The equilibrium magnetization of granular high temperature superconductors in low fields. I. The Meissner effect of YBa 2(Cu 1- xFe x) 3O 7- y
Abstract
We report the temperature and field dependence of the DC Meissner effect of sintered pellets of YBa2( Cu1- xFex) 3O7- y with 0⪕ x⪕0.08. In all samples the spontaneous flux expulsion remains incomplete ( -4πχ≈ {1}/{3}) down to fields, which are much smaller than Hcl, but at H⪕ Hm≈2 Oe, they all exhibit a full Meissner effect and are therefore fully superconducting. The expulsion in 2 Oe decreases to near 30% by aging of the defect surfaces and goes back to near 100% after their reoxidation, all at a fixed and sharp Tc of 93 K. The data are interpreted in terms of equilibrium properties of granular high temperature superconductors on the basis of an analysis worked out in the next paper [1]. From the temperature and field dependence of the spontaneous flux expulsion, an average cross-section of 4μm 2 is extracted for the superconducting blocks bordered by defect surfaces within the much larger grains. The superconducting loops around these surfaces are responsible for the final flux expulsion in H<2 Oe. For the intrinsic transition width one finds Δ Tc⪕1 K in the orthorhombic phase ( x⪕0.02). In the quasitetragonal phase, Δ Tc increases from 1 to 16 K for 0.02< x⪕0.08.
- Publication:
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Physica C Superconductivity
- Pub Date:
- March 1991
- DOI:
- 10.1016/0921-4534(91)90442-2
- Bibcode:
- 1991PhyC..174..233R