Transition between Two Ferromagnetic States Driven by Orbital Ordering in La_0.88 Sr_0.12 MnO₃

A lightly doped manganite 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, orbital ordering is directly detected below T_OOin 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.