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Previous issue date: 2015-03-26 / CAPES / O estudo em sistemas de multicamadas supercondutoras alcan cou grande interesse na pesquisa
de novos materiais, pois al em de apresentarem ampla aplicabilidade tecnol ogica, oferecem
fascinantes possibilidades de observar novos fen^omenos na supercondutividade. Esta ultima caracter
stica e a de maior motiva c~ao para esta tese, uma vez que pouco ainda se tem reportado a
respeito da supercondutividade em multicamadas constitu das por
supercondutor(S)/supercondutor(S0). Neste trabalho, estudamos tr^es novos sistemas nanoestruturados
formados por supercondutores elementares, de baixa temperatura cr tica, tais como
ni obio (Nb), chumbo (Pb) e estanho (Sn), a saber, a tricamada Nb(5)/Pb(500)/Nb(50) e as multicamadas
Nb(100)/[Sn(50)/Nb(50)]7 e Cr(10)/Nb(100)/[Sn(50)/Nb(50)]7, onde o termo entre par^enteses
indica a espessura da referida camada em nan^ometros e os colchetes indicam que a estrutura
Nb/Sn e repetida 7 vezes. Para compara c~ao, tamb em s~ao analisados lmes de refer^encia de Nb,
Pb e Sn. Os lmes e as multicamadas foram crescidos a temperatura ambiente por deposi c~ao via
sputtering, utilizando fontes DC e RF. As amostras foram caracterizadas por difra c~ao de raios
X de baixo ^angulo, microscopia eletr^onica de varredura e por microscopia de for ca at^omica.
As propriedades supercondutoras foram estudadas atrav es de medidas magn eticas e de transporte
el etrico. Magnetiza c~ao e resistividade foram medidas como fun c~oes da temperatura e
do campo magn etico aplicado perpendicular e paralelamente as camadas. Os tr^es sistemas
apresentaram transi c~oes supercondutoras abruptas, tanto na magnetiza c~ao quanto na resistividade
em fun c~ao da temperatura, com os valores de TC obtidos de 7,2 K para o Nb/Pb/Nb,
5,2 K para o Nb/[Sn/Nb]7 e 3,7 K para o Cr/Nb/[Sn/Nb]7. As propriedades supercondutoras
dos lmes de Pb, Nb e Sn, tais como TC, 0Hc2 e comprimentos caracter sticos apresentaram
coer^encia com os valores reportados na literatura, con rmando a boa qualidade das camadas.
Flux jumps s~ao observados nos loops de histerese em todos os sistemas, os quais s~ao atribu dos
a instabilidades termomagn eticas. A amostra Nb/Pb/Nb apresentou uma curvatura positiva
na depend^encia de 0Hc2(T), indicando que a supercondutividade ocorre preferencialmente na
camada de Pb para T > T e preferencialmente na camada de Nb para T < T , de acordo com
a teoria de Takahashi-Tachiki. Os comprimentos caracter sticos s~ao calculados, classi cando
as multicamadas como supercondutores do tipo II. Veri camos grande in
u^encia do material
magn etico Cr na supercondutividade da multicamada, causando a diminui c~ao da temperatura
cr tica do sistema e modi cando o comportamento dos campos cr ticos inferior e superior. A
depend^encia de 0Hc1(T), em todos os sistemas, revelou um comportamento n~ao convencional,
que e atribu do a uma manifesta c~ao de sistemas multicomponentes com componentes espacialmente
separadas. / The study of superconducting multilayers systems has achieved much interest in the research
of new materials. Besides their wide technological applicability, these structures o er fascinating
possibilities to observe new phenomena in superconductivity. The latter characteristic is
the fundamental motivation for this thesis, since a little has been reported about multilayers
constituted of superconductor(S)/superconductor(S0). In this work, we have studied three new
nanostructured systems formed by low critical temperature conventional superconductors, such
as niobium (Nb), lead (Pb) and tin (Sn): the trilayer Nb(5)/Pb(500)/Nb(50) and the multilayers
Nb(100)/[Sn(50)/Nb(50)]7 and Cr(10)/Nb(100)/[Sn(50)/Nb(50)]7. The term in parenthesis indicates
the thickness of the layer in nanometers and the brackets that Nb/Sn structure is repeated 7
times. For comparison, the reference lms of Nb, Pb and Sn were also analyzed. The thin
lms and multilayers were growth at room temperature via sputtering deposition, by using
DC and RF sources. The samples were characterized for small angle X ray di raction, scanning
electron microscopy (SEM), and atomic force microscopy (AFM). The superconducting
properties were investigated through magnetic and electric transport measurements. The magnetization
and resistivity were obtained as functions of temperature and magnetic eld applied
both perpendicularly and parallel to the layers. All the multilayers systems have showed sharp
superconducting transitions in the dependence of magnetization and resistivity with the temperature.
The multilayers critical temperatures were found to be 7.2 K for Nb/Pb/Nb, 5.2 K
for Nb/[Sn/Nb]7 and 3.7 K for Cr/Nb/[Sn/Nb]7. The superconducting properties of reference
lms Pb, Nb and Sn, such as critical temperature, upper critical eld and characteristic lengths
were consistent with the literature values, con rming the good quality of the samples. In the
three systems
ux jumps were observed, which are attributed to thermomagnetic instabilities.
The Ginzburg-Landau parameter is estimated, classifying the multilayers as type II superconductors.
In the case of Nb/Pb/Nb sample, it presented an upward curvature in the 0Hc2(T)
diagrams, which is a signature of superconductivity nucleation in the each layer, in accordance
with Takahashi-Tachiki theory for multilayered systems. We found out a noticeable in
uence
of the magnetic material Cr on the multilayer superconducting properties, reducing the critical
temperature of the system and modifying the lower and upper critical elds behavior. The
dependence of 0Hc1(T), of all the multilayers, revealed a non-conventional behavior feature,
which is consistent with a multicomponent behavior with spatially separated components.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufpe.br:123456789/17342 |
| Date | 26 March 2015 |
| Creators | SANTOS, Flávia Portela |
| Contributors | AGUIAR, José Albino Oliveira de |
| Publisher | Universidade Federal de Pernambuco, Programa de Pos Graduacao em Fisica, UFPE, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Breton |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/masterThesis |
| Source | reponame:Repositório Institucional da UFPE, instname:Universidade Federal de Pernambuco, instacron:UFPE |
| Rights | Attribution-NonCommercial-NoDerivs 3.0 Brazil, http://creativecommons.org/licenses/by-nc-nd/3.0/br/, info:eu-repo/semantics/openAccess |
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