Perovskite Manganite La <sub> 0.7-x </ Sub> Nd <sub> The X </ Sub> Of Mno <sub>, Of Ba <sub> 0.3 </ Sub> 3 </ Sub> And Of La <sub> 0.67 </ Sub> Ca <sub> 0.33 </ Sub> Mn <sub> 1-

In recent years, rare-earth manganite perovskites R_1-xA_xMnO₃(where R = La, Pr.....; A =Ca, Sr, Ba, Pb...) have attracted considerable attention due to their rich physical properties such as magnetocaloric effect and colossal magnetoresistance effect. Since magnetic refrigeration, especially the magnetic refrigeration near room temperature has significant importance to save energy sources and protect environment people relied on living, the large magnetocaloric effect in perovskite provides a new area for the magnetic refrigeration material near room temperature. The application of colossal magnetoresistance is mainly in magnetic recording and sensors. The perovskite-type manganese oxides have attracted more and more attention due to their unique advantages such as high-density, large memory capability, safety and stabilization etc.In this work, two series of samples have been prepared using Sol-Gel technique: 1. Polycrystalline La_0.7-xNd_xBa_0.3MnO₃ (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25 ) samples, which doped a little amounts of Nd in La-site. 2. Polycrystalline La_0.67Ca_0.33Mn_1-xO₃ (x=0.00, 0.02, 0.04 and 0.06) samples with different Mn-site vacancies. The magnetic properties, especially the magnetocaloric effect of these two series have been investigated carefully. X-ray diffraction (XRD) has been used to identify the single phase of perovskite-type oxides and calculate the lattice parameters as well as the cell volume of the samples. The scanning electron microscopy (SEM) has been used to observe the morphology of the samples. The vibrating sample magnetometer (VSM) has been applied to measure the magnetic properties and the magnetocaloric effect of the samples. The main experimental results are as follows: 1. Polycrystalline samples of La_0.7-xNd_xBa_0.3MnO₃.(1) The X-ray diffraction patterns indicate that the series of polycrystalline samples of La_0.7-xNd_xBa_0.3MnO₃ prepared in this work have the single phase of perovskite structure without any other phases.(2) SEM observation shows that the samples sintered at 1273 K are constituted of homogeneous spherical particles, ranging in size from 250 to 400 run.(3) The lattice parameters have been reduced in La_0.7-xNd_xBa_0.3MnO₃ by doping of Nd at La-site.(4) By doping Nd at La site, the Curie temperatures of La0.7-xNdxBa0.3MnO3 have been adjusted from 333 K to 222 K, which covered the whole room temperature range. The magnetic entropy changes of these samples have been enhanced when the doping level x is less then 0.25. A large magnetic entropy change of ΔSM².22 Jkg-K-1 at the Curie temperature 269K under a field of IT has been obtained in La0.7-xNdxBa0.3MnO3(x=0.15). The main reason for the enhancement of the entropy chang in this series is that the Nd moments are primary ordered by a molecular-field-like interaction rather than by usual Nd-Nd exchange interaction. In addition to the Mn spin system, the Nd spin system and the interaction between the two spin systems lead to an extra magnetic entropy, which results inthe increase of the entropy change of the system.(5) The studies of the magnetoresistance show that the magnetoresistance remains approximately a constant when the temperature is lower then 285K. The maximum MR of 21% was obtained for La0.55Nd0.15Ba0.3MnO3 sample under an applied filed of 1.8T.2. Polycrystalline samples of La0.67Ca0.33Mn1-xO3(1) XRD patterns show that La0.67-xCa0.33MnO3 series are orthorhombic perovskite without any other phases.(2) SEM observation shows that the samples sintered at 1273 K are constituted of homogeneous spherical particles, ranging in size from 200 to 450 nm.(3) The experimental results show that the vacancies doping at Mn-site have the great influence on the magnetic properties of the samples.(I) Curie temperature:The Curie temperature 7c decreases with the increase of vacancies doping level at Mn-site. The Curie temperature of La0.67Ca0.33MnO3 (sintered at 1273 K) is 266 K, while the Tc for La0.67Ca0.33Mn1-xO3 (x=0.06) drops to 240K. This phenomenon may be related to the change of Mn-0 bond length and Mn-O-Mn bond angle due to the distortion of the lattice. As a result, the ferromagnetic coupling is weakened and Tc drops.(II) The magnetocaloric effect:The entropy change increases first in La0.67Ca0.33Mn1-xO3 sintered at 1273 K and then decreases with the further increase of vacancies doping level. The magnetic entropy change ΔSM around the Curie temperature for La0.67Ca0.33MnO3 is 2.23 Jkg-1K-1 under the magnetic...