Kapitsa pendulum effects in Josephson junction + nanomagnet under external periodic drive

Abstract

We investigate Kapitsa pendulum-like effects in the magnetic moment dynamics of a nanomagnet coupled to a Josephson junction under external periodic drive. Generated by the Josephson junction and external drive, the magnetic field plays the role of the oscillating force of the suspension point in the Kapitsa pendulum. The high frequency oscillations change the orientation of magnetic moment. The magnetic field of the quasiparticle current of the Josephson junction determines the frequency dependence of the magnetic moment's orientation. We obtain simple analytical formulas for the stable position of magnetic moment, both under the external periodic drive and without it. The influence of external periodic drive on the voltage of complete reorientation have been demonstrated.

Bifurcation structure and chaos in nanomagnet coupled to Josephson junction

“…The other one is related to the oscillating magnetic field generated by the superconducting current. The second one is a manifestation of Kapitza pendulum-like feature which was observed in the magnetization dynamics of the nanomagnet [11,35] and ϕ 0junction [34].…”

“…This shift can be calculated from the vector potential A m (r, t) which takes into account the magnetic field of the nanomagnet created at point r and external magnetic fields if considered (see refs. [12,35] for detail). According to this, the shift is given by [12]:…”

“…where the integration goes from one side of the junction to the other side, µ 0 is the permeability of free space and k play the role of the coupling in the proposed system. The last term which contributes to the total energy is generated by the normal current and is given by [12,35]:…”

“…The nonlinear dynamics of the JJ is sensitive to the orientation of the magnetization [16,[25][26][27][28][29][30][31], and a rich physics has been predicted due to this type of coupling between the Josephson and magnetic subsystems: for example, supercurrent-induced magnetization dynamics [25,32,33]. In the NM-JJ system, the reversal of the magnetic moment by the supercurrent pulse [30], the appearance of Devil's staircase [24] and Kapitza pendulum effects [11,34,35], have been investigated.…”

“…In Ref. [11,35], the authors introduced the Kapitza pendulum as a mechanical analog to the NM-JJ system and demonstrated the reorientation of the easy axis of the magnetic moment of the nanomagnet. In this case, the Josephson to magnetic energy ratio G plays the role of the drive amplitude of the variable force and Josephson frequency Ω J plays the role of the drive frequency in the Kapitza problem.…”