Spin flip scattering engendered quantum spin torque in a Josephson junction

TL;DR

This paper investigates a Josephson junction with ferromagnets and a spin flipper, revealing a novel quantum spin torque mechanism induced by spin-flip scattering, even when ferromagnetic moments are aligned.

Contribution

It uncovers a new quantum spin torque in ferromagnetic Josephson junctions caused by spin-flip scattering, independent of magnetic misalignment.

Findings

Finite equilibrium spin torque occurs with aligned ferromagnets due to quantum spin-flip scattering.

The quantum spin torque can be tuned via exchange coupling and phase difference.

The mechanism differs from conventional spin transfer torque based on misalignment.

Abstract

We examine a Josephson junction with two Ferromagnets and a spin flipper sandwiched between two superconductors. In such Ferromagnetic Josephson junctions, equilibrium spin torque exists only when Ferromagnets are misaligned. This is explained via the "conventional" mechanism of spin transfer torque, which owes its origin to the misalignment of two Ferromagnets. However, we see surprisingly when the magnetic moments of the Ferromagnets are aligned parallel or antiparallel, there is a finite equilibrium spin torque due to the quantum mechanism of spin-flip scattering. We explore the properties of this unique spin-flip scattering induced equilibrium quantum spin torque, especially its tunability via exchange coupling and phase difference across the superconductors.