The Study Of Magnetoresistance Effect And Magnetothermal Effect Of (La_1-x-yCa_1/3MnO₃) System With Rare-earth Vacancies

The perovskite maganese oxides materials are reviewed briefly, the structure, magnetoresistance effect(MRE), the magnetocaloric effect(MCR) of La_(1-x)2/3Ca_1/3MnO₃ and Y-doped (La_0.7-xY_0.3)_2/3Ca_1/3MnO₃ samples with La-vacancy (x=0,0.02,0.04,0.08,0.1) have been investigated. Their forming mechanisms are also dissussed.Our main work includes:1. The GMR effect of La_(1-x)2/3Ca_1/3MnO₃ systemPolycrystalline samples of manganese oxides La_(1-x)2/3Ca_1/3MnO₃ with La-vacancy were prepared by standard solid-state reaction method. All the samples are single phase, with orthorhombic symmetry, analyzed by XRD and with Rietveld Refinement. The electrical properties of La_(1-x)2/3Ca_1/3MnO₃ samples were measured by standard four-probe method. The insulator-metal transition temperature(about 267K) nearly remains unchanged with the increase of vacancy concentration, which indicates small La-vacancy does not destroy the double exchange interaction. The maximum of the MR value (8T) is about 220% at transition temperature(T_MI=268.6K). But the electronic resistance increases and MR value decreases with La-vacancy content x>0.04.2. The CMR effect of (La_0.7-xY_0.3)_2/3Ca_1-3MnO₃ systemPolycrystalline samples of manganese oxides (La_0.7-xY_0.32/3Ca_1/3MnO₃ with La-vacancy were prepared by solid-state reaction method. All the samples are single phase, with orthorhombic symmetry, analyzed by XRD and with Rietveld Refinement. The structure of (La_0.7-xY_0.32/3Ca_1/3MnO₃(x=0, 0.08) samples was analyzed by XRD at various temperature ranging from 10K to 300K. The results shows there is no change in structural symmetry. The electrical properties of (La_0.7-xY_0.32/3Ca_1/3MnO₃ samples were measured by standard four-probe method. The metal-insulator transition temperature(T_MI) shifts to lower temperature, but the corresponding resistivity peak increases with magnetic field of 8T applied. The materils shows strong magnetoresistance effect. The maximum MR value of these samples up to about 10⁶ %. It is suggested that it may be related to the partial substitution of La³⁺ caton with smaller Y³⁺ , which may destroy the double exchange interaction, giving rise to Jahn-Teller effect.