Colossal Magnetoresistance Effect And Related Properties In Perovskite Oxides

Since the discovery of colossal magnetoresistance effect (CMR) in perovskite manganites, it has sparked considerable renewed interests in these long-known materials with an eye towards both an understanding of the CMR and related properties 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 magnetic-field or photo radiation, charge ordering, orbital ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics.In this thesis, the author devoted his effort to the study of the fundamental physics in CMR oxides as well as the exploring of novel materials, by both experimental and theoretical methods.The whole thesis consists of seven chapters.1. A brief overview of magnetoresistance effectThis chapter aims at a brief overview of the history, progress, and current status of all kinds of magnetoresistance materials, such as magnetic multilayers, nano-materials, perovskite oxides, pyrochlore manganites, spinel compounds, magnetite, chromium dioxide, and so on. By these illustration, we may acquire a basic sight on magnetoresistance effect as well as magnetoelectronic.2. An introduction to the physical properties of perovskite manganites.This chapter deals with the influent physics properties and some spectacular phenomenon observed in perovskite manganites, including the structural, magnetic, electronic transport, phase diagram, charge/orbital ordering, and insulator-metal transition induced by applied magnetic field or photo radiation etc. Some special physics concepts,such as double exchange, Jahn-Teller effect, electron-phononcoupling, are interpreted. This part is helping to build up a background for the research on colossal magnetoresistance.3. The effects of A-site/B-site element substitution in perovskite manganites.In this chapter, the effects of A-site or B-site doping by foreign elements were studied by preparing several series of samples, (Lao^GdJSrojMnOj, Lao^Cao^jMn,. .^Oj, Lao^Ca^jMn^C^Oj, and Lao^SrojjMn^CrA. (1) It was found the Curie temperature Tc and magnetization decrease with increasing Gd content, but the insulator-metal transition associated with ferromagnetic ordering transition is greatly enhanced. More important, this enhanced insulator-metal transition is very sensitive to applied magnetic field, which leads to an enhanced CMR effect, for example, the maximum MR ratio of (La03Gd04)Sr03MnO3 is as high as 8x103 in 6 T and above 5xlO2 even in 1 T. Besides, the enhanced insulator-metal transition gives rise to a large temperature coefficient of resistance (TCR), which is beneficial for bolometric application. (2) A large variation of the resistivity coefficient B and the polaronactivation energy Ea with Ga doping was found. The variation of B implies thechange of both polaron concentration and average hopping distance with Ga doping. Considering the possibility of on-site coulomb repulsion and polaron-polaron interactions, we suggest a combination of polaron nearest-neighbor hopping and non-nearest-neighbor hopping dominating the transport process in La^CayMnCv Combining with the data of thermopower, we found a large increase of polaron binding energy with nonmagnetic Ga doping, which strongly suggests the additional magnetic nature of the lattice polaron in Lao^Ca^j MnO3. (3) Extraordinary transport and colossal magnetoresistance behaviors, characterized by double peaks, were observed in Cr-doped samples. The temperature range of CMR response in some samples is greatly broadened, from the lowest temperature to above-room temperature. These results suggest that Cr substitution can be a potent way in tuning CMR response.4. The CMR in cobalt oxides and the effects of Fe doping in La,.xSrxCoO3.Perovskite cobalt oxides have many similar properties such as mixed valence and CMR effect, with perovskite manganites. However, a peculiar feature in cobalt oxides is the thermally induced spin-state transition. In this chapter, we first introduced the basic aspects, especially the magnetic phase diagram, of cobalt oxides. Then westudied the effects of Fe doping in a typical cobalt oxide LaSrCoO. It was found Fe doping weakened the CMR peak near Tc but enhanced the low-temperature MR. By comparing with manganites, we found apparent difference between two systems. The origin of CMR in is proposed to result from the spin-state transition induced by applied magnetic field.5. The study of LaMn^Ci^Oj andThe magnetic, electrical transport, and magnetoresistance properties of direct Mn-site doped systems, LaMn^CUxOj and LaMn,.xCrxO3, were studied by measurments of resistivity, magnetization, electron spin resonance, raman spectroscopy etc. Ferromagnetism and large magnetoresistance were observed in LaM^^C^O,, which is believed to be consistent with Mn3+-O-Mn4+ double exchange mechanism. These results suggest that double exchange and giant magnetoresistance can be obtained by direct Mn-site doping. It was found that the doping of Cr in LaMnO3 introduces ferromagnetism and cluster glass behaviors. Moreover, a close correlation between magnetic state and transport behavior as well as a large magnetoresistance were observed when x ~ 0.3. These results suggest that a ferromagnetic exchange interaction similar to double exchange could occur between Mn and Cr.6. The transport mechanism in mixed-valence manganites.The transport mechanism in mixed valence manganites is attractive but still controversial. In the paramagnetic phase, the carriers are trapped in localized states as small polarons due to the incorporation of three different localization features: (i) strong electron-phonon interaction, (ii) the variations in the Coulomb potential due to the presence of R3+ and A2+ ions in the lattice, (iii) the magnetic localization due to spin disorder on the interatomic scale. When the thermal energy is not enough for small polaron to hop between nearest-neighbour sites, the transport of small polaron could be accomplished by two steps: first, the small polaron is thermally activated into an intermidate state in which the carrier is weakly localized; then it feels the potential fluctuation due to localization feature (2) \& (3) and transport by variable-range hopping. We term this kind of transport mechanism as variable-range hopping of small polaron, and derive the expression of resistivity from this model.