  The dynamics of the coupled electron-nuclear spin system is studied in an
ensemble of singly-charged (In,Ga)As/GaAs quantum dots~(QDs) using periodic
optical excitation at 1\,GHz repetition rate. In combination with the
electron-nuclei interaction, this allows us to lock the electron spins into
magnetic resonance in a transverse external magnetic field. Sweeping the field
to higher values, the locking leads to an effective "diamagnetic" response of
significant strength due to dynamic nuclear polarization, which shields the QD
electrons at least partly from the external field and can even keep the
internal magnetic field constant up to 1.3\,T field variation. We model the
effect through a magnetic field-dependent polarization rate of the nuclei, from
which we suggest a strategy for adjusting the nuclear polarization through the
detuning between optical excitation and electronic transition, in addition to
the magnetic field.
