OXYGEN-ISOTOPE EFFECT OF THE PARAMAGNETIC-INSULATING TO FERROMAGNETIC-METALLIC TRANSITION IN LA1-XCAXMNO3

The oxygen-isotope effect of the ferromagnetic transition in La1-xCaxM nO3 was investigated from x = 20% to x = 43%. This is the range of the conducting ferromagnetic phase. We find that alpha(o) = - Delta In T- c/Delta In m decreases from 0.36 to 0.14 with increasing Ca concentrat ion. A large value of alpha(o) = 0.83 was found for x=20%, it is possi bly connected with excess oxygen content. The isotope effect decreases with increasing tolerance factor, pointing to the importance of doubl e exchange. The isotope effect at 35% Ca is; independent of magnetic f ield.