Spiral State and Giant Magnetoresistance in Perovskite Mn Oxides

A spiral state is proposed to be the stable magnetic structure of perovskite $(\mathrm{L}\mathrm{a}-X){\mathrm{MnO}}_3$ ( $X$: Ba, Ca, or Sr) with a low concentration of $X$ ions, contrary to the canted state predicted before. We use a mean field approximation applied to a model which treats $t_{2g}$ and $e_g$ electrons of Mn ions as localized spins and strongly correlated itinerant electrons, respectively, and includes a strong Hund coupling between them. We find that the Hund coupling is crucial for the giant magnetoresistance observed in $(\mathrm{L}\mathrm{a}-X){\mathrm{MnO}}_3$, indicating that the Hund coupling enhances the dependence of the resistivity on the induced magnetization in agreement with experiments.