Isotope effect studies of the paramagnetic to ferromagnetic conducting transition of the CMR compounds La1-xCaxMnO3

The oxygen isotope effect of the ferromagnetic transition in La1-xCaxMnO3 w as investigated from x = 20 to 43%. This is the range of the conducting fer romagnetic phase. We find that the isotope exponent alpha(0) = -Delta lnT(c )/Delta lnm decreases from 0.34 to 0.11 with increasing Ca concentration. A large value of alpha(0) = 0.84 was found for x = 20%, this is possibly con nected with excess oxygen content. The isotope effect at 35% Ca is independ ent of magnetic field. Raman active lines at 225 and 330 cm(-1) show consid erable hardening below T-c. The isotope effect decreases with increasing to lerance factor, pointing to the importance of double exchange. (C) 1998 Els evier Science Ltd. All rights reserved.