Preparation Of La-ca-mn-o Super-giant Magnetoresistance In Polycrystalline Materials And Nature

Colossal magnetoresistance material of La-Ca-Mn-O is formed by partially doping with divalent alkali metal like Ca, Sr and Ba into 3d transitional metal manganese oxide RE-Mn-O(RE, trivalent rare earth metal, like La, Pr, Sm, Nb and Gd). The doping leads to appearance of mixed valence of Mn3+/Mn4+ which induces hole of eg orbit and changes the electric and magnetic property. Phase transition of paramagnetism to ferromagnetism occurring with metal to insulator transition results in great change of magnetoresistance which brings CMR effect. In peroveskite Ruddlesden-Popper family, La-Ca-Mn-0 is characterized with the formula of (RE,AE) n+1MnnO3n+1, n= , 1, 2 and other integers. Preparation and property of infinitely layered structural polycrystalline La1-xCaxMn03 is mainly researched in this paper.Compared with solid state reaction and sol-gel methods, coprecipitation method is characterized with its simplicity of equipment and preparation, accurate control of the ingredient, small granularity and good homogeneousness, superior activity, low sintering temperature, and preferable repetition of preparation. In this paper polycrystalline La1-xCaxMnO3 was successfully prepared by coprecipitation method with different precipitator, salvolatile and oxalic acid. These samples prepared by copreciptation method were investigated by means of pulverous granularity measurement, X-ray diffraction measurement, polycrystalline density measurement, R-T curve measurement and M-T curve measurement.The result of pulverous granularity measurement shows that the homogeneousness and size of samples prepared by salvolatile are superior to those prepared by oxalic acid. XRD measurement demonstrates that polycrystalline La1-xCaxMnO3 and Ag-doped La0.67Ca0.33MnO3 are homogeneous and single phase. With the increase of Ag-doped, lattice parameters decrease. It is testified by R-T curve measurement that the relative density of polycrystalline La0.67Ca0.33MnO3 is closely associated with the figure of R-T curve and metal-insulator transitional temperature, Tp. M-T curve measurement indicates that at Tc=270K polycrystalline La0.67Ca0.33MnO3 occurs magnetic phase transition from paramagnetism to ferromagnetism, which is coincident with Tp.