Behavior of the irreversibility line in the new superconductor La1.5+xBa1.5+x−yCayCu3Oz

Abstract

The irreversibility properties of high-Tc superconductors are of major importance for technological applications. For example, a high irreversibility magnetic field is a more desirable quality for a superconductor [1]. The irreversibility line in the H–T plane is constituted by experimental points, which divides the irreversible and reversible behavior of the magnetization. The irreversibility lines for series of La1.5+xBa1.5+x−yCayCu3Oz polycrystalline samples with different doping were investigated. The samples were synthesized using the usual solid estate reaction method. Rietveld-type refinement of x-ray diffraction patterns permitted to determine the crystallization of material in a tetragonal structure. Curves of magnetization ZFC–FC for the system La1.5+xBa1.5+x−yCayCu3Oz, were measured in magnetic fields of the 10–20,000Oe, and allowed to obtain the values for the irreversibility and critical temperatures. The data of irreversibility temperature allowed demarcating the irreversibility line, Tirr(H). Two main lines are used for the interpretation of the irreversibility line: one of those which suppose that the vortexes are activated thermally and the other proposes that associated to Tirr a phase transition occurs. The irreversibility line is described by a power law. The obtained results allow concluding that in the system La1.5+xBa1.5+x−yCayCu3Oz a characteristic bend of the Almeida–Thouless (AT) tendency is dominant for low fields and a bend Gabay–Toulouse (GT) behavior for high magnetic fields. This feature of the irreversibility line has been reported as a characteristic of granular superconductors and it corroborates the topological effects of vortexes mentioned by several authors [1,2].