Spin injection in metal films: The bipolar spin transistor

The spin injection technique has been adapted to a thin film, ferromagnetic-nonmagnetic-ferromagnetic metal (F1-N-F2) trilayer geometry, where the thickness of each film is greater than an electronic mean free path. Ferromagnetic film F1 is used to polarize the spin axes of the electrons carrying a bias current. When fabricated on a gold film (N), a non-equilibrium population of spin-polarized electrons builds up in the gold causing a “spin bottleneck”. Using a second ferromagnetic film, a three-terminal device is made with a bipolar output voltage (or current) that depends on the orientation of the spins. This is an active device driven by a thermodynamic force associated with the effective Zeeman energy of spin-polarized conduction electrons in the non-magnetic layer. This force is so large that spin dynamics dominate Coulomb electrodynamics in the system. Measuring the spin-coupled output voltage Vs as a function of the thickness d of N determines the conduction-electron spin-diffusion length, δrms, and the spin relaxation time, T1, in polycrystalline gold films: λs = 1.5 ± 0.4 μm, and T1 = 45 ± 15 ps, for 4 K < T < 70 K. Applications of the device are also discussed.