Doping Effects On Perovskitelike Manganites Colossal Magnetoresistance Material

Since the discovery of colossal magnetoresistance effect (CMR) inperovskitelike manganites, it has sparked considerable renewed interests inthese long-known materials with an eye towards both an understanding of theCMR and related properties and potential applications in magnetic informationstore and low-field magnetic sensors. Beside the CMR effect, these materialsalso exhibit intriguing physical properties such as insulator-metal and structuretransition induced by applied magnetic-field or photo radiation, chargeordering , orbital ordering ,and phase separation etc. The full understanding ofthese properties will definitely stimulate the progress of condensed matterphysics.However, there are still several disadvantages about CMR effect thatseverely limit their practical utilities. For example, large CMR effect occursusually under room temperature and within a narrow temperature range. Thefields needed to saturate CMR effect are as high as several Tesla, which isdifficult to be satisfied. Many researchers have thus been concentrated onfinding new material system that can provides low field magnetoresistance(LFMR) around room temperature. On the other hand, recent efforts tobroaden the CMR temperature range have been made by means of the differentelement substitution.This dissertation aims at investigation of possible physical origins for theobserved semiconductor-metal transition and CMR effect in optimally dopedmanganese perovskites. Based on primary research achievements, this paper furtherdiscussed the effects of A-site and B-site doping by foreign elements. The sampleswere prepared by Glycine-nitrate process (GNP) synthesis technique. Weprepared several series of samples, such as La0.7Sr0.3-xAxMn0.9Fe0.1O3,La0.7-xSmxSr0.3Mn0.9Fe0.1O3, La2/3Pb1/3Mn1-xFexO3, La2/3-xAxPb1/3MnO3. Structure,electron-transport properties, magnetism, magnetoresistance (MR) and size ofparticles in samples have studied.With doping Ca or Ba or Pb , La0.7Sr0.3-xAxMn0.9Fe0.1O3 system exhibit adecrease in Curie Temperature (Tc), conductivity and a increase in MR, but thestructure is not changedSm doping in La0.7-xSmxSr0.3Mn0.9Fe0.1O3 system results decrease ofCurie Temperature (Tc), while resistivity and MR increase. The particles insamples diminish. The samples with x>0.2 show semiconductor-like behaviorover the whole temperature range studied.The La2/3Pb1/3Mn1-xFexO3 system with Fe doping showssemiconductor-like behavior over the whole temperature range studied.Curie temperature (Tc) decreases, while resistivity and MR increase due to Fedoping. The structure is not changed but particles diminish.Structure of La2/3-xAxPb1/3MnO3 system with Ho or Sm doping in a-site is notchanged. Over the whole temperature range studied, magnetoresistance,resistivity and size of particles increased with increasing elements content.For the perovskite-like manganese oxide with A site doping, difference insize of atoms changes crystal type or crystal constant and consequently theelectron-transport properties and magnetic properties. On the other hand disorder of Asite doping causes size disorder, then changes Mn3+-O2-Mn4+ bonds.For perovskite-like manganese oxide with B site doping, bonds ofMn3+-O2—Mn4+ is destroyed to some extent and competition between doubleexchange interaction and other exchange interactions becomes important . Onthe other hand, doping ions at B site strengthens the scattering or trapping ofeg electrons. So eg electrons are localized and the double exchange interactionsis weaken, causing apparent decrease of Tc and increase of electric resistance.