Abstract
A spiral state is proposed to be the stable magnetic structure of perovskite $(\mathrm{L}\mathrm{a}\ensuremath{-}X){\mathrm{MnO}}_{3}$ ( $X$: 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 $(\mathrm{L}\mathrm{a}\ensuremath{-}X){\mathrm{MnO}}_{3}$, indicating that the Hund coupling enhances the dependence of the resistivity on the induced magnetization in agreement with experiments.
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