  We present inelastic light scattering measurements of EuO and
Eu$_{1-x}$La$_{x}$B$_6$ ($x$=0, 0.005, 0.01, 0.03, and 0.05) as functions of
doping, B isotope, magnetic field, and temperature. Our results reveal a
variety of distinct regimes as a function of decreasing T: (a) a paramagnetic
semimetal regime, which is characterized by a collision-dominated electronic
scattering response whose scattering rate $\Gamma$ decreases with decreasing
temperature; (b) a spin-disorder scattering regime, which is characterized by a
collision-dominated electronic scattering response whose scattering rate
$\Gamma$ scales with the magnetic susceptibility; (c) a magnetic polaron (MP)
regime, in which the development of an $H$=0 spin-flip Raman response betrays
the formation of magnetic polarons in a narrow temperature range above the
Curie temperature T$_{\rm C}$; and (d) a ferromagnetic metal regime,
characterized by a flat electronic continuum response typical of other strongly
correlated metals. By exploring the behavior of the Raman responses in these
various regimes in response to changing external parameters, we are able to
investigate the evolution of charge and spin degrees of freedom through various
transitions in these materials.
