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Magnetic and transport measurements have been performed on La 0 . 7 Sr 0 . 3 Mn 1 - x M x O 3 (M = Cr, Ti) in order to study the influence of the substitution on the Mn-site on the magnetotransport properties in the La 0 . 7 Sr 0 . 3 MnO 3 perovskite-like compound. Polycrystalline samples were prepared by a conventional solid-state reaction method in air. The two series of samples show a decrease of the magnetic characteristics (magnetic transition temperature and saturation magnetization) when the level of chromium or titanium increases. The substitution of manganese by other metal (Cr, Ti) also induces strong changes in the transport properties: the metallicity of L 0 . 7 Sr 0 . 3 MnO 3 observed below the room temperature is destroyed and the resistivity increases many orders of magnitude when the ratio of doping metal increases. For low substituting ratios, an electrical transition is observed; the temperature corresponding to the semiconductor-metal transition shifts to a lower values with increasing x, similarly to the T C behaviour. For substituting ratio higher than 10% of Ti and 20% of Cr, the electrical transition disappears and the samples exhibit a semiconductor behaviour in the whole range of temperature. The magnetoresistive (MR) properties have been investigated between 0 and 0.5 T for La 0 . 7 Sr 0 . 3 MnO 3 , La 0 . 7 Sr 0 . 3 Mn 1 - x Cr x O 3 (x = 0.05, 0.1, and 0.2), and La 0 . 7 Sr 0 . 3 Mn 1 - x Ti x O 3 (x = 0.1, 0.2, and 0.3) samples. |
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