Nb / Nb / [Co / Pd] superconducting thin films: nanostructures and structural, magnetic and electric characterization.

Name: Tales Costa de Freitas
Type: PhD thesis
Publication date: 10/03/2017
Advisor:

Name Rolesort descending
Edson Passamani Caetano Advisor *
Jorge Luis Gonzalez Alfonso Co-advisor *

Examining board:

Name Rolesort descending
Edson Passamani Caetano Advisor *
Jorge Luis Gonzalez Alfonso Co advisor *
Waldemar Augusto de Almeida Macedo External Examiner *
Mucio Amado Continentino External Examiner *
Valberto Pedruzzi Nascimento Internal Examiner *

Summary: This thesis deals with the studies of structural, magnetic and electric transport properties of Nb thin films grown directly on Si (100) substrates, by Magnetron Sputtering, and under diferent substrate temperatures TS and Nb thicknesses tNb and also in their hybrid S/F
superconductor/ferromagnetic systems such as Si(100)/Nb(tNb nm)/Pd(1.1 nm)/[Co(0.3 nm)Pd(1.1 nm)]12 and Si(100)/Nb(100 nm)/Pd(tPd nm)/[Co(0.3 nm)Pd(3 nm)]12, WHERE the tNb value is of 15, 20, 50 and 100 nm, and Pd thickness tPd is of 1, 3 ,5 e 10 nm. In
the case of the Nb thin films, the main results show: (i) the dependence of the critical density current JC(T) with the substrate temperature TS, WHERE it was demonstrated the optimum value for the TS (responsible for the highest JC value). The JC(T) values were estimated by mean of magnetic hysteresis loops, assuming the granular character of the
Nb films experimentally observed by atomic force microscopy; (ii) the in
uence of the Nb thickness tNb on the granular features of Nb films; a characteristic that determines the significant reduction of the superconducting critical temperature TC, due to the finite grain size effect; (iii) the semiconducting and negative magnetorresistance behaviors of Nb films around the superconducting transition; properties that were determined, in a small temperature range, by measuring the electrical resistance of granular Nb thinner films (20 e 50 nm); (iv) the dependence of μ0HC2(T) for the 100 nm thick Nb films that present
3D-like behavior (except for their lower TC values), according to empirical models applied to analyze the experimental data of this work. In the case of the hybrid S/F system, the main results demonstrate (I) that the S/F system behaves within an intermediate stage between 2D-3D transition; (II) the in uence of proximity effect of the ferromagnetic F material on the Nb superconducting properties, which causes a reduction and "oscillation" of the critical temperature TC. Empirical models, reported in the literature, help to establish parameter of control of the action of external agents on the dynamic orientation
of the Co/Pd multilayers and the nucleation of Nb superconductor films, establishing, for example, intensities and regions (temperature and magnetic field ranges) for technological applications of the Si/Nb/Pd/[Co/Pd] heterostructures here studied. The films, here studied, were prepared more than one time and their results are reasonable reproducible, indicating that the discussed properties are related to the system and do not to specific samples.

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