Investigation of the Josephson coupling through a magnetoactive barrier (ferrimagnet, paramagnet) in Y3/4Lu1/4Ba2Cu3O7 + Y3(Al1 − x Fe x )5O12 composites

The transport properties of two-phase composites consisting of a high-temperature superconductor and a nonsuperconducting component with magnetic ordering are analyzed. These composites are considered as a network of “superconductor-magnetoactive insulator-superconductor” weak links of the Josephson type. Substituted garnets Y3(Al1 − xFex)5O12 (x = 0, ..., 1.0) are used as a magnetoactive component. The composites under investigation contain 92.5 vol % Y3/4Lu1/4Ba2Cu3O7 (the high-temperature superconductor) and 7.5 vol % Y3(Al1 − xFex)5O12 (x = 0, ..., 1.0). It is shown that an increase in the iron content in the Y3(Al1 − xFex)5O12 garnet leads to a reduction of the Josephson coupling strength: the temperature range in which the electrical resistance of the composites is equal to zero is reduced, and the critical current density at a temperature of 4.2 K decreases exponentially. For composites in which the iron content in the Y3(Al1 − xFex)5O12 garnet is higher than 0.1, the temperature dependence of the electrical resistance R(T) at temperatures below the transition point TC of high-temperature superconductor crystallites has a portion in the range Tm-TC where the resistance R(T) is independent of the transport current and the magnetic field strength. Below the temperature Tm, the dependences of the electrical resistance R(T) of the composites are nonlinear functions of the current and involve a considerable contribution from magnetoresistance. This behavior is characteristic of a network of Josephson junctions. The temperature Tm decreases with an increase in the iron content in the Y3(Al1 − xFex)5O12 garnet. The appearance of the above feature in the temperature dependences of the electrical resistance R(T) is interpreted as complete suppression of the Josephson coupling in the temperature range above Tm due to the interaction of supercurrent carrier pairs with magnetic moments of iron atoms in the dielectric barriers separating high-temperature superconductor grains.