  The temperature dependence of the electron spin $g$ factor in GaAs is
investigated experimentally and theoretically. Experimentally, the $g$ factor
was measured using time-resolved Faraday rotation due to Larmor precession of
electron spins in the temperature range between 4.5 K and 190 K. The experiment
shows an almost linear increase of the $g$ value with the temperature. This
result is in good agreement with other measurements based on photoluminescence
quantum beats and time-resolved Kerr rotation up to room temperature. The
experimental data are described theoretically taking into account a diminishing
fundamental energy gap in GaAs due to lattice thermal dilatation and
nonparabolicity of the conduction band calculated using a five-level kp model.
At higher temperatures electrons populate higher Landau levels and the average
$g$ factor is obtained from a summation over many levels. A very good
description of the experimental data is obtained indicating that the observed
increase of the spin $g$ factor with the temperature is predominantly due to
band's nonparabolicity.
