Spin supercurrent in Josephson contacts with noncollinear ferromagnets

Abstract

We present a theoretical study of the Josephson coupling of two superconductors that are connected through a diffusive contact consisting of noncollinear ferromagnetic domains. The leads are conventional s-wave superconductors with a phase difference of φ. Firstly, we consider a contact with two domains with magnetization vectors misoriented by an angle θ. Using the quantum circuit theory, we found that in addition to the charge supercurrent, which shows a 0–π transition relative to the angle θ, a spin supercurrent with a spin polarization normal to the magnetization vectors flows between the domains. While the charge supercurrent is odd in φ and even in θ, the spin supercurrent is even in φ and odd in θ. Furthermore, with asymmetric insulating barriers at the interfaces of the junction, the system may experience an antiferromagnetic–ferromagnetic phase transition for φ=π. Secondly, we discuss the spin supercurrent in an extended magnetic texture with multiple domain walls. We find the position-dependent spin supercurrent. While the direction of the spin supercurrent is always perpendicular to the plane of the magnetization vectors, the magnitude of the spin supercurrent strongly depends on the phase difference between the superconductors and the number of domain walls. In particular, our results reveal the high sensitivity of spin- and charge-transport in the junction to the number of domain walls in the ferromagnet. We show that superconductivity in coexistence with noncollinear magnetism can be used in a Josephson nanodevice to create a controllable spin supercurrent acting as a spin transfer torque on a system. Our results demonstrate the possibility of coupling the superconducting phase to the magnetization dynamics and, hence, constituting a quantum interface, for example between the magnetization and a superconducting qubit.