Influence of oxygen content on the structural, magnetotransport, and magnetic properties of LaMnO3+delta

A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of LaMnO3+delta samples, with delta=0, 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 LaMnO3 evolves towards a ferromagnetic order as delta increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the MnO6 octahedra. The ferromagnetic coupling weakens for delta greater than or equal to 0.1, The magnetic behavior is interpreted by taking into account two effects caused by the increase in delta: cation vacancies and Mn4+/Mn3+ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for delta greater than or equal to 0.1. The sample delta=0.1 shows a structural transition from rhombohedral to orthorhombic below T-s approximate to 300 K 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 delta=0.15, in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below T-c for delta greater than or equal to 0.07, which could explain the intriguing ferromagnetic insulator state. In the delta=0.07 and delta=0.10 samples we found at temperatures below T-c magnetic and structural anomalies that are characteristic of charge ordering.