The phase diagram and optical properties of La\V Sr V MnO for x) 0.2

摘要: We report on detailed susceptibility, magnetization and magnetoresistance studies in La\V Sr V MnO for x) 0.2. We arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1) x) 0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn} Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appearance of large positive magnetoresistance e! ects. We interpret the rich phase diagram close to x" in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn} Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical conductivities. The temperature dependence of bending and stretching modes clearly indicate the structural and magnetic-phase transitions. The optical conductivities are explained in terms of small polaron absorption.