Estudos de relaxação magnetica em supercondutores do tipo - II

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

We present in this work a study of magnetic relaxation in superconducting Y1Ba2CU3O7-d samples. We have used a single crystal grown in zirconia crucible, with Tc ~ 90 K and D T ~ 2 K; and a directionally solidified MTG sample with 17 % of Y211, Tc ~ 91 K and .D T ~ 2 K for H = 10 Oe. We discuss in a simplified manner the most relevant aspects of the magnetic relaxation process in type-II superconductors, focusing mainly on the collective vortex creep theory. We analyze long duration magnetic relaxation measurements, obtained through M x time curves for the MTG sample, where we find the values of the flux creep exponent µ for the elastic vortex movement regime. We describe an anomalous behavior of the M x T curves, measured on warming (FCW), after lowering the temperature T from a value above Tc to another one well below, under an applied field H. These curves present an increase in diamagnetism (-M) with T, passing through a minimum before rapidly rising again until reaching the normal state base line. We analyze several of these curves measured in the single crystal and the MTG samples, under different fields H. We have tested scaling laws for the magnetization axis derived from the Collective Flux Creep theory. We introduce 3 different time windows, 2, 5 and 10 minutes, to observe small relaxation intervals. This is a novel experimental method to address the collective creep problem, which we believe is very useful to complement the traditional studies based only on long temporal relaxation of the magnetization. We have also observed the occurrence of another type of diamagnetic peak or magnetization minimum, near Tc and only for low fields (H <100 Oe), in policrystaline, granular and low quality single crystal samples. This anomaly is called "dip effect" and is satisfactorily interpreted as a consequence of flux pinning inintergranular regions (weak links) during cooling, with subsequent liberation during the warming measurement (FCW)

ASSUNTO(S)

supercondutividade cristais - propriedades magneticas

ACESSO AO ARTIGO

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