MAGNETIC AND MAGNETORESISTANCE PROPERTIES IN RHOMBOHEDRAL (LA1-XK-MN-X PEROVSKITE-TYPE COMPOUNDS(MNO3))

The effects of monovalent alkali metal substitution into the lanthanid e sites in La1-xKxMnO3 oxides are presented. We report structural feat ures, magnetic and electrical properties of these materials for 0.05 l ess 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 ferrom agnetic, with Curie temperatures strongly dependent on x, reaching a m aximum around 308 K for La0.8K0.2MnO3. A sharp drop of the resistivity is observed just below the Curie temperature. The ferromagnetic-metal lic to paramagnetic-semiconducting transition is explained from the su ggested 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. Pre liminary magnetoresistance measurements (MR) on bulk ceramic samples d isplay an MR effect of the same amplitude as in the case of the alkali ne earth substituted La manganites.