Magnetic instabilities in high-temperature superconductors under rapidly varying magnetic fields

The irreversible magnetization of single-crystalline samples of the high-temperature superconducting compounds ${\mathrm{LaSr}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$O has been studied under rapidly varying magnetic fields. Three types of behavior can be identified for increasing rates of the field variation: first, a stable regime close to the critical state; magnetic instabilities or jumps for intermediate ramp rates; and a second stable regime with a strongly suppressed magnetization for high sweep rates. The conditions for breaking the critical state and the dynamics of the flux penetration at the instability are discussed.