Singlephase polycrystalline samples of La067Pb033MnO3 were prepared by conventional solidstate reaction method. The crystal structure and magnetic and transport properties are investigated. The structure revealed by the room temperature xray powder diffraction is rhombohedral(R3C), and the Curie temperature is very close to TMI(=360K). At the Curie point, the magnetic properties show the transition from ferromagnetic to paramagnetic, and the conductivity changes from metal to semiconducting. The resistivity decreases with increasing magnetic field. Magnetoresistance reaches a maximum at the Curie point. The maximum magnetoresistance is 14\^5% at H=1\^2MA/m, and 9% at H=0\^6MA/m. The transport results show that the transport data satisfy ρ(T)=ρ0+ρT2.5 for TTMI. This phenomenon is due to the magoncarrier scattering.
Magnetic and electrical properties of polycrystalline La1.3Sr1.7Mn2-xCuxO7 prepared by solid state reaction were investigated. The X-ray diffraction pattern shows that the two samples have Sr3Ti2O7 type perovskite structure. The two samples undergo complex transition with lowering temperature. The x=0 samples, have the two-dimensional short-range ferromagnetic order at T*=231 K and then transform to the three-dimensional long-rangs ferromagnetic state at TC=114 K, at last they enter the canted antiferromagnetic state at TN=56K. The T*, TC and TN are all reduced at the 5% Cu doping level. Additionally, the two samples exhibited two distinct resistivity peaks, which is due to coexistence of the two phases. The metal-insulator transition temperature was decreased but the magnetoresistance was enhanced at the 5% Cu doping level.
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