JAHN-TELLER EFFECT AND FERROMAGNETIC ORDERING IN LA0.875SR0.125MNO3 -A REENTRANT BEHAVIOR

The compound La0.875Sr0.125MnO3 undergoes a series of structural phase transitions in which different physical effects are involved. The com pound is rhombohedral above T(RO)similar to 400 K. On cooling, a phase transition to an orthorhombic Pbnm O-phase takes place at T-RO, no si gnificant orbital ordering is observed. At T(O'O '')similar to 270 K a structural phase transition to another orthorhombic (O') form takes p lace, and an orbital ordering appears due to a cooperative JT effect. For this temperature range the compound is a paramagnetic semiconducto r, Upon cooling it becomes ferromagnetic at T-C similar to 180 K and t he orbital ordering is partially suppressed. Below a temperature T(O'O '')similar to 150 K at which the average Mn magnetic moment is about 2 mu(B) the orbital ordering is completely suppressed by a first order structural transition to another orthorhombic phase (O '') similar to the O-phase. Between T-C and T-O'O '', the compound is metallic and b ecomes a ferromagnetic insulator below T-O'O '' where a canting of the ferramagnetic order appears.