  Understanding the coexistence of ferromagnetism and insulating behavior in
manganites is an unsolved problem. We propose a localized-band model involving
effective intermediate-range electron-electron (electron-hole) repulsion
(attraction) generated by cooperative electron-phonon interaction. Double
exchange mechanism, involving holes virtually hopping to nearest neighbors and
back, produces magnetic polarons in an antiferromagnetic environment; when
these magnetic polarons coalesce and percolate the system, we get a
ferromagnetic insulator. Ferromagnetism gets more pronounced when the holes
(doping) increases or when the ratio hopping/polaronic-energy dominates over
superexchange-coupling/hopping.
