Magnetic and magnetoresistance properties in rhombohedral La1-xK+xMnO3 perovskite-type compounds

The effects of monovalent alkali metal substitution into the lanthanide sites in La1-xKxMnO3 oxides are presented. We report structural features, magnetic and electrical properties of these materials for 0.05 less than or equal to x less than or equal to 0.2. The lattice symmetry of the samples is found to be rhombohedral. All the phases are ferromagnetic, with Curie temperatures strongly dependent on x, reaching a maximum around 308 K for La0.8K0.2MnO3. A sharp drop of the resistivity is observed just below the Curie temperature. The ferromagnetic-metallic to paramagnetic-semiconducting transition is explained from the suggested distribution of the cations (La1-x3+Kx+)(A)(Mn1-2x3+Mn2x4+)(B)O-3 by the double exchange of Mn3+-Mn4+ pairs at the B-sublattice. Preliminary magnetoresistance measurements (MR) on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites.