# Spin transfer torque and exchange coupling in Josephson junctions with ferromagnetic superconductor reservoirs

## TL;DR

This paper investigates spin transfer torque and exchange coupling in Josephson junctions with ferromagnetic superconductor reservoirs, revealing novel in-plane STT and exchange coupling behaviors depending on system parameters.

## Contribution

It presents the first analysis of in-plane spin transfer torque in ballistic triplet ferromagnetic superconductor Josephson junctions and explores their exchange coupling characteristics.

## Key findings

- In diffusive structures, torque causes antiparallel/parallel transitions based on parameters.
- In ballistic triplet FS junctions, STT exists in all three spatial directions.
- In-plane STT and beat-like oscillations in out-of-plane STT are reported for the first time.

## Abstract

In this paper, the spin transfer torque (STT) and the exchange coupling of the Josephson junctions containing interesting cases of diffusive/ballistic-triplet/singlet ferromagnetic superconductor materials together with diffusive Josephson junction of the form S$_{1}$/F$_{1}$/I$_1$/N/I$_2$/F$_2$ with I being insulating barrier are investigated. Using the Nazarov quantum circuit theory, it is found that for the diffusive FS$_1$/N/FS$_2$ structure the only emerged torque in normal direction to the plane of the exchange fields of F$_1$ and F$_2$ results in antiparallel/parallel or vice versa parallel/antiparallel transition of favorable exchange coupling depending on the considered parameters of the system, including the nonmagnetic spacer thickness, the superconducting phase difference, the length and the exchange field of the ferromagnets. Furthermore, the analyze of the width of the transitions, the phase difference interval in which an interlayer length-induced antiparallel/parallel transition can be occurred, is performed. For instance, as the exchange field or the temperature increases, the interval of phase difference gets larger. On the other hand, ballistic Josephson junction containing the triplet ferromagnetic superconductor reservoirs solving the 16$\times$16 Bogoliubov-de-Gennes equation is studied. It is found that although the exchange fields of the FS are laid in the z and y direction, the STT interestingly exists in all three directions of x, y and z. This exciting finding suggests that the favorable equilibrium configuration concerning the least exchange coupling occurs in the relative exchange field direction different from 0 or $\pi$. To the best of our knowledge it is for the first time that the occurrence of the in-plane STT is reported. Moreover, the occurrence of the beat like behavior with two oscillation period for the out-of-plane STT is interestingly acquired.