  Various classical models of electrons including their spin degrees of freedom
are commonly applied to describe the electron dynamics in strong
electromagnetic fields. We demonstrate that different models can lead to
different or even contradicting predictions regarding how the spin degree of
freedom modifies the electron's orbital motion when the electron moves in
strong electromagnetic fields. This discrepancy is rooted in the model-specific
energy dependency of the spin-induced Stern-Gerlach force acting on the
electron. The Frenkel model and the classical Foldy-Wouthuysen model are
compared exemplarily in the nonrelativistic and the relativistic limits in
order to identify parameter regimes where these classical models make different
predictions. This allows for experimental tests of these models. In ultrastrong
laser setups in parameter regimes where effects of the Stern-Gerlach force
become relevant, radiation-reaction effects are also expected to set in. We
incorporate the radiation reaction classically via the Landau-Lifshitz equation
and demonstrate that although radiation-reaction effects can have a significant
effect on the electron trajectory, the Frenkel model and the classical
Foldy-Wouthuysen model remain distinguishable also if radiation-reaction
effects are taken into account. Our calculations are also suitable to verify
the Landau-Lifshitz equation for the radiation reaction of electrons and other
spin-1/2 particles.
