Estudo da relaxação magnetica e fluxo retido em Niobio supercondutor : efeitos da superficie

AUTOR(ES)
DATA DE PUBLICAÇÃO

1997

RESUMO

We present in this work a study of the surface influence in magnetic relaxation of superconducting niobium. We also discuss the relevance of the sample surface condition on magnetic flux trapping associated with the occurrence of a paramagnetic frozen state. Magnetization measurements were made, using a SQUID magnetometer, on a single-crystal Nb bar, for different sample surface conditions. Using the model for magnetic relaxation proposed by Anderson-Kim in 1964, we obtained the effective activation energy for some applied magnetic fields and temperatures, for each surface condition. Comparing the results obtained in this way, we tried to separate the surface and bulk contributions to the activation energy. We observed an asymmetry between flux entry and exit in the magnetic relaxation measurements. This asymmetry diminished with surface roughening, which is in agreement with theoretical results obtained by Burlachkov for surface barrier effects. However, the influence of the surface barrier alone cannot explain all of our data. We conclude that the surface roughening with sandpaper created a superficiallayer of damaged material which have a higher Ginzburg-Landau factor (K) than the bulk material. The influence of this high-K, layer masks the effects of the surface barrier in magnetization measurements. We propose the increase of Hc2 with the surface roughening, found in M x H measurements, is a consequence of the high-K, layer. Finally, we discuss the occurrence of a small paramagnetic maximum near the super-conducting transition in M x T data. The surface roughening tends to increase this paramagnetic maximum. We propose that this effect is caused by flux trapping due to the presence of the high-K, layer. To confirm this hypothesis we made a new set of measurements on a polycrystal Nb ring. We found that the paramagnetic maximum is highly dependent of the sample geometry, as expected in the flux trapping picture

ASSUNTO(S)

supercondutividade niobio

ACESSO AO ARTIGO

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