| Perovskite-type manganite La1-xSrxMnO3(LSMO)has electron-phonon coupling effect,double exchange effect and superexchange effect,which lead to the ferromagnetic-paramagnetic phase transition accompanied by metal-insulator transition,phase separation,charge ordering,colossal magnetoresistance effect(CMR)and low-field magnetoresistance effect(LFMR)and other physical phenomena.LSMO materials have practical or potential applications in the fields of solid oxide fuel cell(SOFC)cathode,NTC thermistor components,catalysts(in the determination of automobile exhaust constituents and NOx concentration in the atmosphere,etc.),high-density information storage,random access memory and magnetic sensors,etc.In this thesis,research advances about the crystal structure,electromagnetic structure and the electromagnetic properties of manganites are concluded firstly.Moreover,the research status and application prospect of CMR and LFMR effects are emphatically discussed.On this basis,the research motives of this topic is proposed.In this paper,the effects of B-site doping,oxygen vacancy concentration,grain size,and second phase doping on the structure and electromagnetic transport properties of manganese oxide La0.67 Sr0.33 MnO3(LSMO)were studied systematically.In order to improve the low field magnetoresistance at room temperature,we hope to improve the preparation technology,and reveal the mechanism of the electromagnetic properties of manganites.A series of manganite coatings were prepared by sol-gel method and screen printing method.The phase composition and microstructure of samples were analyzed by means of X-ray diffractometer(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Besides,the vibration sample magnetometer(VSM)and the magnetic physical measurement system(MPMS)were used to detect different experimental conditions on the electromagnetic performances of samples,such as the resistivity,Curie temperature,magnetoresistance,especially the low-field magnetoresistance at room temperature.Major contributions of this thesis are as follows:(1)The relationship between the doped Fe3+ ion concentration and the structure,magnetism,electrical transport and magnetoresistance properties of the La0.67 Sr0.33 Mn1-x FexO3(x=0-0.15)coatings is studied.Results have shown that the partial replacement of Mn ions with Fe3+ ions does not cause obvious lattice distortion,but it inhibits the double exchange between Mn3+-O2--Mn4+,thus significantly reduces the Curie temperature(Tc)and ferromagnetism,and causes that the resistivity increases with the increase of Fe3+ ion doping concentration.When the concentration of Fe3+ ion reaches a certain level,that is,near the critical threshold value(x=0.05),the low-field magnetoresistance at low-temperature and room temperature of the sample is about 1.5 and 2 times that of the undoped sample,respectively.The magnetoresistance enhancement effect can be explained by the phase separation mechanism.(2)The manganite coatings were annealed in the air,vacuum and nitrogen environment,respectively.A comparative analysis of the structure and performances among the several groups of LSMO coatings was performed.Compared with the sample annealed under air condition,the oxygen deficiency concentration and the lattice distortion of vacuum and nitrogen annealing samples increase,and Mn4+/Mn3+proportion and conductive carrier(hole)concentration decrease.As a result,the resistivity increases while TMI decreases considerably.In addition,according to the hypothesis of the competition between positive and negative exchange interaction,oxygen vacancy can lead to the reduction of the ferromagnetic coupling effect and double exchange effect between Mn ions,thus causing lower magnetization and Tc values.The magnetoresistance effects of vacuum and nitrogen annealed samples are significantly enhanced,and the low-field magnetoresistance at room temperature is about 4 times and 3 times that of the sample subjected to annealing in the air ambient,respectively.The results indicate that the electrical,magnetic and magnetoresistance properties of manganite coatings are very sensitive to the oxygen vacancy concentration.(3)A series of polycrystalline LSMO coating samples were prepared by controlling the annealing temperature,and the effect of grain size on the structure,magnetic,electrical and magnetoresistance properties of the samples is analyzed.The results show that the magnetization,TMI and Tc decrease with the decrease of grain size,while the coercivity and resistivity increase.On the basis of the core-shell model theory,the variation of ferromagnetism and electrical transport properties with the change of grain size is elaborated.The conduction mechanisms in three temperature regions by using three conduction models are discussed,and the minimum resistivity in the low-temperature region is explained emphatically.Besides,it is found that the magnetoresistance of the samples with smaller grain size is enhanced,3.5 times as high as that of the sample with largest grain size.This is because the number of grain boundaries increases with the decrease of grain size,which increases the exchange effect between the grains and the electron polarization effect,and thus the magnetoresistance effect enhances.(4)The doping effect of metallic Cu on the microstructure,magnetic,electrical and magnetoresistance properties of the manganite composite coating(1-X)La0.67 Sr0.33MnO3/xCu(x=0-0.15)is studied.Experimental results show that the Cu doping introduces more electron scattering centers and transition energy barriers,which hinder the transport of conduction electrons,causing the increase of resistivity and the reduction of TMI.In addition,the magnetic coupling between intergranular atoms is reduced owing to the introduction of additional grain boundaries,which reduces the ferromagnetism and Tc.When the doping amount of Cu is 15 wt%,Tc is reduced to the value close to room temperature.Moreover,near the percolation threshold(doping amount of Cu is 5 wt%),the low-field magnetoresistance at both low temperature and room temperature of the manganite composite coating are enhanced.The magnetoresistance at room temperature is 4 times that of the non-copper LSMO sample,which is related to the extra local disorder introduced by copper. |
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