Critical state in a circular two-dimensional superconductor and magnetization of thinNd1.85Ce0.15CuO4−δandYBa2Cu3O7−δfilms in a transverse field

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Abstract

Precise measurements of the magnetization of circular epitaxial ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films in a transverse magnetic field are performed and analyzed within the framework of the recently presented critical state model (CSM) in the two-dimensional superconductor. Restrictions on the reliability of the model are considered. It has been found that the original CSM, based on Bean's limitation on the critical current density ${\mathit{J}}_{\mathit{c}}$=const, describes well the temperature and field dependences of the film magnetization in a low field (until the full disappearance of the flux-free region in the film) under the condition ${\mathit{B}}^{\mathrm{*}}$\ensuremath{\gg}${\mathrm{\ensuremath{\mu}}}_{0}$${\mathit{dJ}}_{\mathit{c}}$ln(\ensuremath{\surd}Rd /\ensuremath{\lambda}), where ${\mathit{B}}^{\mathrm{*}}$ is the characteristic field of the critical current drop in a strong field, \ensuremath{\lambda} is the penetration depth, d is the thickness, and R is the radius of the film, respectively. In the strong field an exponential or power suppression of the critical current is observed and the model under question is extended to this region. The problem of a lower critical field related to thin films is also discussed. The paramagnetic response demonstrated by all our films in field-cooled measurements, which has much in common with the Wohlleben effect, was investigated and the conclusion was drawn that this artifact is caused by the temperature-dependent stray field of the magnetometer parts.

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