Magneto-Transport Properties and Hole Self-Doping due to Excess Oxygen Addition in Polycrystalline LaMnO<sub>3</sub>

Abstract

We have presented the study on magneto-transport properties and hole self-doping due to excess oxygen addition in polycrystalline LaMnO3 (LMO). The polycrystalline LMO samples were prepared by use of a solid-state reaction method. Powder mixtures with a molar ratio of 1:1 between La2O3 and Mn2O3 were pre-annealed at 1100oC for 18 hours in the atmospheres of O2, He and vacuum. By this pre-annealing, non-crystalline LMO samples were produced. After that, the non-crystalline LMO samples were grinded and were pressed into pellets at the pressure of 3t/cm3. The pellets were annealed at 1100oC and 1300oC for 18 hours in the same atmospheres as the pre-annealing. Through these processes, crystalline LMO samples were produced. To investigate the crystallographic structure of the LMO samples, X-ray diffraction (XRD) measurements have been performed by use of Cu-K radiation. From the results of XRD measurements, we have found that all LMO samples have perovskite structure and are polycrystalline. In addition, to investigate the surface structure of the LMO samples, scanning electron microscope (SEM) measurements have been carried out. Electrical resistivities (ERs) for the LMO samples have been measured as a function of temperature (4K-300K). The ERs of the LMO samples produced in O2 atmosphere show lower values as compared with other LMO ones produced in He and vacuum atmospheres. Especially, the temperature dependence of the ER for a LMO sample produced at the annealing temperature of 1100oC in O2 atmosphere shows metallic conduction. Thus, we have considered that this LMO sample has the largest hole self-doping concentration in all LMO ones. In addition, the magnetic field dependences of the magneto-resistance (MR) ratios for the LMO samples have been measured at several temperatures. The ferromagnetic behaviors on the MR ratios have been observed for LMO samples produced in O2 and He atmospheres.