Spin injection and spin accumulation in all-metal mesoscopic spin valves

Abstract

We study the electrical injection and detection of spin accumulation in lateral ferromagnetic-metal--nonmagnetic-metal--ferromagnetic-metal (F/N/F) spin valve devices with transparent interfaces. Different ferromagnetic metals, Permalloy (Py), cobalt (Co), and nickel (Ni), are used as electrical spin injectors and detectors. For the nonmagnetic metal both aluminum (Al) and copper (Cu) are used. Our multiterminal geometry allows us to experimentally separate the spin valve effect from other magnetoresistance signals such as the anisotropic magnetoresistance and Hall effects. In a ``nonlocal'' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at $T=4.2\mathrm{K}$ as well as at room temperature (RT). For aluminum we obtain spin relaxation lengths $({\ensuremath{\lambda}}_{\mathrm{sf}})$ of $1.2\ensuremath{\mu}\mathrm{m}$ and $600\mathrm{nm}$ at $T=4.2\mathrm{K}$ and RT, respectively, whereas for copper we obtain $1.0\ensuremath{\mu}\mathrm{m}$ and 350 nm. At RT these spin relaxation lengths are within a factor of 2 of the maximal obtainable spin relaxation length, being limited by electron-phonon scattering. The spin relaxation times ${\ensuremath{\tau}}_{\mathrm{sf}}$ in the Al and Cu thin films are compared with theory and results obtained from giant magnetoresistance (GMR), conduction electron spin resonance, antiweak localization, and superconducting tunneling experiments. The magnitudes of the spin valve signals generated by the Py and Co electrodes are compared to the results obtained from GMR experiments. For the Ni electrodes no spin signal could be observed beyond experimental accuracy.