A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\delta }}$ 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 ${\mathrm{LaMnO}}_3$ evolves towards a ferromagnetic order as δ increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_6$ octahedra. The ferromagnetic coupling weakens for $\delta >~\mathrm{0.1.}$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in δ: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\delta >~\mathrm{0.1.}$ The sample $\delta =0.1\mathrm{}$ shows a structural transition from rhombohedral to orthorhombic below $T_S\approx 300\mathrm{K}$ 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 $\delta >~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\delta =0.07\mathrm{}$ and $\delta =0.10\mathrm{}$ samples we found at temperatures below $T_C$ magnetic and structural anomalies that are characteristic of charge ordering.

• Received 30 January 1997

DOI:https://doi.org/10.1103/PhysRevB.56.8902