The Study Of A Site Doping In Perovskite Manganite

The studies of transition-metal oxides with perovskite structure started in 1950's. In the recent ten.years, the renaissance of these materials is due to the exciting discovery of high- T_C (high temperature superconductivity) in cuprates and CMR (colossal magnetoresistance ) effect in manganites. The CMR materials represented by manganites have attracted the major attention of researchers in recent years due to the fascinating properties such as colossal magnetoresistance, and the potential applications in magnetic devices, e.g., read and/or write heads for magnetic disk drives, magnetic refrigeration, magnetic random access memories as well as magnetic field sensors. These inspire people to investigate it and become one of the hot topics of present physical research. Most importantly, as a strongly correlated electron system, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied field or photo radiation, charge ordering, orbital ordering and phase separation etc. Once the micro mechanism of the CMR effect is fully elucidated, the progress in many fields of condensed matter physics will be definitely stimulated. In this dissertation, the author devoted his effort to the study of the properties and charge ordering in doped perovskite manganites by doping at A-site. The whole dissertation consists of five chapters.The chapter one: a brief overview of the progress of the perovskite manganites and related properties. This chapter devotes to a review of the magnetoresistance phenomena and related physical properties of perovskite manganites, such as the crystal structural, electronic configuration, magnetic property, electronic transport, charge ordering, phase separation. Some physics concept, such as double-exchange, Jahn-Teller effect, etc. are interpreted. This part is helping to build up a background for the research.The chapter two: The magnetic properties of Nd_0.6Ln_0.1Sr_0.3MnO₃ compositions (Ln = La, Pr, Gd, Dy) have been investigated thoroughly. The effect of (r_A) is considered to be profound on the PM-FM phase transition in the manganites. The experiment results indicate that T_C decreases with Ln changing from La to Gd due to the decreases of A> The abnormal increase of T_C in Dy-composition with the smallest A> and largestσ² is attributed to the large magnetic moment of Dy³⁺ ions. The rapid increase of magnetization below 30 K indicates directly the magnetic ordering of rare earth ions at A-site.The chapter three: The electrical and magnetic properties of highly doped manganite Gd_0.4Ca_0.6MnO₃ with a relatively small A> have been investigated thoroughly. Through the M-T, M-H, and ESR measurements it is found that the magnetization in M-T curve originates from the paramagnetic contribution of disordered Mn ions at B-site and Gd³⁺ ions at A-site. The peculiar behavior of M-T curve below T_CO originates from the remaining spin disordered Mn ions after the formation of charge-ordering phase in the manganite, and the AFM charge-ordering state is supposed to be a local short-range ordering state.The chapter four: We investigated the magnetic properties of the Pr_0.5-xNd_xSr_0.5MnO₃ (x = 0, 0.1, 0. 2, 0. 3, 0. 4, 0. 5) system. Since the two end samples Pr_0.5Sr_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ are A-type and CE-type AFM at low temperatures, respectively, due to their different orbital ordering d _x²-y² and d_3x²-r²/d(3y²-r²), the magnetic structure of Pr_0.5-xNd_xSr_0.5MnO₃ is expected to be a mixture of A- and CE-type AFM. Our experiment results show that T_C remains almost constant, while T_N decreases dramatically as Pr³⁺ ions are replaced by Nd³⁺ ions. We suggest that this originates from the orbital ordering instability of the different magnetic structure type rather than from the ionic radius at the A-site.The chapter five: The electrical and magnetic properties of Ln_0.4Ca_0.6MnO₃ compositions (Ln = La, Pr, Nd, Sm) have been investigated. The effect of A> on the charge-ordering behavior of the manganites is considered to be profound. The charge-ordering temperature T_CO increases with decreasing A> , the antiferromagnetic charge-ordering state transforms from long-range into local short-range ordering and the magnetic behaviors for Ln = La, Pr, Nd, Sm are all different. Through the M-T, M-H, and electron spin resonance measurements, it is found that the magnetization in M-T curves comes from the paramagnetic contribution of disordered Mn ions at the B-site and magnetic ions at the A-site. The rise of T_CO is attributed to the fact that e_g electrons are localized by decreasing A>, and the different behavior of M-T curves below T_CO originates from the number of remaining spin disordered Mn ions after the formation of the charge-ordering phase in the manganites.This work is supported by National Natural Science Foundation of China through Grant No. 10334090, No. 10504029, and the State Key Project of Fundamental Research, China, No. 2007CB925001, No. 001CB610604.