Saturation of spin-polarized current in nanometer scale aluminum grains

We describe measurements of spin-polarized tunneling via discrete energy levels of single aluminum grains. In high resistance samples (∼GΩ) , the spin-polarized tunneling current rapidly saturates as a function of the bias voltage. This indicates that the spin-polarized current is carried only via the ground state and the few lowest in energy excited states of this grain. At the saturation voltage, the spin-relaxation rate T₁^-1 of the highest states excited by tunneling is comparable to the electron tunneling rate, T₁^-1≈1.5×10⁶ and 10⁷s^-1 , in two samples. The ratio of T₁^-1 to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet scaling, an evidence that spin relaxation in Al grains is governed by the spin-orbit interaction.