Influence On The Magnetic And Electric Properties Of (Na, Fe)-Doped-LaMnO₃

Mixed valance manganites R_1-xA_xMnO₃ (R stands for rare earth metals and A for divalent metal such as Ca, Sr, and Ba) with perovskite structure have attracted much attention of many researchers due to the observation of colossal magnetoresistance (CMR) close to the Curie temperature (TC). So far most studies have focused on the divalent alkaline-earth-medal doping in R_1-xA_xMnO₃ compounds. Until recent times, alkali ion substituted lanthanum manganites R_1-xA_xMnO₃ (A=Na,K,Rb) have been studied in few reports. In this paper we report our investigations on magnetic and transport properties of the compound La_1-xNa_xMn_1-yFeyO₃ (x=0.17,0.25;y=0,0.05,0.08,0.10,0.12). Polycrystalline samples of La_1-xNa_xMn_1-yFeyO₃ (x=0.17,0.25;y=0,0.05,0.08,0.10,0.12) were prepared by sol-gel method.The study of the samples of La_1-xNa_xMn_0.9Fe_0.1O₃ (x=0.17,0.25) shows that in the high doping regime, due to the increase of Mn₄⁺ concentration, the superexchang contribution becomes significant as compared to the hopping interaction, leading to the change of the TC-value is small. The magnetization of the samples tends to be saturated at a low field. Meanwhile the increase of Na content leads to the increase of the antiferromagnetic interaction. With the increase of Na content, the amount of magnetic impurity increase, resulting in the scatting between impurity and electron enhanced. Therefore the resistivity increase and the temperature TIM shifts slightly to higher temperature with the increase of Na content. All the compounds clearly reveal the existence of a resistivity minimum at low temperature, which is not influenced by an applied magnetic field. The origin of this resistivity is explained in terms of the enhanced electron-electron interaction. Above the metal-insulator transition temperature (TIM), the resistivity is well described by Mott's variable-range hopping model; while in the ferromagnetic region at low temperature, the resistivity is from single-magnon scattering and electron-magnon scattering.The study of the samples of La_0.83Na_0.17Mn_1-yFeyO₃ (y=0,0.05,0.08,0.10,0.12) shows that the increase of Fe content dose not changes the lattice structure. The increase of Fe content leads to the decrease of the ferromagnetic interaction and the increase of the antiferromagnetic interaction. The temperature TC shifts to the lower temperature with the increase of Fe content. The resistivity increase and the temperature TIM shifts slightly to higher temperature with the increase of Fe content. Above the metal-insulator transition temperature (TIM), the resistivity is well described by Mott's variable-range hopping model for y=0.12 sample and by the polaron nearest-neighbor hopping for y=0 sample. The increase of Fe content leads to carrier localization by random fluctuations in Hund's-rule coupling. Localization might lead to variable-range hopping of the eg electrons (small polarons) between the localized states. Above all, the studies of the compounds La_1-xNa_xMn_1-yFeyO₃ (x=0.17,0.25;y=0,0.05,0.08,0.10,0.12) make the mixed valance manganites have more characters. It will be prospective for us to pay more attention to study the magnetoresistance (MR) effect.