Transition between two ferromagnetic states driven by orbital ordering in La_{0.88}Sr_{0.12}MnO_3

A lightly doped perovskite mangantite La 0 . 88 Sr 0 . 12 MnO 3 exhibits a phase transition at T OO = 145 K from a ferromagnetic metal ( T C = 172 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 technique, orbital ordering is directly detected below T OO , in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.