  We investigate electron spin relaxation in GaAs in the proximity of a Fe/MgO
layer using spin-resolved optical pump-probe spectroscopy, revealing a strong
dependence of the spin relaxation time on the strength of an exchange-driven
hyperfine field. The temperature dependence of this effect reveals a strong
correlation with carrier freeze out, implying that at low temperatures the free
carrier spin lifetime is dominated by inhomogeneity in the local hyperfine
field due to carrier localization. This result resolves a long-standing and
contentious question of the origin of the spin relaxation in GaAs at low
temperature when a magnetic field is present. Further, this improved
fundamental understanding paves the way for future experiments exploring the
time-dependent exchange interaction at the ferromagnet/semiconductor interface
and its impact on spin dissipation and transport in the regime of
dynamically-driven spin pumping.

Mesoscale and Nanoscale Physics