Orbital ordering and two ferromagnetic phases in low-doped La1-xSrxMnO3

We present a theory for the transition between two ferromagnetic phases observed experimentally in lightly doped La1-xSrxMnO3. Starting from an electronic model, the instabilities to various types of orbital orderings are studied within the random-phase approximation. In most cases, the instabilities occur in the region of strong correlations. A phase diagram is calculated in the case of strong correlation by means of the projected perturbation technique and the Schwinger boson technique. A phase transition between two types of orbital ordering occurs at a low doping, which may be closely relevant to recent experimental observations.