Giant magnetoresistance in layered Mn oxides

The magnetic phase diagram and giant magnetoresistance in perovskite (LaX)MnO3 with XBa, Ca or Sr are studied by using a model where t2g and eg electrons of Mn ions are treated as localized spins and strongly correlated itinerant electrons, respectively, and a Hund coupling exists between them. It is shown in a mean field approximation that the antiferromagnetism of LaMnO3 changes to a spiral state, not the canted state proposed before, by doping of mobile carriers. Giant magnetoresistance is studied in a strong coupling limit of the model. The strong Hund coupling is shown to account for a strong dependence of the resistivity on the magnetization induced by an external magnetic field in agreement with experimental results.