Paramagneti Meissner Eet and Magneti
Remanene in Granular MgB
2
W. A. C. Passos 1
, P.N. Lisboa-Filho 1
,G. L.Fraga 2
,
F. W. Fabris 2
, P. Pureur 2
, and W. A.Ortiz 1
1
GrupodeSuperondutividadeeMagnetismo,CentroMultidisiplinarpara o
Desenvolvimentode MateriaisCer^amios-Departamentode Fsia,
UniversidadeFederalde S~ao Carlos,Cx.Postal676, 13565-905S~aoCarlos,SP,Brazil
2
InstitutodeFsia,UniversidadeFederaldoRioGrande Sul,
POB15051,91501-970, PortoAlegre,RS,Brazil
Reeivedon28February,2002
Themagneti response of MgB2 samplesprepared bysolid state reation method, werestudied.
Importantfeaturesrevealingthegranularharaterofthespeimenswereveried,as(i)depressed
magnetization versus eld response, when ompared with the orresponding urve for unlinked
powder,(ii)theparamagnetiMeissnereetand(iii)theourreneofamagnetiremaneneina
limitedintervaloftemperatures. Experimentalevideneisaddedtoindiatethattheseeetsare
relatedtouxpinning.
I Introdution
Sine the disovery of superondutivity in MgB
2 [1℄,
there is an inreasing interest on this material, both
forfundamentalstudies andpotentialappliations. As
usual, researh studies are under way to understand
many dierentaspets of superondutivity in MgB
2 .
Most samples produed thus far present a granular
harater, similar to that exhibited byerami
super-ondutors, displaying a two-omponent magneti
re-sponse.
It has been extensively reported [2-5℄ that one of
the inherentproperties ofgranular superondutorsis
theparamagnetiMeissnereet(PME),usually
iden-tied as a positive magnetization exhibited by
er-tainsampleswhenooledthroughtheritial
tempera-ture in the presene of a magneti eld (eld-ooled,
FC). As a matter of fat, the PME has been
sys-tematially detetedwhen samples ofontrolled
gran-ularity were studied, espeially those prepared to
be-haveastridimensionalJosephsonjuntionarrays(JJA)
[3-5℄. The feature has also appeared on the
mag-neti response of good-quality thin-lms of Nb and
YBa
2 Cu
3 O
7 Æg
YBCO)ooledin aeld perpendiular
to theplaneofthesample[2℄. PMEhasalsobeen
ob-served in melt-textured YBCO samples submitted to
high magnetields(H)[6,7℄.
Anotherfeatureofinterestisaremanent
magnetiza-tion(MR), seenin themagnetiresponseofsome
spe-orderedtridimensional Josephsonarraysof Nb[3℄ and
YBCO[4℄. Thisfeaturewasalsoshowntobea
hara-teristiofminimallyorderedgranularsystems[3,4℄.
ExperimentalProedure
ApolyrystallineMgB
2
pelletwaspreparedbysolid
state reation. A stoihoimetri mixture of small Mg
hipsandneBpowderwaswrappedin aTafoil and
sealedunderAratmosphereinaquartzampoule. The
pelletwassinteredat950 o
Cfortwohoursandfurnae
ooled to room temperature. Samples for resistivity
andmagnetimeasurementswereutted fromthe
sin-teredpelletwithadiamondsaw.
Resistivity () measurements of the sample up
to room temperature were arried out using a
low-frequenyAC tehniqueemployinga lok-in amplier
asanulldetetor. Data obtainedat zeroappliedeld
are shown in Fig. 1. The inset shows an amplied
viewof the transition region, evideningthe granular
behaviorharaterizedbyadoubletransition.
Magneti measurementswere onduted using two
instruments provided by Quantum Design: a 5 Tesla
SquidMagnetometeranda9TeslaPPMS.Fig. 2shows
themassmagnetizationversuseld,Mvs. H,urvesfor
twosamplesatlowtemperatures: Oneofthespeimens
isasmallpiee(4.4mg)ofabulksample,whereasthe
otheronsistsofpowder(0.7mg)olletedduring
ma-nipulationofthereasonablybrittlebulksamples. Both
superondu-thebulksamplehassmallerabsolutevaluesofM,
indi-atingthataparamagneti ontributionduetothe
in-tergrainfration ispresent. Measurementsofthe
mag-netiremanene(M
rem
),takenafter appliation ofan
AC-eld (h), as desribed in Refs. [2℄ and [3℄, are in
Fig. 3. Thepositiveremaneneappearingin awindow
oftemperaturesforertainvaluesofhisa
harateris-tiof a minimally ordered granularsystems, similarly
to those reported in Refs. [2℄and [3℄ whih were
pre-paredtobehaveasJosephsonarrays. TheinsetofFig.
3showsthemagnetiresponseofthesamebulksample
submittedtoalowAC-eld,h=0.1Oe.Thisurve
re-semblesthatofanordinarysuperondutingtransition,
withoutanytraeofMR.
0
50
100
150
200
250
300
0
100
200
300
MgB
2
H = 0
ρ
(
µΩ
.c
m
)
Temperature (K)
38
39
40
0
40
80
Figure1. ResistivitymeasurementofabulkMgB2 sample:
themaingraphshowsxTuptoroomtemperature;inset
fouses the doubletransition, revealing agranular
hara-teristi.
-10000
-5000
0
5000
10000
-0.06
-0.04
-0.02
0.00
0.02
0.04
MgB
2
bulk - T = 10 K
powder - T = 2 K
M (
em
u
/g
)
H (Oe)
Figure 2. Mx H urve for bulk and powder MgB2.
Dif-ferenesbetweenvaluesofpowderandbulkaredetailedon
10
20
30
40
0.0
2.0
4.0
6.0
8.0
h = 1.0 Oe
h = 2.0 Oe
h = 3.8 Oe
M
rem
(10
-5
em
u)
T (K)
0
10
20
30
40
50
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
MgB
2
h = 0.1 Oe
Figure3. MagnetiremaneneofabulkMgB2 sampleafter
appliationofanACeldof0.1,1.0,2.0 and3.8 Oe. Inset
showsthemeasurementforsmallesth.
Figure4showshowthePMEisrevealedinthe
stud-ied sample. It is a quite subtle eet, emerging as a
dierene between the magnetization urves taken in
the presene of a magneti eld while ooling
(eld-ooled ooling, FCC) or warming (eld-ooled
warm-ing, FCW)thesample. Interestinglyenough,
depend-ingonthevalueoftheeldtheFCWurvelaysabove
orbelowitsorrespondingFCC.Curveswere seleted
toevidenethisinversionbetweenFCCandFCW
mag-netizations. Measurementstakenat H = 7000Oe are
largelyampliedin theinset. Wehavealsoperformed
timerelaxationmeasurementsofM,whihensuredthat
theeetisnotduetorelaxationofametastablestate.
0
5
10
15
20
25
30
35
40
-30
-20
-10
0
FCC
FCW
MgB
2
50 Oe
1000 Oe
7000 Oe
χ
DC
(10
-7
em
u)
T (K)
10
20
FCC
FCW
Figure 4. DC of a bulk MgB2 sample exhibiting PME.
OneanseetheinversionofFCWandFCCurves,whih
isdepitedintheinset.
An alternativewaytostudy this peuliar behavior
istosanthetemperatureupanddowninthepresene
that these resultsare independent of thetemperature
variationrate. Also,regardlessofthevalueoftheeld,
themagnetizationalwaysendsattheFCWurve,
sug-gestingthattheeet mightberelatedto pinningand
that there might be an ideal \training path" for
op-timizing themagneti response and, onsequently, the
ritialurrentof\dirty"speimensofMgB
2 .
0
10
20
30
40
-1.5
-1.0
-0.5
0.0
FCC
FCC
Granular MgB
2
H = 1000 Oe
H = 7000 Oe
ma
g
n
et
ic
mo
me
n
t (
1
0
-3
em
u)
T (K)
Figure 5. Sans of temperature for a bulk MgB2 sample.
The magneti response suggests a \training path" ending
upattheFCWmagnetization.
Insummingup,wehavepresentedexperimental
re-sults showing that the studied samples of MgB
2
ex-hibitmostofthetypialharateristisofaminimally
orderedarrayofweak-links,namely,asuppressed
mag-spondingpowderresponse,theparamagnetiMeissner
eet and a magneti remanene. Moreover,
temper-aturesans in the preseneof anapplied eld suggest
that pinning might be the ause of the observed
fea-tures,andthatthesystemmagnetizationalwaystends
totheeld-ooledwarmingbranh.
Aknowledgements
Brazilian naning agenies FAPESP, CNPq,
CAPESandFINEPareaknowledgedforpartial
nan-ialsupportto thiswork.
Referenes
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