We examine a Josephson junction with two ferromagnets and a magnetic impurity 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 anti-parallel, 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.
Keywords: Josephson junction; spintronics; torque.