Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8)

It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic insulator (FI) phase. The magnetization shows a sharp jump at the transition field accompanying with a remarkable increase of magnetoresistance. Striction measurements clarified that this transition is associated with the structural change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic phase. These results evidently show that the FI phase with a pseudo cubic symmetry is more stable in high fields than the FM phase due to the double exchange interaction. The driving force of this transition is explained by the enhancement of the ferromagnetic superexchange interaction induced by an antiferromagnetic type orbital ordering in the pseudo cubic phase, which was recently found in the anomalous X-ray scattering experiments.