| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 90 === Abstract
In this study, we synthesized La2/3(Ca1-xBax)1/3MnO3（x=0~0.4）and La2/3Ca1/3(Mn1-yAly)O3（y=0~0.15）with colossal magnetoresistance by citrate process. By replacing Ca2+ with Ba2+ and replacing Mn3+ with Al3+, we discuss the effect of the replacement on the electrical and magnetic properties and magnetoresistance effect of La2/3Ca1/3MnO3.
From the experimental results we find that, substitution of Ca2+ by Ba2+, results in the changing of the average radius of cation on A site and the related position of manganese ion and oxygen ion, enhance the double exchange interaction of Mn3+-O-Mn4+ couple. With the increasing of doping content of Ba2+,both TC and TP increase and the resistivity decrease. At low temperature, because of the tunneling effect at g.b, the Ba-doped samples exhibit a sharp drop in the resistivity at low field followed by a slower background negative MR at higher fields. The MR﹪at room temperature slightly increase with the increasing of doping content of Ba2+.
The doping of Al3+ decreases the number of Mn3+-O-Mn4+ pairs, and substantially decrease TC and TP. When 15﹪manganese ion was replaced by Al3+,there is not a well-defined TC can be found on the M-T curve, and over whole temperature range of 70~300K, it showed the insulator behavior. Without the sharp drop of resistivity under low field at low T, the MR﹪of Al-doped samples reaches about 50﹪under 13.2Koe at low T, which is much greater than MR﹪of the Ba-doped samples under the same field and temperature.
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