Magnetic Polarons and the Metal-Semiconductor Transition in (Eu,La)B6 and EuO: Raman Scattering Studies

We present inelastic light scattering measurements of EuO and Eu1−xLaxB6 (x=0, 0.005, 0.01, 0.03 and 0.05) as functions of doping, 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 decrease in Γ, the carrier scattering rate, with decreasing temperature and a loss of spectral intensity with increasing x; (b) a spin-disorder scattering regime, which is characterized by a collision-dominated electronic scattering response whose scattering rate Γ scales with the magnetic susceptibility; (c) a magnetic polaron (MP) regime, in which a magnetic polaron forms above the Curie transition temperature TC, and grows in energy with decreasing temperature and increasing magnetic field; and (d) a ferromagnetic metal regime, characterized by a flat electronic continuum response typical of other strongly correlated metals. As a result of these studies, we are able to present a qualitative (x,H,T) phase diagram for the Eu1−xLaxB6 system.