Microwave studies of intergranular magnetic flux

Microwave absorption in high-temperature superconductors is a sensitive indicator of the motion of intergranular magnetic flux with dissipation arising from the nucleation and damped motion of fluxons driven by the microwave current. Modulated microwave absorption signals from granular ceramics show hysteresis arising from the development of forward and reverse critical states. Absorption troughs, seen in the scanned microwave response, have the same origin. In single-crystal YBa2Cu3O7-δ, absorption lines periodic in the magnetic field arise from microwave current-driven nucleation of fluxons in Josephson tunnel junctions at domain boundaries. Similar series have been observed in grains of Nb embedded in epoxy, indicating that microwave current-driven nucleation is a general characteristic of intergranular junctions. © 1989.