Extensive measurements of the magnetization of superconducting singlecrystal samples of Bao,6Ko 4Bi03 have been made using superconducting quantum interference device and cantilever force magnetometry at temperatures ranging between 1.3 and 350K and in magnetic fields from near zero to 27T. Hysteresis curves of magnetization versus field allow a determination of the thermodynamic critical field, the reversibility field and the upper critical field as functions of temperature. The lower critical field is measured separately and the Ginzburg-Landau parameter is found to be temperature dependent. All critical fields have higher T = 0 limits than have been previously noted and none of the temperature dependences of the critical fields follows the expected power laws leading to a possible alternative interpretation of the thermodynamic nature of the superconducting transition.f Emaili
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