Abstract: We have synthesized the vacancy manganites La0.77Mg0.23-x?xMnO3 powders, with x?=?0, 0.05, 0.1, 0.15 and 0.2 by the solid state reaction method. X-ray diffraction analysis using Rietveld refinement has proven that all the compounds under investigation crystallize in the orthorhombic structure with Pnma space group. The result of Rietveld refinements shows that the magnesium vacancies modify the structural parameters such as the MnO bond length, the MnOMn angles and the volume. Zero-field-cooled and Field-cooled thermomagnetic curves and the thermal variation of AC-susceptibility show a paramagnetic-ferromagnetic transition for both compounds and exhibit the onset of a spin glass-like state at lower temperatures. The increase of the magnesium-vacancy content lowered the Curie magnetic transition temperature (TC) from TC?=?140.82?K for x?=?0 to TC?=?128.22?K for x?=?0.2. The irreversibility temperature observed from the ZFC and FC magnetization splitting is also modified from 131.65?K for x?=?0 ?118.78?K for x?=?0.20. The irreversibility is associated to the presence of ferromagnetic clusters and spin-glass phase like behavior.

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