Perovskite Manganite La <sub> 0.67 </ Sub> Ca <sub> 0.33 </ Sub> Mn <sub> 1-x </ Sub> The Co. <sub> X </ Sub> O <sub> 3 </ Sub> Series Of Samples Of Magnetocaloric Effect

Recently, considerable renewed interests in magnetic refrigeration technology has been sparked since the discovery of magnetocaloric effect (MCE) in perovskite manganites.In this thesis, polycrystalline samples of La_0.67Ca_0.33Mn_1-xCo_xO₃(x=0.00, 0.01, 0.02, 0.03 and 0.04) with a small amount of Co doping have been prepared by sol-gel method. Their Curie temperatures and magnetocaloric properties have been investigated. The XRD analyses show that the samples are single phase of perovskite-type oxides. The Curie temperatures and the magnetic entropy changes of the samples were measured using a vibrating sample magnetometer (VSM). The M-T curves under a magnetic field H=0.05T and M-H curves at various temperatures around Tc were measured. The main experimental results are as follows:1.The X-ray diffraction patterns indicate that the samples of La_0.67Ca_0.33Mn_1-xCo_xO₃ (x=0.00, 0.01, 0.02, 0.03 and 0.04) have the orthorhombic perovskite structure without any other phases with in the accuracy of the equipment.2. The X-ray data indicate that the doping of Co at Mn-site changes the lattice parameters and the cell volume remarkably. The Increased cell volume in Co doped samples would cause noticeable lattice distortion and might affect the magnetic properties of the materials.3. The experimental results show that the Co doping at Mn-site has great influence on the magnetic properties of the samples. By doping a little Co(x=0.01, x=0.02) at Mn site, the Curie temperatures of La_0.67Ca_0.33Mn_1-xCo_xO₃ decreases. while the magnetic entropy change drops slightly. When the doping level x increased to x=0.03 and x=0.04, the Curie temperatures and magnetic entropy change of La_0.67Ca_0.33Mn_1-xCo_xO₃ drops dramatically. The results indicate that slight Co doping (x≤0.02) on suitable parent sample (such as La1-xSrxMnO3) could decrease the Curie temperature without a notable sacrifice of magnetic entropy change.