Bifurcation structure and chaos in dynamics of nanomagnet coupled to Josephson junction

Abstract

Irregular easy axis reorientation features are observed in numerical simulations of the nanomagnet coupled to the Josephson junction. We study magnetization bifurcations and chaos that appear in this system due to the interplay of superconductivity and magnetism. The bifurcation structure of magnetization under the variation of Josephson to magnetic energy ratio as a control parameter demonstrates several precessional motions that are related to chaotic behavior and orbits with different periodicities in the ferromagnetic resonance region. The effect of an external periodic signal on the bifurcation structure is also investigated. The results demonstrate high-frequency modes of a periodic motion and a chaotic response near resonance. Far from the ferromagnetic resonance, we observe a quasiperiodic behavior. The obtained results explain the irregular reorientation of the easy axis and the transitions between different types of motion.

Josephson current-assisted reversal of a single-domain nanoscale ferromagnet driven by cosine chirp pulse

“…represents the dimensionless magnetic-feedback coupling coefficient [30,[32][33][34], where µ 0 is the permeability of free space. The dc voltage bias V dc is normalized to ℏω c /2e.…”

“…The stable magnetization orientation is no longer along the easy axis, (m x = 0, m y = 1, m z = 0), but changes into new stable position at (0, 0, 1). During this reorientation process, the dynamics of the NM exhibits chaotic response at frequencies near or less than the FMR frequency over a wide range of G [34]. So the improper choice of (G, Ω J ) can lead to complex dynamical behavior for magnetization and unwanted chaotic responses.…”

“…It can be 0.003-0.01 or much larger for the NM at a distance less than 250 nm. Due to the lack of experimental estimations on the coupled system, we consider estimates for the relevant parameters based on experiments of the individual subsystems (see [34] and references therein).…”

“…Instead of the superconducting hybrid junctions in which the proximity effect takes place, the electromagnetic interaction between a NM and JJ has been proposed to manipulate the magnetization and even cause magnetization reversal of the NM via the Josephson current controlled by voltage or current biases [30][31][32][33][34]. In such a coupled system, the Josephson current across the junction generates an oscillating magnetic field that excites the magnetization precession of the NM, whereas the magnetic field by the NM changes the current tunneling through the junction.…”