Evolution of structure and magnetic properties of stoichiometry and oxygen rich LaMnO3 nanoparticles

In this report, the evolution of structure and magnetic properties in stoichiometry and oxygen rich LaMnO3 samples synthesized by a simple, cost effective, conventional co-precipitation technique are systematically studied. While X-ray powder diffraction reveals a monoclinic, I2/a structure in both samples, Reitveld refinement demonstrates higher John-Teller (J-T) distortion in the former one. Higher J-T distortion is accompanied with a large deviation in MnO bond length. FESEM (Field Emission Scanning Electron Microscopy) analysis combined with crystallite size calculated from XRD peak profile suggest that the particles are of 50-60 nm and are polycrystalline in nature. The antiferromagnetism observed in bulk LaMnO3 with TN = 140 K evolves towards a ferromagnetic order in both stoichiometry and oxygen rich samples. However, Curie temperature, TC, is found to be higher in oxygen rich sample due to higher MnOMn bond angle. The large effective magnetic moment calculated from Curie-Weiss plot is found to be higher in stoichiometry sample. Higher moment is ascribed to high oxygen vacancies evidenced from XPS analysis. Dispersion behavior observed in frequency dependent ac susceptibility (χ) indicates the spin-glass, cluster-glass and interacting superparamagnetic behavior. Fitting the susceptibility data with phenomenological models such as Neel-Arrhenius, Vogel-Fulcher and Power law, while canonical spin-glass, cluster-glass and interacting superparamagnetism is ruled out, the spin-glass behavior with a higher relaxation time in oxygen rich sample is confirmed.