Fishtail effect and the superconducting phase diagram of La_1.6-xNd_0.4Sr_xCuO₄ single crystal

Measurements of isothermal magnetic hysteresis loop in c-aligned external field have been performed on superconducting La_1.6-xNd_0.4Sr_xCuO₄ (x = 0.10, 0.20) single crystals, resulting in the H-T phase diagrams for the respective samples. The isothermal magnetization data of Sr-0.10 reveal the existence of fishtail effect in the certain temperature range of measurement (2.5 ≤ T ≤ 5.5 K), terminated at temperature well below the superconducting critical transition temperature (T_c^on ≈ 8 K). Analysis of the irreversibility line shows a sign reversal of its slope at around the crossover field, H_2D, indicating 2D melting at higher field. On the other hand, a single H_irr(T) function of 3D character was found to fit the irreversibility data for the Sr-0.20 sample, indicating the lack of vortex line decoupling effect in the same range. These remarkably different behaviors are suggested to have their origin in the different degrees of anisotropy (γ ≈ 96 for Sr-0.10 vs. γ ≈ 25 for Sr-0.20) as a consequence of different levels of Sr doping.