Understanding the ferromagnetic-insulator phase in manganites through a localized band model

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. A 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 concentration of holes (doping) increases or when the ratio of hopping to polaronic-energy dominates over the ratio of superexchange-coupling to hopping.