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Structural,Magnetic and Magnetocaloric Properties of La-deficient La0.77-xSrxCa0.2MnO2 Perovskites 下载免费PDF全文
La-deficient La0.77-x Ca0.2SrxMnO3 (0 〈 x 〈 0.1) polycrystalline samples are synthesized using the sol-gel technique. The crystal structures of all the samples are single orthorhombic phase with Pbnm space group. Rietveld analysis of x-ray diffraction patterns shows that the Mn-O-Mn bond angle θUn-O-Mn increases whereas the Mn-O bond length dMn--o decreases monotonically with increasing Sr^2+ content, which results in a rich overlap between Mn 3d and O2p orbitals and leads to a systematic increase of the Curie temperature in this compound. It is found that the magnetic entropy change has a maximum at x = 0.06 in La^3+-deficient La0.77-xCa0.2SrxMnO3. This may result from competition between the super-exchange interactions (Mn 4+-O2--Mn4+) and double-exchange interactions (Mn^3+-O^2- -Mn^4+) originating from the appearance of superfluous Mn^4+ ions by substitution of Sr^2+ for La^3+ in this series. Large magnetic entropy changes of 2.32 and 1.83Jkg^-1K^-1 in the x = 0.06 and x = 0.1 samples at their Tcs of 271 K and 303K upon a low magnetic field (10kOe) make these materials promising candidates at near room temperature. 相似文献
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应用物理公式解题,实际上就是把物理问题转化为数学问题,然后运用数学知识和有关的数学技巧来解答物理问题,可以使问题的解答变得更为简单.下面介绍数学方法中的图象法和极值法在物理解题中的应用. 相似文献
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