Irreversibility line of Ba1−xKxFe2As2 (Tc = 36.9 K) superconductor studied with ac-susceptibility measurements

We have studied the phase diagram of a Ba1−xKxFe2As2 (Tc = 36.9 K) single crystal superconductor by employing ac-susceptibility measurements, both as a function of temperature for constant external magnetic field and of magnetic dc-field for constant temperature for angles Θ = ∠(H, c-axis) = 0°, 30°, 45°, 60°, 75° and 90°, between the c-axis and the magnetic field. The irreversibility lines (Hirr(T, Θ)) are estimated from the onset of non-zero values of the amplitude of the third harmonic susceptibility. Hirr-lines for all the studied angles can be reproduced from the equation Hirr = H0(Θ)(1 − T/Tc)n, with n ≈  4/3. From the angular dependence of H0(Θ) parameter, we estimated the anisotropy of the irreversibility lines. In the temperature interval [35, Tc] the anisotropy parameter was estimated γ=Hirrab/Hirrc=2.2±0.1. The measurements of the real part of the fundamental (first harmonic) ac-susceptibility χ′(T) for constant temperature as a function of dc-magnetic field revealed, for high values of the ac-field amplitude, a second peak in the critical current. The second peak line is located far from Hirr-line and exists up to Tc. Although the particular sample has high critical densities, contrary to the predictions of the Bean's model, the maximum of the imaginary part of the fundamental ac-susceptibility χ″ exhibits lower values. This behavior could be explained by assuming a reversible motion of the flux lines around the pinning centers for low values of the ac-magnetic field amplitude.