Magnetization peaks in type II superconductors / Picos da magnetização em supercondutores do tipo II

AUTOR(ES)

Tarciso Mesquita de Oliveira

DATA DE PUBLICAÇÃO

2005

RESUMO

In this thesis we have studied possible origins for the Second Magnetization Peak (SMP) in high and low critical temperature superconductors, using samples of Bi2Sr2CaCu2O8+d and Nb single crystals. To study the SMP we did measurements of magnetization curves (M x H) and AC susceptibility (vs. h and T). We obtained the creep exponent in the SMP region, using a scaling law for the AC susceptibility data. In the Nb samples we have verified the effects on M x H curves due to inhomogeneities like dislocations and interstitial oxygen. In high pure samples (bulk and single crystals) we have observed that the M x H curves do not present either SMP or Peak Effect, near H2c . By adding inhomogeneities in the samples, like doping them with oxygen or deforming by elongation, we have observed the appearance of Peak Effect, thermomagnetic instabilities, and H2c anisotropy. In Bi2Sr2CaCu2O8+d samples we have observed that the SMP is associated with the magnetic field component parallel to the crystal c direction of crystal net. This means that the SMP is related to Abrikosov?s vortices and that Josephson?s vortices seem not to influence the SMP. After cleaving the sample we observed that the SMP intensity was reduced and the temperature window were it occurs was decreased. We have obtained the creep exponent and have observed that in the SMP region the vortices creep diminish for fields below HSPM , but around HSPM the creep increases abruptly and after the SPM it decreases again. We have interpreted the abrupt creep exponent variation as a phase transition in the vortex system, that pass from a quasi-lattice state to an entangled vortex state

ASSUNTO(S)

type ii superconductors bismuto dinamica de vortices niobio magnetização bismuth supercondutividade de alta temperatura niobium vortex dynamics high temperature superconductors supercondutores do tipo ii

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