EFFECT OF ALLOYING WITH CA IN LAMNO3 SYSTEM STUDIED BY KKR-CPA METHODAND GIANT MAGNETORESISTANCE

La1-xCaxMnO3 perovskites show semiconducting properties in the paramag netic range. The gap is reduced near x = 0.33, where ferromagnetic ord ering is observed. The ferromagnetic ordering then induces a semicondu ctor-metal transition, and gives rise to a giant magnetoresistance eff ect. The ground state electronic structure calculations were done with KKR-CPA method for hypothetical cubic anti ferromagnetic LaMnO3 and C aMnO3 compounds, as well as for disordered La1-xCaxMnO3 (x = 0.33, 0.4 , 0.5) alloys with real crystal data. As a result, we get a magnetic m oment per formula 4.00 mu(B) and 3.00 mu(B) and half-metallic behaviou r for end-compounds, respectively. In the ferromagnetic region a linea r decrease in the magnetic moment of La1-xCaxMnO3 is observed, togethe r with the decrease in the gap width for spin-down carriers, if doping Ca in La1-xCaxMnO3. A simple model is developed, which describes magn etic and transport properties as resulting from an exchange-induced ba nd-crossing semiconductor-metal transition, as for instance in EuO.