Phys. Rev. B 59, 436 (1999).
A recently proposed band model to describe metallic ferromagnetism predicts a lowering of the carrier's effective mass upon spin polarization, leading to a gain in kinetic energy. The effect arises from a reduction in bond-charge Coulomb repulsion when spin polarization develops. Recent optical experiments in EuB6 show clear evidence for this effect. Related manifestations of this physics are large negative magnetoresistance, anomalously large decrease in resistivity as the temperature is lowered, and positive pressure dependence of Tc, also seen in EuB6. We explore parameters in the model that can give rise to these effects of the magnitude seen in EuB6, and suggest that qualitatively similar effects should be seen to varying degrees in all ferromagnetic metals. In particular, similar effects of large magnitude are seen in La1 Ð xSrxMnO3. Photoemission experiments should be able to shed light on the validity of different possible explanations for the observed effects.