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 was investigated from x = 20 to 43%. This is the range of the conducting ferromagnetic 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 connected with excess oxygen content. The isotope effect at 35% Ca is independent of magnetic field. Raman active lines at 225 and 330 cm(-1) show considerable hardening below T-c. The isotope effect decreases with increasing tolerance factor, pointing to the importance of double exchange. (C) 1998 Elsevier Science Ltd. All rights reserved.