  We propose the notion of spin-selective Kondo insulator, which provides a
fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice
model with antiferromagnetic coupling. This unveils a remarkable feature of the
ferromagnetic metallic phase: the majority-spin conduction electrons show
metallic- while the minority-spin electrons show insulating-behavior. The
resulting Kondo gap in the minority spin sector, which is due to the
cooperation of ferromagnetism and partial Kondo screening, evidences a
dynamically-induced commensurability for a combination of minority-spin
electrons and parts of localized spins. Furthermore, this mechanism predicts a
nontrivial relation between the macroscopic quantities such as electron
magnetization, spin polarization and electron filling.
