Structure And Electronic Magnetic Properites Study On Polycrystalline Perovskite Manganites

In this paper, we introduces the research progress of the perovskite manganese oxides, including the basic structure, the main theory in a strong correlate electronic system, the principle of samples preparation and experimental mothods. The polycrystalline samples of the ABO₃ and A₃B₂O₇ type perovskite manganese oxides were prepared by traditional high temperature solid state reaction method. The structure, magnetic properites, electric properites and magneto-resistance were characterized by X-ray diffraction (XRD), superconductor quantum interference device (SQUID) and standard four-probe method, respectively. The main results are as follows:The physical properties of ABO₃-type samples ( La_1-xGd_x)_0.5Sr_0.5MnO₃(x=0, 0.1, 0.2, 0.3, 0.4) were studied .The result shows that these samples can be well indexed on the cube perovskite structure with the space group Pm3m. The substitution of a little Gd on La site did not change the structure. When x≤0.2, samples exhibited a transition from a ferromagnetic metal state (FMM) to a paramagnetic insulator state (PMI); the antiferromagnetic insulator (AFMI)-paramagnetic insulator (PMI) transition shows up in the compositions for 0.3≤x≤0.4. All samples, both the curies temperature Tc and the metal-insulator transition temperature Tp decreases with increasing Gd content, the magneto-resistance (MR) effect near Tc could be observed and it get stronger in the low temperature. And the MR effect could be increased with the substitution of a little Gd for La .The physical properties of two series of A₃B₂O₇-typeLa_1.2-xR_xSr_1.8Mn₂O₇(R= Tb,Y, x=0, 0.05,0.1,0.15,0.20) shows that doping with smaller radius Tb and Y ions in La_1.2Sr_1.8Mn₂O₇ caused diffraction peaks shift to high angle. Some samples had an impure diffraction at about 30°, but all samples forms single-phase. Sample can be well indexed on a Sr₃Ti₂O₇-type tetragonal structure with the space group I4/mmm.According to the M-T curves, when x≤0.05, two series of samples shows ferromagnetism in the low temperatures. With increasing temperature, magnetic properties become weak. There are three-dimensional (3D) and two-dimensional (2D) magnetic transition, and appeared a weak transition peak in the high temperature. In addition, the two series of samples shows obvious spin-glass behavior for x = 0.05 in low temperature, When x≥0.1, samples'magnetic properties become weaker and ferromagnetism disappears. Theρ-T curve of La_1.2-xTb_xSr_1.8Mn₂O₇ samples shows the metal-insulator transition for x≤0.1, and the maximum MR values are74%, 94% and 85%, respectively. When x≥0.15, the samples keep in the insulator state in the whole observed temperature range. The maximum MR values which appearances at low temperature are 86% and 89%.The La_1.2-xY_xSr_1.8Mn₂O₇ samples shows a metal - insulator transition for x≤0.05. And the maximum MR values are 91% and 97% near the magnetic transition temperature. When x≥0.1, samples shows the insulator behavior in the whole temperature range and the maximum MR values are discovered at low temperature. The MR value is as high as 99% for x=0.1 sample. We believe that the MR effect around the ferromagnetic-paramagnetic phase transition temperature is due to the intrinsic magnetoresistance CMR. At low temperature, the maximum MR can be ascribed to a non-intrinsic inter grain MR effects, which are related to spin-polarized tunneling effect or grain boundary effect.