Abstract
The investigation of the LaMnO system has allowed manganites La 0.90MnO 3−δ with colossal magnetoresistance properties, near room temperature, to be isolated. These phases exhibit indeed a ferromagnetic metallic to paramagnetic insulating transition, with T C ranging from 230 K to 260 K, depending on the hole carrier density ( δ = 0.05−0.06). A resistance ratio of 6 in a magnetic field of 7 T is observed near room temperature (260 K) for the first time in the LaMnO system. The CMR La 0.9MnO 3−δ phases, can be indexed in a classical rhombohedral cell, but their electron diffraction and X-ray powder study shows that their symmetry is monoclinic I2 a with a = 7.790(1) A ̊ , b = 5.526(1) A ̊ , c = 5.479(1) A ̊ , β = 90.78°. Decreasing slightly the oxygen content leads to orthorhombic phases and kills the CMR properties.
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