Chaotic dynamics from coupled magnetic monodomain and Josephson current

The ordinary (superconductor-insulator-superconductor) Josephson junction cannot exhibit chaos in the absence of an external ac drive, whereas in the superconductor-ferromagnet-superconductor Josephson junction, known as the O0 junction, the magnetic layer effectively provides two extra degrees of freedom that can facilitate chaotic dynamics in the resulting four-dimensional autonomous system. In this work, we use the Landau-Lifshitz-Gilbert model for the magnetic moment of the ferromagnetic weak link, while the Josephson junction is described by the resistively capacitively shunted-junction model. We study the chaotic dynamics of the system for parameters surrounding the ferromagnetic resonance region, i.e., for which the Josephson frequency is reasonably close to the ferromagnetic frequency. We show that, due to the conservation of magnetic moment magnitude, two of the numerically computed full spectrum Lyapunov characteristic exponents are trivially zero. One-parameter bifurcation diagrams are used to investigate various transitions that occur between quasiperiodic, chaotic, and regular regions as the dc-bias current through the junction, I, is varied. We also compute two-dimensional bifurcation diagrams, which are similar to traditional isospike diagrams, to display the different periodicities and synchronization properties in the I-G parameter space, where G is the ratio between the Josephson energy and the magnetic anisotropy energy. We find that as I is reduced the onset of chaos occurs shortly before the transition to the superconducting state. This onset of chaos is signaled by a rapid rise in supercurrent (IS --> I) which corresponds, dynamically, to increasing anharmonicity in phase rotations of the junction.

Automated summary

In the superconductor-ferromagnet-superconductor Josephson junction, the magnetic layer provides additional degrees of freedom that lead to chaotic dynamics, which is not observed in the ordinary superconductor-insulator-superconductor Josephson junction. By studying various parameters, we discovered that chaos occurs when the Josephson frequency is close to the ferromagnetic frequency. Furthermore, the onset of chaos precedes the transition to the superconducting state as the dc-bias current decreases.