The time evolution of a local spin excitation in a (001)-confined two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit interactions of similar strength is investigated theoretically and compared with experimental data. Specific
ally, the consequences of the finite spatial extension of the initial spin polarization is studied for non-balanced Rashba and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction. We show that the initial out-of-plane spin polarization evolves into a helical spin pattern with a wave number that gradually approaches the value $q_0$ of the persistent spin helix mode. In addition to an exponential decay of the spin polarization that is proportional to both the spin-orbit imbalance and the cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the spin polarization by a factor that approaches $exp(-q_0^2 w^2/2)$ at longer times.
Spin injection from Co70Fe30 and Fe contacts into bulk GaAs(001) epilayers is studied experimentally. Using nonlocal measurements, the spin polarization of the differential conductance is determined as a function of the bias voltage applied across th
e injection interface. The spectra reveal an interface-related minority-spin peak at forward bias and a majority-spin peak at reverse bias, and are very similar, but shifted in energy, for Co70Fe30 and for Fe contacts. An increase of the spin-injection efficiency and a shift of the spectrum correlate with the Ga-to-As ratio at the interface between CoFe and GaAs.
The nonlocal spin resistance is measured as a function of temperature in a Fe/GaAs spin-injection device. For nonannealed samples that show minority-spin injection, the spin resistance is observed up to room temperature and decays exponentially with
temperature at a rate of 0.018,K$^{-1}$. Post-growth annealing at 440,K increases the spin signal at low temperatures, but the decay rate also increases to 0.030,K$^{-1}$. From measurements of the diffusion constant and the spin lifetime in the GaAs channel, we conclude that sample annealing modifies the temperature dependence of the spin transfer efficiency at injection and detection contacts. Surprisingly, the spin transfer efficiency increases in samples that exhibit minority-spin injection.
