Detailed magnetization study of superconducting properties ofYBa2Cu3O7−x ceramic spheres

Abstract

We present a magnetization study of low densityYBa2Cu3O7−x ceramics carried outin magnetic fields H such that 0.5 Oe<H<50 kOe. It was demonstrated that superconducting links betweengrains may be completely suppressed either by a magnetic fieldH∼100 Oe (at low temperatures) or by an increase of temperature to above 70 K.This property of the present samples allowed us to evaluate the ratiobetween the average grain size and the magnetic field penetration depthλ. Furthermore,at temperatures T>85 K, using low field magnetization measurements, we were able to evaluate the temperature dependenceof λ, which turned out to be very close to predictions from conventional Ginzburg–Landautheory. Although the present samples consisted of randomly orientedgrains, specifics of magnetization measurements allowed for evaluation ofλab(T). Good agreement between our estimation of the grain size and the real sample structureprovides evidence for the validity of this analysis of magnetization data. Measurements ofthe equilibrium magnetization in high magnetic fields were used for evaluation ofHc2(T). At temperaturesclose to Tc, the Hc2(T) dependence turned out to be linear, in agreement with Ginzburg–Landau theory. The value of the temperatureat which Hc2 vanishes coincides with the superconducting critical temperature evaluated from low fieldmeasurements, which is important evidence of the validity of both approaches to theanalysis of magnetization data.