Abstract
The AC susceptibility, χ, at zero DC magnetic field of a polycrystalline sample of LaFeAsO0.94F0.06 (T c ≈24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H ac =0.003 Oe÷4 Oe) and the frequency (in the range f=10 kHz÷100 kHz). The χ(T) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of χ strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T c down to T=4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.
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