  A novel ferromagnetic transition, accompanied by carrier density collapse, is
found in doped charge-transfer insulators with strong electron-phonon coupling.
  The transition is driven by an exchange interaction of polaronic carriers
with localized spins; the strength of the interaction determines whether the
transition is first or second order. A giant drop in the number of current
carriers during the transition, which is a consequence of bound pairs formation
in the paramagnetic phase close to the transition, is extremely sensitive to an
external magnetic field. This carrier density collapse describes the
resistivity peak and the colossal magnetoresistance of doped manganites.
