| The bilayer manganites not only possess some similar properties with the three-dimensional perovskite manganites, but also show many own unique properties due to the decrease of dimension and particular structure. So, the bilayer manganites offer the opportunity not only to investigate the generic features of the mixed-valence manganites , but also to explore phenomena that are not found in the 3D manganites. In this thesis, we focus on the bilayer manganites La2-2xSr1+2xMn2O7, and choose La1.4Sr1.6Mn2O7 as parent compound to investigate systematically it's A-site and Mn-site doping effect. The major investigations and results are presented as follow:The introduction on the generic properties of the mixed-valence manganites and some important theory model and viewpoints has been presented. Then some features of the bilayer manganites are introduced, and these features include magnetic structure and phase diagram, crystal structure, magnetic and electrical transport properties, magnetoresistance effect, the orbital degrees of freedom etc. On the basis of the review, the basic thinking of subject selection and investigation significance is put forward.The high quality samples have been successfully synthesized by the solid state reaction method and sol-gel method, respectively. The virtue and shortcoming of the two methods is compared. For the sample with nominal composition La4/3Sr5/3Mn2O7, the effect of sintering temperature on the structure and transport properties and magnetoresistance are investigated, and we find the resistivity of the bilayer manganites increases with the increase of sintering temperature. The polycrystalline La2-2xSr1+2xMn2O7 (0.3≤x≤0.5) samples were successfully synthesized by the solid state reaction method and their structure and electrical and magnetic properties were studied and discussed.With the carrier concentration kept the constant, the small ion Y and the big one Ba are chosen to study the A site doping of La1.4Sr1.6Mn2O7 and the main study focus is on the influence of A-site size and mismatch on the system properties. The research includes: Y doping in La1.4-zYzSr1.6Mn2O7, Ba doping in La1.4Sr1.6-yBayMn2O7 and the co-doping of Y and Ba of the same content in La1.4-zYzSr1.6-yBayMn2O7(z=y). The result shows that when z < 0.08,y≤0.3 or z=y < 0.08, the sample is a single phase of bilayer structure. The doping suppresses the magnetic transition and insulator-metal transition (IMT) which results in the increases of the resistivity and the magnetoreaiatance increases near IMT. When z≥0.08,y > 0.4 or z=y≥0.08, the sample is no longer in single phase but includes the perovskite phase of ABO3 type, which shifts IMT towards higher temperature and decreases resistivity. This shows that ionic size mismatch makes it difficult to form bilayer phase and the existence of smaller ions makes it more difficult for tetragonal phase to form. Through the comparison of the experiment results of three samples which are z=0.05, y=0; z=0, y=0.05; z=y=0.05, it is discovered that Y doping has greater doping effect than Ba doping and the co-doping of Y and Ba has the most obvious effect. Because Y doping increases lattice distortion and Ba doping decreases it, Y doping has greater doping effects than Ba doping. Though co-doping of Y and Ba of the same content can keep the average A-site ionic radius nearly constant, it further increases ionic size mismatch and results in the greatest doping effect, which proves that the size mismatch plays a dominant role here.The thesis also studied the doping effects of series transition element on Mn-site of La1.4Sr1.6Mn2O7. The transition elements include Cr,Fe,Co,Ni,Cu,Zn,Ti,Nb,Mo, etc and the doping concentration is fixed as 2%. Doping causes obvious changes of electronic and magnetic properties (such as the three-dimensional magnetic transition temperature TC, the two-dimensional magnetic transition temperature T*, and the insulator-metal transition temperature TIM, resistivity, magnetoreaiatance, high field magnetization, and high temperature conductance). The thesis also analysed and studied the experiment results by considering the size effect of doping ions, the different magnetic interaction between doping ions and Mn ions, the change of Mn3+/Mn4+ ratio and the damage of Mn-O-Mn network and double-exchange interaction etc. Cr has obvious different doping effects. It is observed for the first time that Cr doping increases Tc and TIM at the same time in manganites and resistivity decreases.The thesis studied the electrical and magnetic properties of the La1.4Sr1.6(Mn1-yCry)2O7 (0≤y≤0.1) series and studied magnetic properties especially. We have found that lower Cr doping and higher Cr doping has obviously different effects on electrical and magnetic properties. With the increase of Cr doping, TIM increases first and then decreases while peak resistivity decreases first and then increases. TC also increases first and then keeps almost the same. The change of TC does not correspond to that T*. With Cr doping T* decreases monotonously and almost follows linear relation with the doping concentration. All these results can not be explained well by double-exchange interaction. Through considering the different influences of Cr doping on intrabilayer and interbilayer magnetic interaction, we have explained reasonably its doping effect.
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