Magnetic properties and magnetocaloric effects in GdCo9Si2 compound with multiple magnetic phase transitions

Magne ic p ope ies and magne ocalo ic e ec s in GdCo9Si2 compound wi h mul iple
magne ic phase ansi ions
Z. G. Zheng, X. C. Zhong, J. L. Zhang, Z. W. Liu, V. F anco, and D. C. Zeng
Ci a ion: Jou nal o Applied Physics 113, 17A938 (2013); doi: 10.1063/1.4799973
View online: h p://dx.doi.o g/10.1063/1.4799973
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Magne ic p ope ies and magne ocalo ic e ec s in GdCo
9
Si
2
compound
wi h mul iple magne ic phase ansi ions
Z. G. Zheng,
1
X. C. Zhong,
1
J. L. Zhang,
2
Z. W. Liu,
1,a)
V. F anco,
3
and D. C. Zeng
1,a)
1
School o Ma e ials Science & Enginee ing, Sou h China Uni e si y o Technology, Guangzhou 510640,
People’s Republic o China
2
Depa men o Physics and Ma e ials Science, Ci y Uni e si y o Hong Kong, Kowloon Tong, Hong Kong
3
Depa men o Condensed Ma e Physics, ICMSE-CSIC, Se illa Uni e si y, P. O. Box 1065, 41080 Se illa,
Spain
(P esen ed 18 Janua y 2013; ecei ed 5 No embe 2012; accep ed 22 Janua y 2013; published
online 10 Ap il 2013)
The s uc u e and magne ic p ope ies o polyc ys alline GdCo
9
Si
2
compound ha e been
in es iga ed. I has a BaCd
11
s uc u e and unde goes wo magne ic phase ansi ions: an
an i e omagne ic o e imagne ic ansi ion occu ing a 93 K, and a e imagne ic o
pa amagne ic ansi ion a 420 K, which esul s in a posi i e and a nega i e magne ic en opy
change, espec i ely. The wo peak alues o magne ic en opy change a e 0.6 and 1.1
Jkg
1
K
1
o DH¼5 T. Fu he mo e, he e exis s a me al-semiconduc o ansi ion empe a u e
(T
P
), below which he esis ance inc eases wi h inc easing empe a u e, while he semiconduc o
cha ac e is ic is obse ed abo e T
P
. The magne ic domain s uc u es a e cha ac e ized by s ipe and
g id s uc u es 1 lm wide. Al hough he MCE is small o applica ions, i s s udy is use ul o clea ly
unde s and he na u e o mul iple magne ic phase ansi ions in he GdCo
9
Si
2
compound. V
C2013
Ame ican Ins i u e o Physics.[h p://dx.doi.o g/10.1063/1.4799973]
Magne ic ma e ials wi h la ge magne ocalo ic e ec
(MCE) ha e a ac ed conside able a en ion o po en ial
applica ions in magne ic e ige a ion. In ecen yea s, excel-
len magne ocalo ic p ope ies ha e been obse ed in Gd and
i s compounds.
1
Due o hei la ge magne ic momen s and
low aniso opy, Gd-based compounds a e de ini ely p ime
candida es o achie ing la ge MCE.
2
In a p e ious wo k, we
ha e in es iga ed he magne ic p ope ies and magne o-
calo ic e ec o compounds (Gd
12x
Tb
x
)Co
7
(x ¼0, 4, and
8),
3
amo phous Gd
4
Co
3
alloys,
4
and Gd
65
Mn
35x
Ge
x
(x ¼0,
5, and 10).
5
All amilies exhibi ed la ge magne ic en opy
change (DS
M
). Howe e , he high con en s o ansi ion me -
als gene ally lead o he dec ease o sa u a ions because o
hei e imagne ic na u e. This poin is clea ly seen om he
high sa u a ed momen o Co-de icien compounds, such as
Gd
5
CoSi
2
.
6
The compound shows a e omagne ic o de ing
a 168 K and has la ge MCE wi h adiaba ic empe a u e
change DT
ad
¼5.9 K and magne ic en opy change DS
M
¼8.7 Jkg
1
K
1
o a magne ic ield change o 4.6 T.
Compa ed wi h Gd, ansi ion me als show many p ac ical
ad an ages, such as lowe ma e ial cos and la ge co osion
esis ance. The e a e p e iously published esul s o com-
pounds in his sys em, such as Gd
6
Co
2
Si
3,
7
GdCo
13x
Si
x,
8
and Gd
6
Co
1.67
Si
3.
9
They ha e gained mo e a en ion as
po en ial highly e icien magne ocalo ic ma e ials and gian
iso opic magne os ic ion ma e ials.
10
This inspi es us o
in es iga e he magne ic beha io s o Co- ich compounds in
he Gd-Co-Si sys em. Howe e , excep o abo e men ioned
Gd-based compounds, only a ew epo s a e a ailable on he
s uc u e and magne ocalo ic s udies on his se ies. In his
wo k, we ha e syn hesized he e na y silicide GdCo
9
Si
2
,
and in es iga ed i s s uc u e, magne ic p ope ies and mag-
ne ocalo ic e ec in de ail.
The compound GdCo
9
Si
2
was p epa ed by a c-mel ing a
mix u e o pu e Gd (99.95 w . %), Co (99.99 w . %), and Si
(99.99 w . %) in a gon a mosphe e. To ensu e composi ional
homogenei y, he ingo s we e epea edly mel ed a leas ou
imes. Be o e cha ac e iza ion, he ingo s we e w apped in Ta
oil and annealed a 1000 C o 5 days. The s uc u e o he
samples was iden i ied by Philips X’pe P o MPD X di ac-
ome e . The mic os uc u e was obse ed using scanning
elec on mic oscope (SEM) wi h ene gy dispe si e spec os-
copy (EDS) a ached. The empe a u e and magne ic ield
dependences o magne iza ion we e measu ed by a physical
p ope ies measu emen sys em (PPMS-9, Quan um Design
Co.). The elec ical esis ance measu emen was done by he
ou p obe me hod using PPMS. The magne ic o ce mic os-
copy (MFM) and a omic o ce mic oscopy (AFM) measu e-
men s we e pe o med wi h a comme cial scanning p obe
mic oscope (Asylum Resea ch, Cyphe ).
The X- ay powde di ac ion pa e n indica es ha
BaCd
11
- ype GdCo
9
Si
2
(space g oup I41/amd) is he majo
phase, whils he e is s ill a mino phase o Co
9
Si (space
g oup P63/mmc). The e ined uni cell pa ame e s o
GdCo
9
Si
2
a e: a¼7.932(2) A
˚and c¼6.287(8) A
˚, espec-
i ely. The es ima ed con en o Co
9
Si is abou 7 w . %. The
small amoun o Co
9
Si, wi h a Cu ie empe a u e highe han
900 K, should make negligible con ibu ion o he magne ic
beha io o samples.
To con i m he con en s o he impu i y phase, he sam-
ple was also inspec ed using SEM/EDS. The back-sca e ed
elec ons image p esen ed in Figu e 1con i ms he composi-
ion o he majo phase as 9.1 a . % o Gd, 73.8 a . % o Co,
a)
Au ho s o whom co espondence should be add essed. Elec onic
add esses: [email p o ec ed] and [email p o ec ed].
0021-8979/2013/113(17)/17A938/3/$30.00 V
C2013 Ame ican Ins i u e o Physics113, 17A938-1
JOURNAL OF APPLIED PHYSICS 113, 17A938 (2013)
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and 17.1 a . % o Si, close o he exac o mula o GdCo
9
Si
2
.
The s oichiome y o he g ay zone is ound o be Co
9
Si. The
black ound a eas co espond o he open po osi ies, which
may be o med in a c mel ing p ocesses. The ac ion o
Co
9
Si impu i y was also es ima ed based on he SEM images
o be less han 7%, in good ag eemen wi h esul s om
X- ay di ac ion.
Figu e 2shows he empe a u e dependence o ze o
ield cooled (ZFC) and ield cooled (FC) magne iza ions o
GdCo
9
Si
2
compound. The ansi ion empe a u es a e
de i ed om he ex eme alues o dM/dT s Tcu es shown
as he inse o Fig. 2. I is clea ha he compound exhibi ed
magne ic o de ing below 420 K (T
SR2
), and a sha p d op o
magne ic suscep ibili y was also e iden a 93 K (T
SR1
). A
small he mal i e e sibili y be ween ZFC and FC cu es is
obse ed below T
SR2
, which may be a ibu ed o he
domain-wall pinning e ec . I is indica ed ha he compound
unde goes wo magne ic phase ansi ions wi h inc easing
empe a u e: an an i e omagne ic o e imagne ic nea T
SR1
(Re . 11) and hen o pa amagne ic ansi ion a T
SR2
. Abo e
T
SR2
, pe ec ly pa amagne ic beha io was obse ed,
sugges ing he negligible magne ic con ibu ion o he Co
9
Si
impu i y e en hough i s Cu ie empe a u e is highe han
900 K. In compounds in ol ing la e a e-ea h me als
(REM) and ansi ion me als (TM), he in e ac ion be ween
TM 3d elec ons and REM 5d elec ons domina es he mag-
ne ic beha io . Such in e ac ion is gene ally nega i e and
makes he o e all 3d-4 spin coupling an i e omagne ic. I
has been epo ed ha in he Co- ich Co-REM compounds
he onse o long- ange magne ic o de ing is induced by he
3d-3d in e ac ion in ansi ion me als.
10
The e o e, he ab up
inc ease o suscep ibili y when empe a u e is dec eased
below 420 K should esul om he o de ing o Co momen s.
Fu he dec ease o empe a u e enhances he in e ac ion
o Gd la ice, bu he an i e omagne ic in e ac ion be ween
Gd la ice and Co subla ice educes he ne momen . A low
empe a u es, bo h subla ices a e sa u a ed and he ne
momen is lowe ed o a minimum and in a iable.
The sa u a ion magne iza ion o GdCo
9
Si
2
a 5 K is 9.4
emu/g, which equals 0.09 l
B
pe o mula uni , smalle han
he alue o a ee Gd
3þ
ion (7l
B
), indica ing an i e omag-
ne ic coupling o he Gd and Co momen s. Wi h inc easing
empe a u e, he sa u a ion magne iza ion g adually inc eases
up o 29.5 emu/g a 300 K, hen dec eases a highe empe a-
u es. The inse s o Figu e 3show he iso he mal magne iza-
ion cu es M(H) o GdCo
9
Si
2
wi h empe a u e om 34 K
o 316 K and om 329 K o 585 K. Based on he iso he mal
M(H) cu es and he Maxwell ela ion, he alues o DS
M
as a unc ion o empe a u e o a maximum ield o 5 T a e
calcula ed and shown in Figu e 3. I is ob ious ha he posi-
i e and nega i e DS
M
peaks o GdCo
9
Si
2
compounds co -
espond o he i s (T
SR1
) and second ansi ions (T
SR2
),
espec i ely. The alue o DS
Max
a T
SR2
eaches 1.1
Jkg
1
K
1
unde 5 T. I is compa able wi h hose o some
e omagne ic ma e ials, such as Nd
0.9
Dy
0.1
Co
4
Al
12
and
Fe
75
Nb
10
B
15
alloys.
13
The o he peak alue o DS
Max
a
T
SR1
is 0.6 Jkg
1
K
1
unde 5 T. The e ige an capaci y
(RC) is ob ained by in eg a ing he a ea unde he DS
M
(T)
cu es, using he ull empe a u e wid h a hal maximum
FIG. 1. Mic os uc u e o he annealed sample. The majo phase GdCo
9
Si
2
and he impu i ies Co
9
Si a e indica ed. The black ound a eas co espond o
he opened po osi ies.
FIG. 2. Tempe a u e dependences o ZFC and FC magne iza ion o
GdCo
9
Si
2
compound unde a magne ic ield o 0.05 T. The inse is he di -
e en ial o ze o ield cooling M–T cu e.
FIG. 3. Magne ic en opy change DS as a unc ion o empe a u e in a i-
ous applied magne ic ields change o GdCo
9
Si
2
compound. The inse s a e
iso he mal magne iza ion cu es.
17A938-2 Zheng e al. J. Appl. Phys. 113, 17A938 (2013)
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o he DS
M
peak as he in eg a ion limi s. The alues o RC
a T
SR2
a e 22 and 62 Jkg
1
in a ield o 2 T and 5 T,
espec i ely.
I is wo h men ioning ha empe a u e dependen
elec ical esis ance (R) measu emen s show a me al-
semiconduc o ansi ion a ound 260 K, as e idenced by a
ansi ion om an inc easing Rwi h inc easing empe a u e
(T), o a dec easing R(T) cu e, showing a peak a 260 K.
This peak is shi ed o highe Twhen magne ic ield is
applied, displacing he maximum o highe empe a u es ou -
side he expe imen al empe a u e ange o H¼2T.Amo e
de ailed analysis o he possible in luence o his ansi ion
on he magne ocalo ic esponse is cu en ly being unde -
aken and will be epo ed elsewhe e.
F om abo e discussion, one can jus i y ha he e is a
e imagne ic phase nea oom empe a u e in GdCo
9
Si
2
compound. In o de o unde s and he magne ic domain
s uc u e, AFM and MFM expe imen s we e pe o med a
oom empe a u e in he apping o li mode, as shown in
Figu e 4. Figu e 4(a) p esen s he AFM image o he sample.
The co esponding MFM image, shown in Figu e 4(b),is
cha ac e ized by he s ipe and g id s uc u es o 1 lm wide,
as shown in blue dashed box. Wi h knowledge abou he e -
imagne ic cha ac e iza ion and he p esen ed AFM mic o-
g aphs, he magne ic domains a e ske ched schema ically as
a op iew and c oss-sec ion in Figu e 4( ). The magne ic
momen s migh display al e na ing in- and ou -o -plane.
Excep o hese s uc u es, he e a e o he ype domain
a eas, such as ed solid box, which could be asc ibed o he
al e na ing magne ic momen s up- and down-o -plane, as
shown in Figu e 4(e). Figu es 4(b) and 4(d) e eal he AFM
and MFM images a o he zones in he same sample,
espec i ely. In addi ion o he s ipe domain, he e is a zone
labeled “A” in which he magne ic domains become e y
s ong. I could be associa ed o he e omagne ic impu i y
phase Co
9
Si.
The polyc ys alline compound GdCo
9
Si
2
wi h BaCd
11
ype s uc u e (space g oup I41/amd) has been p epa ed by
a c-mel ing. The s uc u e, magne ic, and magne ocalo ic
p ope ies o he compound ha e been in es iga ed. Two
magne ic ansi ions a e obse ed: one om an i e omag-
ne ic o e imagne ic (T
SR1
¼93 K) and ano he o pa amag-
ne ic o de (T
SR2
¼420 K). The e a e wo peak alues in
DS
M
(T), co esponding o T
SR1
and T
SR2
, wi h magni udes
0.6 and 1.1 Jkg
1
K
1
, espec i ely, o DH¼5 T. The
magne ic domain s uc u es show s ipe and g id s uc u es
wi h abou 1 lm wide. I is in e es ing ha he esis i i y
shows a me al-semiconduc o ansi ion a 260 K and o
de e mine i s o igin heo e ical calcula ions, X- ay pho o-
elec on spec oscopy, and q(T) s udies unde di e en mag-
ne ic ield a e in p og ess.
This wo k was inancially suppo ed by he Guangzhou
Municipal Science and Technology P og am (G an No.
12F582080022), he Scien i ic Resea ch Founda ion o he
Re u ned O e seas Chinese Schola s, S a e Educa ion Minis y
(G an No. x2clB7120290), he Fundamen al Resea ch Funds
o he Cen al Uni e si ies (G an Nos. 2011ZM0014 and
2012ZZ0013), and he Guangdong P o incial Science and
Technology P og am (G an Nos. 2010B050300008 and
2009B090300273).
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FIG. 4. The AFM (a), (b) and co esponding MFM (c), (d) mo phology a
oom empe a u e, espec i ely. (e) and ( ) The domain s uc u e is ske ched
schema ically as a op iew and a c oss-sec ion co esponding o ed solid
and blue dashed box, espec i ely.
17A938-3 Zheng e al. J. Appl. Phys. 113, 17A938 (2013)
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