec o he as-cas sample, eaches a maxi-
mum alue o he ini ial-nanoc ys alline sample 共⬃ 60%兲
and dec eases o he inal-nanoc ys alline sample. Howe e ,
o he highe Co con en alloys, he maximum GMI e ec
appea s o he as-cas samples, dec easing as he annealing
empe a u e inc eases.
The magne oimpedance scans 共Fig. 3兲show a single-
peak s uc u e o all he as-cas samples s udied and o he
ini ial-nanoc ys alline sample o he 18 a . % Co alloy; how-
e e a double peak s uc u e can be obse ed o all he o he
FIG. 2. Magne oimpedance modulus o Fe60Co18Nb6B15Cu1alloy: 共a兲as-
cas , 共b兲s uc u ally elaxed, 共c兲ini ial-nanoc ys alline, and 共d兲 inal-
nanoc ys alline samples.
FIG. 3. Magne oimpedance a io o he di e en alloys s udied: 共a兲as-cas ,
共b兲s uc u ally- elaxed, 共c兲ini ial-nanoc ys alline, and 共d兲 inal-
nanoc ys alline samples
FIG. 4. In luence o he mic os uc u e on he maximum magne oimpedance
a io.
2174 J. Appl. Phys., Vol. 93, No. 4, 15 Feb ua y 2003 Blazquez
e al.
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samples s udied. This kind o wo-peak s uc u e and he
hys e e ic beha io o magne oimpedance a e dele e ious o
he applicabili y o he alloys.
IV. DISCUSSION
The beha io o he pe meabili y and magne oimpedance
is co ela ed wi h he p e iously epo ed beha io o coe -
ci i y and magne os ic ion.6,26 As he ac o s ha a ec he
magne ic p ope ies depend s ongly on he mic os uc u e, i
is sui able o di ide he samples s udied in amo phous
samples 共as-cas and s uc u ally elaxed samples兲and nano-
c ys alline samples 共annealed a he beginning and a he end
o he nanoc ys alliza ion兲.
A. Amo phous samples
Figu e 5 shows he ini ial pe meabili y measu ed a 400
K共maximum empe a u e below which
␮
emains app oxi-
ma ely cons an o all he alloys s udied and no p oducing
u he s uc u al ans o ma ion o he samples兲 e sus
maximum achie ed empe a u e in he p e ious hea ea -
men o he con inuous hea ing-cooling cycle expe imen s. In
his igu e i is possible o obse e he e olu ion o
␮
wi h
he mic os uc u e. An inc ease in pe meabili y wi h he
s uc u al elaxa ion phenomena can be ound o all he
s udied alloys, being he e ec la ge as he Co con en de-
c eases in he alloy. This e olu ion di e s om he one e-
po ed o coe ci i y26 and om he obse ed e olu ion in
MIR 共Fig. 4兲. In ac , only o he 18 a . % Co alloy is a clea
so ening o he magne ic p ope ies obse ed a e s uc u al
elaxa ion, e lec ed in he magne oimpedance esul s as an
inc ease in he GMI e ec in s uc u ally elaxed samples.
This di e ence can be unde s ood in e ms o he p e i-
ously p oposed mechanism o he inc ease in oom empe a-
u e coe ci i y: he domain wall s abiliza ion phenomena.6
Pe meabili y measu emen s and hea ea men s o he
samples we e made simul aneously, hus he annealing was
unde he p esence o an ac ield. I has been epo ed ha
annealing unde ac ield diminishes 共o e en nulls兲 he do-
main wall s abiliza ion e ec ,33 which can explain he di e -
en e olu ion o he pe meabili y and he coe ci i y wi h he
s uc u al elaxa ion.
As i has been poin ed be o e, he beha io o MIR wi h
he s uc u al elaxa ion ag ees wi h he beha io ound o
he coe ci i y.6,26 Howe e , he domain wall s abiliza ion
phenomena would be a sui able explana ion o he magne ic
ha dening o he highe Co con en alloys only i i is pos-
sible o con i m ha he domain wall mo ion is a con ibu ing
mechanism o he magne iza ion p ocess a he equency
used in he magne oimpedance expe imen s 共1 MHz兲. The
equency e olu ion o MIR in nanoc ys alline alloys shows
ha he maximum alue o MIR inc eases up o a ce ain
alue o equency, a e which i dec eases. This all in he
GMI e ec has been associa ed wi h he disappea ance o he
con ibu ion o he domain wall mo ion o he magne iza ion
p ocess a high equencies.15,34 The e o e, he measu ing
equency was changed om 0.5 o 5 MHz in he magne-
oimpedance expe imen s. A con inuous inc ease in MIR was
obse ed as he applied equency is inc eased, con i ming
ha he domain wall mo ion is a con ibu ing mechanism o
he magne iza ion p ocess a 1 MHz and, he e o e, he do-
main wall s abiliza ion is a sui able explana ion o he mag-
ne ic ha dening o he high Co con en alloys.
The wo-peak s uc u e obse ed in he magne oimped-
ance scans is ela ed o he magne ic aniso opy ield, HK,
being equal o he obse ed shi o he maximum alue o
MIR wi h espec o H⫽0.34 In ou case, his is ue o he
18 a . % Co con aining alloy, howe e , because he domain
wall s abiliza ion phenomena a ec he posi ion o he maxi-
mum alue o MIR,35 i is no possible o a i m he same o
he 39 and 60 a . % Co alloys.
In amo phous samples, he main con ibu ion o he an-
iso opy ield is he magne oelas ic aniso opy, and HKcan
be exp essed as
HK⫽2K
␮
0MS
⫽3␭S
␴
␮
0MS,
whe e K⫽共3/2兲␭S
␴
, is he magne oelas ic aniso opy con-
s an , ␭S he sa u a ion magne os ic ion cons an ,
␴
he in-
e nal s esses,
␮
0 he acuum pe meabili y, and MS he sa u-
a ion magne iza ion. Fo he s uc u ally elaxed sample o
he 18 a . % Co alloy HK, ob ained om he posi ion o
maximum alue o MIR, is ⬃200 A/m. The alues o ␭Sand
␮
0MS o his sample a e 30 ppm and 1.1 T, espec i ely,
which gi es a alue o ⬃1 MPa o he in e nal s esses and
an aniso opy cons an alue compa able o hose ound in
o he me allic glasses wi h simila sa u a ion magne os ic-
ion cons an s.36
B. Nanoc ys alline samples
Bo h he pe meabili y and magne oimpedance esul s
ag ee wi h he epo ed beha io o coe ci i y a e he
nanoc ys alliza ion p ocess,26 in which he coe ci i y de-
c eases a he beginning o he nanoc ys alliza ion only o
he alloy wi h 18 a . % Co. Fo his alloy, he ini ial pe me-
abili y shows high alues a low empe a u es 共⬍500 K兲in
nanoc ys allized samples 共⬃ 10 500兲bu low alues can be
ound in he o he alloys 共⬃ 1000兲. The e y low alues
ob ained o samples annealed a e he second ans o ma-
FIG. 5. Ini ial pe meabili y measu ed a 400 K s maximum eached em-
pe a u e in he p e ious ea men . A ows indica e he c ys alliza ion p o-
cesses de ec ed by DSC.
2175J. Appl. Phys., Vol. 93, No. 4, 15 Feb ua y 2003 Blazquez
e al.
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ion s ages a e due o he o ma ion o bo ide- ype phases
which d as ically ha den he magne ic p ope ies.
Fo o he HITPERM- ype composi ion, Fe44Co447
B4Cu1,8 he maximum ini ial pe meabili y measu ed a oom
empe a u e was 1800, ob ained a 0.4 kHz. A 6 kHz 共mea-
su ing equency in ou expe imen s兲 he alue o
␮
is
⬃1000, simila o he ob ained alue o ou nanoc ys allized
alloy Fe39 Co39Nb6B15Cu1, bu 1 o de o magni ude lowe
han he maximum alue exhibi ed by he nanoc ys alline
samples o Fe60 Co18Nb6B15Cu1共⬃10 500兲.
The e olu ion o pe meabili y and he dec ease o esis-
i i y wi h nanoc ys alliza ion mus a ec he e olu ion o
he GMI e ec .37 Fo samples o he di e en alloys a he
same annealing s age 共s uc u ally elaxed, ini ial nanoc ys-
alline and inal nanoc ys alline兲, he 兩Z兩 alues a he maxi-
mum applied ield a e simila , indica ing ha esis i i y does
no di e conside ably be ween hese samples. The e o e, he
composi ional e ec on GMI o he se ies s udied migh be
asc ibed o he di e en alues o pe meabili y.
The GMI e ec inc eases a he beginning o he nanoc-
ys alliza ion only o he alloy wi h 18 a . % Co, as i has
been p e iously obse ed o FINEMET and NANOPERM
composi ions.37 Fo hese alloys, bo h he enhancemen o
magne ic so ness and he dec ease o esis i i y con ibu e
o he inc ease in he GMI e ec . Howe e , o he 39 and 60
a . % Co alloys he GMI e ec is lowe in he nanoc ys al-
lized samples han in he amo phous samples, despi e he
dec ease o he esis i i y bu in ag eemen wi h he obse ed
beha io o he ini ial pe meabili y and he epo ed e olu-
ion o he coe ci i y.26 This kind o co ela ion be ween he
e olu ion o esis i i y and GMI e ec has also been ob-
se ed o o he nanoc ys alline alloys, whe e a dec ease o
esis i i y a ad anced s ages o nanoc ys alliza ion canno
a oid he dec ease o he GMI e ec .37
The di e en e olu ion o he magne os ic ion cons an
o he di e en alloys can explain he di e en obse ed be-
ha io . In ac , he nanoc ys alline
␣
-FeCo phase ha ap-
pea s in he 39 and 60 a . % Co alloys wi h composi ions
close o Fe61 Co39 and Fe40 Co60 , espec i ely,38 shows a
highe ␭S alue 共⬃65 ppm兲39 han ha co esponding o he
amo phous alloy 共⬃10–15 ppm兲6and his makes he ne
magne os ic ion cons an inc ease as he nanoc ys alliza ion
p og esses. Howe e , o he alloy wi h 18 a . % Co, he
magne os ic ion cons an is ⬃15 ppm o bo h he
amo phous6and
␣
-Fe82Co18 c ys alline phases.39 Fo
FINEMET and NANOPERM alloys, ␭Sis nega i e in he
nanoc ys als and posi i e in he amo phous phase, p oducing
a dec ease o he ne magne os ic ion o he sys em and,
consequen ly, an enhancemen o he magne ic p ope ies a
oom empe a u e.3
An impo an ea u e o possible applica ions o hese
ma e ials is he empe a u e ange o cons an pe meabili y.
S udies on he empe a u e dependence o he ini ial pe me-
abili y in nanoc ys allized FINEMET- ype alloys40 show ha
␮
ab up ly dec eases a empe a u es highe han 575 K. To
cha ac e ize quan i a i ely he empe a u e s abili y o
␮
,
he empe a u e coe icien o pe meabili y can be used as a
pa ame e o compa ison. Whe eas in FINEMET alloys pe -
meabili y dec eases a a a e o ⬃0.3%/K in a empe a u e
ange om 400 o 500 K,40 in he Fe39 Co39Nb6B15Cu1alloy
he empe a u e coe icien is ⬃0.02%/K in a empe a u e
ange om 400 o 900 K, indica ing an imp o emen in he
high empe a u e applica ions o his alloy wi h espec o
FINEMET. This pa ame e is e y impo an o some ech-
nological applica ions as LC il e s, whe e e i es a e usually
used. A ypical alue o he empe a u e coe icien o pe -
meabili y in Mn e i es is ⬃0.3%/K bu hey a e limi ed o
low empe a u e applica ions due o hei low Cu ie empe a-
u es 共⬃450 K兲.
V. CONCLUSIONS
Two cha ac e is ic p ope ies ha a e in e es ing o so
magne ic applica ions o HITPERM- ype alloys ha e been
s udied: pe meabili y and magne oimpedance. The beha io
is in line wi h p e ious esul s on coe ci i y, and can be
explained in e ms o domain wall s abiliza ion phenomenon,
he di e ence be ween he magne os ic ion cons an o he
cons i u ing wo phases and g ain coa sening. The oom em-
pe a u e so ness is enhanced in he alloy wi h he lowes Co
con en which exhibi s
␮
⬃10 500 and a maximum alue o
MIR⬃60% 共a 1 MHz兲 o nanoc ys alline samples wi h low
c ys alline olume ac ion. Al hough he ob ained alues o
he o he composi ions 共39 and 60 a . % Co兲a e
␮
⬃1000
and a maximum alue o MIR⬍10% 共a 1 MHz兲, high em-
pe a u e applicabili y is clea ly enhanced in hese alloys,
showing a wide empe a u e ange wi h app oxima ely con-
s an pe meabili y. This sui abili y o high empe a u e ap-
plica ions can be exp essed h ough he empe a u e coe i-
cien o he pe meabili y 共⬃0.02%/K om oom empe a u e
up o 900 K o he alloy wi h 39 a . % Co兲, 1 o de o
magni ude lowe han hose ound in FINEMET and Mn e -
i es and ex ended o e a much wide empe a u e ange han
in hese ma e ials.
ACKNOWLEDGMENTS
This wo k was suppo ed by he Spanish Go e nmen
and EU FEDER 共P ojec No. MAT 2001-3175兲, he PAI o
he Jun a de Andalucı
´a, he Hispano-Hunga ian Bila e al Co-
ope a ion P og am 共G an No. TE
`T E-2/2001兲and he Hun-
ga ian Scien i ic Resea ch Fund 共OTKA兲 h ough G an No.
T-30753. J.S.B. acknowledges a esea ch ellowship o he
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