Magnetic localization and magnetoresistance in mixed-valence manganites and related ferromagnetic oxides

The electronic structure and properties of ferromagnetic mixed-valence manganites with the perovskite structure are briefly reviewed. The importance of disorder induced localization is emphasized, and it is proposed that magnetic disorder is effective in this sense on three different length scales. First, spin disorder on the interatomic scale (ca. 1 nm) contributes to localization in the paramagnetic state above the Curie point. Second, fluctuations in the ferromagnetic order and electron density (ca. 10 nm) contribute to localization in the 'bad metal' state found in most manganites below the Curie point. Third, in granular systems (ca. 100 nm), electrons are inhibited from tunnelling between grains by misalignment of their ferromagnetic axes. The latter effects are studied in pressed powder compacts of (La0.7Ca0.3)MnO3 and the other half-metallic systems Fe3O4 and CrO2. Here the resistance is entirely due to the interparticle contacts. The new phenomenon of powder magnetoresistance is presented, where magnetoresistance ratios of 50% are observed in pressed powder.