Transition between Two Ferromagnetic States Driven by Orbital Ordering inLa0.88Sr0.12MnO3

A lightly doped manganite ${\mathrm{La}}_{0.88}{\mathrm{Sr}}_{0.12}{\mathrm{MnO}}_3$ exhibits a phase transition at $T_{\mathrm{OO}}=145\mathrm{K}$ from a ferromagnetic metal ( $T_C=172\mathrm{K}$) to a novel ferromagnetic insulator. We identify that the key parameter in the transition is the orbital degree of freedom in $e_g$ electrons. By utilizing the resonant x-ray scattering, orbital ordering is directly detected below $T_{\mathrm{OO}}$, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present studies uncover a crucial role of the orbital degree of freedom in the metal-insulator transition in lightly doped manganites.