Chemical size effect on the magnetic and electrical properties of colossalmagnetoresistance La-1.2(Sr1.8-xCax)Mn2O7 materials

Chemical size effects on the magnetic and electrical properties of the colo ssal magnetoresistance La-1.2(Sr1.8-xCax)Mn2O7 materials have been investig ated. Based on the chemical shifts of the first unoccupied excited states i n the core-level X-ray absorption spectrum correlated to the charge, the va lence of Mn has been determined to be 3.43 +/- 0.05 for all x in La-1.2(Sr1 .8-xCax)Mn2O7. The magnetoresistance ratio [rho(0)/rho(H)] can be efficient ly increased from approximate to 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.