Chemical size e ff ect on the magnetic and electrical properties of colossal magnetoresistance

Chemical size effects on the magnetic and electrical properties of the colossal magnetoresistance La1.2(Sr1.8 2 xCax)Mn2O7 materials have been investigated. Based on the chemical shifts of the first unoccupied excited states in the core-level X-ray absorption spectrum correlated to the charge, the valence of Mn has been determined to be 3.43 ± 0.05 for all x in La1.2(Sr1.8 2 xCax)Mn2O7. The magnetoresistance ratio [ρ(0)/ρ(H)] can be efficiently increased from ≈192% (135 K, 1.5 T) for x = 0 to 208% (102 K, 1.5 T) for x = 0.4. On further increase in the concentration of Ca (x = 0.6 and 0.8) the samples only show a spin-glass behavior.