Spinel And Perovskite System Giant Magnetoresistance Effect And Related Materials

Rich physics has been found in strongly correlated electron systems. Especially, chalco-genide spinels and perovskite mangnites have attracted considerable interests with an eye towards both an understanding of the colossal magnetoresistance (CMR) mechanism and potential applications in magnetic information store and low-field magnetic sensors. Beside the CMR effect, 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 mechanism of the CMR effect is fully elucidated, the progress in many fields of condensed matter physics will be definitely stimulated.In this thesis, the author devoted his effort to the study of the fundamental physics in Cr-based chalcogenide spinels and perovskite mangnites, by both experimental and theoretical methods.The whole thesis consists of six chapters.1. A brief introduction to the condensed matter physics and strongly correlated electron systems.In this chapter, some basic concepts, such as paradigm, symmetry broken, Fermi liquid theory and Mott insulator, are introduced. By these illustration, we may acquire a basic sight of the correlated electron physics.2. A brief overview of the progress of the CMR effect and related properties.This chapter devotes to to a review of the magnetoresistance phenomena and related physical properties of chalcogenide spinels and perovskite mangnites, such as the structural, magnetic, electronic transport, charge/orbital ordering and phase separation, etc. 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 of the CMR effect and related properties of chalcogenide spinels and perovskite mangnites.3. The effect of Fe site In doping in Fe_1-xIn_xCr₂S₄.The Fe site doping is an effective method to explore the CMR effect in chalcogenide spinels. In this chapter, the effect of Fe site In doping in Fe_1-xIn_xCr₂S₄ is investigated. The results prove that In ions occupy Fe sites in the +2 valence state. The magnetic fluctuation is induced in the paramagnetic matrix with the In doping. Thus, the magnetic random potential appears and trapped the carrier. As a result, the transport mechanism in the high temperature region changes from small polaron hopping to variable-range hopping (VRH) with the introduce of In. By the aids of the angular dependent of the electron-...