Spin injection in metals and semiconductors

Abstract

The propagation of spin-polarized currents from a magnetic to a nonmagnetic material is fundamental to spintronics and, furthermore, has become a topic of great interest for basic research. The techniques of electrical spin injection and detection in bulk metals and thin metal films are reviewed, along with recent studies of spin injection between a ferromagnetic metal and a two-dimensional electron gas (2DEG). Electric current crossing the interface between a ferromagnet (F) and nonmagnetic metal (N) is also a current of magnetization, JM, and provides a source for spin diffusion in N. When the spin relaxation rate 1/T2 in N is relatively slow, a nonequilibrium population of polarized spins accumulates in a region characterized by the interfacial area and the spin diffusion length δs. The spin accumulation, equivalently called nonequilibrium magnetization M̃, can be detected by application of a transverse external magnetic field which causes a Hanle effect. Spin–orbit effects are the dominant spin relaxation mechanism in nonmagnetic metals. In a noncentrosymmetric 2DEG, unusual spin–orbit effects cause a spin splitting of the electron energy band. A bias current applied to such a 2DEG can generate nonequilibrium spin magnetization, which can be detected by a ferromagnetic electrode. Preliminary experiments on the injection and detection of spin polarized current at a F–2DEG are relevant to the realization of a spin injected field effect transistor (FET).