Perovskite Manganese Oxides La <sub> 0.7-x </ Sub> Ca <sub> 0.28 </ Sub> Of Mno <sub>, Of Sr <sub> 0.02 </ Sub> 3 </ Sub> And Of La <sub> Of Mno <sub>, 0.77-x </ Sub> Ca <sub> 0.2 <

In this work, the polycrystalline manganese perovkites samples of La_0.7-xCa_0.28Sr_0.02MnO₃ (x=0.00-0.10) with different La-site vacancies and La_0.77-xCa-(0.2)Sr_xMnO₃ (x = 0.00-0.10) doped with suitable amount of Sr in La-site have been fabricated by the sol-gel technique. The magnetic properties, magnetocaloric effect and magnetoresistance of these two series have been investigated. The main experimental results are as follows:1. La_0.7-xCa_0.28Sr₍0.02MnO₃ Polycrystalline series(1) The X-ray diffraction analysis shows that the La_0.7-xCa_0.28Sr_0.02MnO₃ series are orthorhombically distorted perovskite. The lattice parameter and the cell volume have been changed a lot by doping of vacancies in La-site.(2) The observation of SEM shows that the samples prepared by sol-gel method are quite homogenous and the particles are nearly spherical. The grain sizes of La_0.7-xCa-(0.28)Sr_0.02MnO₃ are between 200 to 350 nm.(3) The studies of magnetoresistance show that the vacancies in La-site are propitious for enhancement of magnetoresistance in this series. The maximum MR of 36% was obtained for La_0.64Ca_0.28Sr_0.02MnO₃ sample under an applied filed of 1.8 T.(4) The experimental results indicate that doping of small amount of vacancies in La-site is favorable to the enhancement of the Curie temperature. The Curie temperature of La_0.7Ca-(0.28)Sr_0.02MnO₃ is 227 K. With the addition of the vacancy, the Curie temperature rises to 257 K for x = 0.02 sample and 270 K for x = 0.10 sample, respectively.At the same time, the magnetic entropy changes of the samples have been increased by small amount of vacancy doping in La-site. The maximum magnetic entropy change 3.0 Jkg^-1K^-1 was obtained in La_0.64Ca_0.28Sr_0.02MnO₃ sample at its Curie temperature (264 K) in an applied magnetic field of 1T. The studies show that the first-order transition has occurred in th -is sample at its Curie temperature and a very large change of the cell volume has been observ -ed. This may contribute to the large entropy change in this sample.2. La_0.77-xCa_0.2Sr_xMnO₃ Polycrystalline series(1) XRD results show that the samples are the single phase of perovskite structure without any other phases. The cell volume decreases linearly with Sr²⁺doping level. SEM observations show that Lao.77-xCao.2Sr.,Mn03 samples are constituted of homogeneous spherical particles and the distribution of the particle sizes is between 100 to 350 nm.(2) The Curie temperatures of the samples have been adjusted from 236 K for x = 0.00 sample to room temperature 300 K for x = 0.10 sample without the sacrifice of entropy change. A large magnetic entropy change of 2.32 Jkg"'K"' at its Curie temperature 271 K under a field of 1 T was obtained in x = 0.06 sample.The large magnetic entropy change in these two series maybe originated from the great changes of magnetization near Tc and the variation of the double exchange coupling interactio -n between Mn3+ and Mn4+ in these systems. The results show that these materials have large entropy changes and their Curie temperatures are near room temperature. They are suitable for the candidate of room temperature refrigerant materials.