Magnetic properties and the phase diagram of La_1-<i>x</i>Sr_<i>x</i>MnO₃for<i>x</i>⩽0.2

Abstract

We report on susceptibility, magnetization, magnetoresistance, and ultrasonic studies of La1-xSrxMnO3 for x⩽0.2. The magnetic susceptibilities in the different high-temperature paramagnetic phases are studied in detail. We provide experimental evidence for the occurrence of short-range ferromagnetic order within the paramagnetic phase. Furthermore, we can exclude the possibility of electronic phase separation for x<0.1. Variable-range-hopping conductivity dominates in the paramagnetic and magnetically ordered phases. From the anomalies at the structural and magnetic phase transitions, 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 effects. We interpret the rich phase diagram close to x = 1/8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn-Teller distortions.