Transition between two ferromagnetic states driven by orbital ordering in La0.88Sr0.12MnO3

A lightly doped manganite La0.88Sr0.12MnO3 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 orderings is directly detected below Too, 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. [S0031-9007(99)09213-3].