  A novel spin orientation mechanism - dynamic electron spin polarization has
been recently suggested in Phys. Rev. Lett. $\mathbf{125}$, 156801 (2020). It
takes place for unpolarized optical excitation in weak magnetic fields of the
order of a few millitesla. In this paper we demonstrate experimentally and
theoretically that the dynamic electron spin polarization degree changes sign
as a function of time, strength of the applied magnetic field and its
direction. The studies are performed on indirect band-gap (In,Al)As/AlAs
quantum dots and their results are explained in the framework of a theoretical
model developed for our experimental setting.
