Abstract
A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of samples, with 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1–320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in evolves towards a ferromagnetic order as δ increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the octahedra. The ferromagnetic coupling weakens for The magnetic behavior is interpreted by taking into account two effects caused by the increase in δ: cation vacancies and ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for The sample shows a structural transition from rhombohedral to orthorhombic below with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below for which could explain the intriguing ferromagnetic insulator state. In the and samples we found at temperatures below magnetic and structural anomalies that are characteristic of charge ordering.
- Received 30 January 1997
DOI:https://doi.org/10.1103/PhysRevB.56.8902
©1997 American Physical Society