Short-time magnetization insuperconducting thin films

REVISTA MEXICANA DE FíSICA 43 SUPLEMENTO
3. 193-195
DICIEMBRE 1998
Short-time magnetization in superconducting thin films
H. Ferrari. S.O. Valenzuela. and V. Bekeris
Laboratorio de Bajas Temperaturas, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Buenos Aires
Argefllina
J. Guimpel and F. de la Cruz
Cetltro Atómico Bariloche Qnd Instituto Balseiro Comisión Nacional de Energía Atómica
8400 San Carlos de Bariloche. R.N.. Argemina
Recibido ellO de enero de 1998; aceptado e16 de marzo de 1998
We havc investigated the magnctization
in thin (Y;Gd)Ba2Cu30¡_<l
superconducting
films in lhe lOO J.ls time !'calc. Magnetic moment was
measured with a non-conventional tcchnique lhat combines lhe use of a pulsed magnctic ficld and a synchronized pulsed laser. BOlh virgin
and magnetically cyeled samples werc examined and rcsl1lts are dcsnibed within Bean's critical state rnodel.
Krywords: dc-magnetization;
short.time; high Te superconductors;
thin films
Hemos investigado la magnetización dc de películas delgadas superconductoras de (Y;Gd)Ba2Cu307_ó) en la escala temporal de lOO lIS. El
momento magnético en función del campo aplicado fue medido con una técnica no convencional que combina el uso de un campo magnético
pulsado y un láser pulsado sincroni7,ado. Se examinaron tanto mucstras vírgenes cOl11omagnéticamcnte cicladas, y los resultados se describen
muy satisfactoriamcnte en el marco del modelo de estado crítico de Bean para un disco en un campo magnético transversal.
Descriptores: magnetización dc; tiempos cortos; supcrconductividad
rAes: 74.60.Ge:
de alta Te; películas delgadas
74.60.1g; 74.76.Bz
J. Introduction
The current and field dislribulion in thin supcrconducting
tilms in transverse magnetic ficlds has ocen un arca nI' inlerest in lhe pust years. Analytical Solulions for the critical
stale in circular disks [1] ao<llong strips [21. as well as numerical calculalions for square films [31, have been reported.
Nevertheless, when high Te superconductors in the mixed
stale are investigated with conventional slow temporal response magnelization leehniques. highly relaxed lIux diSlribution may lead to wrong estimates 01' lhe critical state parameters. We have measured lhe short-timc mugnetic momenl
01' (Y;Gd)Ba,Cu307_á
epilaxial supereondueling films wilh
a non-conventional technique Hl with an overall response
time 01' 10-5 s. six orders of magnilude below conventional
lechniques. Thc use of this technique may provide a way of
avoiding lhe relaxation drawbacks.
Both virgin samples and magnetically cycled samplcs
wcre examined, and the results are dcscribed within the
framework 01' Bean's critical state model. We report the temperalure dependencc of the apparenl erilieal eurrenl density
measured 100 JIS after having induced the critical slale.
2. Experimental
Brief1y, the magnetization measurements are performed as
follows. First. a transverse pulsed magnetic field wilh a
1 J1s rising elige is applied and then, after a delay time
t d = 100 J1s, a svnchronizcd shorl laser pulse ('" 10 ns) is
lriggered producing a rapid optieal healing of lhe sample lo
the normal slale [51. The metastable distribution of vonices al
id complelcly relaxes (magnetic Ilux penetrates the sample)
and the resulting flux variation is dcteclcd with a pick-up coi!.
Thc lime inlegraled signal thus oblained is proponional to lhe
magnetizalion al td. M(td). A seeond laser pulse is shotlo
leave lhe sample in a elean stale (B
O) before performing the next measurement al a higher magnetic field. This is
schemalieally indicated in I'ig. la [zero field eooled configuration (ZI'C»).
=
However. if lhe second laser pulse is not Iriggered. lhe
sample is left in a field eooled state before removing Ihe field
(FCR) ando in Ihe nexl laser shol. a eyeled FCR-ZFC sample
will be measured, as shown in Fig. lb.
We investigated lhe low field magnetization of two e-axis
orienled epitaxial RBa2Cu30i_ó films for magnetic f1eld Ha
perpendicular lo Ihe CuO, layers: a 0.5 x 0.7 em2• 300 nm
lhiek Ylla,Cu30'_á
íilm deposiled on SrTiO by laser ablaIion 01' bulk largels [61; and a 0.5 x 0.5 em2 film 300 nm thiek
GdBa,Cu30'_á
deposiled on (100) MgO by magnetron DC
spultering [7J. BOlh films were eharaelerized by AC suseeptibility rneasurerncnts, showing sharp transitions al Te equal
lo 90.3 K and 88.5 K. respeelively.
The fihns were altaehed lo the sample holder 01' an oplical cryostat. Two coaxial coils (primary and secondary) were
fixed lo lhe cryostat's cold f1nger. wilh their axis perpendicular lo Ihe íilm [4J. A Q-switehed Nd:YAG laser provided 10
ns FWIlM pulses at 532 nm. The spatial profile 01' ¡he pulse
is llaussian providin1! a uniform illumination of Ih{~"amnlf'
194
H. FERRARI
el /JI
ZFC
1,0
H
o
o
0,8
Laser
;; 0,6
::;; •
<al
-::¡¡
FCR - ZFC
0,4
0,2
H
0,0
Laser
O
(b)
FIGURE 1. Schematic temporal scqucnce for magnetization mea.
surcments. Two lascr pulses are shot for zero field coolcd samplcs
(ZFC). The magnctically cycled samples (FCR.ZFC) are rncasured
with one laser pulse. (FCR stands for lIcld cool reduced. sec (ex!).
The laser eomplelely ilJuminated the film sUffaee with normal
incidence aod a nuence of 2 mI/cm:.! . Magnetic moment was
Jctermincd with an overall resolution of 10-3 e.m.u. which,
for our set-up. is equivalent lo a Hall probc with 0.1 Gauss
rcsolution.
234567
HI H
d
FIGURE 2. Normalized time.intcgraled pick-up coil signals for dif.
ferenl lempcratures as a function of HJ Hd(T). for T = 81.5.
83.5, 84.5. 85.5 and 86.5 K. The measurements were performed
100 liS aftcr applying the magnctic lield. for ZFC (open symbols)
and FCR-ZfC (full symbols) samplcs. H.(T) was obtaincd by fitting lhcsc mcasuremenls to Eq. (1) and Eq. (2) rcspectively. The
conlinuous line shows the normalizcd fl!.
3. Result~ and discussion
o
uo
•
FCR lFe
-JlOl('.TIT,l'"
10
The virgin magnctic moment 01' a circular disk in a transvcrsc
magnelic field (ZfC case) can be wrillen as (ll:
M(T)
S(T)
=
8R'
-3HnS(T)
= ~2x
{areeos
[
cos
=
~()]
x
+ ros
sin
:,<~\}
x
-,
(I)
u
=
where T
Ha/ Hd(T), Hd(T)
j,(T)d/2 is a eharaelerislie field. R lhe film radius and d its lhiekness.
This exprcssion was uscd lo fit the normalized timeinlegrated piek-up coil signal wilh Ifd(T) as lhe only filling
parameter. To the best of our knowlcdge, analytical results
for lhe geometry of our films have not been reported.
The resulls for the YBCO film are shown in Fig. 2 (open
symbols) as a funelion of If / Hd(T) for different lemperatures. They are nonnalizcd by their corresponding saturation
values and plolled wilh the filling curve (fuJl line).
The experimental data for aJl lemperalures foJlow lhe lhemetical prediction for lhe lransvcrse Bcan model. This seems
lo imply lhal surfaee barriers 181 have a seeondary effeel
when eompared wilh bulk pinning and also lhal we are working in the single vortcx pinning rcgimc [9].
The magnelie momenl of a FCR-ZFC circular disk, is
given by lhe folJowing express ion (I]:
8R'
¡'vI(x) = -3Ha[S(T)
- 5(T/2)1,
(2)
whieh was used to fit the normalized results (Fig. 2, fuJl sym-
081
82
83
84
85
86
87
T(K)
FIGURE 3. je 1'5. T obtaincd from lhe filting parameter H,¡(T) for
ZFC (orcn syrnhols) and FCR-ZFC (full symbols) samplcs. The
contilluous line is given by )('(0)(1 - T/Te)3/2
(Te = 90.3 K).
The valuc of lhe critical current densily al T = O K is estimaled lO
be jorO) = 3.2 x lO' Alcm'.
bols) wilh Hd(T) as lhe filling parameler (fuJl line). Again,
resulls are welJ deseribed by lhe mode!.
Jt should be nOled ¡hal although previous relaxation experiments
in lhe same samples
[41 have shown lhat the flux
dislribution induced by removing the magnetic field relaxes
in aboul 20% before lhe oexl magnetie field pulse is applied (- I s), ealeulalions have shown lhal filling our dala
to Eg. (2) underestimates the critical current density in approximately 10%. However, this effecl could not be observed
with our presenl dala dispersion.
Rev. Mex. Fis. 4353 (1998) 193-195
SHORT-T1ME MAGNETlZATION
IN SUPERCONOUCTING
Figure 3 shows the temperalure dependenee of the eritieal eurrent density obtained from Hd(T) and lhe Ihickness of the film, je(T)
= 2Hd(T)/d.
Open (full) symbols eorrespond lo the ZFC (FCR-ZFC) data. If we assume
je = je(O)(1 - T /Te)3/2 as in ReL 10 (fullline), we obtain
je(O)
3.2 X 107 Nem' as an estimate of the eritieal eurrenl density al T = O K. Similar results wcrc obtained ror the
GBCO film.
=
195
THIN F1LMS
verse magnetic fields was reported. The critical statc was
induced in the J-lS time scale. and magnctic moment was
measured at 1001's following lhe magnetie f1eld rising edgc.
Films were examined in two differcnl magnctic configuratians, and results were satisfactorily describcd within Bean's
critical statc for a superconducting circular disk in a Lransverse magnetic fleld.
Acknowledgments
4, Conclusions
The low field dependenee of the magnetizalion of high temperature (Y;Gd)Ba,Cu307_á supereondueling films in trans-
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We are grateful lo S.L Krasnosvobodtsev fm providing the
YBCO film. This work was partially supporled by Fundación
Sauberán, EXO 18-VBA and EX285-VBA granls.
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e
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Rev. Mex. Fís. 4383 (1998) 193-195