  The spin dynamics in a broad range of systems can be studied using circularly
polarized optical excitation with alternating helicity. The dependence of spin
polarization on the frequency of helicity alternation, known as the spin
inertia effect, is used here to study the spin dynamics in singly-charged
(In,Ga)As/GaAs quantum dots (QDs) providing insight into spin generation and
accumulation processes. We demonstrate that the dependence of spin polarization
in $n$- and $p$-type QDs on the external magnetic field has a characteristic V-
and M-like shape, respectively. This difference is related to different
microscopic mechanisms of resident carriers spin orientation. It allows us to
determine the parameters of the spin dynamics both for the ground and excited
states of singly-charged QDs.
