Study On Magnetic And Transport Properties Of Manganites With Inhomogeneous Structure

The colossal magnetoresistance (CMR) effect in rare-earth manganese perovskites has aroused considerable interest in scientific studies and potential technological applications. CMR effect, the intrinsic characteristic of manganites, is considered from the spins ordering of manganese under an applied field. However, the CMR effect can only be obtained under a field range of several teslas, which is way too high for practical requirements. In order to put the CMR materials into use, researchers thus focus on another type of magnetoresistance (MR) effect—extrinsic MR effect, which is often attributed to the spin-dependent scattering process. By controlling the state of spins disordering near the grain boundaries or the heterostructure interface between manganites and other materials, under a lower applied field, the spin-dependent scattering process would be weakened and thus the low field magnetoresistance (LFMR) achieved. One advantage of manganites is their nearly 100% spin polarization which may boost structural magnetoresistance associated with the spin-memory contribution to charge transport (e.g., spin-dependent tunneling) across the interfaces. In this letter, we studied the transport properties of La2/3Ca1/3Mn1-xO3 prepared by different ways in detail. We came to the conclusion that the non-stoichiometric La_2/3Ca_1/3Mn_1-xO₃ samples prepared by the sol-gel method possess inhomogeneous structure inside their grains. The inhomogeneous structure would enhance the spin-dependent scattering process and further improve the LFMR effect. This paper will discuss the transport properties of non-stoichiometric La_2/3Ca_1/3Mn_1-xO₃ samples in different way, as they are listed as follow: In chapter one, the study background and application perspectives of manganites are introduced. In chapter two, the preparation process of La_2/3Ca_1/3Mn_1-xO₃ samples is introduced. In chapter three, the transport properties of La_2/3Ca_1/3Mn_1-xO₃ samples prepared by sol-gel method and sintered at 14500C are discussed in detail. The conclusions are listed as follow: 1). The micro-structure of La2/3Ca1/3Mn1-xO3 is consisted of crystalline La2/3Ca1/3MnO3 with more or less manganese vacancies, less mount disordered rock-salt layers and the regions with spins disorder caused due to the distorted crystalline La2/3Ca1/3MnO3. 2) With the increase of x, accompanying with the enhanced double-peak effect, the Curie temperature of samples shifts to a lower temperature region. Meanwhile, LFMR caused due to the enhanced spin disorder is augmented. 3) Although all the samples are sintered at same temperature, grain size of non-stoichiometric samples decreases with the increase of x. This should be a new way to control the grain size. In chapter four, the transport properties of La2/3Ca1/3Mn1-xO3 samples prepared by sol-gel method and sintered at 1100 and 9000C are discussed in detail. We come to the conclusions listed as follow: 1) Despite of the decrease of grain size, the constitutes of grains of the samples sintered at 1100 and 9000C are similar with that sintered at 14500C,. 2) There is only one peak in the ρ-T curves of the samples. 3) LFMR effect of the samples is much higher than that sintered at 14500C. 4) Based on the possible structure of the samples, the transport properties of the samples are discussed in detail. In chapter five, the transport properties of the La2/3Ca1/3Mn1-xO3 samples made by traditional solid reaction or fabrication are discussed. The main conclusions are listed as follow: 1) The surplus ions of La and Ca in the samples made by solid reaction or fabrication tend to be mass among the grains of La2/3Ca1/3MnO3. However, the conglomeration of the La or Ca would diffuse to the La2/3Ca1/3MnO3 grain boundaries and thus cause a distortion layer. 2) There is no double-peak can be observed in their ρ-T curves. LFMR effect caused due to the distortion layer at the grain boundaries can be obtained. In chapter six, the mechanism of electrical transport property has been discussed. The main conclusions are listed as follow: I. When T>TC: 1) Electrical transport properties of the samples are controlled by the variable-range hopping (VRH) model. 2) With the increase of x, the increase of the amount of disordered rock-salt layers andthe distortion region of La2/3Ca1/3MnO3 would cause the increase of barrier height. 3) For the sample with x>0, inhomogeneous structure of the grain would has strain increase when temperature decreases, which would further cause the increase of barrier height. II. When T<