Millisecond spin-flip times of donor-bound electrons in GaAs

We observe millisecond spin-flip relaxation times of donor-bound electrons in high-purity $n\text{\ensuremath{-}}\mathrm{GaAs}$. This is three orders of magnitude larger than previously reported lifetimes in $n\text{\ensuremath{-}}\mathrm{GaAs}$. Spin-flip times are measured as a function of magnetic field and exhibit a strong power-law dependence for fields greater than $4\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. This result is in qualitative agreement with previously reported theory and measurements of electrons in quantum dots.