Preparation And Properties Of The Non-stoichiometric And Different A-site Size Perovskite Manganites Of ABO₃

Colossal magnetoresistive (CMR) materials such as rare-earth manganites are still a hot topic up to now. For one thing, they are expected to find a number of potential applications for their CMR effect; for another, the essence of the CMR is related to the basic matters in strongly correlated physics.The main purpose of this paper is detailed giving how the A-site effects influence the properties of the manganites. Subsequently we focus on the magnetic and electrical transport properties of non-stoichiometric La_2/3Ca_1/3Mn_1-xO₃ samples. To obtain manganites with inhomogeneous structure by preparing samples with manganese deficiencies, we studied extrinsic low field magnetoresistance effect in a new way.The dissertation consists of 4 chapters. First, in introduction the properties and the A-site effects are briefly summarized. The physical mechanisms of the CMR including double exchange (DE) theory and phase separation model are also reviewed; then the intrinsic and extrinsic magnetoresistance are described; finally, based on these mentioned above ,we further bring forward thesis of this letter.As the basis of this paper, the preparation process and the principle of the four-probe method are introduced in chapter 2.Chapter 3 is a kernel of this paper. we prepared a series of samples by decreasing the or by fixing the value of and increasingσ².The measurement of resistivity versus temperature dependence for magnetic field indicates that the insulator-metal transition temperature moves towards the low temperature regime with increasingσ². The measurement of susceptibility as a function of temperature is performed on polycrystalline La_(0.7-x)Y_xCa_(0.3-y)Sr_yMnO₃ samples.In order to obtain the low field magnetoresistance, we prepared non-stoichiometric La_2/3Ca_1/3Mn_1-xO₃ samples with Sol-Gel method and studied the effects of manganese deficiencies on samples'micro-structure ,magnetic and electrical transport properties as well as MR effect.