Spin transport driven by emergent magnetic fields

Abstract

Spin current is a key concept in spintronics. In nonmagnetic metals, the spin-orbit coupling has been utilized for the spin current generation. Recently, an alternative scheme using emergent magnetic fields in moving media is demonstrated [1-5]. In this talk, we will present our recent results on spin transport in nonmagnetic metals driven by a variety of emergent magnetic fields originating from:1. Liquid metal flow [2,3].2. Elastic motion [2,4].3. Rigid rotation [1,5].4. Non-uniform conductivity of gradient material [6].5. Surface plasmon polaritons [7,8].Firstly, we consider the spin-current generation in moving media. In a moving media, such as elastic materials with the Rayleigh type surface acoustic wave is excited or a pipe flow of liquid metal, the vorticity field couples to conduction electron spin. As a result, the mechanical angular momentum of the moving media is converted into spin angular momentum. In particular, a spin current is generated along the vorticity gradient [2,3,4]. It should be noted that the mechanism using the spin-vorticity coupling is free from the strength of the spin-orbit coupling, and thus, weak spin-orbit coupling materials such as Cu can be utilized for the spin-current source [4].In a rigid rotating metal, the conventional spin-orbit coupling is augmented due to the emergent magnetic field originating from mechanical rotation [1]. The mechanically induced spin-orbit coupling leads to the spin Hall effect [1]. We introduce a theory of the mechanical analog of the spin Hall effect and an experimental demonstration using magneto-optical detection of spin accumulation under the influence of mechanical rotation [5]. Next, we will discuss our recent experimentally demonstration of the nonreciprocal generation of spin current in a surface-oxidized copper film [6]. This nonreciprocity relies on the transfer of angular momentum from the velocity field of conduction electrons' collective motion. Finally, we consider a mechanism of angular momentum conversion from a transverse optical spin in surface plasmon polaritons (SPPs) to conduction electron spin. Conduction electrons in the metal follow the SPP's SPP's transversally spinning electric field, and the resulting orbital motions create inhomogeneous static magnetization in the metal. We will also present an experimental demonstration of the SPPs-induced spin current generation [8]. Such conversion from light to a spin current can be used as a coupler in next-generation spintronic computing with optical data transfer or storage. **