Effects Of Magnetic Inhomogeneity On The Magnetoresistance Property Of Perovskite Manganites

Because of their particular colossal magnetoresistance effect (CMR), perovskite manganites have great potential for practical applications. But usually, CMR can only be observed in a narrow temperature region, which seriously restricted its applications. Spin-polarized tunneling or spin-dependent scattering can make this material show low-field magnetoresistance (LFMR) in a wider temperature region. But generally, LFMR achieves large MR value in low temperature and attenuates quickly with temperature increasing, so LFMR is not effective enough to improve the performance of manganites. This dissertation gives an alternative way to improve the performance of perovskite manganites by broadening the intrinsic CMR through inducing magnetic inhomogeneity in the material.The dissertation consists of seven chapters. In Chapter One, several main magnetoresistance effects and the colossal magnetoresistance effect of perovskite manganites are introduced. Then the basic properties, conducting mechanism and low-field magnetoresistance effect of perovskite manganites are reviewed. In the end, the comparison of CMR and LFMR comes to the origin of the dissertation. Chapter Two introduces the preparation and the experimental equipments of the material detailedly.In Chapter Three, the effects of doping Na on the structure, electrical and magnetic properties of La2/3Ca1/3MnO3 are studied. The results indicate that two ferromagnetic phases with close Curie temperature coexist in the samples because of the difference of chemical valence between Na+ and Ca2+. In a certain doping concentration, the competition of the ferromagnetic phases results in two intrinsic CMR peaks in the MR-T curves and the superposition of the peaks makes the samples show larger magnetoresistance in a wider temperature region.In Chapter Four, the properties of La1-x(Ca1-yKy)xMnO3 samples sintered in high temperature are investigates. Because of the serious loss of K, the magnetic property of the sample surface is different from the main phases in the interior. In the y=0.4 samples, the surface phase can compete with the main phase, which leads to the broadening of the metal-insulator transition. And accordingly, with the decrease of temperature, the intrinsic magnetoresistance of the surface phase makes the sample show a magnetoresistance plateau after the CMR peak and finally broadens the temperature region in which the sample shows intrinsic magnetoresistance.In Chapter Five, the effect of grain boundaries on the magnetoresistance of La1-x(Ca1-yNay)xMnO3 and La1-x(Ca1-yKy)xMnO3 are examined. The final results show that the y=0.4, 0.5, 0.6, 0.75 samples of La1-x(Ca1-yKy)xMnO3 system have magnetoresistance plateaus near room temperature. The analysis indicates that the inhomogeneous magnetic phases in the grain boundaries are helpful to the formation of the magnetoresistance plateaus.In Chapter Six, granular system with nominal chemical composition La2/3Ca1/3BixMn1-xO3 is studied. The structure measurement indicates that only a small amount of Bi3+ ions can not enter the A-site of La2/3Ca1/3MnO3 lattice. Because of the low melting behavior of bismuth oxide, the superfluous Bi will segregate as insulating bismuth oxides and bismuthate. Because of the segregation of Bi, the surface of the main phase has different ferromagnetic property from that of the core, namely the magnetic inhomogeneity of the samples is enhanced, which results in the broader ferromagnetism-paramagnetism transition and magnetoresistance plateaus in samples with the high Bi content.The last chapter is the conclusion of all the research work mentioned in this dissertation. In the end of the dissertation, the author's published and submitted papers are listed.